Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and...

159
Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping Effects By Waqar Mahmood CIIT/FA08-PPH-004/ISB PhD Thesis In Physics COMSATS Institute of Information Technology Islamabad-Pakistan Spring, 2014

Transcript of Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and...

Page 1: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization of II-VI

Semiconductor Thin Films and the Study of Post

Doping Effects

By

Waqar Mahmood

CIIT/FA08-PPH-004/ISB

PhD Thesis

In

Physics

COMSATS Institute of Information Technology

Islamabad-Pakistan

Spring, 2014

Page 2: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

ii

COMSATS Institute of Information Technology

Fabrication and Characterization of II-VI

Semiconductor Thin Films and the Study of Post

Doping Effects

A Thesis Presented to

COMSATS Institute of Information Technology, Islamabad

In partial fulfillment

of the requirement for the degree of

PhD (Physics)

By

Waqar Mahmood

CIIT/FA08-PPH-004/ISB

Spring, 2014

Page 3: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

iii

Fabrication and Characterization of II-VI

Semiconductor Thin Films and the Study of Post

Doping Effects

A Post Graduate Thesis submitted to the Department of Physics as partial

fulfillment of the requirement for the award of Degree of PhD (Physics).

Supervisor:

Dr. Nazar Abbas Shah

Associate Professor

Department of Physics

COMSATS Institute of Information Technology (CIIT)

Islamabad

Co-Supervisor:

Dr. Ashraf Atta

Advisor

Department of Physics

COMSATS Institute of Information Technology (CIIT)

Islamabad

Name Registration No.

Waqar Mahmood CIIT/FA08-PPH-004/ISB

Page 4: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

iv

Final Approval

This thesis titled

Fabrication and Characterization of II-VI

Semiconductor Thin Films and the Study of Post

Doping Effects

By

Waqar Mahmood

CIIT/FA08-PPH-004/ISB

Has been approved

For the COMSATS Institute of Information Technology, Islamabad

External Examiner

Dr. Zulfiqar Ali

Deputy Chief Scientist

Optics Laboratories, PAEC Islamabad

External Examiner

Dr. Nadeem Younas

Bahaud-din-Zakariya University

Multan

Supervisor

Dr. Nazar Abbas Shah

Associate Professor,

Department of Physics, Islamabad

HoD

Dr. Sadia Manzoor

Department of Physics, Islamabad

Dean, Faculty of Sciences

Prof. Dr. Arshad Saleem Bhatti

Page 5: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

v

Declaration

I Waqar Mahmood, CIIT/FA08-PPH-004/ISB, hereby declare that I have produced the

work presented in this thesis, during the scheduled period of study. I also declare that I

have not taken any material from any source except referred to wherever due that

amount of plagiarism is within acceptable range. If a violation of HEC rules on research

has occurred in this thesis, I shall be liable to punishable action under the plagiarism rules

of the HEC.

Date: ____________________

Waqar Mahmood

CIIT/FA08-PPH-004/ISB

Page 6: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

vi

Certificate

It is certified that Mr. Waqar Mahmood, Registration number CIIT/FA08-PPH-004/ISB

has carried out all the work related to this thesis under my supervision at the Department

of Physics, COMSATS Institute of Information Technology, Islamabad and the work

fulfills the requirement for award of PhD degree.

Date: _________________

Supervisor:

_________________________

Dr. Nazar Abbas Shah

Associate Professor

Submitted through:

Head of the Department:

_____________________________

Dr. Sadia Manzoor

Head, Department of Physics

COMSATS institute of Information

Technology, Islamabad

Page 7: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

vii

Dedicated to

My beloved son

Mohammad Ans Bin Waqar

&

Sweetest daughter

Kinza Waqar

Page 8: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

viii

Acknowledgements In the name of Allah, the most Gracious, the ever Merciful

All praise belongs to Allah Almighty, „He who taught man which he did not know‟ and

countless prayers (Daru‟d) upon LAST Prophet Muhammad (P.B.U.H). First and

foremost I would like to thank Allah almighty for blessing me with a loving, supportive

and patient family. In particular I would like to thank my parents. My father helped me to

develop logical and critical thinking and he is my first teacher. My mother taught me

discipline, good manners and respect for humanity. Today I am a balanced individual

because of their efforts. I would like to thank my brother and two sisters for supporting

me throughout my studies. I would like to acknowledge the sacrifices my wife and son

had to make, by making sure I wasn‟t disturbed while studying. Without their support and

patience, I would not be completing my PhD. Finally, this moment reminds me of my late

grandparents, who always said I would become a person of distinction- may Allah grant

them highest ranks in paradise.

Words cannot do justice to the appreciation I have for my supervisor Dr. Nazar Abbas

Shah and co-supervisor Prof. Dr. Ashraf Atta for their constant guidance and support

with the research work.

I would like to thank the supervisory committee members, Dr. Waqas Khalid, Dr.

Ghazanfar Abbas for their positive influence and deep interest in my research work also

Dr. Umair Hassan for his technical support during thesis write up.

From the department of physics, COMSATS, I would like to acknowledge the efforts of

Prof. Dr. Arshad Saleem Bhatti (Dean of Science) and Prof. Dr. Sajid Qamar (Chairman

Dept. of Physics) for their persistent efforts in improving the reputation of the department

at the international level. I would like to thank the former HoDs Prof. Dr. Mehnaz Q.

Haseeb and Prof. Dr. Ishaq Ahmed for guidance in official matters. I highly acknowledge

COMSATS for providing me scholarship for PhD studies.

The Higher Education Commission, Pakistan is highly acknowledged for its financial

support through project # 20 – 1187 / R&D / 09, which has granted funding of my PhD

research work. The International Research Support Initiative Program (IRSIP) is thanked

for funding my visit to Photon Science Institute (PSI), the University of Manchester UK-

Page 9: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

ix

a valuable experience and Dr. Andrew Thomas for his support at PSI. Pakistan Science

Foundation (PSF), Pakistan and Volkswagen Foundation, Germany is also acknowledged

for providing travel grants during this project.

Special thanks to my fellow PhD student, Mr. Ali Zaman for their assistance, support and

friendship. I am indebted to Dr. Anis-ur-Rehman for facilitating my access to use XRD

and giving me lifts home when I left CIIT late at night. I am thankful to the staff of

Department of Physics, especially Mr. Tanveer Baig, Mr. Khalil, Mr. Khizar and Mumtaz

for facilitating me in different matters.

I am thankful to the Department of Higher Education; Govt. of the Punjab, for providing

me an opportunity to complete my PhD. I would like to thank Prof. Tariq Habib Khan

(Principal Govt. Postgraduate College Murree, RWP) for his patronage. Without his

support it would not have been possible for me to complete the research work. I am also

thankful to my colleagues Mr. Yasir Rafiq, Mr. Sajid Rafiq, Mr. Muttaher Bashir and Dr.

Gulfraz Abbasi for their moral support.

The Acknowledgements remain incomplete without mentioning some of my close family

members-my aunty (in fact my Nani), Zaheer bhai, Asif bhai, baji, bhabi, Mano, Chand,

Mesa, Falaq, Ibrahim, Toheed, Umar, Maryam (B-Api) Musa and youngest and

naughtiest of all-Haroon. Their love, affection and support, was of immeasurable value to

me. My brother in law and childhood best friend Shaheer A. Aziz is highly acknowledged

for his technical support during this research project and helping me to settle in the UK. I

will never forget the hospitality he offered while I was in Luton, UK and for being my

personal chauffer.

Finally, I would like to remember and pray for my cousin M. Zareef who was suffering

with blood cancer and died last year. He always welcomed me with a smiling face and

prayed for my future. He is no longer amongst us but his memories will always be with

me.

I am grateful for the opportunity to do a PhD, it has truly been a fulfilling and rewarding

journey.

Waqar Mahmood

CIIT/FA08-PPH-004/ISB

Page 10: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

x

ABSTRACT

Fabrication and Characterization of II-VI Semiconductor Thin

Films and the Study of Post Doping Effects

II-VI semiconductors have great importance in solar cell applications due to their

excellent optical and electrical properties. This thesis is mainly concerned with the study

of II-VI semiconductor thin films with a particular interest in their potential application in

solar cells. A coating system based on close spaced sublimation (CSS) has been

developed and thin films of zinc telluride (ZnTe), Zn and Te enriched ZnTe, cadmium

sulfide (CdS) and cadmium zinc sulfide (CdZnS) were fabricated.

CdS is transparent to electromagnetic radiations; in particular to the visible and infra-red

regions and are highly resistive materials. This research work pertained to improve CdS

thin films as window materials using close spaced sublimation technique. The

optimization of deposition parameters including vacuum in the chamber, distance

between source-substrate, source and substrate temperatures are all investigated.

ZnTe has been used as a buffer layer between CdTe and the metal back contact in II-VI

semiconductor solar cells due to its compatibility with p-type cadmium telluride (CdTe).

The purpose of the buffer layer is to help CdTe form a good Ohmic contact with the

metal back contact, however, ZnTe itself is highly resistive. The main goal is to reduce

electrical resistivity of ZnTe for its use as buffer layer in the back contact. The resistivity

of the ZnTe thin film is modified by doping with silver (Ag) and/or copper (Cu). The

compositions of zinc (Zn) and tellurium (Te) in the enriched (Zn or Te) ZnTe thin film is

also explored to lower the resistivity of ZnTe film, which has not been reported earlier

using CSS technique. In all these processes, the structural, surface, electrical and optical

properties are studied for strong correlation. Ion exchange process is adopted for Ag and

Cu doping in as-deposited ZnTe thin films with subsequent annealing.

CdS is a potential candidate for window layer due to its suitable and tunable energy band

gap (2.42 eV). Effects of doping are investigated on the structural, electrical and optical

Page 11: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

xi

properties of CdS thin films fabricated by the CSS technique. These properties of

fabricated CdS thin films are found to be suitable for solar cell applications. To enhance

band gap, CdS and Zn powder are mixed mechanically with different weight percentages

to deposit thin films CZS fabricated by CSS technique that has not been documented

earlier. The increased energy band gap for CZS is 2.57 eV, which has improved the

window region.

Page 12: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

xii

List of Abbreviations

ΔGv Volume free energy

σ Surface free energy

CSS Close spaced sublimation

ZnTe Zinc telluride

CdS Cadmium sulfide

Ag Silver

Cu Copper

CdZnS Cadmium zinc sulfide

CdTe Cadmium telluride

ZnS Zinc sulfide

XRD X-ray diffraction

β Full width at half maximum

ϴ Bragg‟s angle

D Crystallite size

λ Wavelength

a Lattice parameter

hkl Miller indices

ε Strain

SEM Scanning electron microscope

EDX Energy dispersive X-rays spectroscopy

UV Ultra violet

Vis Visible

NIR Near infrared

IPA Iso-propanol alcohol

Page 13: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

xiii

List of publications

The thesis is based on the following publications

1. Effects of metal doping on the physical properties of ZnTe thin films

Waqar Mahmood, Nazar Abbas Shah

Current Applied Physics, 14 (2014) 282-286.

2. CdZnS thin films sublimated by close spaced using mechanical mixing: A

new approach

Waqar Mahmood, Nazar Abbas Shah

Optical Materials, 36 (2014) 1449-1453.

3. Investigation of substrate temperature effects on physical properties of ZnTe

thin films by close spaced sublimation technique

Waqar Mahmood, Nazar Abbas Shah, Sohaib Akram, Usman Mehboob, Umair

Sohail Malik, Muhammad Umer Sharaf

Chalcogenide Letters, 10 (2013) 273 – 281.

4. Physical properties of sublimated zinc telluride thin films for solar cell

applications

Nazar Abbas Shah, Waqar Mahmood

Thin Solid Films, 544 (2013) 307-312.

5. Study of cadmium sulphide thin films as a window layers

Waqar Mahmood, Nazar Abbas Shah

AIP Conf. Proc., 1476 (2012) 178-182.

Page 14: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

xiv

Other Publications

1. Physical properties of silver doped ZnSe thin films for photovoltaic

applications

Nazar Abbas Shah, Musarat Abbas, Waqar A. Adil Syed, Waqar Mahmood

Iranica Journal of Energy & Environment, 5 (2014) 87 - 93.

2. Cu-doping effects on the physical properties of cadmium sulfide thin films

N. A. Shah, R. R. Sagar, W. Mahmood, W. A. A. Syed.

J. of Alloys and Compd, 512 (2012) 185 - 189.

3. Physical properties and characterization of Ag-doped CdS thin films.

N. A. Shah, A. Nazir, W. Mahmood, W. A. A. Syed. S. Butt, Z. Ali, A. Maqsood

J. of Alloys and Compd, 512 (2012) 27 – 32.

4. Nanostructure cadmium sulphide thin films as window material in optical

devices by close spaced sublimation technique

Nazar A. Shah, W. Mahmood, W. A. Syed, M. Abbas and I. Haq

Proceedings of the 4th International Conference on Nanostructures (ICNS4)

12-14 March, 2012, Kish Island, I.R. Iran

Page 15: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

i

Table of Contents

Table of Contents ........................................................................................... i

List of Figures .............................................................................................. vi

List of Tables .................................................................................................. x

Chapter 1 Introduction ................................................................................. 1

1.1 Introduction .......................................................................................................... 2

1.2 CdTe/CdS based solar cells .................................................................................. 4

1.2.1 Cell‟s Architecture ........................................................................................ 4

1.2.2 Working principle ......................................................................................... 6

1.3 Aims of research work ......................................................................................... 7

1.4 Preface .................................................................................................................. 8

References: .................................................................................................................... 10

Chapter 2 Literature Review .....................................................................12

2.1 Role of thin film ................................................................................................. 13

2.2 Fabrication of thin films and mechanisms ......................................................... 16

2.3 Deposition techniques ........................................................................................ 17

2.3.1 Physical vapor deposition ........................................................................... 18

Page 16: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

ii

2.4 Close spaced sublimation (CSS) technique ........................................................ 20

2.5 II-VI Semiconductors ......................................................................................... 22

2.5.1 ZnTe thin films ........................................................................................... 23

2.5.2 CdS thin films ............................................................................................. 23

References: .................................................................................................................... 26

Chapter 3 Fabrication and Characterization Techniques ......................32

3.1 Design and assembly of CSS developed at COMSATS, Islamabad .................. 33

3.2 X-rays diffraction ............................................................................................... 36

3.2.1 Bragg‟s law ................................................................................................. 36

3.3 Surface morphological study .............................................................................. 38

3.3.1 Energy dispersive X-rays spectroscopy ...................................................... 39

3.4 Optical measurements ........................................................................................ 40

3.5 Electrical measurements ..................................................................................... 44

3.5.1 Hall effect.................................................................................................... 44

References: .................................................................................................................... 48

Chapter 4 Fabrication of ZnTe Thin Films ..............................................51

4.1 Samples preparation ........................................................................................... 52

4.2 Results and discussion ........................................................................................ 52

4.2.1 Structural study ........................................................................................... 52

Page 17: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

iii

4.2.2 Optical study ............................................................................................... 55

4.2.3 Electrical study............................................................................................ 60

4.3 Summary ............................................................................................................ 61

References: .................................................................................................................... 63

Chapter 5 Effect of Ag Doping on ZnTe Thin Films ...............................65

5.1 Silver doping procedure ..................................................................................... 67

5.2 Results and discussion ........................................................................................ 67

5.2.1 Structural study ........................................................................................... 67

5.2.2 Surface morphological study ...................................................................... 69

5.2.3 Electrical study............................................................................................ 71

5.2.4 Optical study ............................................................................................... 72

5.3 Summary ............................................................................................................ 75

References: .................................................................................................................... 76

Chapter 6 Cu-Doping Effects on ZnTe Thin Films .................................78

6.1 Copper doping procedure ................................................................................... 80

6.2 Results and discussion ........................................................................................ 80

6.2.1 Structural study ........................................................................................... 80

6.2.2 Surface morphological study ...................................................................... 82

6.2.3 Optical study ............................................................................................... 84

Page 18: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

iv

6.2.4 Electrical study............................................................................................ 86

6.3 Summary ............................................................................................................ 89

References: .................................................................................................................... 90

Chapter 7 Zn and Te Enrichment in ZnTe Thin Films ..........................92

7.1 Samples preparation ........................................................................................... 94

7.2 Results and discussion ........................................................................................ 96

7.2.1 Structural study ........................................................................................... 96

7.2.2 Surface morphological study ...................................................................... 99

7.2.3 Optical study ............................................................................................. 101

7.2.4 Electrical study.......................................................................................... 103

7.3 Summary .......................................................................................................... 105

References: .................................................................................................................. 106

Chapter 8 Fabrication of CdS Thin Films ..............................................108

8.1 Samples preparation ......................................................................................... 109

8.2 Results and discussion ...................................................................................... 110

8.2.1 Structural study ......................................................................................... 110

8.2.2 Optical study ............................................................................................. 111

8.2.3 Electrical study.......................................................................................... 114

8.3 Summary .......................................................................................................... 115

Page 19: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

v

References: .................................................................................................................. 116

Chapter 9 Fabrication of CdZnS Thin Films .........................................118

9.1 Samples preparation ......................................................................................... 120

9.2 Results and discussion ...................................................................................... 121

9.2.1 Structural study ......................................................................................... 121

9.2.2 Surface morphological study .................................................................... 123

9.2.3 Optical study ............................................................................................. 125

9.3 Summary .......................................................................................................... 128

References: .................................................................................................................. 129

Summary and future recommendations ..................................................131

Recommendations for future work ............................................................................. 133

Page 20: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

vi

List of Figures

Figure ‎1.1 Spectral irradiation as a function of wavelength from sun to earth

atmosphere [4] ............................................................................................... 3

Figure ‎1.2 Schematic and working of solar cell .............................................................. 5

Figure ‎1.3 Working principle of pn junction ................................................................... 7

Figure ‎2.1 Solid-liquid system and free energy [8] ....................................................... 15

Figure ‎2.2 Growth mechanism of thin films ................................................................. 17

Figure ‎2.3 Schematic of sputtering................................................................................ 18

Figure ‎3.1 Schematic of CSS technique at CIIT, Islamabad ......................................... 35

Figure ‎3.2 Diffraction pattern of X-rays from the lattice planes of crystal ................... 37

Figure ‎3.3 Use of diffraction curve to determine the particle size ................................ 38

Figure ‎3.4 Schematic of SEM ....................................................................................... 39

Figure ‎3.5 Transmission spectrum as a function of wavelength to measure the thickness

and refractive index...................................................................................... 42

Figure ‎3.6 Schematic of spectrophotometer for transmittance ...................................... 43

Figure ‎3.7 Schematic of Hall effect............................................................................... 44

Figure ‎3.8. Contact formation on thin film .................................................................... 45

Figure ‎4.1 XRD patterns of ZnTe films deposited at different substrate temperatures 53

Figure ‎4.2 Transmission spectra of ZnTe films deposited at different substrate

temperatures as a function of wavelength .................................................... 55

Page 21: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

vii

Figure ‎4.3 Transmission spectra of ZnTe films as a function of wavelength with angle

variation (a) 300ºC and (b) 330ºC ................................................................ 57

Figure ‎4.4 Optical band gap of ZnTe films deposited at (a) 300ºC, (b) 330ºC and (c)

360ºC ............................................................................................................ 58

Figure ‎4.5 (a) Optical density and (b) optical conductivity of ZnTe films deposited at

three substrate temperatures (300, 330 & 360ºC) ........................................ 60

Figure ‎5.1 X-ray diffraction patterns of (a) as-deposited (b) Ag-doped ZnTe sample . 68

Figure ‎5.2 SEM images of Ag-doped ZnTe samples .................................................... 70

Figure ‎5.3 (a) No. of grains of ZnTe and Ag vs. immersion time (b) atomic % of Ag vs.

immersion time ............................................................................................ 70

Figure ‎5.4 (a) Atomic % of Ag vs. resistivity, (b) atomic % of Ag vs. carriers

concentration ................................................................................................ 72

Figure ‎5.5 Transmission spectra as a function of wavelength of as-deposited and Ag-

doped ZnTe samples .................................................................................... 73

Figure ‎5.6 Transmission spectra as a function of wavelength of as-deposited, immersed

before annealing and after annealing ZnTe samples .................................... 73

Figure ‎5.7 (a) Energy band gap of as-deposited ZnTe thin film and (b) 30 min Ag-

doped ZnTe sample ...................................................................................... 74

Figure ‎5.8 Transmission spectra as a function of wavelength with variable angles of as-

deposited ZnTe sample ................................................................................ 75

Figure ‎6.1 X-rays diffraction patterns of (a) as-deposited ZnTe thin film (b) Cu-doped

ZnTe sample................................................................................................. 81

Figure ‎6.2 Variation in crystallite size as a function of Cu immersion time ................. 81

Page 22: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

viii

Figure ‎6.3 SEM images of (a) as-deposited ZnTe thin film, (b) 30 min Cu-doped, (c) 35

min Cu-doped and (d) 40 min Cu-doped ZnTe samples.............................. 83

Figure ‎6.4 Transmission spectra as a function of wavelength of as-deposited ZnTe thin

films and Cu-doped ZnTe samples .............................................................. 84

Figure ‎6.5 Energy band gap of as-deposited ZnTe thin film......................................... 85

Figure ‎6.6 Energy band gap of 40 min Cu-doped ZnTe sample ................................... 86

Figure ‎6.7 Variations in resistivity vs. Cu immersion time........................................... 87

Figure ‎6.8 Variations in mobility vs. Cu immersion time ............................................. 88

Figure ‎6.9 Variations in sheet concentration as a function of Cu immersion time ....... 88

Figure ‎7.1 Schematic of Zn-enriched ZnTe thin films sandwich structure ................... 95

Figure ‎7.2 X-ray diffraction patterns of (a) Zn-enriched ZnTe thin films and (b) Te-

enriched ZnTe thin films .............................................................................. 98

Figure ‎7.3 SEM micrographs of (a) as-deposited ZnTe thin film, (b) 3 min deposition

of ZnTe (c) 5 min deposition of ZnTe thin film, and (d) 7 min deposition of

ZnTe thin film .............................................................................................. 99

Figure ‎7.4 SEM micrographs of (a) as-deposited ZnTe thin films, (b) after annealing at

275 C for 1 hour, (c) 300 C for 1 hour and (d) 300 C for 2 hour ............... 100

Figure ‎7.5 Transmission spectra as a function of wavelength of as-deposited and Zn-

enriched ZnTe thin films ............................................................................ 102

Figure ‎7.6 Transmission spectra as a function of wavelength of as-deposited and Te-

enriched ZnTe thin films ............................................................................ 103

Figure ‎7.7 Variations in resistivity and mobility vs. atomic % of Zn ......................... 104

Page 23: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

ix

Figure ‎8.1 XRD patterns for CdS 01 to CdS 05 thin films ......................................... 110

Figure ‎8.2 Transmission vs wavelengths of as-deposited CdS thin films ................... 112

Figure ‎8.3 Variations in energy band gaps of as-deposited CdS thin films ................ 112

Figure ‎8.4 Variations in angle from zero to 60 degrees of CdS thin films ................. 113

Figure ‎8.5 Direct band gap transition type in as-deposited CdS thin films................. 113

Figure ‎8.6 Thicknesses vs. resistivity of CdS thin films ............................................. 114

Figure ‎9.1 X-ray diffraction patterns of (a) as-deposited CdS and (b) CZS thin films

.................................................................................................................... 122

Figure ‎9.2 (002) peak shift of as-deposited CdS and CZS thin films in the XRD

patterns ....................................................................................................... 122

Figure ‎9.3 SEM images of (a) as-deposited CdS (b) CZS (10 % Zn) (c) CZS (15 % Zn)

(d) CZS (25 % Zn) thin films ..................................................................... 124

Figure ‎9.4 Optical spectra of as deposited CdS and CZS thin films with different

composition of Zn ...................................................................................... 126

Figure ‎9.5 (a) Energy band gap of as-deposited CdS thin film and CZS thin film (25 %

Zn) (b) direct band gap transition type in CZS thin film .......................... 127

Figure ‎9.6 Angle resolved spectra of 25 % Zn CZS thin film ..................................... 127

Page 24: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

x

List of Tables

Table 4.1 Structural parameters of ZnTe thin films deposited at different substrate

temperatures (300, 330 & 360ºC) ............................................................... 55

Table 4.2 Energy band gap and refractive index of the ZnTe thin films deposited at

different substrate temperatures (300, 330 & 360ºC) ................................. 59

Table 4.3 Carrier concentrations and resistivity of ZnTe thin films deposited at

different substrate temperatures ................................................................. 61

Table 6.1 EDX study of as-deposited ZnTe thin film and 10-40 min Cu-doped ZnTe

samples ....................................................................................................... 83

Table 7.1 EDX study of as-deposited ZnTe thin film with Zn and Te enriched ZnTe

thin films ................................................................................................... 101

Table 9.1 Elemental composition of as-deposited CdS thin film ............................. 124

Table 9.2 Elemental composition of CZS (10 % Zn, 15 % Zn and 25 % Zn) .......... 125

Page 25: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 1 Introduction

Page 26: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

2

In this chapter a brief introduction of renewable energy, CdTe/CdS based solar cell, its architecture and

working principle are given. It also discusses the aim of this research work.

1.1 Introduction

World population is increasing day by day and has crossed figure of 6 billion till now.

The main problem is energy, required to sustain life on earth. The electricity requirement

for the whole world was about 18 trillion kWh in 2006 [1, 2]. The energy covered with

combustion of fossil fuels like gas, oil and coal is harmful for environment. The

increasing demand of energy makes humans innovate alternative sources of energy that

will fulfill the requirements and will not be harmful for environment. The renewable

energy sources are the best option according to the requirements for harvesting energy

from sun and solar cell is a potential candidate to meet the challenges. The energy

produced by renewable sources are favorable for environment and don‟t have chemical

hazards which makes environment clean, friendly and healthy. Renewable energy is also

named as clean energy due to these reasons [3-5]. Many energy sources alternate to fossil

fuels can be used, like solar, wind, geothermal and hydro etc. [6]. The development of

these sources will be helpful to wean us off from fossil fuels [7, 8]. Solar energy resource

has the amount of 2730000 x 1018

J per year and only 0.3 x 1018

J per year currently is in

use; a significant amount is being wasted [9].

The sun is the past, present and future of life. Solar energy is the primary source of

energy. Without this source nothing would exist on the earth mainly by providing energy

to plants for photosynthesis [10, 11]. Only a part of sun‟s energy reaches the earth‟s

atmosphere almost 30% is reflected back into space and ~ 20% is absorbed in the clouds

and air. Only ~ 0.01% of solar energy reaching the earth‟s surface is required to fulfill the

world energy needs. The Spectral irradiation as a function of wavelength from sun to

atmosphere is shown in Fig. 1.1, where sun is assumed as a black body having a

temperature of 5800 K. At this temperature of the sun surface, the solar spectrum has a

maximum value in the visible part of the solar spectrum [4].

Page 27: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

3

Among various types of solar cells, the most common type of solar cell is based on

silicon technology and recognized as a first generation solar cell due to its manufacturing

infrastructure on large area. The efficiency of these cells is also limited due to narrow

energy band gap. Silicon single crystal is expensive and due to its high manufacturing

cost, the single crystal silicon is replaced by a variety of polycrystalline solar cells. The

polycrystalline solar cell materials are the promising candidates in photovoltaic energy

conversion. Polycrystalline copper indium gallium disellenide (CIGS), amorphous silicon

and cadmium telluride (CdTe) based solar cells are some of the types of polycrystalline

solar cell. These polycrystalline / amorphous solar cell are cost effective but are

comparatively less efficient to silicon single crystal solar cells [12].

Figure 1.1 Spectral irradiation as a function of wavelength from sun to earth

atmosphere [4]

Page 28: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

4

1.2 CdTe/CdS based solar cells

1.2.1 Cell’s Architecture

CdTe belongs to II-VI semiconductors family and gained attention of researcher due to

high absorption [13]. It is a direct band gap semiconductor with an optimum energy band

gap 1.45 eV [14]. CdTe is used as an absorbent layer in the second generation solar cell.

The role of CdTe is to absorb maximum light coming from the window layer and

generate significant amount of electron hole pairs to obtain high performance of the cell

[15]. Schematic of CdTe based solar cell with working principle is shown in Fig 1.2. The

other parts of the solar cell containing window layer and back contact also have

significant importance in the functionality and enhancement of the efficiency of the cell.

The main role of the window layer is to form p-n junction between the window and the

absorbent layer. It has to pass/transmit maximum light coming from the sun to reach at

junction region. For this purpose, the window layer should have a wide energy gap as

compared to the absorbent layer. The lattice mismatch should be minimum with

absorbent layer material. More importantly, it should not capture photons [16].

The back contact formation is difficult in CdTe based solar cell due to its high work

function and thus formation of metal Ohmic contact with it. This problem is overcome by

introducing an interfacial or buffer layer between CdTe and the metallic back contact,

which helps to form a good Ohmic contact. The back contact assembly should be low

resistive and having p-type conductivity.

Page 29: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

5

Figure 1.2 Schematic and working of solar cell

It is superstrate arrangement, in which soda lime glass on the top having transparent

conducting oxide coating. The transparent conducting oxide coating makes the glass

conducting as it is low resistive (10-40 Ω) and optically transparent (˃ 95 % T). The

second layer is CdS window layer having small thickness to allow maximum light for

transmission through it. The third layer is the CdTe absorbent layer; it absorbs maximum

light and creates electron hole pairs. Finally, the buffer layer ZnTe (part of the back

contact) is deposited.

Window layer material is important part of solar cell as more and more light is required

to create electron hole pairs in absorbent layer. Cadmium sulfide is usually used as a

window layer deposited by chemical bath deposition (CBD) technique in maximum

reported efficiency (16.5 %) [17, 18] CdS has energy band gap 2.42 eV at 300 K and

absorption edge on 516 nm [19]. The solar spectrum peak intensity in visible region start

about 500 nm, it shows more than 20 % light is absorbed in window layer. Many

attempted have been done to improve or replace CdS window layer to any other material

[20-25].

Page 30: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

6

It is desirable to enhance the photon transmission through window layer by tuning the

energy band gap towards lower wavelength and transmit the light which was absorbed.

CdZnS is a direct band gap ternary compound with energy band gap larger than CdS and

behaves like n-type as reported earlier [26].

Back contact layer is another important part of CdTe based solar cells. CdTe has a high

electron affinity value and thus poses problems to form direct Ohmic contact with metal.

This problem is overcome by introducing another p-type semiconductor layer in the

assembly of the back contact. Zinc telluride (ZnTe) is a p-type material and used as an

interfacial/buffer layer between CdTe and the metal back contact in cells formation. Due

to self-compensation effect it remains a p-type material [27]. The resistivity of ZnTe is of

the order 107-10

9 Ω-cm [28-30].

1.2.2 Working Principle

The principle of solar cells based on photoelectric effect and lies in clean energy region

due to no waste products. Solar cells mostly made of semiconductor materials having

large area p-n junction. P-type material absorbs photons from sun light fall on it, the

electrons absorbs these photons and move through the band gap of semiconductor due to

excitation. The electron hole pairs are generated in the result of excitation of electrons.

The electron from n side and hole from p side moves and recombine at the interface. The

recombination results the formation of depletion region and hence the electric field is

established across the junction [12]. Electric field is responsible to separate the charge

carriers and these carriers are collected on the two contacts as shown in Fig. 1.3.

Page 31: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

7

Figure 1.3 Working principle of pn junction

1.3 Aims of research work

Main focus of this research is to attain low resistive back contact buffer layer and to

minimize the window layer absorption losses for CdTe based solar cell. ZnTe is used as a

buffer layer between the CdTe layer and the metal back contact material, while CdS as an

active window layer material. The window layer should have high transparency. The

buffer layer back contact should be low resistive. Many attempts have been made to

achieve good transparency with low resistivity, yet industry demands further

enhancements. Primary focus is to optimize the deposition parameters using CSS

technique to fabricate ZnTe and CdS thin films.

Doping of Ag, Cu and fabrication of Zn and Te enriched ZnTe thin films as interfacial

layer between CdTe and metal back contact with reduced material‟s resistivity is

obtained. This research opens a path way and helps to improve the physical properties of

ZnTe thin films. Correlation between the optical, electrical along with structural and

surface properties is also studied.

The addition of Zn in CdS (CdZnS) thin films using CSS technique enhanced the energy

band gap to make it as a good window layer material.

Page 32: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

8

In order to achieve the set goals, a system based on close spaced sublimation was

designed and installed in Thin Films Technology Research Lab at CIIT, Islamabad.

1.4 Preface

This thesis consists of nine chapters and the conclusions.

Chapter one is a brief introduction to solar cells, working principle and issues related to

efficiency discussed.

Chapter two deals with the history of thin film and describe their importance. The

physical processes involved during fabrication of thin film are discussed. The chapter

also presents brief review of the literature available on II-VI semiconductors especially of

ZnTe and CdS semiconductors.

Chapter three covers the fabrication technique that has been developed by the author in-

house, the close spaced sublimation (CSS) technique. It explains the important organs of

the system. Characterization techniques including X-rays diffraction (XRD), scanning

electron microscopy (SEM) and energy dispersive X-rays spectroscopy (EDX) along with

their working principle have been discussed. The calculations of refractive index,

thickness and energy band gap with Swanepoel model are described in this chapter with

electrical characterization parameters.

Chapter four deals with the optimization of the fabrication of ZnTe thin films using CSS

technique. This research work has been published in Chalcogenide Letters (Investigation

of substrate temperature effects on physical properties of ZnTe thin films by close spaced

sublimation technique, Chalcogenide Letters, 10 (2013) 273 – 281).

Chapter five explains the effects of silver doping on structural, surface, optical and

electrical properties of ZnTe thin films used as the buffer layer to back contact in second

generation solar cell. Effect of silver doping on ZnTe thin films have been published in

Thin Solid Films (Physical properties of sublimated zinc telluride thin films for solar cell

applications. Thin Solid Films, 544 (2013) 307-312)

Page 33: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

9

Chapter six deals with effects of copper doping on the physical properties of ZnTe thin

films. Some of the results are published in Current Applied Physics (Effects of metal

doping on the physical properties of ZnTe thin films, Current Applied Physics, 14 (2014)

282-286.) whereas the remaining part is under review.

Chapter seven contains the study on enrichment of Zn and Te on ZnTe thin films using

CSS technique, the research work is not reported earlier with the CSS technique. The

research work in this chapter opens a new gateway for researcher to control the

stoichiometric arrangement using CSS technique.

Chapter eight describes the fabrication of CdS thin films. The effects of thickness on

optical and electrical properties are discussed. The research work is published in AIP

conference proceedings (Study of Cadmium Sulphide Thin Films as a Window Layers,

AIP Conf. Pro. 1476 (2012) 178-182).

Chapter nine shows the properties of CdZnS thin films fabricated using CSS of a

mechanically premixed source. Such mechanical premixing for II-VI materials is not

documented earlier with CSS technique. This work has been published in Journal of

Optical Materials (CdZnS thin films sublimated by closed space using mechanical

mixing: A new approach, Optical Materials, 36 (2014) 1449-1453).

Conclusions from this study are given at the end of the thesis along with the suggested

future recommendations.

Page 34: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

10

References:

[1] V. Quaschning, James and James (Science Publishers) Ltd, London, United

Kingdom, 2005.

[2] P. Y. Yu, M. Cardona, Springer-Verlag Berlin Heidelberg New York, 1996.

[3] D. Seifried, W. Witzel, Earthscan Ltd, London, United Kingdom, 2010.

[4] P. Wurfel, WILEY-VCH Verlag, Weinheim, Germany, 2005.

[5] S. Adachi, John Wiley & Sons Ltd, Chichester, United Kingdom, 2009.

[6] R. E. Hester, R. M. Harrison, Royal Society of Chemistry, United Kingdom,

2003.

[7] B. Sorensen, Elsevier Science, United Kingdom, 2004.

[8] H. Girardet, M. Mendonca, Green Books Ltd, Foxhole, United Kingdom, 2009.

[9] C. J. Chen, John Wiley & Sons Inc., New Jersey, 2011.

[10] F. H. Cocks, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, United

Kingdom, 2009.

[11] A. L. Fahrenbruch, R. H. Bube, Academic Press, New York, 1983.

[12] A. S. Gilmore, PhD Thesis, Colorado School of Mines, Golden, CO 2003.

[13] R. Yang, D. Wang, L. Wan, RSC Adv., 4 (2014) 22162.

[14] S. G. Kumar, K. S. R. K. Rao, Energ. Environ. Sci., 7 (2014) 45.

[15] B. A. Korevaar, R. Shuba, A. Yakimov, H. Cao, J. C. Rojo, T. R. Tolliver, Thin

Solid Films, 519 (2011) 7160.

[16] N. R. Paudel, C. Xiao, Y. Yan, J. Mater. Sci; Mater. In Electron, 25 (2014)

1991.

Page 35: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Introduction

11

[17] C. Narayanswamy, T. A. Gessert, S. E. Asher, Photovoltaic Prog. Rev. Meeting

Denver, (1998).

[18] X. Wu, R. G. Dhere, D. S. Albin, T. A. Gessert, C. DeHart, J. C. Keane, A.

Duda, T. J. Coutts, S. Asher, D. H. Levi, H. R. Moutinho, Y. Yan, T. Moriarty,

S. Johnston, K. Emery, P. Sheldon, NREL Conf. Pro., 520 (2001) 31025.

[19] B. N. Patil, D. B. Naik, V. S. Shrivastava, Chalcogen. Lett., 8 (2011) 117.

[20] R. Panda, V. Rathore, M. Rathore, V. Shelke, N. Badera, L. S. Sharath Chandra,

D. Jain, M. Gangrade, T. Shripati, V. Ganesan, Appl. Surf. Sci., 258 (2012)

5086.

[21] R. Xie, J. Su, M. Li, L. Guo, Int. J. of Photoenergy, (2013) 620134.

[22] R. Ganesh, V. S. Kumar, K. Panneerselvam, M. Raja, Arch. of Phys. Res., 4

(2013) 97.

[23] M. Elango, D. Nataraj, K. P. Nazeer, M. Thamilselvan, Mater. Res. Bull., 47

(2012) 1533.

[24] T. D. Dzhafarov, F. Ongul, S. A. Yuksel, Vacuum, 84 (2010) 310-314.

[25] H. Khallaf, G. Chai, O. Lupan, L. Chow, S. Park, A. Schulte, J. Phys. D: Appl.

Phys., 41 (2008) 185304.

[26] D. Xia, T. Caijuan, T. Rongzhe, L. Wei, F. Lianghuan, Z. jingquan, W. Lili and

L. Zhi, J. Semicond., 32 (2011) 1.

[27] A. E. Rakhshani, Thin Solid Films, 536 (2013) 88.

[28] K. R. Murali, M. Ziaudeen, N. Jayaprakash, Solid State Electron., 50 (2006)

1692.

[29] A. M. Salem, T. M. Dahy, Y. A. El-Gendy, Physica B, 403 (2008) 3027.

[30] J. Pattar, S. N. Sawant, M. Nagaraja, N. Shashank, K. M. Balakrishna, H. M.

Mahesh,Int. J. Electrochem. Sci., 4 (2009) 369.

Page 36: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 2 Literature Review

Page 37: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

13

This chapter includes a brief review of thin films, deposition techniques, importance of close spaced

sublimation (CSS) technique and II-VI semiconductor materials involved in this research work.

2.1 Role of thin film

Thin film semi-conductor is of the order of micro to nano-meters having smooth layer

formed by the growth and nucleation processes. The nucleation process depends on

condensing individual interacting atoms or ions on the substrate. The deposition

parameters and thickness of the thin film influence strongly on the physical as well as

chemical characteristic of the films [1-3].

The physical properties of thin films which prominently include electrical and optical

characteristics are examined by the chemical composition and morphology (structure,

interface with surface and uniformity). These properties are highly dependent on the

fabrication technique of the film, deposition parameters and post deposition treatments.

Bulk and the thin film properties such as optical, electrical, thermal and magnetic differ

greatly with each other [4-10], mainly due to the significant difference in surface to

volume ratio. The substrate and the thin film interface also affect the surface mobility,

adsorption of gases, surface topology, chemical reactions and crystallographic orientation

[1].

The first deposition technique for thin films was developed in 1850 by M. Faraday and T.

A. Edison, Arago, Fizeau, Wernicks and Wiener. They also contributed to find the

thickness of thin films. In electrochemistry, the gold plating was used by Galvanics on

commercial bases. Dennis Taylor published his book on testing and adjustment of

telescope objectives in 1891. In 1899, Fabry-Perot designed a new interferometer which

later became a thin film filter.

Rouard observed, in 1932, a very thin metallic film may reduce internal glass plate‟s

reflectance. John Strong 1936, introduced anti reflection coating of fluorite by

evaporation inhomogeneous films which remarkably reduced the reflectance of glass

slide by 89 %. The first thin film metal-dielectric interface filter was invented by

Geffcken in 1939. Mechanical and optical coating applications were carried out mostly

Page 38: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

14

military on industrial bases in 1940. The thin films industry was revived in mid-sixties as

an integral part of optical and semiconductors. Thin films were used as surface decoration

items in 1960s. In 1970s, physical vapor deposition (PVD), chemical vapor deposition

(CVD) was developed. Since then, use of thin film use has increased dramatically to most

fields. The tailoring of microstructures of atomic and mesoscopic scale e.g. Cu-

technology in electrochemistry and quantum dots using PVD was developed in 1995 and

used as integrated circuits [11-20].

As the year 2000‟s turn, the defined structure and composition of nano-crystalline

material was manufactured for protective coating. The sizes were at nm range with highly

ordered deposition in two and three dimension. The further up scaling of thin film coating

for industrial applications on glass involving the ternary and quaternary material systems

was started in 2004. The organic electronics led by organic coating was developed in

around 2006. On the basis of these developments, nanotechnology established.

Physics of thin films is divided into two parts

Thermodynamics and kinetics

Solid state physics

Thermodynamics is a bridge between microscopic parameters e.g. internal energies and

macroscopic parameters like temperature, heat, volume and pressure. Kinetics is the

study of energy conversion between heat and the mechanical work. The important aspects

of thermodynamics are mainly involved with thermal equilibrium, entropy and laws of

thermodynamics shown in Fig. 2.1. Thermodynamics of films is mainly classified into

two types:

Phase transition: It is the transformation of liquid to solid and solid to liquid. In thin

film‟s formation, gas molecules are condensed into solid due to the temperature, volume

and pressure, causing phase transformation. Phase transition is expressed in phase

diagram represents the equilibrium conditions as a function of temperature, composition

or pressure of system

Page 39: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

15

Figure 2.1 Solid-liquid system and free energy [8]

Nucleation: The production of a new phase is called nucleation. As the liquid cools down

just below its freezing point, the energy difference between liquid and solid is volume

free energy ΔGv. This free energy increases as the solid grows and an interface is

established between solid and liquid. The decrease in free energy makes possible for

nucleation. The surface formation and volume transition happens at energy minimization.

A surface free energy „σ‟ is related to its interface. As the surface free energy increases

the solidification also increases [3]. The change in free energy in homogenous nucleation

is

(

)

Where

r is the radius of embryo sphere and (4/3)πr3 is the volume of sphere, also 4πr2 is the

area of spherical embryo [4]. Free energy is negative for volume transition if vapor phase

is in super saturation and positive for surface formation.

Page 40: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

16

Main classes of solid state physics are:

Crystalline

Amorphous

The regular pattern of atoms is crystalline solids, the study known as Crystallography and

the irregular arrangement is amorphous.

Crystallography is the study of arrangement of atoms in solids. Lattice and the basis

combination make the crystal structure. Lattice is the regular arrangement of atoms

whereas basis is a group of atoms located in lattice at each point. Primitive cell is a

building block of crystalline solid.

When all points in a lattice are identical, it is known as Bravais lattice e.g. Na, Cl etc. If

the atoms are not of the same kind, it is a non-Bravais lattice e.g. NaCl. There are

fourteen Bravais lattices that have been classified into seven sets in three dimensional

arrays.

2.2 Fabrication of thin films and mechanisms

Fabrication of thin films is the process of growth and nucleation on the substrate. The

crystallinity and the microstructure are determined by the nucleation as a result. The

initial nucleation is an important process for the thickness of films in the nano meter

range. The interaction between film and the substrate are important in order to determine

the initial nucleation and film growth. There are three basic modes of nucleation listed

below [5] as shown in Fig. 2.2.

Layer or Frank-van der Merwe growth

Island or Volmer-Weber growth

Sland-layer or Stranski-Krastonov growth

Frank-van der Merwe growth: where the growth, species have strong bond with substrate

than each other. Epitaxial growth of single crystal is the example of Frank-van der

Merwe growth.

Page 41: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

17

Island or Volmer-Weber growth: Where the growth species have much stronger bond as

compared to the substrate. The metals on insulator substrate like alkali halides, graphite

and mica substrate showed such type of nucleation during initial film growth.

Stranski-Krastonov growth: Stranski-Krastonov growth is an intermediate combination of

island and Frank-van der growth. It involves the stress, which is created during the

formation of film.

Figure 2.2 Growth mechanism of thin films

2.3 Deposition techniques

There are different techniques for fabrication of thin films, each technique has its

advantages and disadvantages to optimize desired film properties. These techniques are

mainly divided into different classes [20-30]:

Chemical vapor deposition (CVD)

Physical vapor deposition (PVD)

Electro-chemical deposition (ECD)

Page 42: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

18

Electrolyses or solution growth

The main emphasis will be on the physical vapor deposition, in particular - thermal

evaporation.

2.3.1 Physical vapor deposition

It is the process in which growth species transfer from source and deposit on the substrate

to fabricate the film. It does not have a chemical reaction and it proceeds atomically. The

thickness can vary from several angstroms to millimeter. This method is further divided

into two parts

Sputtering: It is the process, where atoms are ejected from solid target due to energetic

bombardments of particles. It is useful for compounds or mixture, where various

components tend to evaporate at different rates. Targets are kept at relatively low

temperatures. Sputtering is not the kind of evaporation, which makes it a flexible

deposition technique. Schematic of sputtering is shown in Fig. 2.3.

Figure 2.3 Schematic of sputtering

Evaporation: The atoms or molecules are accommodated by heat from a material in

liquid or solid phases, which has some equilibrium pressure (saturated vapor pressure) in

a closed system. The temperature T at equilibrium and the number of particles having

molecular weight M is evaporated using kinetic theory [9]:

Page 43: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

19

Where Ne is the molecular flux and Pe is vapor pressure at equilibrium (torr). If the

equilibrium is not maintained in the system there will be a temperature difference in some

parts. The vapors will be condensed at the cooler parts and the transfer of material will

take place from evaporation source to cooler substrates. So the deposition by evaporation

of a film is a non-equilibrium process. The energetic particles move along a straight line

until they collide with atoms of residual gas. The space between source and substrate

must be evacuated in order to provide long mean free path [6-8].

Heat is supplied for vaporizing and maintaining the charge to produce required vapor

pressure. An electric heater is used as a thermal evaporator to melt the material and raise

its vapor pressure to a desire range. It could be achieved at high vacuum, to reduce the

impurities from residual gas in vacuum chamber.

An ablation process can be achieved by Pulse Laser Deposition (PLD); a laser light

vaporizes the material from the surface and converts it into plasma. This usually changes

to gaseous state before sticking onto the substrate [10].

The vacuum has great importance in fabrication or deposition of thin films. A vacuum

assures collision free transfer of material; it reduces energy transfer and provides a

contamination free region. Vacuum can also reduce boiling points of desired materials

[31-34]. Different levels of vacuum are achieved by using different vacuum pumps.

In a common vacuum pump, the gas particles are pumped from chamber to the

atmosphere in one or more steps. In the other type, the particles are chemically condensed

to remove at a solid wall, which is a part of boundary of chamber. Here are the main

types of pumps according to their working principle.

Vapor-steam pump: Vapor-steam pumps are used to pump out gas molecules that are

directed towards the substrate. It can be achieved with the help of heavy molecules from

vapor pump. A liquid reservoir is boiled to produce heavy vapor molecules.

Mechanical pump: Mechanical pumps trap the gas, compress it, and move it from low to

high pressure. The gas is moved directly or indirectly through a second back up pump.

Page 44: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

20

Chemical pump: In chemical pumps, molecules are removed from the chamber,

chemically combines with substances like titanium (getter). These are converted and

trapped in solid phase. Evaporator or stutterer is used to disperse the getter from the

pump‟s surface.

Vacuum gauges are used to measure the pressure inside the chamber. Pirani gauge can

read the chamber upto10-3

mbar. The working principle of Pirani gauge is thermal

conductivity of gases. For higher vacuum, penning gauge is used for a vacuum up to 10-6

mbar. Penning gauge works on the principle of momentum transfer.

2.4 Close spaced sublimation (CSS) technique

Close spaced sublimation (CSS) technique is one of the best techniques for fabricating

good quality films. Maximum reported efficiency (16.5 %) of II-VI solar cell is due to

CSS and chemical bath deposition technique. The main advantage is the simplified

deposition with high transport rate under low vacuum conditions. The source material

sublimates and condenses due to vapor on substrate, which is very close to source.

Commonly 15-20 mg material is used for deposition of film; rough films can be

fabricated due to high flux as compared to other technique [35, 36]. The main advantages

of CSS are given here;

The coating system is simple and easy to handle.

Under low vacuum conditions, high transport efficiency can be achieved.

The heating arrangement of source and substrate is directly and separately. The

temperature gradient is maintained by mica sheet between source and substrate

which enhances the stickiness of atoms to the substrate. The space between source

and substrate (5 mm) is very small but can be varied.

A very small loss of material as compared to other techniques in PVD.

The vapor pressure is extremely high as compared to thermal evaporation, two

source and e-beam evaporation techniques. Knudsen evaporation expression

explains this effect;

Page 45: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

21

( )

( )

where dN/dt gives number of atoms per unit surface area (cm-2

), C is constant

depends upon degree of rotational freedom, P* is vapor pressure of material at

temperature T and P denotes the vapor pressure above surface [8].

The deposition through CSS technique depends upon the transportation of gas from

source to substrate and the sublimation on the surface of source as well as on the

substrate [37]. High deposition rate is one of the main features of CSS which makes it

distinguish from the others techniques. Our experience on CSS tells us that growth and

material transport atom strongly depend upon source-substrate distance and temperature,

ambient pressure and also on source material [38].

The source and substrate spacing should be very small for good quality film, allowing

increasing the deposition rate and reducing the material consumption. It is also favorable

that source material should completely be deposited on to substrate. Distance between

source and substrate are inversely proportional to the growth rate [39].

The deposition rate increased as the source temperature is increased under vacuum [40].

The temperature of substrate affects the resistivity of the films; increase in substrate

temperature decreases the resistivity of the films. The grain sizes can be increased with

higher temperature of substrate. The surface mobility of deposited species increases due

to less nucleation sites and hence the larger grains are formed [41].

Some disadvantages are also mentioned as:

No shutter arrangement in this system to control the deposited atoms or

molecules.

Growth rate cannot be observed during evaporation.

No thickness monitor due to small distance between source and substrate to

control the thickness and at one time, only one film can be fabricated.

Page 46: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

22

Vacuum also plays an is important role to enhance the transportation of molecules from

source to substrate; higher growth rates are obtained due to larger mean free path as

compared to gas filled chamber. Vacuum helps to grow of relatively thick films as

compared to gas filled chamber without large number of cycles. The most importantly

another advantage of vacuum is that, thin films can be grown at lower temperature due to

inter-diffusion processes [42-43]. Complete CSS setup was erected in-house, see details

in chapter 3.

2.5 II-VI Semiconductors

The electrical properties of semiconductors have great importance, and can be tailored to

the requirements. The energy band gap of semiconductor material ( 1 eV) varies with

temperature. These features make semiconductor materials an integral part in most

electronic devices. Semiconductors are doped with other elements to provide desired

electrical conductivity and/or optical energy band gap. Research of new materials or

improvements in the properties of current semiconductor materials is always welcomed.

Semiconductors are classified into elemental and compound semiconductors.

Semiconductors composed of group V and III or group II and VI of periodic table are

called compound semiconductors.

Semiconductors consisting of Group II and VI elements have a wide energy band gap,

generally these are known as II-VI semiconductor material. Cadmium sulfide (CdS),

cadmium selenide (CdSe), zinc telluride (ZnTe), zinc sulfide (ZnS) and cadmium

telluride (CdTe) are the potential semiconductors for electrical and optical study. CdS has

hexagonal structure and ZnTe mostly is in cubic structure. II-VI semiconductor materials

are most commonly used as light emitting diodes, solar cells and detectors etc. Due to the

tunable energy band gap, high melting point, favorable electrica1 and optical properties,

these semiconductors are contenders to commonly used semiconductors such as Si and

Ge.

Page 47: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

23

2.5.1 ZnTe thin films

Zinc telluride is an attractive II-VI semiconductor material for optoelectronics devices

and solar cells etc. It has a tunable band gap of 2.26 eV at room temperature which

ensures high transparency in the visible spectral range [44-47]. Owing to this important

property, p-type ZnTe has been used in CdS/CdTe/ZnTe solar cells [48]. ZnTe thin films

can be prepared by various methods, most commonly chemical vapor deposition and

physical vapor deposition techniques. However, deposition under vacuum has many

advantages over the other techniques, because film properties can be precisely controlled

[49]. ZnTe thin films can be fabricated by using close spaced sublimation (CSS)

technique [50], sputtering method [51], electro-deposition process [52], electron beam

evaporation [53] and pulse laser deposition technique [54]. The CSS has an advantage of

efficient utilization of material due to small distance between the source and substrate

[55]. Doping can be done by using thermal evaporation [56] as well as solution based

methods like ion exchange method. The ion exchange is one of the most convenient

methods for doping II-VI semiconductor thin films [57, 58].

ZnTe is an ideal intermediate layer between the absorbent layer of CdTe and metal back

contact to improve the performance of heterostructure solar cell. Efficiency of solar cell

can be enhanced by making low resistive back contact. Stability of the cell over a long

period is another important factor in polycrystalline thin film solar cells

(ZnTe/CdTe/CdS/ITO) [59]. The electron affinity is about 4.28 eV for CdTe and for a

good back contact layer of ZnTe, the required work function should be near 5.78 eV (i.e.

electron affinity plus energy band gap of CdTe 1.5 eV) therefore heavily doped p-type

material is required for this purpose [60]. Silver (Ag) and copper (Cu) may be

incorporated as accepters dopant in II-VI semiconductors to improve their functionality.

2.5.2 CdS thin films

CdS is an n-type material with energy band gap of 2.42 eV. Since 1950, out of many

materials, CdTe has been considered as an extraordinary compound for solar cell

applications. It can be doped n and p type and has a very suitable energy band gap (1.45

eV) to convert solar energy [61, 62]. After a few decades of research, it was discovered

Page 48: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

24

that solar cells become more efficient when CdS thin film is used, increasing their

performance between 5 % to 8 % [63-66]. In the 1980s, II-VI thin film solar cells were

fabricated using CSS technique which enhanced their output to 12 % [67]. After that, it

was realized that by reducing the thickness of the CdS layer, filtration from CdS thin film

can be prevented. Further research was based on the deposition of CdS/CdTe on a

conducting glass layer.

In 1991 Ting L. Chu obtained 15 % efficiency of solar cell. This boosted the commercial

interest; dozens of new companies were interested in solar cell production. In 2001

National Renewable Energies Laboratories reported highest obtained efficiency of

CdS/CdTe solar cells as 16.5 %. After many improvements in the way of commercially

stable cell preparation, in 2005, a production capacity of 25 MW / year was attained [68].

A very thin layer of CdS was evaporated on a glass substrate and on the top of it another

relatively thicker layer, about 2 µm of CdTe was deposited. Often the cell is treated with

CdCl2 flux for a short time at about 450oC and causesing partial crystallization of the

semiconductors. Meanwhile copper doping of CdS is also carried out using the same

procedure. These processes are conducted on a semiautomatic production line.

Many attempts were made to replace CdS with other compounds in the last 20 years to

attain better efficiency than CdS. These attempts did not show significant enhancements

in CdTe based solar cell [69-71].

This leads to a question whether CdS is a good partner with CdTe? With increase optical

transmission by placing a layer of CdS on CdTe. This is mainly due to comparatively

lower index of refraction of CdS with CdTe. Causing low reflection of light from the

surface. Thus the output efficiency of the cells is almost doubled as they are illuminated

to solar light [72-77].

This improvement in the conversion efficiency has been known for 30 years; however,

recent detailed analysis of Cu-doped CdS has helped to explain the cause of improved

efficiency [78, 79].

Page 49: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

25

Attempts were continued to replace CdS with other compound, these were not successful

and this re-asserts the superiority of CdS in efficient conversion of solar power. Since

manufacturing processes are quite complicated, it is difficult to think of a reasonable

computational model which agreeing with experimental observations especially with

measured current-voltage characteristics. The thickness of the CdS layer can be

controlled to about 2000 nm [77].

From 1950s to 1960s many experiments were performed in order to explain the behavior

of CdS [80, 81]. It was found that CdS is an n-type semiconductor and becomes more

photoconductive when doped with Cu. Its electron density reaches 1018

cm-3

when

exposed to sun light. The photo conductivity can be quenched quite easily using the low

energy excitation like infra-red light. It may become important in solar cell by the

intermediate E-field.

CdS/CdTe solar cells can be made more efficient if the following properties are taken

into account. If the grain size is large one, has good quality material for PV cells and

reduces the resistance to charge motion at grain boundaries. Hence more sulfur is

diffused at CdTe surface and thereby enhancing the rate of production of the compound

CdSxTe1-x, which has a wider band gap than CdTe. This improves the absorption

characteristics. To have very efficient and long life stable molecules of CdS/CdTe, the

material used for back contact has great influence and to have the desired effect the way,

it is deposited.

By mixing any material with CdTe, will create Schottky barrier as CdTe to electron ratio

is quite large. So in order to create ohmic contact, a material with activation energy

greater than 5.7 eV is needed. Till now, the popular and tested elements for the use of

back contact are Cu/Au [62], Cu/graphite [63], Cu-doped ZnTe with Au or Ni [63, 64],

and Cu/Mo [65]. Contacts of Cu are more efficient; however diffusion of Cu at joint

creates shunt affect and the efficiency of the cell decreases. Thus for stability and durable

solar cells, the use of Cu is mostly avoided [66]. Metals diffuse more rapidly across grain

boundaries which degrades the cell. In the window layer, no photocurrent is generated

therefore CdS is a preferred as window layer material with a direct band gap type.

Page 50: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

26

References:

[1] J. Poortmans, and V. Arkhipov, Thin Film Solar Cells Fabrication,

Characterization and Applications, John Wiley & Sons Ltd, England, 2006

[2] J. Villain, J. Phys. I France, 1 (1991) 19.

[3] J. Krug, Adv. Phys. 46 (1997) 139.

[4] C. Ratschand, J. Venables, J. Vac. Sci. Technol. A21 (2003) 2101.

[5] G. Cao, Imperial College Press, (2004).

[6] M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego,

New York, Boston, London, Sydney, Tokyo, Toronto, (1992).

[7] L. Eckertova, Physics of Thin Films, Plenum Press, New York, (1977).

[8] K. L. Chopra and S. R. Das, Thin Film Solar Cells, Plenum Press, New York

(1983).

[9] Z. Ali, Ph.D Thesis, Quaid-i-Azam University, 2005.

[10] A. Ali, Ph.D Thesis, Quaid-i-Azam University, 2006.

[11] H. Ibach, Physics of Surfaces and Interfaces, Springer-Verlag, Berlin, 2006.

[12] L. Yu, S. Misra, J. Wang, S. Qian, M. Foldyna, J. Xu, Y. Shi, E. Johnson, P. R.

Cabarrocas, Sci. Reports, 4, (2014) 4357.

[13] C. Yeo, J. H. Choi, J. B. Kim, J. C. Lee, Y. T. Lee, Opt. Mater. Expr., 4 (2014)

346.

[14] M. Xu, A. J. H. Wachters, J. V. Deelen, M. C. D. Mourad, P. J. P. Buskens, Opt.

Express, 22 (2014) 425.

[15] N. R. Paudel, Y. Yan, Appl. Phys. Lett., 104 (2014) 143507.

Page 51: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

27

[16] I. M. Dharmadasa, Coatings, 4 (2014) 282.

[17] L. Fraas, L. Partain, John Wiley & Sons, Inc., New Jersey, 2010.

[18] I. Repins, M. A. Contreras, B. Egaas, C. DeHart, J. Scharf, C. L. Perkins, B. To,

R. Noufi, Prog. Photovolt. Res. Appl., 16 (2008) 235.

[19] U. P. Singh, S. P. Patra, Int. J. Ph. energ., 2010 (2010) 468147.

[20] S. H. Moon, S. J. Park, Y. J. Hwang, D. K. Lee, Y. Cho, D. W. Kim, B. K. Min,

Sci. Reports, 4 (2014) 4408.

[21] T. P. Dhakal, C. Y. Penga, R. R. Tobias, R. Dasharathy, C. R. Westgate, Sol.

Energy, 100 (2014) 23.

[22] D. Bonnet, H. Rabenhorst, Proc. 9th

IEEE Photovoltaic Specialists Conf., New

York, (1972) 129.

[23] X. Mathew, G. W. Thomson, V. P. Singh, J. C. M. Clure, S. Velumani, N. R.

Mathews, P. J. Sebastian, Sol. Energy Mater. Sol. Cell., 76 (2003) 293.

[23] C. Gretener, J. Perrenoud, L. Kranz, L. Kneer, R. Schmitt, S. Buecheler, A. N.

Tiwari, Prog. Photovolt: Res. Appl., 21 (2013) 1580.

[25] E. R. Shaaban, I. S. Yahia, N. Afify, G. F. Salem, W. Dobrowolski, Mater. Sci.

Semi. Process., 19 (2014) 107.

[26] W. L. Rance, J. M. Burst, D. M. Meysing, C. A. Wolden, M. O. Reese, T. A.

Gessert, W. K. Metzger, S. Garner, P. Cimo, T. M. Barnes, Appl. Phys. Lett.,

104 (2014) 143903.

[27] X. Wu, Sol. Energy, 77 (2004) 803.

[28] K. T. R. Reddy, N. K. Reddy, R. W. Miles, Sol. Energy Mater. Sol. Cell., 90

(2006) 3041.

Page 52: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

28

[29] T. S. Moss, Butterworths Publications, London, 1959.

[30] D. K. Schroder, Semiconductor Material and Device Characterization, John

Wiley & Sons, Hoboken, New Jersey, 2006.

[31] N. A. Shah, A. Ali and A. Maqsood, J. Non-Cryst. Solids, 355 (2009) 1474.

[32] A. Bayer, D. S. Boyle, M. R. Heinrich, P. O‟Brien, D. J. Otway, O. Robbe,

Green Chem. 79 (2000) 554.

[33] N. A. Shah, A. Ali and A. Maqsood, J. Electron. Mater., 37 (2008) 145.

[34] A. Ali, N. A. Shah, A. K. S. Aqili and A. Maqsood, Semicond. Sci. Tech., 21

(2006) 1296.

[35] N. A. Shah, W. A. Adil Syed, M. A. Atta, M. Ajmal, A. Ali and A. Maqsood,

Nano sci. Nanotech. Letter., 1 (2009) 62.

[36] J. Britt, C. Ferekides, Appl. Phys. Lett. 62 (1993) 2851.

[37] B. R. Wakeling, Ph.D Thesis, Cranfield University, 2009 .

[38] V. Kumar, Ph.D Thesis, University of South Florida, 2003.

[39] T. Anthony, A. Fahrenbruch, R. Bube, J. Vac. Sci. Technol. 3 (1984) 1296.

[40] P. M. Martin, Handbook of Deposition Technologies for Films and Coatings,

The Boulevard, Langford Lane, Oxford, United Kingdom, 2010.

[41] C. S. Ferekides, D. Marinskiy, V. Viswanathan, B. Tetali, V. Palekis, P.

Selvaraj, D. L. Morel, Thin Solid Films, 361 (2000) 520.

[42] S. Larramendi, F. C. Zawislak, M. Behar, E. Pedrero, M. Herna´ndez Ve´ lez,

O. de-Melo, J. Cryst. Growth, 312 (2010) 892.

[43] L. B. Freund, and S. Suresh, Thin Film Materials, Cambridge University Press,

United Kingdom, 2004.

Page 53: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

29

[44] L. Feng, L. Wu, Z. Lei, W. Li, Y. Cai, W. Cai, J. Zhang, Q. Luo, B. Li, J.

Zheng, Thin Solid Films 515 (2007) 5792.

[45] K. R. Murali, M. Ziaudeen, N. Jayaprakash, Solid State Electron., 50 (2006)

1692.

[46] A. Erlacher, A. R. Lukaszew, H. Jaeger, B. Ullrich, Surf. Sci., 600 (2006) 3762.

[47] P. K. Kalita, B. K. Sarma, H. L. Das, Indian J. Pure Ap. Phy., 37 (1999) 885.

[48] T.A. Gessert, P. Sheldon, X. Li, D. Dunlavy, D. Niles, R. Sasala, S. Albright, B.

Zadler, 26th IEEE Photovoltaic specialist Conference, Sept. 29-Oct. 03, 1997,

Anaheim, California.

[49] A. A. Ibrahim, N. Z. El-Sayed, M. A. Kaid, A. Ashour, Vacuum, 75 (2004) 189.

[50] A. K. S. Aqili, A. J. Saleh, Z. Ali, S. Al-Omari, J. Alloys Compd. 520 (2012)

83.

[51] H. Bellakhder, A. Outzourhit, E. L. Ameziane, Thin Solid Films 382 (2001) 30.

[52] V. S. John, T. Mahalingam, J. P. Chu, Solid State Electron., 49 (2005) 3.

[53] A. M. Salem, T. M. Dahy, El-Gendy, Physica B, 403 (2008) 3027.

[54] S. Bhumia, P. Bhattacharya, D. N. Bose, Mater. Lett., 27 (1996) 307.

[55] O. de Melo, E. M. Larramendi, J. M. Duart, M. H. Velez, J. Stangl, H. Sitter, J.

Cryst. Growth, 307 (2007) 253.

[56] P. U. Bhaskar, G. S. Babu, Y. B. K. Kumar, V. S. Raja, Appl. Surf. Sci., 257

(2011) 8529.

[57] N. A. Shah, A. Nazir, W. Mahmood, W. A. A. Syed, S. Butt, Z. Ali, A.

Maqsood, J. Alloy Compd., 512 (2012) 27.

Page 54: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

30

[58] N. A. Shah, R. R. Sager, W. Mahmood, W. A. A. Syed, J. Alloy Compd., 512

(2012) 185.

[59] T. A. Gessert and T. J. Coutts, AIP Conf. Pro., 306 (1994) 345.

[60] M. S. Hossain, R. Islam, K. A. Khan, J. Ovonic Research, 5 (2009) 195.

[61] J. I. Cisnors, Appl. Optics, 37 (1984) 896.

[62] U. Pal. S. Saha. A. K Chaudhuri, V. V. Rao and H. D. Banerjee, J. Phys. D:

Appl. phys., 22 (1989) 965.

[63] J. B. Webb, D. E. Brodie, Can, J. Phys., 53 (1975) 1415.

[64] Y. Namba, T. Mori, Thin Solid Films, 39 (1976) 119.

[65] H. M. Browm, D. E. Brodie, Can. J. Phys., 50 (1972) 2512.

[66] S. Kobayashi, N. Saito, T. Shibuya, Jpn. J. Appl. Phy., 19 (1980) 1199.

[67] C. J. Moore, B. S. Bharaj, D. E. Brodie, Can. J. Phy., 59 (1981) 924.

[68] X. Wu, R. G. Dhere, D. S. Albin, T. A. Gessert, C. DeHart, J. C. Keane, A.

Duda, T. J. Coutts, S. Asher, D. H. Levi, H. R. Moutinho, Y. Yan, T. Moriarty,

S. Johnston, K. Emery, P. Sheldon, NREL Conf. Pro., 520 (2001) 31025.

[69] G. K. M., S. G. Tomlin, J. Phys. D: Appl. Phys., 9 (1976) 1639.

[70] S. Nam, J. Rhee, B. O., K. Lee, Y. D. Choi, J. Cryst. Growth, 180 (1997) 47.

[71] R. C. Tu, Y. K. Huang, F. R. Chien, J. Cryst. Growth, 180 (1997) 506.

[72] H. Bellakhder, F. Debbagh, A. Outzourhit, A. Bennouna, M. Brunel, Sol. Energ.

Mat. Sol. Cell., 45 (1997) 361.

[73] T. Baron, K. Saminadayar, S. Tatarenko, J. Cryst. Growth, 159 (1996) 271.

Page 55: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Literature Review

31

[74] W. Kuhn, H. P. Wagnert, H. Stanzi, K. Wolf, S. Lankes, Semicond. Sci.

Technol., (1991) 105.

[75] N. B. Chaure, J. P. Nair, R. Jayakrishnan, V. Ganesan, R. K. Pandey, Thin Solid

Films, 324 (2001) 447.

[76] S. Stolyarova, N. Amir, Y. Nemirovsky, J. Cryst. Growth, 186 (1998) 55.

[77] T. Matsumota, T. Ishida, J. Cryst. Growth, 186 (1998) 185.

[78] H. Wang, J. Opt. Soc. Am. A, 12 (1995) 769.

[79] S. Ikegami, Sol. Cells, 23 (1988) 89.

[80] S. I. Gheyas, S. Hirano, M. Nishio, H. Ogawa, Appl. Surf. Sci.,100 (1996) 634.

[81] L. Feng, D. Mao, J. Tang, R. T. Collne, J. U. Trefny, J. Electron. Mater., 25

(1996) 1422.

Page 56: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 3 Fabrication and

Characterization

Techniques

Page 57: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

33

The chapter under consideration relates to the fabrication and characterization techniques used during this

research work

3.1 Design and assembly of CSS developed at COMSATS, Islamabad

CSS fabrication unit is assembled and installed at Thin Film Technology (TFT) lab,

COMSATS I.I.T., Islamabad as shown in Fig. 3.1. The CSS technique is internationally

recognized but the important aspect is that the whole apparatus was designed locally with

the help of KRL, Islamabad. The main components and their functions are described

below;

Cylindrical bell jar,

Base plate (Flange)

Three rod and holder for source and substrate holding

Graphite boat and slab

Vacuum pumps

Vacuum gauges

Heating assembly including halogen lamps, thermocouples and temperature controller,

Base plate is also known as flange, where the whole deposition assembly is placed on it.

It is made of stainless steel (SS 304 L), with no magnetic charge, for construction of

chamber with a diameter of 330 mm. One of the main reasons to use SS 304 L is that it

does not require post weld annealing due to low carbon. It is called 18-8 related to the

chromium, nickel contents. Cylindrical bell jar is made of a transparent pirated glass of

thickness 7 mm used to observe the deposition phenomenon inside the chamber. The

height of the jar is 360 mm and its internal diameter 290 mm, O-ring is used to contact

the bell jar and flange, which was made airtight connect and to sustain the vacuum inside

the chamber. The flange has some feed throughs for vacuum, thermocouple and

electronic wiring with external setup.

The source and substrate holders inside the chamber are placed with three vertical rods of

height 255 mm and diameter of 11 mm fitted on to the flange. Graphite boat is used as a

source material holder with base thickness of 15 mm, length 60 mm, depth 7.5 mm and

Page 58: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

34

width 33 mm. The substrate is covered with graphite slab of dimension 78 mm x 42 mm

x 5 mm for uniform heating. Mica sheet of thickness 1 mm is used as a thermal insulator

between source and substrate. The distance between the source and substrate is 5 mm.

Two halogen lamps of 1000 W and 500 W are placed inside the chamber for direct

heating of the source and substrate. K-type thermocouples are attached with the source

graphite boat and substrate slab for measurement of the temperatures. These

thermocouples are attached with the temperature controllers (SG-632). These controllers

have algorithm control to adjust the parameters which may enhance the effectiveness of

the controllers.

Rotary pump (Edwards) is usually used for creating vacuum upto 10-3

mbar and then a

diffusion pump is attached for higher levels of vacuum (upto 10-6

mbar). Pirani gauge is

connected with rotary pump to check its vacuum level; similarly penning gauge is used to

measure the vacuum level of a diffusion pump.

Rotary vane pump has mounted rotor with spring loaded vanes. Vanes slide moves back

and forth inside the cylindrical container to confine the gas from the chamber to compress

it and discharge through exhaust valve to the atmosphere. The pumping action is

achieved as the spring loaded blades follow the walls of container while the rotor

rotating. Oil is used as a lubricant and as a sealant between the components.

The pumping rate is controlled by the valve between pump and vacuum chamber, another

valve called vent valve is used to break the vacuum when required. Vent valve is also

used as an inlet for inert gas when an ambient gas is required.

Page 59: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

35

Figure 3.1 Schematic of CSS technique at CIIT, Islamabad

The following characterization techniques were used to determine structural, surface

optical and electrical parameters of the fabricated thin films;

X-rays diffraction (XRD)

Scanning electron microscopy (SEM)

Optical measurements

Electrical measurements

XRD patterns were recorded on a PANanalytical spectrometer model X‟PERT PRO. The

operating conditions were 40 keV at 30 mA with Cu-Kα line (λ= 1.5406 Å), increment

0.5o at a scan speed 1 sec/step, X ray incidence angle = 2° and scan range was 20°–80°.

The surface morphologies of the films were observed by SEM images attached with EDX

using LSM – 6490 an analytical scanning microscope. The energy variations for SEM

Page 60: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

36

were 15 keV to 20 keV with 8 nm resolutions and the applied EDX voltage was 15 kV

with standard less analysis.

The optical parameters were recorded using ultraviolet, visible and near infrared (UV-

VIS-NIR) spectrophotometer (Perkin Elmer LAMDA 950) with UV Win lab software.

The electrical properties were measured using a Hall measurement system (HMS-5000)

Ecopia, applying nA current with constant magnetic field of 0.5 T at room temperature.

3.2 X-rays diffraction

XRD is a non-destructive, most suitable and a precise method to analyze the structure of

a crystal. The preparation of sample for using this technique is very simple and there is

no need for special attention [1, 2]. XRD provides structural information regarding

phases, crystal orientation etc [3, 4]. Using the data attained from the XRD, crystal

defects, stress, strain and particle size can be calculated [5-15]. The underlying physics

involved in XRD is the diffraction through crystal lattice. The condition of constructive

diffraction is „nλ = 2dsinө‟, where „λ‟ the wavelength of light used and „d‟ the obstacle

width through which light passed. X-rays photon used in XRD is of the order of 100 eV

to 100 keV and spacing between the atoms of the crystals is comparable with wavelength.

The X-rays enter in the material and provide information about the structure of the

material.

3.2.1 Bragg’s law

X-rays are incident on the surface of the crystal and they are reflected back. The

reflection only takes place when angle of incidence is from a certain range of values. The

wavelength and the lattice constant are dependent on these values. It is also possible to

examine the selective reflectivity in terms of interference effects.

If the spacing between the atoms of the crystals is „d‟ and angle of the incident beam is

ϴ, „λ‟ is the wavelength of X-rays as shown in Fig. 3.2. The condition for constructive

interference is set with Bragg‟s law

Page 61: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

37

Using the information from Bragg‟s law and the data from the XRD machine,

information on the lattice parameter „a‟, „b‟, „c‟, for (hkl) has been calculated by the

following expression, Eq. 1 [5].

(1)

Where d is the inter-planar spacing of the atomic plane whose Miller indices are (hkl).

Figure 3.2 Diffraction pattern of X-rays from the lattice planes of crystal

Scherrer‟s formula as mentioned in Eq. 2 is used to calculate the average crystallite size

(D).

(2)

Where k = 0.9 constant of machine, λ is X-ray wavelength (1.5406 Ǻ), where full width

half maximum is denoted by β and measured in radians also ϴ is Bragg angle

respectively as shown in Fig. 3.3. The dislocation density (ρ), which represents the

amount of defects in the crystal, is estimated from the following Eq. 3 [5]

(3)

Strain (ε) of the thin film is determined from Eq. 4 [5]:

Page 62: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

38

(4)

Figure 3.3 Use of diffraction curve to determine the crystallite size

The lattice constant (a), average crystallite size (D), dislocation density (ρ), strain (ε) are

calculated. The measurements are taken at room temperature. The factors including „λ‟

(1.5406 Ǻ), „β‟ (FWHM) and ϴ (radians) have very small machine error so results have

not much effected.

3.3 Surface morphological study

Surface study of thin films at micro to nano level is an important aspect. The surface

study directly correlates with the optical and electrical properties of thin films.

Morphology of the film surface provides important and useful information of the size,

shape and the particle arrangement. The morphological aspects are characterized by

SEM. The basic information about shape, size of particle and surface features are

obtained by use of SEM. The element and compound made up of their relative ratios in

area 1 micrometer diameter. The resolution of SEM is around 8 nm. The schematic of

SEM is shown in Fig. 3.4. The beam of electrons is generated under vacuum in a

scanning electron microscope. A collimated beam is produced by the condenser lenses

Page 63: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

39

and is focused by the objective lens onto the sample surface. Secondary electrons,

released by the sample are collected by primary images. A scintillation material is used to

detect secondary electrons which are produced as a flash of light. A photomultiplier tube

detects the amplified flashed light, an image is formed which can be observed through an

optical microscope.

Figure 3.4 Schematic of a scanning electron microscope

Energetic electrons used for backscatter images that penetrate about 180 degrees from the

direction of illuminating beam. The composition information can be found by the

backscatter electrons, which are function of atomic number of each point on the sample.

X-rays analyzer is attached with scanning electron microscope; the sample interaction

with the beam of energetic electron produces X-rays which are characteristic X-rays of

the elements in the samples [16]. Detectors are of two types, wave dispersive X-rays and

energy dispersive X-rays (EDX).

3.3.1 Energy dispersive X-rays spectroscopy

The interaction of primary electrons with the generated X-rays also detected in scanning

electron microscope equipped for EDX. It is used for the identification of composition of

elements. It is an integrated function of SEM [17]. An electron beam is bombarded on

sample for compositional study in SEM. Some of the electrons are knocked out when

they collide with bombarded electrons. The energetic electrons from loosely bound shells

Page 64: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

40

occupy the vacancy of ejected electron from inner shell. The outer electron emits X-rays.

Every element emits a unique energy in the form of X-rays during this process. Finally

the X-ray energy identifies the atoms in samples. The EDX spectra is the output in EX

study, it gives information about the energy level received by X-rays. The corresponding

peaks in EDX spectrum normally indicate the most received X-rays. Every peak

corresponds to an element and peak intensity represents the concentration of elements in

the sample.

3.4 Optical measurements

Transmission, reflectance, refractive index and absorption coefficient are known as

optical properties. The properties are measured using absorption spectrometry. It is

simple and determined optical parameters using only the transmission spectrum.

Swanepoel model is usually used to calculate the optical parameters, which is the

modified form of model given by Manifacier et. al. [18]. Manifacier et. al. used the

infinite substrate approximation, which gives more than 10% error in the calculated

absorption coefficient. Many interesting optical phenomenon are studied using absorption

or transmission spectroscopy of the thin films. If the maximum of valence band and the

minimum of conduction band lie at the same k-value; electrons migrate between the

bands without any change in the momentum; this is known as a direct band gap transition

type. Most of the II-VI semiconductors are direct band gap materials. In the optical

properties, transmission can be obtained directly from the films as shown in Fig. 3.5.

Perkin Elmer Lamda 950 spectrophotometer is used to record the transmission or

absorption data using UV-Win Lab software, the schematic diagram is shown in Fig. 3.6.

The graph between % T verses wavelength is plotted. The oscillations show the

constructive and destructive interference phenomenon in thin films. The optical

parameters like refractive index, absorption coefficient, optical thickness and energy band

gap etc can be calculated using transmission data. The transmission T = T (λ, n, s, α and

d) for complex function. Here if the value of s (refractive index of glass) is known, than T

= T (n, x). These parameters were calculated using transmittance data and fitting the

transmittance curve using a well-known Eq.5 [18]

Page 65: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

41

2)cos( DxCxB

AxT

(5)

Where „T‟ is the transmittance of the thin film on a transparent substrate placed in air, the

refractive index of air is taken as one. The other parameters are defined as A = 16n2s, B =

(n+1)3(n+s

2), C = 2(n

2-1)(n

2-s

2), D = (n-1)

3(n-s

2), /4 nd , x = exp(-αd),

4/k Where „n‟ and„s‟ are the refractive indices of the film and glass substrate

respectively. The estimated value „n‟ and„s‟ can be calculated directly from the optical

data attained by spectrophotometer. i.e.

[ ( )

]

and

(

)

„α‟ is the absorption coefficient of the film, „λ‟ the wavelength and „d‟ is the thickness of

the thin film. Refractive index is calculated using n = a + b/λ2.where „b‟ and „a‟ are

constants. Absorption of light in the transparent region could be due to the Urbach tail,

multi-phonon absorption, defect absorption or light scattering [19-30].

Page 66: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

42

Figure 3.5 Transmission spectrum as a function of wavelength to measure the

thickness and refractive index

Absorption process dependent upon the wavelength is complicated, so Taylor series is

used. If the absorption is small around phonon energy (which is far from any absorption

lines) „α‟ could be expressed using the relation α = c + f/λ + g/λ2

+…, here, „f ‟and „g‟ are

the constants for refractive index involved.

For the calculation of „α‟ in the high absorption region, „n‟ and„d‟ are used in the curve

fitting. In the medium to high absorption regions, the exact solution for „x‟ of equation 5

gives the value for at every wavelength.

/ / 2 1/ 2( / ) [( / ) 4 ]

2

C A T C A T B Dx

D

(6)

Where

cos/ CC

and

Page 67: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

43

dx /)ln(

The calculation for energy band gap was done by using the Tauc relation, Eq. 7 [20].

n

gEhAh )( (7)

Where „hυ‟ is the photon energy and „n‟ is related to the transition type, i.e., for direct

allowed transition type it is equal to

; and for indirect allowed transition, it is taken as 2

and for direct forbidden its value

. „α‟ is absorption coefficient its value was calculated

by the famous relation,

(

) (

)

where „d‟ was thickness and „T‟ the transmission of the sample. [31]. Thickness (d) was

calculated by using Eq. 8

)(4 minmax

minmax

nd

(8)

Here „n‟ is refractive index, λmin is minimum wavelength and λmax is consecutive

maximum wavelength.

Figure 3.6 Schematic of spectrophotometer for transmittance

Page 68: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

44

3.5 Electrical measurements

The electrical properties of semiconductors are of great importance due to the correlation

with the optical and surface properties. Resistivity, mobility, carrier concentration and the

type of semiconductor are the major concerns in electrical measurements. Generally two

types are considered for electrical measurements; two point and four point probes. Four

point probe method at room temperature was used in Hall effect measurements.

3.5.1 Hall effect

Edwin H. Hall discovered the Hall effect in 1879 for finding the carrier concentration,

mobility and resistivity. Magneto-sheet resistance and Hall coefficient for

semiconductors can also be calculated. An electric current and a perpendicular magnetic

field are applied through a semiconductor specimen as shown in Fig. 3.7. A Hall voltage

is generated due to the setup of this field, which is perpendicular to the flowing current.

The resistivity of arbitrary shaped specimen with ohmic contact can be measured using

Van der Pauw technique. Sheet resistance „Rs‟ can also be calculated using this technique

[32].

Figure 3.7 Schematic of Hall effect

There are two characteristic resistances RA and RB associated with four point probe

terminals as shown in Fig. 3.8. (The positive magnetic field is applied in the first step,

current applied between terminal 1 & 3 and measured the voltage dropped i.e. V24. Now

Page 69: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

45

reverse the direction of current and measure the V42. By applying the current between the

terminals of 2 & 4, the voltage drop V13 is measured. The voltage dropped V31 is

measured by reversing the current on the terminals. In next step, the values of V13, V24,

V42 and V31 is measured by reversing the direction of magnetic field. So with the help of

these eight values of voltages, resistances are calculated.)

12

43

I

VRA

and

23

14

I

VRB

For RA the voltage across terminal 4 and 3 is V43 and current I12 is across terminal 1 and 2.

Similarly voltage across terminal 1 and 4 is denoted by V14 with current through terminal

2 and 3 is I23. The sheet resistance of Van der Pauw can be related using;

Figure 3.8. Contact formation on thin film

1)exp()exp(

S

B

S

A

R

R

R

R

(9)

Eq. 9 can be solved numerically for RS. The equation for the calculation of resistivity;

where „t‟ is the thickness of the film.

tRS (10)

The resistivity of an arbitrary shaped sample can be calculated using:

Page 70: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

46

ρA =

fA tS

( )

and ρB =

fB tS

( )

, here ρA and ρB are the

resistivites measured in Ω-cm, the average of these value leads to ρave. „V1‟ to „V8‟ are

potential differences in volts and „I‟ is the measured current in amperes. fA and fB are

geometric factors depend upon the shape of the sample. For symmetric sample, fA and fB

have unit value otherwise fA and fB are calculated by QA =

and QB =

, the

relation between Q and f is given by the expression

=

( ) cos

-1h(

( )

).

A set of voltage is measured against Hall voltage measurements by taking current and

perpendicular B-field. The value of sheet carrier concentration can be calculated using

the relation;

H

SVq

IBn

(11)

In above expression „q‟ is charge and „I‟ is the amount of current flowing through the

sample. The thickness and uniform specimen is important to overcome errors, size and

quality of ohmic contacts.

The basic physics involved in the Hall measurements are the equilibrium between

magnetic and electric force; i.e. Fm = Fe.

So

Where „vd‟ is the drift velocity, the current is also expressed in terms of vd; i.e. I = nqAvd.

In this expression A is the area, n is density of charges. Now the expression of Fm = Fe

can be written as:

(12)

Here „VH‟ is Hall voltage and „w‟ is width.

Now

Page 71: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

47

finally

The information about type of conductivity (n-type or p-type) can be attained from the

Hall voltage „VH‟. If „VH‟ has positive value it confirms p-type conductivity and vice

versa.

Resistance „R‟ can be calculated using the relation

In this expression „ρ‟ is resistivity, „A‟ is area, „L‟ is length, „ ‟ is width and „t‟ is

thickness of the film.

Electrons and holes mobility „µ‟ can be calculated using the Eq. 13 [33]:

(13)

Parameters involved in above expression have the usual meanings.

Page 72: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

48

References:

[1] K. L. Chopra and S. R. Das, Thin Film Solar Cells, Plenum Press, New York

(1983).

[2] D. K. Schroder, Semiconductor Materials and Device Characterization (2nd

Edition), A Wiley-Inter Science Publication, John Wiley & Sons, Inc., (1998).

[3] B. D. Cullity, S. R. Stock, Elements of X-Ray Diffraction 3rd Edition, Prentice

Hall, New York, 2001.

[4] B. E. Noltingk, Instrumentation Ref. Book (2nd

Edition), Butterworth

Heinemann Ltd. 92 (1995).

[5] J. Pattar, S. N. Sawant, M. Nagaraja, Shashank, K. M. Balakrishna, G. Sanjeev,

H. M. MaheshInt. J. Electrochem. Sci., 4 (2009) 369.

[6] G. B. Williamson, R. E. Smallman, III. Dislocation densities in some annealed

and cold-worked metals from measurements on the X-ray debye-scherrer

spectrum, Phil. Mag., 1 (1956) 34.

[7] C. R. Brundle, C. A. Evans, J. S. Wilson, Encyclopedia of Materials

Characterization, Butterworth-Heinemann, USA, 1992.

[8] Y. Leng, Materials Characterization, John Wiley & Sons (Asia), Singapore,

2008.

[9] G. Gordillo, M. Botero, J. S. Oyola, , Micro. J., 39 (2008) 1351.

[10] R. Amutha, Adv. Mat. Lett., 4 (3) (2013) 225.

[11] K. Ersching, C. E. M. Campos, J. C. de Lima, T. A. Grandi, S. M. Souza, D. L.

da Silva, P. S. Pizani, J. Appl. Phys., 105 (2009) 123532.

[12] M. Patel, I. Mukhopadhyay, and A. Ray, Opt. Mater., 35 (2013) 1693.

Page 73: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

49

[13] D. Papadimitriou, N. Esser, C. Xue, Phys. Status Solidi (b), 242 (2005) 2633.

[14] K. Prabakar, S. K. Narayandass, D. Mangalaraj, Physica B, 328 (2003) 355.

[15] F. de M. Flores, J. G. Q. Galvan, A. G. Cervantes, J. S. Salazar, A. H.

Hernandez, M. de la L. Olvera, M. Z. Torres, M. M. Lira, AIP Advances, 2

(2012) 022131.

[16] R. Howland and L. Benatar, A practical guide to Scanning Probe Microscopy,

(2000).

[17] M. Ohring, Academic Press, San Diego, New York, Boston, London, Sydney,

Tokyo, Toronto, (1992).

[18] R. Swanepoel, J. Phys. E. Sci. instrum., 6 (1983) 1214.

[19] A. Ali, N. A. Shah, A. Maqsood, Solid State Electron., 52 (2008) 205.

[20] W. Mahmood, N. A. Shah, AIP Conf. Proc., 1476 (2012) 178.

[21] S. S. Hegde, A. G. Kunjomana, K. A. Chandrasekharan, K. Ramesh, M.

Prashantha, Physica B: Cond. Matt., 406 (2011) 1143.

[22] T. S. Moss, Optical Properties of Semiconductors, Butterworths Publications,

London, 1959.

[23] S. M. Sze, Physics of Semiconductor Devices, 2nd Edition, Wiley& sons, New

York, 1981.

[24] S. Sohila, M. Rajalakshmi, C. Ghosh, A. K. Arora, and C. Muthamizhchelvan, J

Alloys Compd., 509 (2011) 5843.

[25] S. M. Sze, Physics of Semiconductor Devices 3rd

Edition, John Wiley & Sons,

New Jersey, 2007.

Page 74: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication and Characterization Techniques

50

[26] J. Singh, Optical Properties of Condensed Matter and Applications, John Wiley

& Sons Ltd, England, 2006.

[27] O. Stenzel, The Physics of Thin Film Optical Spectra, Springer-Verlag, Berlin,

2005.

[28] A. Luque, and S. Hegedus, Handbook of Photovoltaic Science and Engineering,

John Wiley & Sons Ltd, England, 2003.

[29] R. H. Bube, Photovoltaic Materials, Imperial College, London, 1998.

[30] P. Bhattacharya, Semiconductor Optoelectronic Devices 2nd Edition, Prentice

Hall, New Jersey, 1996.

[31] N. A. Shah, R. R. Sagar, W. Mahmood, W. A. A. Syed, J Alloys Compd., 512

(2012) 185.

[32] L. J. van der Pauw, et al, Philips Research Reports, 13 (1958) 1; L. J. van der

Pauw, Phillips Tech Rev. 20 (1958) 220.

[33] J. A. Mccormack, J. P. Fleurial, Conf. Proc., (1991) 135.

Page 75: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 4 Fabrication of

ZnTe Thin Films

Page 76: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

52

The chapter describes the fabrication of ZnTe thin films and the optimization of deposition parameters. The

work has been published in international journal of Chalcogenide letters with the title:

“INVESTIGATION OF SUBSTRATE TEMPERATURE EFFECTS ON PHYSICAL PROPERTIES OF

ZnTe THIN FILMS BY CLOSE SPACED SUBLIMATION TECHNIQUE”

4.1 Samples preparation

ZnTe fine powder of 99.99 % purity was used as the base material for deposition of ZnTe

thin films on soda lime glass substrates (2.5 cm x 5 cm). Before the deposition, the

substrate slides were cleaned with iso-propanol (I.P.A) in an ultra-sonic bath (60°C bath

temperature and sonication for 20 minutes) and dried in air. A small amount (20 mg) of

ZnTe fine powder was placed in a graphite boat and was heated by 1000 W halogen

lamp. The substrate and source heaters were linked through separate triacs to the main

supply and gate triacs linked directly to the temperature controllers. In both, graphite

substrate slab and source graphite boat, K-type thermocouples were inserted. The

distance between the source and the substrate was optimized and kept at 5 mm for good

quality films. The chamber was evacuated with the help of rotary and diffusion pumps to

∼1 x 10−4

mbar prior to deposition. The substrate slides were placed at three different

optimized temperatures 300˚C, 330˚C and 360˚C in three separate sets of experiments.

The source temperature was kept at 450˚C and deposition was carried out for three

minutes each. After depositing the films, the source heater was switched off; while the

substrate heater was kept on to avoid further deposition. In order to avoid oxidation, films

were cooled to room temperature in vacuum. As-deposited ZnTe thin films showed good

adhesion and uniformity.

4.2 Results and discussion

4.2.1 Structural study

XRD patterns of ZnTe thin films fabricated at three substrate temperature (300, 330 &

360ºC) are shown in Fig. 4.1. All ZnTe thin films had cubic structure, polycrystalline

nature with preferred orientation in [111] direction.

Page 77: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

53

The lattice parameter calculated from these scans is 6.0980 Å, as compared with the

International Card of Diffraction Data (ICDD) reference card No. 01-089-3054. The

average crystallite size of thin films, calculated by using Scherrer‟s formula is shown in

Table 4.1 (Eqn. 2, 3 and 4 in Ch. 3) [1-15]. The deposited thin films compositions depend

on the number of Zn and Te atoms reaching the surface and diffusing on the substrate.

The film‟s crystallinity improves with the increase in substrate temperature as reported

earlier [16]. At low temperatures the available thermal energy is much less than the

required for the inter-diffusion of atoms which also results in a slow reaction between Zn

and Te. In this case, the temperature gradient between the substrate and source results in

the quenching of the atoms on the substrate with a reduced diffusion length and also

mobility of adatoms [16-18]: As the substrate temperature increases, there is sufficient

thermal energy available for arriving atoms resulting in increase in the reaction between

Zn and Te. This, in effect becomes the recipe to achieve good quality film.

Figure 4.1 XRD patterns of ZnTe films deposited at different substrate temperatures

The vertical composition of the deposited film evolves with time – that is, at early stages

of deposition, the material having high partial pressure (Te in this case) will be in excess.

Page 78: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

54

This results in rapid consumption of Te. As the time passes by, the amount of Te will

become considerably lesser than Zn. At this stage, Zn partial pressure will supersede the

Te vapor pressure, resulting in increasing concentration of Zn which moves towards the

surface. Although, the above discussion is generic and applicable to other deposition

techniques as well, such as sputtering and e-beam evaporation, I notice interesting effects

of increasing substrate temperature. In other words, in parallel to the universal

phenomenon discussed above, the substrate temperature also dictates the overall

stoichiometry. At low substrate temperature, the vapor pressure of Te is higher than that

of Zn. Due to this, Te remains in excess in these films. Typically, at low vacuum (10-2

bar), the surface evaporation rate for Zn is between 10-2

to 10-3

g-cm-2

sec-1

at T < 500 C,

whereas, that for Te is less than Zn at this temperature. High vapor pressure material

vaporizes more rapidly than low vapor pressure material [17]. Also, a smaller

atom/molecule will be more mobile at any substrate temperature. Regardless of the

quantity of Zn atoms in the film, more mobile Te atoms have the larger probability of

finding Zn atoms and bond there – that is, at higher substrate temperature the effect will

be more pronounced, resulting in better crystallinity. Notice that, the distance between

source and the substrate was kept 5 mm in our technique. Therefore, vapor pressures (and

flux) of Zn and Te are very high due to close proximity. In contrast, other techniques that

incorporate comparatively larger source-to-substrate distance (sputtering and e-beam

evaporation), such high vapor pressure is not possible. Consequently, the flux of

evaporated species is not very high. Here, by increasing the substrate temperature, the

stoichiometry is improved by increasing the Zn-Te reaction. Thereby, their inter-sticking

is enhanced.

Crystallite size in ZnTe thin films increases from ~ 37 to 63 nm because of increase in

the diffusion length and mobility of the arriving species at higher temperature: an

elevated temperature increases the surface diffusion and gives a larger relaxing time to

the mobile atoms before they settle at a crystal site. Small crystallites merge and

rearrange to form a larger crystallite at high temperature [17]. By the same token, the

dislocation density decreased from ~ 7.24 x 10-4

to 2.49 x 10-4

nm-2

, and strain is reduced

from 9.32 x 10-4

to 5.47 x 10-4

, Table 4.1.

Page 79: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

55

As a final note to this section, in CSS technique, a high vapor pressure at an elevated

substrate temperature dictates the adatoms to move to the lattice sites energetically more

favorable, results in larger crystallite size and reduces imperfections in as-deposited ZnTe

thin films.

Table 4.1 Structural parameters of ZnTe thin films deposited at different substrate

temperatures (300, 330 & 360ºC)

Substrate

temperature (˚C)

Crystallite Size

„D‟(nm)

Dislocation Density

(x10-04

nm-2

) ± 0.01

Strain

(x10-04

) ± 0.01

300 37 7.24 9.32

330 44 5.12 7.84

360 63 2.49 5.47

4.2.2 Optical study

Transmission spectra for ZnTe thin films deposited at different substrate temperatures are

recorded at room temperature as shown Fig. 4.2 for the percentage transmission plotted

against the incident wavelength.

Figure 4.2 Transmission spectra of ZnTe films deposited at different substrate

temperatures as a function of wavelength

Page 80: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

56

The oscillations show the constructive and destructive interference phenomenon in thin

films. Almost similar behavior was observed in all sets of as deposited ZnTe thin films

with a small change in peak position towards lower wavelengths. A sharp decrease in

oscillations occurred below 500 nm wavelength was due to absorption edge. The

absorption edge reflects the crystalline or amorphous nature of thin films in transmission

spectra [19]. The transmission in visible region was between 60 to 80 % and more than

90 % in infra-red (IR) region. The variations in transmission caused by higher substrate

temperature may be due to enhanced structural order, removal of defects, residual stresses

and improved stoichiometry formed during the deposition of thin films.

The absorption co-efficient „α‟ is related to the optical energy band gap (Eg) by the

relation [19, 20],

( ) (1)

In Eq. 1, „n‟ tells about the transition type; for direct allowed transition type its value is

½, for indirect transition, the allowed value of n is 2 and for direct forbidden transition,

its value is 3/2. For allowed optical transition type is direct for ZnTe films,

(

)

where, mo is free electron mass and n is refractive index, ν is frequency of the radiation

and h is Plank‟s constant [21, 22].

The transmission spectra with variation in angle (zero to 50 degrees) of incident light on

the samples are shown in Fig. 4.3. A slight increase in the transmission of light through

the samples at higher angles in IR region was observed. The oscillations moved slightly

through lower wavelengths as angle of incident light is increased. The samples are more

transparent at higher angles in IR region. The variations in transmission might be due to

trapping of light through the ZnTe thin films which affects the optical properties of ZnTe

thin films.

Page 81: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

57

Figure 4.3 Transmission spectra of ZnTe films as a function of wavelength with

angle variation (a) 300ºC and (b) 330ºC

Fig. 4.4 shows a graph between ( hν)2 as a function of incident photon energy (hν). The

energy band gap was determined by extrapolating linear part of the curves (Fig. 4.4). As

the substrate temperature increased, optical band gap increased from 2.173 ± 0.001 eV to

2.233 ± 0.001 eV. The variation in energy band gap of different substrate temperatures

were due to band tailing caused by disorderedness [23]. The optical scattering reduced as

the structure improved at higher substrate temperature. At lower substrate temperature,

the variation in the band gap was due to quantum confinement of charge carriers in

crystallites. The narrow band gap was due to reduction in the defects in the films. At

higher substrate temperature, the films were homogeneous with fewer defects. The band

gap was reduced as the decrease in localized density states occurred in the band structure.

The effect of band gap is also related to the crystallite size. The crystallite size with

packing density was improved. The defects, lattice strain and decreased structural

disorder results in optical study strongly correlated with structural study [16, 22].

Page 82: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

58

Figure 4.4 Optical band gap of ZnTe films deposited at (a) 300ºC, (b) 330ºC and (c)

360ºC

A slight decrease in refractive index of the films observed because the films were more

homogeneous and uniform at higher substrate temperatures as discussed in XRD study.

The enhancement in crystallanity of the films at elevated substrate temperature

consequently shifted the optical band gap as shown in Table 4.2. The thicknesses of the

films were 500 to 1000 nm. The films which were deposited at relatively higher

temperature of substrate are almost stoichiometric due to high vapor pressure of Zn and

Te during the sublimation process so their band gap was nearly equal to that of bulk

material.

Page 83: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

59

Table 4.2: Energy band gap and refractive index of the ZnTe thin films deposited at

different substrate temperatures (300, 330 & 360ºC)

Substrate temperature (˚C) Band Gap

± 0.001 (eV)

Refractive Index

± 0.001

300 2.227 2.572

330 2.236 2.535

360 2.173 2.504

Optical density (O.D) is the measure of transmittance of an optical medium for a given

wavelength [17]. Optical density was calculated by Eq. 2.

(2)

Here „T‟ is the transmittance. Higher the optical density, lower the transmittance. At

lower substrate temperature, the transmission of light through ZnTe thin films was

decreased as compared to higher substrate temperature which may increase O.D of these

thin films. At higher value of substrate temperature, the transmission was high so O.D

was decreased in these films. The transmission is correlated to the refractive index of thin

films which results at higher value of refractive index means higher O.D as shown in Fig.

4.5 (a).

Optical conductivity of ZnTe films can be calculated using Eq. 3.

(3)

Where α is the absorption co-efficient, n is refractive index and c is the speed of light. At

higher substrate temperature, ZnTe thin films were more homogeneous and showed

enhanced optical conductivity Fig. 4.5 (b) [17]. The optical scattering reduced as the

structure improved at higher substrate temperature. At lower substrate temperature, the

variation in the band gap was due to quantum confinement of charge carriers in

crystallites.

Page 84: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

60

Figure 4.5 (a) Optical density and (b) optical conductivity of ZnTe films deposited at

three substrate temperatures (300, 330 & 360ºC)

On the concluding note, the narrow band gap was due to reduction in the defects in the

films. At higher substrate temperature the films were homogeneous with fewer defects.

The band gap was reduced as the decrease in localized density states occurred in the band

structure and transmission is enhanced. The effects of band gap also related to crystallite

size. The crystallite size with packing density was improved. The defects, lattice strain

and structural disorder decreased results in optical study strongly correlated with

structural study [16-22].

4.2.3 Electrical study

Four probe technique was used to measure the electrical properties of ZnTe thin films

fabricated by CSS technique. As-deposited ZnTe thin films are of p-type confirmed by

Hall measurements. Similar results were reported earlier [22-24]. Resistivity, mobility

and carrier concentrations of the films are given in Table 4.3. At higher substrate

temperature, a slight decrease in resistivity of as-deposited ZnTe thin films was observed

due to decrease in the defect states. The lowest value of resistivity ~ 1.58 x 105 Ω-cm was

achieved. ZnTe thin films were grown at lower substrate temperature had large number of

defects due to greater disorder in structure [25, 26]. The lowest value of resistivity using

other techniques were 5.5 x 106 [26] and 2.5 x 10

7 Ω-cm [9] which is much higher than the

Page 85: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

61

CSS technique. The carrier concentration had values of 1.44x1011

cm-3

was achieved with

CSS technique. The values are more favorable for back contact solar cell as compared to

other techniques [9, 26].

Table 4.3 Carrier concentrations and resistivity of ZnTe thin films deposited at

different substrate temperatures

Substrate

temperature (˚C)

Carrier concentration

(cm-3

)

Resistivity

(Ω-cm)

300 4.01x109 2.3x10

5

330 3.11x1010 1.72x10

5

360 1.44x1011

1.58x105

4.3 Summary

Good quality ZnTe thin films were deposited on soda lime glass substrates by close

spaced sublimation (CSS) technique due to efficient use of materials and high

throughput. The high vapor pressure of source material in CSS technique made it

significant compared to other techniques. Temperature of the substrate was the main tool

to couture the structural, optical and electrical properties of the deposited films. The films

exhibited a cubic structure and were polycrystalline. The crystallite size increased with

increase in the substrate temperature. The transmission value was more than 70 % in

visible region and 90 % in IR region. The transmission was found to be strongly affected

by the substrate temperature due to tellurium micro crystallites presence in the films and

having greater absorbance in the visible domain. These effects consequently lead to

increase in the optical energy band gap width with increasing temperature of the

substrate. The energy band gap study showed that the optical data was allowed direct

transition type. The energy band gap increases from 2.17 eV to 2.23 eV by increasing the

temperature of the substrate. Larger crystallite sizes were noticed in the thin films

deposited at higher substrate temperatures and nearly had homogeneous composition.

The lowest value of resistivity was about 105 Ω-cm. Optical and electrical properties of

films were found to be favorable by the improvements in substrate temperature. Thus

Page 86: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

62

homogeneous composition and good crystallanity were achieved by using CSS technique

for ZnTe thin films by increasing the temperature of substrate in a suitable manner, which

are the basic requirement for the device and quality of the ZnTe thin films.

Page 87: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

63

References:

[1] A. Erlacher, A. R. Lukaszew, H. Jaeger, B. Ullrich, Surf. Sci., 600 (2006) 3762.

[2] K. R. Murali, M. Ziaudeen, N. Jayaprakash, Solid State Electron., 50 (2006)

1692.

[3] T. Barona, K. Saminadayarb, N. Magnea, J. Appl. Phy., 83 (1998) 1354.

[4] J. T. Trexler, J. J. Fijol, L. C. Calhoun, R. M. Park, P. H. Holloway, J. Cryst.

Growth, 159 (1996) 723.

[5] H. Bellakhder, A. Outzourhit, E. L. Ameziane, Thin Solid Films, 382 (2001) 30.

[6] A. M. Salem, T. M. Dahy, Y. A. El-Gendy, Physica B, 403 (2008) 3027.

[7] V. S. John, T. Mahalingam, J. P. Chu, Solid-State Electronics 49 (2005) 3.

[8] S. S. Kale, R. S. Mane, H. M. Pathan, A. V. Shaikh, Oh-ShimJoo, S. Han, Appl.

Surf. Sci., 253 (2007) 4335.

[9] A. K. S. Aqili, Z. Ali, A. Maqsood, J. Cryst. Growth, 317 (2011) 47.

[10] A. A. Ibrahim, Vacuum 81 (2006) 527.

[11] J. Pattar, S. N. Sawant, M. Nagaraja, N. Shashank, K. M. Balakrishna, H. M.

Mahesh,Int. J. Electrochem. Sci., 4 (2009) 369.

[12] D. Noda, T. Aoki, Y. Nakanishi, Y. Hatanaka, Vacuum 51 (1998) 619.

[13] A. K. S. Aqili, A. Maqsood, Appl. Opt., 41 (2002) 218.

[14] K. Sato, S. Adachi, J. Appl. Phys., 73 (1993) 2.

[15] M. Nishio, Q. Guo, H. Ogawa, Thin Solid Films, 343 (1999) 512.

[16] S. Venkatachalam, D. Mangalaraj, S. Narayandass, J. Phys. D Appl., 39 (2006)

4777.

Page 88: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of ZnTe Thin Films

64

[17] K. L. Chopra and S. R. Das, Thin Film Solar Cells, Plenum Press, New York

(1983).

[18] A. H. Moharram, F .M. Abdul Rahim, Vacuum, 73 (2003) 113.

[19] A. Saeed, N. Ali, W. A. A. Syed, Chalcogenide Letters, 10 (2013) 143.

[20] N. A. Shah, R. R. Sager, W. Mahmood, W. A. A. Syed, J. Alloy. Compd., 512

(2012) 185.

[21] M. T. Bhatti, M. I. Raza, A. M. Rana, J. Res. Sci., 15 (2004) 369.

[22] A. K. S. Aqili, A. Maqsood, Z. Ali, Appl. Surf. Sci., 191 (2002) 280.

[23] L. J. van der Pauw, et al, Philips Research Reports, 13 (1958) 1; L. J. van der

Pauw, Phillips Tech Rev. 20 (1958) 220.

[24] J. A. Mccormack, J. P. Fleurial, Conf. Proc. (1991) 135.

[25] N. Ali, S. T. Hussain, M. A. Iqbal, Z. Iqbal, D. Lane, Chalcogenide Letters, 9

(2012) 435.

[26] T. Mahalingam, J. Semicon. Sci. Technol. 17 (2002) 465.

Page 89: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 5 Effect of Ag

Doping on ZnTe

Thin Films

Page 90: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

66

This chapter discusses the effect of silver doping on structural, surface, optical and electrical properties of

ZnTe thin films grown by close spaced sublimation (CSS) technique. Some of these results have been

published in Journal of Thin Solid Films entitled as: “Physical properties of sublimated zinc telluride thin

films for solar cell applications”.

Zinc telluride (ZnTe) thin films have been fabricated using PVD and CVD techniques for

solar cell applications due to its tunable optical and electrical properties [1-12]. The

resistivity of as deposited ZnTe thin films is very high. However, the process of doping

can reduce their resistivity. Doping can be achieved via thermal evaporation [13], as well

as solution based methods, like ion exchange method in II-VI semiconductor thin films.

The ion exchange process is recently utilized – one of the most convenient and cost

effective methods for doping in the II-VI semiconductor thin films [14, 15].

In solar cell applications, low resistive p-type ZnTe is required as an interfacial or buffer

layer between the CdTe layer and the back metal contact for cadmium telluride (CdTe)

solar cells. ZnTe has small valence offset (E < 1 eV for a ZnTe/CdTe heterojunction)

and thus considered as an ideal intermediate layer between the CdTe based absorber and

metallic back contact in order to improve the performance of cell [1, 2]. Due to self-

compensating effect, conduction type is p-type in nature in ZnTe thin films: doped

monovalent atoms substitute the Zn ion and create one hole per substitution, which is

required for shallow acceptor level. Silver (Ag) (group-I element) may be incorporated as

an acceptor dopant in the II-VI semiconductors [16], because it gives rise to the hole

concentration and influences the electrical conductivity as well as optical properties of

materials of interest. Furthermore, the optical properties can also be tuned via copper

doping into II-VI semiconductors thin films, as reported earlier [17]. The purpose of this

study is to tailor the material with doping and to get the low resistive ZnTe thin films for

the use of interfacial layer between absorbent CdTe and metal back contact in solar cells.

Page 91: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

67

5.1 Silver doping procedure

The as-deposited ZnTe thin films were cut into small pieces and were immersed in low

concentrated AgNO3 solution for different times (5, 10, 20, 25 and 30 minutes) made

from 1 gram of AgNO3 in 1 litter of distilled water at room temperature. In AgNO3

solution due to ion exchange process, Ag diffusion could be possible in ZnTe thin films.

Substitution of Zn2+

with Ag1+

may be possible due to following reaction.

( ) ( )

It may be possible that Ag incorporation acts as interstitial or as formation of Ag2Te in

ZnTe thin films [7, 17]. Afterwards, the Ag-doped thin films were cleaned with IPA and

dried. Ag layer was formed on the ZnTe thin films surface due to the ion exchange

method. These films were annealed at 380 0C for one hour under vacuum of 10

-2 mbar to

diffuse Ag into the films. Different AgNO3 immersion times lead to different composition

of Ag in ZnTe thin films. The doped and un-doped thin film samples were characterized

for their structure, surface morphology along with composition, electrical and optical

properties. The comparison was made in order to understand the effects of such doping

with the earlier reported results.

5.2 Results and discussion

5.2.1 Structural study

The as-deposited ZnTe thin films and annealed Ag-doped ZnTe samples exhibited

polycrystalline structure as shown in XRD patterns of Fig. 5.1. The crystal structure was

found to be cubic with preferred growth orientation along [111]. Crystallite size was

determined using Scherrer‟s formula and micro strain and dislocation density was also

determined [18, 19]. Reflections peaks (111), (220) and (311) obtained from the XRD

pattern for as-deposited ZnTe thin film was confirmed by using ICDD card number 01-

089-3054. Furthermore, the reflection peaks of Ag-doped ZnTe sample was confirmed by

using ICDD card number 00-004-0783. In Ag-doped ZnTe thin film samples, Ag diffused

Page 92: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

68

interstitially or substituted Zn+2

, which was evidenced by the absence of AgTe compound

peak. The slight shifting of peaks towards large 2θ values in the XRD patterns could be

evidence of the increase in volume of unit cells. The crystallite size of as-deposited

samples was 23 to 27 nm which increased up to 48 nm after doping as shown in Fig. 1 (a)

& (b) respectively, but preferred orientation was still [111]. The crystallite size of Ag

varied from 17 nm to 28 nm. The decreasing trend in lattice constant of doped samples

(05 min, 15 min and 30 min) might be due to the substitution of Zn+2

(Zn+2

ionic radius

0.074 nm) with Ag+1

as Ag has smaller ionic radius of 0.0126 nm. In other samples (10

min, 20 min and 25 min) the d-spacing increased gradually which could be due to

interstitial placement of Ag into ZnTe structure [18]. The difference in intensity levels of

as-deposited and Ag-doped samples is shown in Fig. 5.1. A decreasing trend in strain and

change in crystallinity of the films after Ag immersion was also observed.

Figure 5.1 X-ray diffraction patterns of (a) as-deposited (b) Ag-doped ZnTe sample

Page 93: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

69

5.2.2 Surface morphological study

SEM micrographs of annealed Ag-doped ZnTe thin film samples 05, 10, 20 and 30 min

are shown in Fig. 5.2. The SEM images clearly show different surface morphologies as

the number of grains decreases with an increase in the doping time. Average lengths of

elongated Ag grains were found to be 280 ± 20 nm. The Ag-doped ZnTe samples showed

that for increased immersion time, a decrease in the length of Ag grains occurred. It is an

advantage of the ions exchange process that after annealing the AgNO3 layer diffused

into ZnTe thin films. Furthermore, the shape of grains also changed significantly by

increasing immersion time because more ions were exchanged and also Ag composition

increased with time. The average grain size of as-deposited ZnTe thin film was found to

be 350 nm. After annealing, the Ag-doped ZnTe samples showed larger grain sizes as

more and more Ag is diffused in ZnTe thin films with increasing immersion time. The

Ag-doped samples have larger grain size due to stress produced in the layer of the thin

films for different times. By gradually increasing the doping time, the grain size of the

Ag-doped ZnTe samples increased from 400 nm to 710 nm. The generation of larger

grains was due to the coalescence of Ag grains into ZnTe grains as depicted by Fig. 5.2.

This is also supported by the XRD results. In SEM images, it can be seen that the

numbers of Ag grains reduced as shown in Fig. 5.3 (a) and size of Ag-doped ZnTe

samples increased. It was assumed that Ag has diffused in the films during the annealing

process and findings from the EDX measurements confirm the claim, where the

composition of Ag up to 6 atomic % increased, respectively. The EDX results shown in

Fig. 5.3 (b) confirmed the Ag contents increased with immersion time. It appears that

annealing in the presence of Ag facilitates the grain growth in ZnTe thin films in such a

manner that some small grains coalesce together and reorient themselves. The re-

orientation of the grain‟s growth in metal doped II-VI semiconductor compounds has

been reported earlier [20, 21] which is in good agreement with XRD study. The

composition of Ag in the doped ZnTe sample was about 6 atomic % attained after 30

minutes of immersion in (0.1/100 ml) AgNO3 solution at room temperature as shown in

Fig. 5.3 (b). The results obtained with similar ZnTe samples fabricated by using two

Page 94: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

70

source evaporation technique was 4 atomic % even in higher concentrated (0.4/100 ml)

AgNO3 solution [22]. The compositional study shows that by increasing the Ag

composition relatively, the Zn composition decreased from 53 to 49 atomic %. The ion

exchange process involves both Ag as well as Zn ions. Therefore, a decrease in the

composition of one element coincides with increase of the other as shown in Fig. 5.3.

Figure 5.2 SEM images of Ag-doped ZnTe samples

Figure 5.3 (a) No. of grains of ZnTe and Ag vs. immersion time (b) atomic % of Ag

vs. immersion time

Page 95: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

71

5.2.3 Electrical study

The electrical properties including resistivity, mobility, carrier concentrations of as-

deposited and Ag-doped ZnTe thin films samples at room temperature was measured by

the Hall measurement‟s system.

The resistivity was calculated using the relation,

In this expression „ρ‟ is resistivity, „A‟ is area, „L‟ is length, „ ‟ is width and „t‟ is

thickness and R is resistance of the film.

The mobility of electron and holes was calculated by the expression

(13)

Where VH is the Hall voltage used to find the type of conductivity, q is charge and R is

resistance of the films (as discussed in Ch. 3),

These results are shown in Fig. 5.4. The resistivity of as-deposited ZnTe thin film was

105Ω-cm and decreased to 10

3 Ω-cm after 30 minutes Ag doping. The mobility of the as-

deposited ZnTe thin film was found to be 101 cm

2/Vs and increased to 10

2 cm

2/Vs after

30 minutes of Ag doping due to excess charge carrier provided by Ag atoms. The carrier

concentration increased with the increase in Ag compositions in ZnTe thin films.

The decrease in resistivity arises due to an increase in Ag composition which increases

charge carrier concentration in the Ag-doped ZnTe samples. The electrical resistivity of

as-deposited ZnTe thin films by two source evaporation technique reported earlier [23],

was in the range of 107

to 109 Ω-cm, which is 10 to 10

3 times greater than the thin films

deposited by the CSS technique in this experiment.

Page 96: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

72

Figure 5.4 (a) Atomic % of Ag vs. resistivity, (b) atomic % of Ag vs. carriers

concentration

5.2.4 Optical study

The optical parameters like energy band gap and refractive index were determined by

applying the Swanepoel model [24] from the transmission spectra. The oscillations were

due to constructive and destructive interference phenomenon in thin film. The

transmission of as-deposited ZnTe thin film was more than 85 % as shown in Fig.5.5.

The transmission spectra of as-deposited ZnTe thin film, Ag-doped ZnTe samples before

and after annealing are shown in Fig. 5.6. The transmission increases with annealing as

evident from Fig. 5.6.

The dependence of energy band gap on immersion time is shown in Fig. 5.7. The energy

band gap decreases in Ag-doped ZnTe samples which is due to change in the grain size

[25]. The energy band gap for as-deposited thin film was found to be 2.22 eV, where the

thickness of the same thin film was 2399 nm. Similar results on the energy band gap have

been observed for ZnTe thin films by using r. f. sputtering deposition technique [25]. For

Ag-doped ZnTe samples, the energy band gap decreased up to 2.09 eV as shown in Fig.

5.7, while Aqili et al [7] have reported 2.16 eV for Ag-doped ZnTe samples fabricated by

CSS technique. The energy band gap has reduced in these experiments as compared to

previous reports.

Page 97: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

73

Figure 5.5 Transmission spectra as a function of wavelength of as-deposited and Ag-

doped ZnTe samples

Figure 5.6 Transmission spectra as a function of wavelength of as-deposited,

immersed before annealing and after annealing ZnTe samples

Page 98: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

74

Figure 5.7 (a) Energy band gap of as-deposited ZnTe thin film and (b) 30 min Ag-

doped ZnTe sample

In order to study the angle resolved transmission, the transmission spectra of the as-

deposited ZnTe thin film was also taken at different angles ranging from 10 to 50

degrees. The angle resolved spectra showed an interesting behavior, which has not been

reported earlier. As the angle of incidence is increased, the transmission decreases in the

UV and VIS range due to trapping of light in ZnTe thin films. However, in the IR region,

transmission increases as the incident angle increases as shown in Fig. 5.8. This indicates

that at larger incident angles, ZnTe thin film becomes more transparent (about 5 %) in the

IR region.

Page 99: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

75

Figure 5.8 Transmission spectra as a function of wavelength with variable angles of

as-deposited ZnTe sample

5.3 Summary

On the basis of these results, it is concluded from the XRD patterns, that the ZnTe thin

films fabricated by the CSS technique have preferred orientation of [111] and shift in

peaks in Ag doping samples, indicates the diffusion of Ag into the ZnTe thin films. Due

to Ag diffusion, the grain size of the samples increases after annealing as observed in the

SEM images. The resistivity of ZnTe thin films decreases with the increase of Ag

compositions, which is also supported by the SEM results. The composition of Ag varies

from 0 to 6 atomic % as observed by the EDX results. The optical transmission of as-

deposited thin film is more than 85 % and energy band gap 2.22 eV. After Ag doping, the

transmission decreases to 70 %, energy band gap to 2.09 eV and the resistivity reduces

down to 103 Ω-cm. The transmission decreases in UV and visible regions but increases in

the IR region with the angle variations. These results show that ZnTe thin films

fabricated by this technique and doped with low concentrated AgNO3 solution by the ion

exchange method can be used as interfacial layer between CdTe absorbent layer and

metal back contact in the second generation solar cells.

Page 100: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

76

References:

[1] L. Feng, L. Wu, Z. Lei, W. Li, Y. Cai, W. Cai, J. Zhang, Q. Luo, B. Li, J.

Zheng, Thin Solid Films, 515 (2007) 5792.

[2] K. R. Murali, M. Ziaudeen, N. Jayaprakash, Solid State Electron., 50 (2006)

1692.

[3] A. Erlacher, A. R. Lukaszew, H. Jaeger, B. Ullrich, Surf. Sci., 600 (2006) 3762.

[4] P. K. Kalita, B. K. Sarma, H. L. Das, Indian J. Pure Appl. Phy., 37 (1999) 885.

[5] J. De-Merchant, M. Cocievera, Int. J. Electrochem. Sc., 143 (1996) 4054.

[6] A. A. Ibrahim, N. Z. El-Sayed, M. A. Kaid, A. Ashour, Vacuum, 75 (2004) 189.

[7] A. K. S. Aqili, A. J. Saleh, Z. Ali, S. Al-Omari, J. Alloys Compd., 520 (2012)

83.

[8] H. Bellakhder, A. Outzourhit, E. L. Ameziane, Thin Solid Films, 382 (2001) 30.

[9] V. S. John, T. Mahalingam, J. P. Chu, Solid State Electron., 49 (2005) 3.

[10] A. M. Salem, T. M. Dahy, El-Gendy, Physica B, 403 (2008) 3027.

[11] S. Bhumia, P. Bhattacharya, D. N. Bose, Mater. Lett., 27 (1996) 307.

[12] O. de Melo, E. M. Larramendi, J. M. Duart, M. H. Velez, J. Stangl, H. Sitter, J.

Cryst. Growth, 307 (2007) 253.

[13] P. U. Bhaskar, G. S. Babu, Y. B. K. Kumar, V. S. Raja, Appl. Surf. Sci., 257

(2011) 8529.

[14] N. A. Shah, A. Nazir, W. Mahmood, W. A. A. Syed, S. Butt, Z. Ali, A.

Maqsood, J. Alloy Compd., 512 (2012) 27.

Page 101: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Effect of Ag-Doping on ZnTe Thin Films

77

[15] N. A. Shah, R. R. Sager, W. Mahmood, W. A. A. Syed, J. Alloy Compd., 512

(2012) 185.

[16] H. Wolf, T. Filz, V. Ostheimer, J. Hamann, S. Lany, Th. Wichert, J. Cryst.

Growth., 214 (2000) 967.

[17] N. A. Shah, A. Ali, A. Maqsood, J. Non-Cryst. Solids, 355 (2009) 1474.

[18] R. Amutha, A. Subbarayan, R. Sathamoorthly, K. Natarajan, S. Velumani, J.

New Mat. Elect. Sys., 10 (2007) 27.

[19] J. Pattar, S. N. Sawant, M. Nagaraja, Shashank, K. M. Balakrishna, G. Sanjeev,

H. M. MaheshInt. J. Electrochem. Sci., 4 (2009) 369.

[20] A. Ali, N. A. Shah, A. K. S. Aqili, A. Maqsood, Semicond. Sci. Technol., 21

(2006) 1296.

[21] N. A. Shah, J. Alloys Compd., 506 (2010) 661.

[22] A. K. S. Aqili, A. Maqsood, Z. Ali, Appl. Surf. Sci., 191 (2002) 280.

[23] A. K. S. Aqili, Z. Ali, A. Maqsood, J. Cryst. Growth, 317 (2011) 47.

[24] R. Swanepoel, J. Phys. E. Sci. Instrum., 6 (1983) 1214.

[25] H. Bellakhder, A. Outzourhit, E. L. Ameziane, Thin Solid Films, 382 (2001) 30.

Page 102: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 6 Cu-Doping Effects

on ZnTe Thin

Films

Page 103: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

79

Copper doping effects on physical properties including structure, surface, optical and electrical properties

is discussed in this chapter. Some of the results were published in Current Applied Physics with the title:

“Effects of metal doping on the physical properties of ZnTe thin films”.

Zinc telluride (ZnTe) is a II-VI compound semiconductor material with a direct band gap

of 2.26 eV at room temperature [1-4]. ZnTe is an ideal intermediate layer between CdTe

absorber and metallic back contact for the improvement of heterojunction‟s performance

of ITO/CdTe/CdS thin films solar cell [5]. It promotes the formation of low resistive,

stable back contact for the achievement of high efficiency with long term stability of

polycrystalline ITO/CdTe/CdS solar cells.

ZnTe has been used in many other applications [6-8]. It is a p-type semiconductor so it is

used as a background layer in solar cells. It can be doped easily, for this reason, it is one

of the more common semiconducting material used in optoelectronics [9]. ZnTe has been

fabricated using many techniques [10-17]. The simple, easy and low cost method for

doping in II-VI semiconductors is ion exchange process. Cu, Ag and Al were doped using

this method [18-20].

Zinc telluride (ZnTe) thin films were deposited on soda lime glass substrate in vacuum

using close spaced sublimation technique (CSS). The ion exchange process was adopted

for copper doping on ZnTe thin film. X-rays diffraction (XRD) technique was used for

structural analysis. Scanning electron microscope (SEM) images were taken to estimate

the grain boundaries; Energy dispersive X-rays (EDX) results confirmed the copper

composition in ZnTe samples. Optical parameters like refractive index and energy band

gap were calculated using Swanepoel model. Electrical properties were measured using

Hall effect and Van der Pauw measurements to find the resistivity, mobility and carriers

concentration.

Page 104: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

80

6.1 Copper doping procedure

In the present research work, ZnTe thin film was sublimated using CSS technique. Cu

was doped in ZnTe as-deposited thin film by ion exchange process. Low concentration

(0.1/100 ml) solution of copper nitrate Cu (NO3)2 with distilled water was prepared, and

the as-deposited ZnTe thin films were immersed in the solution at 80˚C for varying times:

(05 to 40 min) and later dried in air. The doped films were annealed at 350˚C for 1 hour

to diffuse Cu into the immersed ZnTe samples. Following reaction may be possible due

to ion exchange process

6.2 Results and discussion

6.2.1 Structural study

XRD reflection peaks of ZnTe thin films showed polycrystalline behavior before and

after doping as shown in Fig. 6.1. The preferred orientation of plane was [111] with cubic

phase. No reflection peak in XRD pattern was found that corresponded to the Cu

compound after Cu immersion.

Crystallite size of ZnTe thin film was calculated using Scherrer‟s formula [21, 22] and

was 27 nm which increased slightly with Cu immersion time 05 min to 30 min as shown

in Fig. 6.2. The 40 min Cu-doped ZnTe sample showed a significant increase in

crystallite size, which was 52 nm. The slight shift in peaks towards higher 2θ values was

observed and an increase in the crystallite sizes, produced a stress in volume of unit cell

of Cu-doped ZnTe samples

Page 105: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

81

Figure 6.1 X-rays diffraction patterns of (a) as-deposited ZnTe thin film (b) Cu-

doped ZnTe sample

Figure 6.2 Variation in crystallite size as a function of Cu immersion time

Page 106: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

82

The peak intensity of as-deposited ZnTe thin films was very high as shown in Fig. 6.1,

but after doping, the relative intensity of Cu-doped ZnTe samples decreased significantly.

In un-doped and Cu-doped ZnTe thin film samples, the ratios of planes between (111)

and (311), (111) and (240), (240) and (311) was reduced, suggesting that (311) and (240)

planes reoriented after Cu immersion as compared to (111) planes. The decrease in

intensity of the doped samples and intensity ratio of different planes was due to Cu

atoms‟ diffusion in ZnTe thin films. The stress was produced, as Cu diffused interstitially

or substituted the Zn atoms. The XRD study showed that the crystallinity of ZnTe thin

films was affected and planes reoriented after immersion of Cu into ZnTe thin films.

6.2.2 Surface morphological study

SEM micrographs of as-deposited and Cu-doped ZnTe samples are shown in Fig. 6.3;

doping time was increased from 05 min to 40 min. The average grain size of as-deposited

ZnTe thin films was 138 ± 10 nm, which increased to 470 ± 10 nm after 40 min doping.

The Cu-doped samples had larger grains as compared to as-deposited ZnTe thin films;

this was due to stress produced by Cu atoms on ZnTe grains. The coalescence was also

responsible for larger grains in Cu-doped ZnTe samples [23]. Coalescence is the

phenomenon of merging the small grains into larger grain, due to this effect, the grain

sizes were increased after Cu doping.

The elemental composition of the as-deposited ZnTe thin film and Cu-doped ZnTe

samples were determined from EDX analysis and given in Table 6.1. By increasing the

Cu immersion time, the Cu composition increased in ZnTe samples. In Table 6.1, it is

very clear that Cu substituted Te composition. So Te was decreasing and the composition

of Zn was increasing after Cu immersion; this was responsible for the change in grain

size of doped samples and the conductivity was still p-type. The atomic weight of Zn is

65.37 u and Te 127.60 u so the diffusion rate of Zn is high. Due to the high diffusion rate

of Zn atoms, it is in larger amount as compared to Te atoms as shown in Table 6.1.

Page 107: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

83

Figure 6.3 SEM images of (a) as-deposited ZnTe thin film, (b) 30 min Cu-doped, (c)

35 min Cu-doped and (d) 40 min Cu-doped ZnTe samples

Table 6.1 EDX study of as-deposited ZnTe thin film and 10-40 min Cu-doped ZnTe

samples

Element As dep. 10 min Cu 20 min Cu 30 min Cu 40 min Cu

Atom %

Atom %

± 0.02

Atom %

± 0.02

Atom %

± 0.02

Atom %

± 0.02

Cu K 0 1.67 2.01 2.33 2.98

Zn K 52 53.69 52.80 52.70 54.52

Te L 48 44.64 45.19 44.96 42.51

Total 100 100 100 100 100

As-deposited ZnTe thin film Cu-doped ZnTe samples

Page 108: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

84

Tang et al. reported the compositional study of Cu-doped ZnTe samples used to check

the efficiency of solar cell as a back contact from 1 to 8 atomic % . The efficiency of

solar cell was higher for 2 atomic % of Cu in ZnTe samples. Cu diffuses in the junction

of solar cell which degrades the efficiency of solar cell [24].

6.2.3 Optical study

The optical properties are important for photovoltaic materials. The Swanepoel model

was used to calculate the thickness and refractive index of these samples [25]. The

oscillating behaviors of thin films were considered to be interference phenomenon in it.

The absorption coefficient, energy band gap and thickness were calculated using the

transmission spectra of the as-deposited ZnTe films and doped samples given in Fig. 6.4.

Figure 6.4 Transmission spectra as a function of wavelength of as-deposited ZnTe

thin films and Cu-doped ZnTe samples

(c) (d)

Page 109: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

85

The transmission in the as-deposited ZnTe was 85 %, which reduced down to 65 % for

40 min Cu-doped ZnTe sample in the visible region. The thickness of the samples was

800 to 1000 nm, refractive index 2.03 and the absorption edge lies in the visible range.

The transmission reduced due to the Cu immersion in ZnTe samples. As-deposited ZnTe

thin film has band gap of 2.24 eV as shown in Fig. 6.5, which reduced to 2.20 eV due to

Cu composition as shown in Fig. 6.6. The variation in energy band gap relates to grains

as well as carriers concentration. In previous reports, the variation of band gap of as-

deposited ZnTe thin film was 2.24 eV reducing to 2.20 eV after Cu doping [26]. The

defects level could be created near the valence band of ZnTe thin films after the Cu

immersion [27].

Figure 6.5 Determination of energy band gap of the as-deposited ZnTe thin film

Page 110: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

86

Figure 6.6 Determination of energy band gap of the 40 minutes Cu-doped ZnTe sample

6.2.4 Electrical study

Electrical properties of as-deposited ZnTe and Cu-doped ZnTe thin films were calculated

using Van der Pauw technique [28]. The electrical properties were measured at 300 K

and all measurements were repeated several times to minimize experimental error. The

resistivity of as-deposited ZnTe thin film was 105 Ω-cm. The resistivity of ZnTe samples

reduced down to 1 Ω-cm after 40 min Cu immersion, which is correlated with increase in

grain sizes of Cu-doped samples. In Fig. 6.7, the increase in immersion time of Cu results

in the reduction of resistivity. Tariq et al. reported the resistivity of Cu-doped ZnTe

samples upto 30 Ω-cm using thermal evaporation technique [18]. Aqili et al. found the

10.5 Ω-cm resistivity of Cu-doped ZnTe samples by two-source evaporation method [19].

Resistivity changed due to increase in charge carriers resulting from an increase in Cu

composition of doped ZnTe sample [27].

The mobility of as-deposited ZnTe thin film was 16 cm2/Vs and increased up to 3.5x10

2

cm2/Vs for 40 min doped ZnTe samples as shown in Fig. 6.8. The mobility increased

gradually with the increase in immersion time, which was due to the increase in carriers

which is good agreement with previous reports [29, 30]. The value of sheet concentration

Page 111: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

87

for as-deposited ZnTe thin film was found to be 4x109/cm

2 which increased to

1.4x1011

/cm2 for 40 min doped ZnTe sample. The value of sheet concentration increased

gradually up to 40 min doped ZnTe sample as shown in Fig. 6.9. The increase in sheet

concentration was due to the increase in copper composition with the increase of

immersion time. As the copper elemental composition increased, the value of resistivity

decreased while the mobility and sheet concentration increased. The lowest value of

resistivity after Ag doping was about 103 Ω-cm in previous chapter while in Cu doping it

reduced down to 1 Ω-cm which shows Cu is comparatively good dopant as compared to

Ag.

These effects were due to the Cu immersion in ZnTe thin films, and these doped samples

could be used as a back contact for second generation solar cells.

Figure 6.7 Variations in resistivity vs. Cu immersion time

Page 112: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

88

Figure 6.8 Variations in mobility vs. Cu immersion time

Figure 6.9 Variations in sheet concentration as a function of Cu immersion time

Page 113: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

89

6.3 Summary

ZnTe thin films sublimated by CSS technique exhibited polycrystalline nature with

preferred growth orientation in [111] direction. XRD showed that average crystallite size

increased from 27 nm to 52 nm. In Cu-doped ZnTe samples, it was observed that

morphology and microstructure were correlated as grain sizes were increased after Cu

immersion in low concentration copper nitrate solution. EDX results showed the

increased Cu composition immersed in ZnTe thin films. Optical study showed that

transmission decreased with increase in Cu atomic % because Cu acts as a reflecting

material. Transmissions decreased from 85 % to 65 % after Cu doping. A minute

decrease in band gap was observed after Cu doping. In electrical measurements, it was

observed that resistivity of as-deposited ZnTe thin film was 105 Ω-cm, which reduced to

1 Ω-cm after Cu immersion in low concentrated solution. The value of resistivity after

doping was reduced by several orders of magnitude and the Cu-doped ZnTe sample

showed semiconducting behavior. ZnTe could be used as an interfacial layer between

CdTe and metallic back contact for solar cells.

Page 114: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

90

References:

[1] K. Sato, M. Hanafusa, A. Noda, A. Arakawa, M. Uchida, T. Asahi, O. Oda, J.

Cryst. Growth, 214 (2000) 1080.

[2] K. R. Murali, M. Ziaudeen, N. Jayaprakash, Solid State Electron., 50 (2006)

1692.

[3] A. Erlacher, A. R. Lukaszew, H. Jaeger, B. Ullrich, Surf. Sci., 600 (2006) 3762.

[4] L. Feng, L. Wu, Z. Lei, W. Li, Y. Cai, W. Cai, J. Zhang, Q. Luo, B. Li, J.

Zheng, Thin Solid Films, 515 (2007) 5792.

[5] T. A. Gessert ad T. J. Coutts, AIP, Conf. Pro., 306 (1994) 345.

[6] H. Zhou, A. Zebib, S. Trivedi, W. M. B. Duval, J. Cryst. Growth, 167 (1996)

534.

[7] J. Britt, C. Ferekids, Appl. Phys. Lett., 62 (1993) 2851.

[8] O. Hayden, A. B. Greytak and D. C. Bell, Thin Solid Films, 385 (2001) 220.

[9] M. S. Hossain, R. Islam, K. A. Khan, J. Ovonic Res., 5 (2009) 195.

[10] T. Ishizaki1, T. Ohtomo, A. Fuwa, J. Phys. D: Appl. Phys., 37 (2004) 255.

[11] A. A. Ibrahim, Vacuum, 81 (2006) 527.

[12] K. R. Murali, M. Ziaudeen, N. Jayaprakash, J. Mater. Sci., 41 (2006) 1887.

[13] H. Bellakhder, A. Outzourhit, E. L. Ameziane, Thin Solid Films, 382 (2001) 30.

[14] A. M. Salem, T. M. Dahy, Y. A. El-Gendy, Physica B, 403 (2008) 3027.

[15] V. S. John, T. Mahalingam, J. P. Chu, Solid State Electron., 49 (2005) 3.

Page 115: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Cu-Doping Effects on ZnTe Thin Films

91

[16] S. S. Kale, R. S. Mane, H. M. Pathan, A.V. Shaikh, Oh-Shim Joo, S. Han, Appl.

Surf. Sci., 253 (2007) 4335.

[17] O. de Melo, E. M. Larramendi, J. M. Duart, M. H. Velez, J. Stangl, H. Sitter, J.

Cryst. Growth, 307 (2007) 253.

[18] G. H. Tariq, M. A. Rehman, Key Eng. Mater., 510 (2012) 89.

[19] Z. Ali, A. K. S. Aqili, M. Shafique, A. Maqsood, J. Non-Cryst. Solids, 352

(2006) 409.

[20] G. H. Tariq, M. A. Rehman, Key Eng. Mater., 510 (2012) 156.

[21] B. D. Cullity, S. R. Stock, Elements of X-Ray Diffraction 3rd Edition, Prentice

Hall, New York (2001).

[22] G. B. Williamson, R. E. Smallman, Dislocation densities in some annealed and

cold-worked metals from measurements on the X-ray debye-scherrer spectrum,

Phil. Mag., 1 (1956) 34.

[23] N. Abbas Shah, J. Alloys Compd., 506 (2010) 661.

[24] J. Tang, D. Mao, J. U. Trefny, AIP Conf. Proc., 394 (1997) 639.

[25] R. Swanepoel, J. Phys. E. Sci. Instrum., 6 (1983) 1214.

[26] V. S. John, T. Mahalingam, J. P. Chu, Sol. Stat. Electron., 49 (2005) 3.

[27] J. A. Mccormack, J. P. Fleurial, Conf. Proc., (1991) 135.

[28] L. J. van der Pauw, et al, Philips Research Reports, 13 (1958) 1; L. J. van der

Pauw, Phillips Tech Rev. 20 (1958) 220.

[29] M. Aven, Journal of Applied Physics, 38 (1967) 4421.

[30] T. Baron, K. Saminadayar, N. Magnea, Journal of Applied Physics, 83 (1998) 1354.

Page 116: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 7 Zn and Te

Enrichment in

ZnTe Thin Films

Page 117: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

93

In this chapter, for Zn enrichment in ZnTe thin films, a new method was adopted for fabrication, the atomic

% of Zn and Te was controlled and variation in physical properties was examined for solar cell

applications.

The purpose of this research work is to study the Zn and Te enriched ZnTe thin films by

close spaced sublimation (CSS) technique for low resistive back contact. The fabrication

conditions play a vital role on the physical properties [1-6]. In this context, many

researchers have reported good quality thin films by using various fabrication conditions

[5-7]. ZnTe thin films have been prepared using vapor phase epitaxy [5], molecular beam

epitaxy [8], hot wall epitaxy [9], metal organic vapor phase epitaxy [10], radiofrequency

sputtering [11], electro-synthesis [12], two source evaporation [13], thermal evaporation

[14] and CSS technique [15]. Among all of the techniques, CSS is one of the best

techniques for sublimation of II-VI solar cell materials due to some unique features such

as; a small distance between source and substrate, resulting in a rough film for light

trapping, efficient use of material (20 mg) with high throughput, a beneficial crystallite

orientation and large grain size. The vapor pressure is extremely high which makes

stoichiometric films as compared to other technique that are suitable for solar cell

applications [16, 17].

ZnTe thin films have been used as a buffer layer between CdTe and metal back contact in

CdS/CdTe solar cells. A stable and low resistance back contact is required for high

efficiency solar cells [16]. The electron affinity for CdTe is 4.28 eV; the work function

required for good ohmic contact with a metal is nearly equal to the electron affinity plus

the band gap of CdTe (i.e. 5.78 eV). Many attempts have been made to produce a good

back contact to CdTe using metal doping, but this requires a high work function. An

alternative way to make a good back contact is to use a heavily doped p-type

semiconductor. ZnTe has a valence band offset of 0.2 eV from CdTe [18]. The tunneling

of carriers takes place when the interface between the potential barrier of doped ZnTe and

CdTe is low [19]. The resistivity of ZnTe decreases with metal doping, as reported earlier

[13, 15], and must be decreased further to produce low resistive back contact.

Page 118: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

94

Stoichiometric change in Zn and Te atoms may enhance the defects in ZnTe thin films

and also reduces the resistivity of these thin films.

In this study, a new layer by layer method is adopted to produce Zn, Te-enriched ZnTe

thin films which exhibit both low resistivity and are stable as an interfacial layer used in

back contacts.

7.1 Samples preparation

Zinc telluride (ZnTe) powder of high purity (99.99 %) was used as a source material. A

glass slide was used as substrate onto which ZnTe thin film was deposited by CSS

technique. The substrate was cleaned by isopropyl alcohol (IPA) in an ultrasonic cleaner

for 20 min at 60oC, and dried in air. A cleaned graphite boat was used to sublimate the

source material (20 mg). The distance between source and substrate was set to 5 mm in

order to obtain a good quality thin film. A mica sheet was introduced with a rectangular

wide hole between source and substrate to produce a temperature gradient. The top of the

glass substrate was covered with a graphite slab for uniform heating. A combination of

rotary and diffusion pump were used to evacuate the chamber up to 10-4

mbar. Once the

vacuum level was obtained, 1000 W and 500 W halogen lamps were switched ON to heat

the source and substrate respectively. Two K-type thermocouples with temperature

controllers were inserted into the source boat and substrate slab to monitor the

temperatures. The temperatures of source and substrate were gradually increased up to

450oC and 360

oC respectively. After fabrication of a film, the lamps were switched off.

The chamber was left to cool down to room temperature.

For making Zn-enriched ZnTe thin films, a layer by layer method was adopted in which

the 1st layer of ZnTe thin film was followed by the Zn as a 2

nd layer, evaporated by the

CSS technique onto it. After the Zn evaporation, a third layer of ZnTe, was evaporated to

sandwich the Zn layer as shown in Fig. 7.1. As the sticking coefficient of Zn on the

substrate is not good and therefore Zn cannot be deposited as 1st layer on the glass

substrate [20]. In addition, pure zinc (99.99 %) was treated as a source material. Zn was

Page 119: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

95

evaporated on to the ZnTe thin film‟s substrate. The same procedure as mentioned above

was adopted for making Zn film, the only difference was source and substrate

temperatures, which were 425oC and 300

oC, respectively for sublimation of Zn. To form

a third layer of ZnTe on the 2nd

layer, the substrate temperature was kept at the source

temperature of 2nd

layer (i.e. Zn sublimation temperature 425oC) to avoid the re-

evaporation of Zn. The third layer was sublimated for 1 min, 3 min, 5 min and 7 min. The

prepared ZnTe thin films in such a way were annealed for 1 hour at 350oC, to improve

the Zn diffusion and for further study.

Figure 7.1 Schematic of Zn-enriched ZnTe thin films sandwich structure

For the enrichment of tellurium (Te) in ZnTe thin films, pure Te powder (99.99 %) was

used as a source material by using the CSS technique. Deposition was carried at a source

temperature of 350oC and substrate temperature of 250

oC. The procedure was the same as

that described for Zn enrichment. The Te-enriched ZnTe thin films were subsequently

Page 120: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

96

annealed at different temperatures (225oC, 250

oC, 275

oC and 300

oC for 1 hour and at

300oC for 2 hours) to investigate the diffusion of Te in the ZnTe thin films.

Finally, films were characterized structurally, compositionally, optically, and electrically

as described below. The effect of annealing for the applications in optoelectronics and

renewable energy technologies was investigated.

7.2 Results and discussion

7.2.1 Structural study

XRD patterns of Zn-enriched and Te-enriched ZnTe thin films are shown in Fig. 7.2 (a)

& (b) respectively. XRD patterns of the as-deposited ZnTe thin films showed the

polycrystalline nature. The preferred orientation was found to be [111] with a cubic

phase. The peak at 2Ө = 27.27 has a full-width at half-maximum (FWHM) 0.3542, which

matched with ICDD reference card No. 00-089-3054. This corresponds to a lattice

parameter of 6.0980 Å.

The crystallite size (D) was calculated using the Scherrer‟s formula [21]. The average

crystallite size for the as–deposited ZnTe thin films was found to be 28 nm in case of Zn

enrichment. After 1 min deposition of the 3rd

layer of the ZnTe thin film, the strongest

diffraction peak appears at 2Ө = 25.28 with a FWHM of 0.2755. When matched with

reference card No.00-015-0746, the lattice constant is determined to be 6.1026 Å and the

average crystallite size is 38 nm. Following enrichment of Zn, after depositing the 3rd

layer of ZnTe for 3 min, 5 min and 7 min, the values of 2Ө are 25.35, 25.15, 25.37

degrees and corresponding values of FWHM of 0.2362, 0.5510 and 0.3936 were obtained

as a strongest peak. The ICDD reference card No. 01-071-8947 was matched with 3 min

deposition, and for 7 min deposition with a difference of 0.01 degree in the value of 2Ө,

corresponding to a lattice constant of 6.0790 Å. The 5 min deposition was matched with

ICDD reference card no. 01-071-5965 with a difference of 0.01 degree in the value of

2Ө. The values of crystallite size obtained for these samples were 46 nm, 17 nm and 24

Page 121: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

97

nm, respectively due to the diffusion of Zn in ZnTe thin films by substitution or

interstitial.

The phase was found to be cubic after the Zn enrichment process in all the samples. It can

be observed in the XRD data that the number of different planes (peak intensity) is

reduced after enrichment. Seven peaks were observed in the as-deposited ZnTe thin film,

and six for the 1 min deposition of a 3rd

layer of ZnTe. For 3 min and 5 min, there were

four peaks and, after 7 min deposition, the number of peaks was three because of

orientation of peaks. The composition of Zn and Te in atom % shown in Fig. 7.2 is taken

from the EDX spectroscopy.

The ratio of planes (111) / (220), (111) / (311) and (220) / (311) was found to be in

decreasing trend as the composition of Zn increased, which shows that the planes reorient

after Zn diffusion. The value of 2Ө was found to increase corresponding to an increase in

crystallite size for the as-deposited films. The FWHM decreased, which indicated

improved crystallinity.

The decrease in the value of the lattice constant, suggested that the ZnTe thin films, after

the Zn enrichment process, might be subject to a tensile stress due to the planes of Zn and

ZnTe thin films. Pure Zn peak was not observed in Zn enriched ZnTe thin films. A color

change was also observed after the Zn-enrichment process, from brown to dull brown

which might be due to the dissipation of Te or increasing Zn composition. These results

are consistent with those of E. Bacaksiz et al. [22].

Fig. 7.2 (b) shows the XRD data for the Te-enrichment process. The annealing

temperature was increased to 250oC for 1 hour, preferred orientation peak (111) was

observed at 2Ө = 27.64 with a FWHM of 0.47. Annealing at 250oC for 2 hours gave an

amorphous film as no peak was observed in XRD data. A further increase in temperature

up to 275oC, peak was observed at 2Ө of 27.51

o with a FWHM of 0.24. The above values

of 2Ө could not be matched with any reference cards for ZnTe. Annealing to a

temperature of 300oC for 1 hour, gave a 2Ө value of 25.33

o with FWHM = 0.27.

Page 122: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

98

Figure 7.2 X-ray diffraction patterns of (a) Zn-enriched ZnTe thin films and (b) Te-

enriched ZnTe thin films

This was matched with ICDD card No. 03-065-0149 and corresponds to a lattice constant

of 6.0890 Å and a crystallite size of 46 nm. Further annealing optimization of the film

was done by increasing the annealing time up to 2 hours at 300oC. This resulted peak at

2Ө value of 25.19o, with FWHM 0.16. This matched with ICDD card No. 01-071-5965

with a crystallite size of 38 nm. The peaks ratio of (111) / (220) and (111) / (311) was

decreasing; the ratio of peak (220) / (311) was unchanged. The change in peak ratio

might be due to the planes reorientation. Mainly, (111) plane reoriented with respect to

(220) and (311) planes. The preferred orientations become shifted towards higher angular

positions with increase in Zn and Te composition. This effect was related to the reduction

in bond length of the ZnTe lattice by Zn or Te interstitials, the lattice constant also

decreased after Zn and Te enrichment. Similar results were found in the already reported

research work on CdTe [23, 24].

Page 123: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

99

7.2.2 Surface morphological study

The effect of Zn enrichment on the morphology of the ZnTe thin films is shown in Fig.

7.3. As-deposited ZnTe thin film resulted in a smooth surface, containing fine grains of

different sizes as shown in Fig. 7.3 (a). The presence of larger grains as a part of the

surface structure indicate that many changes occurred in the microstructure after

enrichment of Zn on the as-deposited ZnTe thin film. Fig. 7.3 (b), (c), (d) shows the SEM

micrographs of the samples enriched with Zn for 3 min, 5 min and 7 min respectively.

Figure 7.3 SEM micrographs of (a) as-deposited ZnTe thin film, (b) 3 min deposition

of ZnTe (c) 5 min deposition of ZnTe thin film, and (d) 7 min deposition

of ZnTe thin film

Page 124: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

100

The images show re-orientation after Zn enrichment which indicates grain merging. It

was observed that in Zn-enriched ZnTe thin films, the grain sizes were significantly

reduced. After annealing, Zn-enriched ZnTe thin films showed non-homogeneous grains

and some discontinuities. These discontinuities affect the mobility of charge carriers.

This effect occurred might be due to the separation of grains.

SEM images of Te enriched ZnTe thin films are shown in Fig. 7.4. Different annealing

temperatures were investigated as described in section 7.1. The SEM images confirm that

the grain sizes increase after Te enrichment and annealing at 300oC for 1 hour and for 2

hours. In addition, the shapes of the grains appeared to form rod like structures. A similar

approach was used on CdTe by M. A. Khan et al. [24].

Figure 7.4 SEM micrographs of (a) as-deposited ZnTe thin films, (b) after annealing

at 275oC for 1 hour, (c) 300

oC for 1 hour and (d) 300

oC for 2 hour

Page 125: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

101

The maximum Te content was found for the film annealed at 3000C for 1 hour. The

annealing process facilitates the movement of Te atoms in ZnTe thin films and is also

responsible for the grain growth in ZnTe thin films resulting in coalescence of the small

grains to bigger one and the reorientation of the grains. This coalescence is believed to

play a vital role in compound semiconductors [23]. As the surface diffusion increases,

due to the increase in annealing time and temperature, the mobility of Zn or Te impurity

atoms increases and the coalescence to form larger grains can occur.

Table 7.1 shows the elemental composition of the films obtained from EDX study. The

atomic % of Zn increases as the enrichment time in the third layer of ZnTe increases.

Similarly the Te content in ZnTe thin films also increases after annealing.

Table 7.1 EDX study of as-deposited ZnTe thin film with Zn and Te enriched ZnTe

thin films

Zn-enrichment Zn

(Atomic %)

± 0.01

Te

(Atomic %)

± 0.01

Te-enrichment Zn

(Atomic %)

± 0.01

Te

(Atomic %)

± 0.01

As-deposited

ZnTe thin film

51 49 As-deposited

ZnTe thin film

51 49

1 min Zn 52.09 47.91 2500C for 1 hour 44.12 55.88

3 min Zn 54.02 45.98 2750C for 1 hour 39.89 60.11

5 min Zn 56.34 43.66 3000C for 1 hour 37.42 62.58

7 min Zn 67.91 32.09 3000C for 2 hour 38.78 61.22

7.2.3 Optical study

The optical properties of the films were measured using UV-VIS-NIR spectrophotometer

(LAMDA 950) in transmission mode at room temperature. The energy of the band gap

was measured by using the Swanpoul model [25]. Oscillations (shown in Fig. 7.5.) were

due to interference phenomenon in these films. The as-deposited ZnTe thin film

displayed greater than 80 % transmission. A graph comparing transmission of the as-

deposited ZnTe thin film and Zn-enriched ZnTe thin films is shown in Fig. 7.5.

(a) (a) (a) (b) (c) (d)

Page 126: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

102

As the composition of Zn is increased, the transmission decreases which might be due to

the fact that optical scattering increases as the crystal structure becomes distorted. A very

small decrease in energy band gap is observed which is attributed to Zn diffusion. It may

also be related to the change in grain sizes. Transmission spectra of Te-enriched ZnTe

thin films are shown in Fig. 7.6 and show that the presence of Te affects the transmission.

As mentioned above, the annealing process leads to diffusion of Te into the ZnTe thin

films [23]. The thicknesses of the Zn and Te enriched samples were 300 nm to 500 nm in

ZnTe thin films.

Figure 7.5 Transmission spectra as a function of wavelength of as-deposited and Zn-

enriched ZnTe thin films

Page 127: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

103

Figure 7.6 Transmission spectra as a function of wavelength of as-deposited and Te-

enriched ZnTe thin films

7.2.4 Electrical study

In order to measure the electrical properties of Zn, Te enriched ZnTe thin films, the Van

der Pauw technique was used at room temperature. The resistivity of as-deposited ZnTe

film was measured as 105 Ω-cm. As the composition of Zn increased in the ZnTe thin

films, the resistivity of the films increased to 108 Ω-cm. The mobility reduced from 10

3 to

101 cm

2/Vs as shown in Fig. 7.7. Resistivity increases due to increase in micro-structural

defects and the grain boundary discontinuities. The resistivity of Te-enriched ZnTe thin

Page 128: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

104

films was observed up-to 103 Ω-cm. P-type conductivity was observed in Zn and Te

enriched ZnTe thin films due to self-compensation effects. The excess of Te in ZnTe thin

films might be expected to reduce the resistivity as the optical band gap energy was

reduced [26]. However the increase in resistivity is observed, possibly due to the fact that

the stoichiometry changed as indicated in EDX results, leading to defects at grain

boundaries. Similar results using two-source evaporation were observed by A. Ali et al.

for CdTe thin films. The elemental composition of Cd varied from 47 wt. % to 61 wt. %

and Te varied from 53 wt. % to 39 wt. %. The value of sheet resistance was 109 which

reduced to 103 Ω/sq as reported earlier [27]. To best of our knowledge no literature is

available on Zn and Te enrichment using CSS technique.

Figure 7.7 Variations in resistivity and mobility vs. atomic % of Zn

Page 129: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

105

7.3 Summary

The effects of Zn and Te diffusion on the structure, surface, optical and electron transport

properties of ZnTe thin films were studied. XRD revealed that the preferred orientation

was [111] and as-deposited ZnTe, Zn/Te-enriched thin films exhibited a polycrystalline

structure. The crystal structure was found to be cubic with both Zn and Te enriched ZnTe

thin films. XRD showed that enrichment affected the crystallinity due to reorientation of

planes in Zn and Te enriched ZnTe thin films. The crystallite size after Zn enrichment

decreased. In addition, the value of the lattice constant was reduced showing that bond

length between Zn and Te was changed. The as-deposited ZnTe films were larger in grain

sizes and more tightly packed relative to the enriched films. Zn-enriched ZnTe thin films

showed smaller grains sizes and discontinuities between the grain boundaries occured.

Te-enriched ZnTe thin films showed rod-like grains. The optical energy band gap

increased after Zn-enrichment showing that the defect states were produced and the

transmission decreased as the composition of Zn was increased in ZnTe thin films

indicating that Zn is a good reflector for light in UV-VIS-NIR regions. The transmission

also decreased for Te-enriched ZnTe samples. The electrical resistivity increased as Zn

composition increased which may be the effect of discontinuities of grains. EDX analysis

also showed a change in stoichiometry after the enrichment process. The electrical data

suggest that Zn-enriched ZnTe thin films could not be used as the back contact in solar

cells because of increase in resistivity. The resistivity is decreased down to 103 Ω-cm, in

Te-enriched ZnTe thin films might be suitable for back contact for solar cells instead of

ZnTe thin films having 105 Ω-cm. The decrease in resistivity of Te-enriched ZnTe thin

films was due to the fact that defect states are generated in energy band gap which might

have shifted the Fermi level and hence the transition of electron possible having less

energy.

Page 130: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

106

References:

[1] K. Sato, M. Hanafusa, A. Noda, A. Arakawa, M. Uchida, T. Asahi, O. Oda, J.

Cryst. Growth, 214 (2000) 1080.

[2] M. Nishio, K. Hayashida, Q. Guo, H. Ogawa, Appl. Surf. Sci., 169 (2001) 223.

[3] A. B. Kashyout, A. S. Arico, P. L. Antonucci, F. A. Mohamed, V. Antonucci,

Mater. Chem. Phy., 51 (1997) 130.

[4] T. Barona, K. Saminadayarb, N. Magnea, J. Appl. Phy., 83 (1998) 1354.

[5] L. Feng, L. Wu, Z. Lei, W. Li, Y. Cai, W. Cai, J. Zhang, Q. Luo, B. Li, J.

Zheng, Thin Solid Films, 515 (2007) 5792.

[6] A. E. Rakhshani, Thin Solid Films, 536 (2013) 88.

[7] T. Mahalingam, V. S. John, S. Rajendran, G. Ravi, P. J. Sebastian, Surf. Coat.

Tech., 155 (2002) 245.

[8] N. A. Bakr, J. Cryst. Growth, 235 (2002) 217.

[9] J. D. Merchant, M. Cocivera, J. Electrochem. Soc., 143 (1996) 4054.

[10] K. Wolf, H. Stanz, A. Naumov, H. P. Wagner, W. Kuhn, B. Hahn, W. Gebhardt,

J. Cryst. Growth, 138 (1994) 412.

[11] Y. Tokumitsu, A. Kawabuchi, H. Kitayama, T. Imura, Y. Osaka, F. Nishiyama,

J. Appl. Phy., 29 (1990) 1039.

[12] C. Konigtein, M. Neumann-Spallart, J. Electrochem. Soc., 145 (1998) 337.

[13] C. S. Ferekides, D. Marinskiy, V. Viswanahan, B. Tetali, V. Palekis, P.

Selvaraj, D. L. Morel, Thin Solid Films. 361 (2000) 520.

[14] A. A. Ibrahim, Vacuum, 81 (2006) 527.

Page 131: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Zn and Te Enrichment in ZnTe Thin Films

107

[15] J. Li, Y. F. Zheng, J. B. Xu, K. Dai, J. Semicond. Sci. Technol. 18 (2003) 611.

[16] H. Moualkia, S. Hariech, M. S. Aida, Thin Solid Films 5 (2009) 450.

[17] S. J. C. Irvine, V. Barrioz, A. Stafford, K. Durose, Thin Solid Films, 480 (2005)

76.

[18] D. Rioux, D. W. Niles, H. Hochst, J. Appl. Phy., (1993) 8381.

[19] B. Spath, J. Fritsche, F. Sauberlich, A. Lien, W. Jaegermann, Thin Solid Films,

480 (2005) 204.

[20] W. C. Hsu, I. Repins, C. Beall, C. DeHart, G. Teeter, B. To, Y. Yang, R. Noufi,

Sol. Energ. Mater. Sol. Cell., 113 (2013) 160.

[21] R. Mariappana, M. Ragavendarb, V. Ponnuswamy, J. Alloys Compd. 509

(2011) 7337.

[22] E. Bacaksiz, S. Aksu, N. Ozer, M. Tomakin, A. Ozcelik, Appl. Surf. Sci., 256

(2009) 1566.

[23] N. A. Shah, J. Alloys Compd., 506 (2010) 661.

[24] M. A. Khan, N. A. Shah, A. Ali, M. Basharat, M. A. Hannan, A. Maqsood, J.

Coat. Tech., 6 (2009) 251.

[25] R. Swanepoel, J. Phy. E, 6 (1983) 1214.

[26] N. A. Shah, A. Ali, Z. Ali, A. Maqsood, A. K. S. Aqili, J. Cryst. Growth, 284

(2005) 477.

[27] A. Ali, N. A. Shah, A. K. S. Aqili, A. Maqsood, J. Cryst. Growth & Design, 6

(2009 2149.

Page 132: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Chapter 8 Fabrication of

CdS Thin Films

Page 133: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

109

The present chapter describes the fabrication of CdS thin films using CSS technique; the work is published

in AIP conference proceeding entitled as:

“Study of Cadmium Sulfide thin films as a window layers”

Cadmium sulfide (CdS) is prominent material in II-VI semiconductors. It is used as a

window layer material for CdTe based solar cells [1]. Commonly, it is n-type material

with energy band gap 2.42 eV at room temperature. CdS was fabricated using many

techniques including thermal evaporation [1] chemical bath deposition technique [2],

sputtering [3], close spaced sublimation (CSS) [4] etc. CdS is a highly resistive material

so the main problem with this material is to control its resistivity but not on the loss of its

optical properties including transmission for window layer. Many attempts have been

made so far to achieve low resistivity [5-10], improvement in its electrical and optical

properties is still necessary for efficiency of CdTe based solar cells.

CdS thin films of different thicknesses were deposited on corning glass by close spaced

sublimation technique under vacuum. All the deposition parameters were optimized to

achieve good quality films. Structural, electrical and optical properties were investigated.

8.1 Samples preparation

Cadmium sulfide powder was put in a graphite boat, heated by a halogen lamp (1000W)

connected to the main power through temperature controller with K-type thermocouple.

The substrate was fixed at a distance of about 5 mm from the source material. It was

heated by another halogen lamp (500W), while the thermocouple was placed over the

substrate to monitor its temperature. Source and substrate temperatures were 550oC and

400 oC respectively. The chamber was evacuated 10

-4 mbar with the help of rotary as

well as diffusion pump. The deposition time was varied from 1 to 5 minutes at the source

temperature of 550 oC, then the source and substrate lamps were switched off to allow

cool down to room temperature before opening the chamber. All thin films were annealed

at 400 0C temperature for 30 minutes under the same vacuum. The samples denoted by

Page 134: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

110

CdS 01 to CdS 05 were prepared by the CSS technique. The physical properties of these

samples are presented here.

8.2 Results and discussion

8.2.1 Structural study

XRD pattern clearly shows polycrystalline behavior of the films. CdS has been reported

in cubic as well as hexagonal phases; here the structure is cubic matched with ICDD

reference card and preferred orientation of planes [111] direction as shown in Fig. 8.1.

Figure 8.1 XRD patterns for CdS 01 to CdS 05 thin films

The crystallite size was calculated by using Scherrer‟s formula [11-13]:

It was observed that different diffraction peaks mainly (111), (101) and (103) in which

intensity of (111) planes was high, compared to other peaks as film thickness increases

Page 135: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

111

the intensity ratio of C (111) to C (103) and C (111) to C (101) varied which indicates the

reorientation of planes and improvement in crystal structure [14-17].

By increasing the thickness, it is observed that average crystallite size increases and

dislocation density, strain decrease [13]. For the thickness of 1600 nm, the sample shows

better crytallinity.

8.2.2 Optical study

Optical spectra of CdS thin films have high transmission in visible and infra-red region

also a steep edge at about 515 nm wavelengths is shown in Fig. 8.2. The oscillations are

due to constructive and destructive interference phenomenon. Transmission is in between

60 % to 80 % in the visible region and becomes 90 % in the IR region. The transmission

varied with thickness as thickness increased transmission decreased. The band gap of

these samples was deduced from the pot of (αhυ)2 Vs. hυ [18-20]. The energy band gap

was 2.42 eV in these CdS thin films. More than 20 % light is absorbed in the visible

region due to this energy band gap edge. The energy band gap values are not much

affected by increase in thickness.

The samples were placed at different angles from zero to 60 degree, which show that

transmission decreases by increasing angles variation by 10 degrees step as shown in Fig.

8.4., but there is no change in band gap. More light trapped in CdS thin films at higher

angles suggests the incident angle for window layer should be normal to surface of films.

Page 136: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

112

Figure 8.2 Transmission vs wavelengths of as-deposited CdS thin films

Figure 8.3 Variations in energy band gaps of as-deposited CdS thin films

Page 137: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

113

Figure 8.4 Variations in angle from zero to 60 degrees of CdS thin films

Using equation (hυ-Eg)1/2

vs αhυ, the direct band gap was observed in CdS thin films [21,

22] as shown in Fig. 8.5. The direct band gap transition type is one of the important

parameter for window layer material to minimize the energy loss in terms of lattice

phonon absorption.

Figure 8.5 Direct band gap transition type in as-deposited CdS thin films

Page 138: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

114

8.2.3 Electrical study

For CdS 01 to 05 samples, the data were taken from the Hall measurement system,

having optical thickness which was calculated by the spectrophotometer. Current of 1 nA

is applied with 0.5 T magnetic field, for the calculation of Hall co-efficient, mobility and

resistivity. The mobility was calculated using the relation,

q is the charge, n denotes the charge density and R shows resistance of the sample. Fig.

8.6 shows that there is decrease in the resistivity and increase in the mobility with the

increase in thickness of the samples. As the thickness increases more charges are

available for conduction. The resistivity in all CdS thin films was of the order of 105 Ω-

cm which is in good agreement with previous reports available on CdS thin films.

Resistivity of CdS thin films was reported of the order of 107 Ω-cm by thermal

evaporation technique [13]. Low resistivity is favorable for window layer material as it

may enhance the electron density [4].

Figure 8.6 Thicknesses vs. resistivity of CdS thin films

Page 139: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

115

8.3 Summary

As a result, the XRD patterns show polycrystalline behavior and crystallite size changes

with thickness variations. In electrical properties, resistivity decreases as thickness

increases. The mobility increases with the increase of thickness of the samples. % T

decreases by giving angle variation but band gap remains the same. The absorption edge

of CdS thin films should move towards lower wavelength to minimize absorption losses.

Optical data confirms the direct band gap in CdS thin films. CdS thin films follow

semiconductor properties and may be used as window layer. Further enhancement in

energy band gap of CdS thin films is favorable for window layer and can be achieved by

doping of Zn in CdS thin films as discussed in next chapter.

Page 140: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

116

References:

[1] N. R. Paudel, C. Xiao, Y. Yan, J. Mater. Sci: Mater. in Electron., 25 (2014)

1991.

[2] A. P. Samantilleke, M. F. Cerqueira, S. Heavens, P. Warren, I. M. Dharmadasa,

G. E. A. Muftah, C. J. R. Silva, B. Mari, Thin Solid Films, 519 (2011) 7583.

[3] G. P. Hernandez, J. P. Enriquez, B. E. Morales, D. M. Hernandez, L. L. D.

Flores, C. R. Jimenez, N. R. Mathews, and X. Mathew, Thin Solid Films, 535

(2013) 154.

[4] C. S. Ferekides, D. Marinskiy, V. Viswanathan, B. Tetali, V. Palekis, P.

Selvaraj, D. L. Morel, Thin Solid Films, 361 (2000) 520.

[5] R. Panda, V. Rathore, M. Rathore, V. Shelke, N. Badera, L. S. Sharath Chandra,

D. Jain, M. Gangrade, T. Shripati, and V. Ganesan, Appl. Surf. Sci., 258 (2012)

5086.

[6] R. Xie, J. Su, M. Li, L. Guo, Int. J. Photoenergy, 34 (2013) 6201.

[7] R. Ganesh, V. S. Kumar, K. Panneerselvam, M. Raja, Arch. Phys. Res., 4

(2013) 97.

[8] M. Elango, D. Nataraj, K. P. Nazeer, and M. Thamilselvan, Mater. Res. Bull.,

47 (2012) 1533.

[9] T. D. Dzhafarov, F. Ongul, and S. A. Yuksel, Vacuum, 84 (2010) 310.

[10] E. M. Feldmeier, A. Fuchs, J. Schaffner, H. J. Schimper, A. Klein, W.

Jaegermann, Thin Solid Films, 519 (2011) 7596.

[11] K. Ravichandran, P. Philominathan, Appl. Surf. Sci., 255 (2009) 5736.

Page 141: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdS Thin Films

117

[12] P. Roy, S. Kumar Srivistava, Mater. Chem. Phy., 95 (2005) 235.

[13] G. H. Tariq, M. A. Rehman, Key Eng. Mater., 510 (2012) 156.

[14] A. Bayer, D. S. Boyle, M. R. Heinrich, P. O‟Brien, D. J. Otway, O. Robbe,

Green Chem., 2 (2000) 79.

[15] H. Moualkia, S. Hariech, M. S. Aida, Thin Solid Films, 5 (2009) 450.

[16] R. Mendoza Perez, J. Sastre Hernandez, G. Puente, O. Vigil Galan, Sol. Energ.

Mater. Sol. Cell., 8 (2009) 79.

[17] N. A. Shah, R. R. Sagar, W. Mahmood, W. A. A. Syed, J. Alloys Compd., 512

(2012) 185.

[18] N. A. Shah, A. Ali and A. Maqsood, J. Electron. Mater., 37 (2008) 145.

[19] J. T. Hu, T. W. Odom, C. M. Lieber, Acc. Chem. Res., 32 (1999) 435.

[20] I. P. Mcclean, C. B. Thomas, Semicond. Sci. Technol., 7 (1992) 1394.

[21] N. Romeo, A. Bosio, A. Romeo, Sol. Energ. Mater. Sol. Cell.,

doi:10.1016/j.solmat.2009.06.001

[22] X. Wu, Sol. Energy, 77 (2004) 803.

Page 142: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

118

Chapter 9 Fabrication of

CdZnS Thin Films

Page 143: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

119

In this chapter fabrication of CdZnS through mechanical mixing is achieved using CSS technique. The

work is published in Journal of Optical Materials entitled as:

“CdZnS thin films sublimated by close spaced using mechanical mixing: A new approach”

II-VI semiconductors have achieved attention of scientists and researcher due to the

tunable energy band gaps [1]. II-VI semiconductor compounds have been reported

intensively on theoretical and experimental aspects of materials and devices as well.

Cadmium sulfide (CdS) is an important II-VI material for the use of window layer in

CdS/CdTe solar cells [2-8]. Zn substitution in CdS forms the CdZnS (CZS) ternary

compound, which has a direct energy band gap over the whole alloy composition. The

varied compositions can be used to fabricate the hetero-junction photovoltaic devices.

CZS thin films were used as a window layer in solar cells [9-19]. The addition of Zn in

CdS thin films was responsible of enhancement in energy band gap of CdS, which was

used in heterojunction solar cells.

CdS and zinc (Zn) powders were mixed mechanically with different weight percentages

to make CdZnS powder. CZS as-deposited thin films were characterized for structural,

surface morphology with energy dispersive X-rays (EDX) and optical properties for the

use of window layer in CdS/CdTe based solar cells. The different Zn compositions in

CZS played a vital role on crystallite size in structural analysis and optical properties e.g.

transmission, absorption coefficient and energy band gap is the primary focus for window

layer. The crystallite size of as-deposited CZS thin films were increased as Zn

composition increased. The energy band gap varied from 2.42 eV to 2.57 eV for as-

deposited CZS thin films with increasing Zn compositions also surface morphology

changed. These changes occurred due to zinc substitution in CdS thin films. An angle

resolved transmission data was taken to check the behavior of CZS thin film at different

angles.

In the present study, CZS thin films have been fabricated using CSS technique by

mechanical mixing of CdS and Zn powders to raise the energy band gap. The

Page 144: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

120

transmission of light has been increased in the window layer which is not reported earlier

as per our knowledge for CdTe based solar cells. The composition of Zn is not controlled

in CSS technique due to unavailability of shutter arrangement [13, 14, 16-18].

9.1 Samples preparation

Cadmium sulfide powder (Aldrich 99.99 %) was used with pure zinc powder by weight

percentage using electronic balance to fabricate CdZnS thin films. Different weight

percentages 5 %, 10 %, 15 %, 20 % and 25 % Zn were mechanically mixed with the CdS

powder. The mechanically mixed CdZnS powder was used as a source material. A glass

slide was used as a substrate which was cleaned in ultrasonic bath with isopropyl alcohol.

A cleaned graphite boat having 20 mg source material was put in a chamber with a

substrate. The source and substrate distance was fixed at 5 mm. The vacuum of the

chamber was 10-4

mbar, attained with the help of rotary and diffusion pump. Source and

substrate were heated directly with halogen lamps and the temperatures were controlled

by two temperature controllers connected with K-type thermocouples. A mica sheet was

introduced between graphite boat and glass slide to create temperature gradient between

the source and substrate. The substrate was covered with graphite slab for uniform

heating. The vacuum was measured by pirani and penning gauges connected with the

vacuum chamber. The lamps were switched ON; the source and substrate were heated

slowly up to 550oC and 350

oC respectively. The deposition time was kept at 3 min for

CZS thin films to control the thickness. As the deposition time was attained, the lamps

were switched OFF and the chamber was left for cooling to room temperature. The same

process was adopted for the fabrication of CdS thin films as discussed in previous

chapter.

Page 145: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

121

9.2 Results and discussion

9.2.1 Structural study

The crystal structure and crystalline quality of CZS thin films were examined by using X-

ray diffraction technique with diffraction angle ranging from 20 to 80 degrees. The CdS

thin films exhibited hexagonal phase which was indicated by the scattering from (002),

(101), (102), (103), (201), (203) planes matched with ICDD reference card No. 00-049-

1302, Fig. 9.1 (a). The XRD patterns revealed that the most intense peak was (002)

corresponding to preferred orientation of CdS thin films at 26.840.

CZS thin films showed mixed phases of cubic and hexagonal structures with preferred

orientation of [002] at 27.01o. Two new peaks (100) and (110) that are related to cubic

phase shown in Fig. 9.1 (b), matched with ICDD reference card No. 00-040-0836. The

diffraction angle was shifted towards higher 2θ values with the increase in Zn

compositions as shown in Fig. 9.2 which confirmed that (002) lattice constant decreased.

The shift was due to the residual strain. The change in peak position was also due to

lattice stress. As Zn composition was increased in CZS thin films, Zn2+

substituted Cd2+

site in the CdS lattice [15]. The hexagonal phase of CdS and CZS thin films showed the

general tendency that c-axis of CZS was perpendicular to the film surface [17]. The

intensity decreased after Zn composition, indicating that crystalline quality of CdS thin

films was good as compared to CZS thin films [15]. The crystalline quality was attributed

using the crystallite size calculations by using Scherrer‟s formula,

(1)

Where „λ‟ is the wavelength, „β‟ full width at half maximum (FWHM) expressed in

radians and „θ‟ the Bragg‟s angle [20, 21]. The dislocation density was calculated using

1/D2

[22].

Page 146: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

122

Figure 9.1 X-ray diffraction patterns of (a) as-deposited CdS and (b) CZS thin films

Figure 9.2 (002) peak shift of as-deposited CdS and CZS thin films in the XRD

patterns

Page 147: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

123

Crystallite size of as-deposited CdS thin films was about 20 nm, which was increased up

to 25 nm after increasing Zn composition in CZS thin films. So the dislocation density

was decreased by increasing the Zn composition, which may be attributed to

improvement of the crystal structure. These results are matched with the results achieved

by Bhavanasi et al. [23].

9.2.2 Surface morphological study

The SEM is the most suitable technique to study the microstructure of thin films. In Fig.

9.3, the morphology of CdS and CZS thin films with different Zn composition is

presented. CdS and CZS thin films showed different shapes of grains including

triangular, round and elongated. CdS thin films also indicate the roughness in the

micrograph which was reduced as the Zn composition increased which is good for

window layer material. Low concentration of Zn in the CZS thin films showed larger

grains as shown in Fig. 9.3 (b) in which 10 % Zn was mechanically mixed in CdS

powder. The increase in composition of Zn in CdS powder with the same method up to

15 % clearly indicated the process of coalescence took place as observed in Fig. 9.3 (c).

The further increase of Zn quantity in the CdS pow

der showed that grains sizes decreased and the number of grain boundaries increased as

shown in Fig. 9.3 (d). The re-orientations of grain‟s growth in II-VI semiconductors are

well reported [21].

Page 148: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

124

Figure 9.3 SEM images of (a) as-deposited CdS (b) CZS (10 % Zn) (c) CZS (15 %

Zn) (d) CZS (25 % Zn) thin films

EDX was carried out in atomic % composition for the mechanically mixed CZS and CdS

thin films. Table 9.1, shows the EDX of as-deposited CdS thin films while the Table 9.2

shows the Zn composition in CZS thin films.

Table 9.1: Elemental composition of the as-deposited CdS thin film.

Element Atom %

S 40

Cd 60

EDX indicated that Zn replaced Cd and the composition of Cd was decreased after Zn

diffusion with a slight increase in S as shown in Table 9.2.

Page 149: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

125

Table 9.2 Elemental composition of CZS (10 % Zn, 15 % Zn and 25 % Zn)

thin films by mechanical mixing method

9.2.3 Optical study

The transmission spectra of CdS and CZS thin film samples were recorded in the range

of ultraviolet, visible and near infrared (UV-VIS-NIR) (250 nm to 2000 nm) as shown in

Fig. 9.4. The oscillations show the interference phenomenon in these thin films. The CdS

and CZS films are highly transparent in the visible region (more than 80 %).

These spectra clearly indicate the effects of Zn on the optical parameters like refractive

index, absorption coefficient and energy band gap etc. These parameters were calculated

by using transmittance data and fitting the transmittance curve by well-known equations

[24, 25]. The increase in Zn composition showed variation in refractive index and energy

band gap. The energy band gap of as-deposited CdS thin films was 2.42 eV and average

refractive index was 2.65, after increasing the Zn composition to 25 %, the energy band

gap increased to 2.57 eV and refractive index to 3.25.

10 % Zn 15 % Zn 25 % Zn

Element Atom % Atom % ± 0.05 Atom %± 0.05

S 40 43.13 54.87

Zn 8 11.08 15.76

Cd 52 45.79 29.37

Total 100 100 100

Page 150: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

126

Figure 9.4 Optical spectra of as depositedCdS and CZS thin films with

different composition of Zn

As the composition of Zn was increased the absorption edge shifted toward lower

wavelength. The energy band gap of CdS thin film was shown in Fig. 9.5 (a) and was

increased after Zn diffusion. CdS thin films were direct band gap transition type as

reported earlier [3]. After Zn substitution it was observed that the CZS thin films

remained as direct band gap material as shown in Fig. 9.5 (b).

An angle resolved optical transmission data was carried out for CZS thin films ranging

from zero to 60 degrees as shown in Fig. 9.6. As the angle of incident light was

increased, the transmission for CZS thin films was decreased in UV and visible regions,

which may be due to the trapping of light. In IR region, the transmission was increased as

the angle increased which showed that at higher angles, CZS thin films are more

transparent in this region.

Page 151: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

127

Figure 9.5 (a) Energy band gap of as-deposited CdS thin film and CZS thin film (25

% Zn) (b) direct band gap transition type in CZS thin film

Figure 9.6 Angle resolved spectra of 25 % Zn CZS thin film

Page 152: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

128

9.3 Summary

The effects of Zn mixing in CdS were studied by structural, surface morphology and the

optical properties. In XRD patterns, CdS and CZS thin films showed polycrystalline

behavior with preferred orientation [002]; the peaks shifted towards higher angles after

Zn diffusion and the decrease in lattice parameter observed. The increase in crystallite

size indicates the improvement in crytallinity. SEM images indicated that the grains were

increased after Zn diffusion in the CZS thin films. Zn composition observed in the EDX

results confirmed the presence of Zn in CZS thin films. The optical parameters like

energy band gap of CZS thin films was increased up to 2.57 eV, which reflected the

wavelength of 482 nm after Zn substitution and the transition type remained as direct. An

angle resolved transmission showed a very interesting behavior that at higher angles, the

transmission was decreased in UV and VIS regions but increased in the IR region, which

confirmed that these thin films are more transparent in the IR range at higher angles.

These results including structural, surface morphology and the optical properties are

strongly correlated which validate this argument that Zn can be diffused by mechanical

mixing method and the CZS thin films could be used as a window layer instead of CdS

having wide band gap. The energy band gap of CZS was successfully tailored towards

lower wavelength which enhances the transmission in window layer. Before that Zn was

added in CdS by chemical bath deposition in which maximum efficiency of 16.5 % was

reported for CdTe based solar cells.

Page 153: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

129

References:

[1] D. Xia, T. Caijuan, T. Rongzhe, L. Wei, F. Lianghuan, Z. jingquan, W. Lili and

L. Zhi, J. Semicond., 32 (2011) 1.

[2] J. Britt, C. Ferekides, Appl. Phys.Lett., 62 (1993) 2851.

[3] W. Mahmood, N. A. Shah, AIP Conf. Proc., 1476 (2012)178.

[4] M. Rehman, A. K. S. Aqili, Z. Ali, A. Maqsood, J. Mater. Sci. Lett., 2 (2003)

127.

[5] M. Urbanczyk, W. Jakubik, E. Maciak, J. Mol. Q. Acou., 26 (2005) 273.

[6] Y. K. Liu, J. A. Zapien, C. Y. Geng, Y. Y. Shan, C. S. Lee, Y. Lifshitz, S. T.

Lee, Appl. Phys. Lett., 85 (2004) 3241.

[7] S. Kar, S. Chaudhuri, Met. Org. Nano-Metal Chem., 36 (2006) 289.

[8] M. D. G. Potter, M. Cousins, K. Durose, D.P. Halliday, J. Mater. Sci. Mater.

Electron., 11 (2000) 525.

[9] J. H. Lee, W. C. Song, J. S. Yi, K. J. Yang, W. D. Han, J. Hwang, Thin Solid

Films, 431 (2003) 349.

[10] C. S. Ferekides, D. Marinskiy, V. Viswanathan, B. Tetali, V. Palekis, P.

Selvaraj, D. L. Morel, Thin Solid Films, 361 (2000) 520.

[11] S. C. Ray, M. K. Karanjai, D. D. Gupta, Thin Solid Films, 322 (1998) 117.

[12] B. J. Wu, H. Cheng, S. Guha, M. A. Hasse, J. M. D.Puydt, G. M. Haugen, J.

Qiu, Appl. Phys. Lett., 63 (1993) 2935.

[13] P. Kumar, A. Misra, D. Kumar, N. Dhama, T. P. Sharma, P. N. Dixit, Opt.

Mater., 27 (2004) 261.

Page 154: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

Fabrication of CdZnS Thin Films

130

[14] V. Kumar, V. Singh, S. K. Sharma, T.P. Sharma, Opt. Mater., 11 (1998) 29.

[15] G. G. Rusu, M. Rusu, M. Girtan, Vacuum, 81 (2007) 1476.

[16] D. S. Rane, L. A. Patil, AIP Conf. Proc., 1451 (2012) 295.

[17] T. Prem Kumar, K. Sankaranarayanan, Chalcogenide Lett., 6 (2009) 555.

[18] S. Yamag, A. Yoshikawa, H. Kasai, J. Cryst. Growth., 99 (1990) 432.

[19] N. A. Shah, R. R. Sagar, W. Mahmood, W. A. A.Syed, J. Alloys Compd., 512

(2012) 185.

[20] N. A. Shah, A. Nazir, W. Mahmood, W. A. A. Syed, S. Butt, Z. Ali, A.

Maqsood, J. Alloys Compd., 512 (2012) 27.

[21] N. A. Shah, J. Alloys Compd., 506 (2010) 661.

[22] N. A. Shah,W. Mahmood, Thin Solid Films, 544 (2013) 307.

[23] V. Bhavanasi, C. B. Singh, D. Datta, V. Singh, K. Ahahi, S. Kumar, Opt.

Mater., (2013)

[24] R. Swanepoel, J. Phys. E: Sci. instrum., 6 (1983) 1214.

[25] A. Ali, N. A. Shah, A. Maqsood, Solid State Electron., 52 (2008) 205.

Page 155: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

131

Chapter 10 Summary and

future

recommendations

Page 156: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

132

Close spaced sublimation technique has been successfully constructed and installed at

Thin Films Technology (TFT) Research Laboratory, CIIT, Islamabad. By using the CSS

technique, the deposition parameters of ZnTe thin films were optimized. It was observed

that the higher substrate temperature had influenced the structural, optical and electrical

properties of thin films. ZnTe thin films had cubic structure with [111] as a preferred

orientation. The energy band gap of ZnTe thin films was 2.24 eV. The electrical

resistivity of the order of 105 Ω-cm was achieved which is 10

2 times less than other

techniques.

Ion exchange process was adopted for Ag and Cu doping onto ZnTe thin films to achieve

low resistive back contact. After Ag immersion, the resistivity of ZnTe thin films reduced

to 103 Ω-cm, the energy band gap was 2.09 eV and more than 65 % transmission was

observed in visible region. The resistivity was reduced to 1 Ω-cm by Cu doping. The

energy band gap was reduced to 2.20 eV in this case after Cu doping. Strong correlations

between structural, surface, optical and electrical properties were observed in the light of

previous reports available in the literature.

The Zn and Te-enriched ZnTe thin films were successfully fabricated by using the CSS

technique. The composition of Zn and Te were enhanced by using CSS technique which

was not available previously in literature as per our knowledge. The structural,

morphological, optical and electrical properties were examined. The resistivity of Zn-

enriched ZnTe thin films increased up to 108 Ω-cm but the transmission was more than

70 % in the visible region. The Te-enriched ZnTe thin films showed low resistivity of 103

Ω-cm with more than 70 % transmission in the visible region. It was concluded that the

Te-enriched ZnTe thin films were suitable for interfacial layer between absorbent layers

to metallic back contact in II-VI semiconductor solar cells instead of as-deposited ZnTe

thin films with high resistivity.

Fabrication of CdS thin films was carried out using CSS technique as a window layer.

The structure showed hexagonal phase with (002) as preferred orientation. The effects of

variation in thickness on optical and electrical properties were observed. The resistivity

Page 157: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

133

of as-deposited CdS thin films was 105 Ω-cm. The transmission of as-deposited CdS thin

films was more than 80 % in the visible region. The energy band gap was 2.40 eV.

Angle resolved transmission showed the decrease in transmission as angle of incident

light increased.

CdZnS thin films were fabricated successfully using mechanical mixing method. Mixed

phase of cubic and hexagonal structure were observed in CZS thin films with (002)

planes as preferred orientation. The energy band gap enhanced from 2.42 eV to 2.57 eV

after Zn diffusion. Variation in energy band gap of CZS thin films indicated that these

films were more transparent (10 %) for visible spectrum as compared to CdS thin films.

The Transmission about 80 % and n-type conductivity was observed. CZS thin films can

be used as a window layer instead of CdS having wide energy band gap and can be

fabricated using CSS technique.

Recommendations for future work

For further experiments and developments in second generation based solar cells,

suggestions and recommendations are given below:

Parameters optimization may enhance the physical properties of ZnTe thin films used as

a buffer or interfacial layer between the CdTe and back metal contact and CZS thin films

for window layer in CdTe based solar cell. Doping should be done using CSS technique.

The composition could be controlled by using CSS technique; it will be helpful to

fabricate thin films with desire compositions.

Mechanical mixing method could be adapted to fabricated II-VI semiconductors thin

films using CSS technique.

This study should be applied in the fabrication of CdTe based solar cells.

Page 158: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

134

CONFERENCES, WORKSHOPS AND SEMINARS ATTENDED

o Oral Presentation “Physical properties of sublimated ZnTe thin films for solar

cell applications” Joint International Workshop on Nanotechnology: Policy

Ethics and Science 25-27th

March-2013, Islamabad PAKISTAN.

o Poster Presentation “Investigation of Physical Properties of Doped ZnTe Thin

Films for Photovoltaic Applications” 1st Herrenhausen Conference

Downscaling Science, 12-14th

Dec. 2012, Hannover GERMANY.

o Oral Presentation “Study of Cadmium Sulphide Thin Films as a Window

Layers” 2nd

International Advances in Applied Physics & Material Science

Congress (APMAS-2012) 26-29th

April-2012, Antalya TURKEY.

o Workshop on Thin film Deposition and Characterization techniques COMSATS,

(April 9-11, 2012), Islamabad PAKISTAN.

o Poster Presentation “Fabrication Of CdS Thin Films by Close Spaced

Sublimation Technique and study of its Physical Properties for Solar Cell

Applications” International Conference on Nanomaterials & Nanoethics

(December 1-3, 2011) Lahore, PAKISTAN.

o Oral Presentation “Physical Properties of CdS Thin Films Fabricated Under

Vacuum” International Symposium on Vacuum Science and Technology (ISVST

- 2010), Islamabad PAKISTAN.

o Oral Presentation “Doping Effects on Physical Properties of ZnTe Thin Films

for Solar Cell Applications” 2nd

National Conference on Physics & Emerging

Sciences, (April 07-08, 2014) AIOU, Islamabad PAKISTAN.

Page 159: Fabrication and Characterization of II-VI Semiconductor Thin … · 2019-06-01 · Fabrication and Characterization of II-VI Semiconductor Thin Films and the Study of Post Doping

135

SCHOLARSHIPS

Award of scholarship for PhD studies from COMSATS Institute of Information

Technology (CIIT) includes monthly stipend and full fee waiver.

Award of scholarship for PhD research work from Higher Education Commission

(HEC) under International Research Support initiative Program (IRSIP).

Award of scholarship for presentation of research work in 2nd

International Advances in

Applied Physics & Material Science Congress from Pakistan Science Foundation (PSF)

Islamabad.

Award of scholarship for presentation of research work in 1st Herrenhausen Conference

Downscaling Science from Volkswagen Foundation GERMANY.

PROJECT

Research Associate (2009-2012) in HEC project # 20 – 1187 / R&D / 09.