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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
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
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
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
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
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
vii
Dedicated to
My beloved son
Mohammad Ans Bin Waqar
&
Sweetest daughter
Kinza Waqar
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-
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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
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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
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.
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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
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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.
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
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
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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
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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
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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
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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
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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
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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
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
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
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
Chapter 1 Introduction
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].
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]
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.
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].
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.
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.
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)
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.
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.
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.
Chapter 2 Literature Review
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
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
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.
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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.
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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)
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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]:
√
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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.
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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;
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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.
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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.
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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
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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].
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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.
Literature Review
26
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Chapter 3 Fabrication and
Characterization
Techniques
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
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.
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
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
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]:
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
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
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]
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].
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
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
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
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:
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
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.
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.
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.
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.
Chapter 4 Fabrication of
ZnTe Thin Films
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.
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.
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.
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
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.
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].
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.
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.
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
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
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.
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.
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.
Chapter 5 Effect of Ag
Doping on ZnTe
Thin Films
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.
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
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
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
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
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.
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.
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
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.
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.
Effect of Ag-Doping on ZnTe Thin Films
76
References:
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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.
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[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.
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(2011) 8529.
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Maqsood, J. Alloy Compd., 512 (2012) 27.
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.
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Growth., 214 (2000) 967.
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[18] R. Amutha, A. Subbarayan, R. Sathamoorthly, K. Natarajan, S. Velumani, J.
New Mat. Elect. Sys., 10 (2007) 27.
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H. M. MaheshInt. J. Electrochem. Sci., 4 (2009) 369.
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(2006) 1296.
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[25] H. Bellakhder, A. Outzourhit, E. L. Ameziane, Thin Solid Films, 382 (2001) 30.
Chapter 6 Cu-Doping Effects
on ZnTe Thin
Films
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.
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
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
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.
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
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)
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
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
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
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
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.
Cu-Doping Effects on ZnTe Thin Films
90
References:
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Cryst. Growth, 214 (2000) 1080.
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Cu-Doping Effects on ZnTe Thin Films
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Chapter 7 Zn and Te
Enrichment in
ZnTe Thin Films
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.
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
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
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
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.
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].
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
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
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)
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
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
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
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.
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.
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Mater. Chem. Phy., 51 (1997) 130.
[4] T. Barona, K. Saminadayarb, N. Magnea, J. Appl. Phy., 83 (1998) 1354.
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Zheng, Thin Solid Films, 515 (2007) 5792.
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Tech., 155 (2002) 245.
[8] N. A. Bakr, J. Cryst. Growth, 235 (2002) 217.
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[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.
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.
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Sol. Energ. Mater. Sol. Cell., 113 (2013) 160.
[21] R. Mariappana, M. Ragavendarb, V. Ponnuswamy, J. Alloys Compd. 509
(2011) 7337.
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(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.
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[26] N. A. Shah, A. Ali, Z. Ali, A. Maqsood, A. K. S. Aqili, J. Cryst. Growth, 284
(2005) 477.
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(2009 2149.
Chapter 8 Fabrication of
CdS Thin Films
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
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
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.
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
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
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
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.
Fabrication of CdS Thin Films
116
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Fabrication of CdS Thin Films
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118
Chapter 9 Fabrication of
CdZnS Thin Films
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
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.
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].
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
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].
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.
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
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.
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
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.
Fabrication of CdZnS Thin Films
129
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131
Chapter 10 Summary and
future
recommendations
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
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.
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.
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.