(NEW IC NO. /PASSPORT NO.)

PSZ 19:16 (Pind. 1/07)

UNIVERSITI TEKNOLOGI MALAYSIA

DECLARATION OF THESIS / UNDERGRADUATE PROJECT PAPER AND

COPYRIGHT Author’s full name : Mohamad Syah Bin Abu Bakar

Date of birth : 08 December 1991

Title :

Practical Nano Characterization By Microscopy Of

High Quality Aluminium Nitride Thin Film

Academic Session : 14/15 2 I declare that this thesis is classified as :

CONFIDENTIAL (Contains confidential information under the Official Secret

Act 1972)*

RESTRICTED (Contains restricted information as specified by the

organization where research was done)*

OPEN ACCESS I agree that my thesis to be published as online open access

(full text)

I acknowledged that Universiti Teknologi Malaysia reserves the right as follows:

1. The thesis is the property of Universiti Teknologi Malaysia. 2. The Library of Universiti Teknologi Malaysia has the right to make copies for the

purpose of research only. 3. The Library has the right to make copies of the thesis for academic exchange.

Date:

Prof. Dr. Noriyuki Kuwano

Date:

Notes: * If the thesis is CONFIDENTIAL or RESTRICTED, please attach with the letter

from the organization with period and reasons for confidentially or restriction.

Certified by:

SIGNATURE

NAME OF SUPERVISOR

911208-05-5121

“I hereby declare that I have read this thesis and in my opinion this thesis is sufficient

in term of scope and quality for the award of the degree of Bachelor Mechanical

Precision Engineering.”

Signature : ……………………............

Name of supervisor : Prof. Dr. Noriyuki Kuwano

Date : ……………………………

PRACTICAL NANO CHARACTERIZATION BY MICROSCOPY FOR HIGH

QUALITY OF ALUMINIUM NITRIDE THIN FILMS

MOHAMAD SYAH BIN ABU BAKAR

A report submitted in partial fulfilment of the requirements for the award of the

degree of Bachelor of Mechanical Precision Engineering

Malaysia-Japan International Institute of Technology

Universiti Teknologi Malaysia

JUNE 2015

ii

I declare that this report entitled “Practical Nano Characterization by Microscopy for

High Quality of Aluminium Nitride Thin Films” is the result of my own research except

as cited in the references. The thesis has not been accepted for any degree and is not

concurrently submitted in candidature of any other degree.

Signature : ……………………....................

Name : Mohamad Syah Bin Abu Bakar

Date : …………………………………

iii

To my family, friends and lecturers.

iv

ACKNOWLEDGEMENT

First of all, I would like to express my gratitude to my final year project’s

supervisor, Prof. Noriyuki Kuwano. Without his guidance and patient, I do not think

that this report can be completed.

Not to forget, my family who had supported me throughout this four years of

my study. Without them, I would not be here completing this report. Thank you to

them for their love, encouragement and financial.

My gratitude also goes to iKohza members, Dr Anthony Centeno, Mrs. Marina,

my senior, Jesbain Kaur and Sarah Azlan who help me a lot in accomplishment of this

research. Last but not least, to all my friend who have been with me through up and

down. Who help me a lot along this research. Thank you so much everyone.

v

ABSTRACT

Thin films is known as the base to fabricate a semiconductor. A good quality

of thin film will increase the performance of the semiconductor. A very thin

Aluminium Nitride (AlN) layer is grown on the sapphire substrate as a buffer layer to

form a base. This is where the problem occur. When AlN is grown on the sapphire

substrate, there is defect occur which is lattice mismatch between AlN and substrate.

This is the reason why it is hard to develop a high quality of thin films. Annealing

treatment is used to overcome this defect. This technique is used to see whether it is

effective to reduce the lattice mismatch in the structure. Annealing is a heat treatment

that will alter the material lattice structure. The specimen is heated for two hour at high

temperature. Then it will leave cooled. Transmission Electron Microscope (TEM) and

Scanning Electron Microscope (SEM) is the method that used to get the image of the

cross section of the specimen. Then, the images from the result will be analyzed in

detail.

vi

ABSTRAK

Filem nipis digunakan sebagai asas untuk menghasilkan semikonduktor. Filem

nipis yang mempunyai kualiti yang baik akan meningkatkan prestasi semikonduktor.

Lapisan Nitride Aluminium (AlN) yang sangat nipis akan ditanam pada substrat

sapphire sebagai lapisan pengantara untuk digunakan di semikonduktor. Di sinilah

masalah akan timbul. Apabila AlN ditanam pada substrat sapphire, terdapat kecacatan

berlaku iaitu ketidakpadanan structur atom antara AlN dan substrat. Ini adalah sebab

mengapa ia adalah sukar untuk mendapatkan filem nipis yang berkualiti tinggi.

‘Annealing’ telah digunakan untuk mengatasi kecacatan ini. Teknik ini digunakan

untuk melihat keberkesanannya dalam mengatasi masalah ketidakpadanan antara

struktur atom. ‘Annealing’ adalah rawatan haba yang akan mengubah struktur bahan

sesuatu itu. Bahan kajian akan dipanaskan selama dua jam pada suhu tinggi. Kemudian

ia akan dibiarkan untuk penyejukan. Transmission Electron Microscope (TEM) dan

Scanning Electron Microscope (SEM) adalah teknik yang digunakan untuk

mendapatkan imej keratan rentas bagi bahan kajian. Seterusnya, imej-imej yang telah

diperolehi akan di analisis dan dikaji lebih mendalam.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

TITLE i

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF SYMBOLS xiv

1 INTRODUCTION 1

1.1 Background of research 1

1.2 Problem statement 5

1.3 Research question 5

1.4 Objective of research 5

1.5 Research scope 5

1.6 Significant of study 6

1.7 Outline of thesis 6

1.8 Summary of work 7

2 LITERATURE REVIEW 9

2.1 Mechanism of dislocation 9

viii

2.1.1 Edge dislocation 9

2.1.2 Screw dislocation 10

2.2 Effect of AIN buffer layer on crystallographic

structure 12

2.3 Structural and optical properties of AlN thins films

deposited by pulsed dc magnetron sputtering 14

2.4 Electron channeling 15

2.5 Determination of Burgers Vector,b Dislocated Crystal

Structure 16

3 METHODOLOGY 21

3.1 Convergent beam electron electron diffraction(CDED) 21

3.2 Metal Organic Vapour Phase Epitaxy

(MOVPE) 22

3.3 Focused Ion Beam (FIB) 22

3.4 Transmission electron microscope (TEM) 23

3.5 Scanning Electron Microscope (SEM) 24

3.6 Bright Field Image (BF) 24

3.7 Dark Field Image (DF) 25

3.8 Sample preparation for FIB 25

3.8.1 Sample preparation for annealing

temperature 1650℃ 27

3.8.2 Sample preparation for annealing

temperature 1500℃ 30

3.9 Procedure on how to use TEM 36

4 RESULTS AND DISCUSIION 40

4.1 Annealing temperature: 1500°C 40

4.2 Annealing temperature: 1550°C 42

4.3 Annealing temperature: 1600°C 44

ix

4.4 Annealing temperature: 1650°C 47

5 CONCLUSION AND RECOMMENDATION 55

5.1 Conclusion 55

5.2 Problems 56

5.3 Recommendations 56

REFERENCES 58

x

LIST OF TABLES

TABLE NO. TITLE PAGE

1 The lattice and thermal mismatch between the nitride

films and sapphire substrate

4

2 Comparison between edge and screw dislocation 11

3 Colour variation with different ratio on Nitrogen 15

4 Comparison between annealed surface and normal

surface

50

5 Microstructure changes and observation 53

xi

LIST OF FIGURES

FIGURE NO. TITLE PAGE

1.1 Crystal structure of AlN 2

1.2 Application of UV light sources 2

1.3 Structure of LED 3

1.4 Gantt chart for semester 1 7

1.5 Gantt chart for semester 2 8

2.1 The movement of edge dislocation 9

2.2 The movement of screw dislocation 10

2.3(a) Schematics of the sample structure without an

Aluminium nitride interlayer

12

2.3(b) Schematics of the sample structure with a 10 nm-thick

Aluminium nitride interlayer

12

2.3(b) Schematics of the sample structure with a 30 nm-thick

Aluminium nitride interlayer

12

2.4(a) GaN deposited with AlN buffer layer 13

2.4(b) GaN deposited without AlN buffer layer 13

2.5 Graph of deposition rate against flow ration of Nitrogen 14

2.6 An electron beam is project to the lattice of a material 16

2.7 The correlation of step spiral geometry with the

direction of Burgers Vector

17

2.8(a) Burgers Circuit in perfect crystal 18

2.8(b) Burgers circuit in dislocated crystal (edge dislocation) 18

2.9 Burgers Circuit for screw dislocation 19

3.1 Flow chart of the overall process that take place during

research

20

3.2 CBED mechanism 21

xii

3.3 Focused Ion Beam 23

3.4 Sample of image taken by TEM 24

3.5 Sample image taken by SEM 24

3.6 Cutting machine 26

3.7 AlN specimen after carbon coating 26

3.8 Specimen that will be wax 27

3.9 Wax used to attach the specimen to glass 27

3.10 Etched and carbon deposited specimen 27

3.11 Specimen is picked by W Needle 28

3.12 Specimen is deposited to TEM mesh before thinning 29

3.13 Specimen undergo thinning using U fine 29

3.14 Specimen is picked by using W Needle 30

3.15 Specimen that is attached to TEM mesh 31

3.16 Sample mesh 31

3.17 Thinning after using mid beam condition 32

3.18 Plan view of after mid beam condition 32

3.19 Thinning using fine beam condition 33

3.20 Thinning using u fine condition 33

3.21 Plan view of the U fine beam condition 34

3.22 Specimen when using 15kV of beam current 34

3.23 Specimen when using 3kV beam current 35

3.24 Specimen after Argon milling 35

3.25 JEM-2100 36

3.26 Setting up astigmatism 38

3.27 TEM-2000EX 39

4.1 Bright Field (BF) Image and Diffraction Pattern AlN 40

4.2 Dark Field image cross sectional TEM image and

diffraction pattern of AlN

41

4.3 Bright Field image and diffraction pattern of AlN 42

4.4 Dark Field image and diffraction pattern of AlN 43

4.5 The CBED pattern identifies inverted polarity regions 45

4.6 Bright Field and Dark Field Image and Diffraction

pattern

45

xiii

4.7 Bright Field and Dark Field Image of AlN 47

4.8 Dark Field image and diffraction pattern 48

4.9 Changes in microstructure for different annealing

temperature

49

4.10 Schematic diagram for microstructure changes 49

4.11 Sample of ID 51

4.12 Surface of AlN buffer layer without annealing 51

4.13 Surface of AlN buffer layer with annealing at 1500℃ 52

4.14 Surface of AlN buffer layer with annealing at 1600℃ 52

*Some of the figure have the same caption but has different purpose

xiv

LIST OF SYMBOLS

°C - Degree Celsius, common temperature scale

N/m - Unit of force

λ - Wavelength

θ - Angle of incidence

kgf - Unit of pressure, kilogram force

µm - Micrometer

nm -Nanometer

kV - Kilovolt

SE2 - Secondary electron

Å -Angstrom

CHAPTER 1

INTRODUCTION

1.1 Background of Research

Aluminum Nitride (AlN) was discovered over 100 years ago, and it has been

developed into a commercial product with controlled and reproducible properties

within the last 20 years [1]. Some of the general properties of AlN are, good dielectric

properties, high thermal conductivity, low thermal expansion coefficient and a non-

reactive with normal semiconductor process chemical and gases. AlN is a material that

is widely used in various field such as in electronic, acoustic and many more. Then

again, the presence of defect is a problem that curb the development for mass

production of AlN based product. Thin films are normally used as a base in

semiconductor production. In other word, thin films is used as a substrate for

semiconductor during its fabrication. However, there is some problem is occurred in

order to produce a high quality of AlN thin films. That is thin film growth and

dislocation of material crystal structure. Figure 1.1 shows the crystal structure of AIN.

2

Figure 1.1 Crystal structure of AlN

The need of semiconductor nitride such as Galium Nitride (GaN), AlN (AlN)

and Aluminium Gallium Nitride (AlGaN) has rising since 1990s as a new assuring for

optical devices field. Nitrides material has certain properties that fulfil the requirement

for development of nitride semiconductor thin films. The characteristic are large direct

band gap energy such as, GaN has 3.4eV at room temperature while AlGaN has up to

6eV. Band gap is the minimum amount energy needed to excite an electron from

valence band to conduction band. So the electron can participate in conduction. Due

to this characteristic has make semiconductor nitride as one of leading material in

developing for various photoelectric devices, such as Light Emitting Diodes (LED)[2-

3]. Figure 1.2 shows the application of UV light sources.

Figure 1.2 Application of UV light sources

3

Light with a shorter wavelength than 40nm is called ultraviolet (UV) light. In

a semiconductor, light is produced when electron (negative charges) fill a hole

(positive charges). Band gap energy of a material will affect the wavelength of the

light emitted. The wavelength is inversely proportional to the band gap energy, the

higher the band gap energy, the shorter the wavelength.

Figure 1.3 Structure of LED

Figure 1.3 shows the structure of LED. LEDs create light through

electroluminescence in a semiconductor. Electroluminescence is a material emits a

light when electric current is passed through. As electron a pass through one crystal to

other crystal, it will fill the hole present. Through that phenomena, a light or photon

are emitted.

However, the performance of these nitride semiconductor is disturbed by

lattice defect in the nitride material itself. The example of defect are threading

dislocation (TDs), partial dislocation (PDs) and stacking fault (SFs). Many research

has been done in order to a high quality of nitride thin films. Several method also been

reported on growth a single crystal films such as Hydride Vapour Phase Epitaxy

(HVPE), Molecular Beam Epitaxy (MBE) and Metal Vapour Phase Epitaxy (MOVPE)

using sapphire as the subtrate. However it is very difficult to grow high quality thin

films with a smooth surface free from cracks due to the large lattice and thermal

mismatches between nitride films and sapphire substrate and other several issue

regarding thin-film growth. Table 1 shows the lattice and thermal mismatch between

the nitride films and sapphire substrate.

4

Table 1: The lattice and thermal mismatch between the nitride films and sapphire

substrate[4]

Lattice Constant (Å) Thermal Expansion

Coefficient x 10-6 (K-

1)

GaN a 3.189 5.59

c 5.182 7.75

AlN a 3.111

5.3

c 4.980 4.2

Sapphire a 4.758

7.5

c 12.991 8.5

There are a few negligible lattice mismatch, but threading dislocation is

required. In order to reduce TDs effort in this line have been focused on (i) researching

modification of parameter or steps in the growth process, (ii) using additional buffer

layer to induce the recombination of these defect[5]. Then it will undergo annealing

process. As for AlGaN/GaN system, GaN layer thickness is increased to reduce the

dislocation density[6]. Recent development has succeeded in improving the surface

morphology of nitride films by adding a thin AlN layer as a buffer layer on the

sapphire[7-10].

In order to reduce the density of dislocation on the thin films, annealing process

is tested whether it is effective to decrease the dislocation in the nitride material. A

starting temperature is 1500°C and final temperature is 1650°C by using four sample.

1.2 Problem statement

When AlN or GaN thin films is grown on a substrate such as sapphire substrate

usually will contains with large lattice mismatch and large difference in thermal

expansion that cause to have defects especially dislocations, so the quality of thin films

is low. In order to grow a high quality of thin films, the behavior of lattice defect is

studied. Therefore, the current study is to identify, analyze and characterize the

5

dislocation in order to reduce the dislocation density so that high quality of thin films

can be formed. The growth process of thin films of different parameter and condition

will be analyze in detail.

1.3 Research Questions

1. What are the condition of growth process of nitride semiconductor thin

films?

2. How to develop high quality nitride semiconductor which free from

defect to be used in application of semiconductor devices?

1.4.1 Research Objective

As reported from other research, annealing process for metallic material is

widely used while annealing for semiconductor material is rarely used. So the objective

of this research is:

To determine whether annealing process is effective for semiconductor

material in reducing lattice mismatch or dislocations.

1.5 Research Scope

This research is conducted with collaboration of Mie University Japan who is

responsible in thin film growth. While all result in this report were obtained by the

experiments performed at Kyushu University, Japan.

The scope of this research is as follows:

6

AlN grown by MOVPE on a sapphire substrate that has been annealed at

variety temperature.

After all the information regarding the lattice defect formation is determined,

the condition to growth a high quality of AlN thin films is proposed to the research

group of crystal growth to get their feedback. All the result are presented to them

including preliminary result of specimen using transmission electron microscope

(TEM) and scanning electron microscope (SEM). The cross sectional microstructure

of the nitride semiconductor films were observed and the result is analysed and

characterized. To be brief, this research is more focused on the analysing the defect

especially lattice mismatch in nitride semiconductor films which is grown on sapphire

substrates.

1.6 Significance of Study

The analysing and characterizing that has been done in this research will

indirectly contribute in further research especially in improvement of growth process

and condition for developing a high quality of AlN thin films for application in

semiconductor devices.

1.7 Outline of Thesis

This report consist of four chapter. In first chapter consist of objective, scope

and the significance of this research. In the second chapter the discussion is on the

paper, journal that has been used as a references throughout this research. More

likely is discuss about the effect of buffer layer, mechanism of dislocation and type

of experiment that has been carried out that has the same purpose with research.

In the third chapter, the discussion is on methodology, procedure that was

along this research. In Chapter 4 consist of result and discussion. In the last chapter

7

which id Chapter 5 containing conclusion for this research and recommendation that

should be consider for future work.

1.8 Summary of Work

All the research flow is presented in Figure 3.1. Gantt chart that shown in

Figure 1.4 and Figure 1.5 will show, when each of stage for this research take place

during the first and second semester.

Figure 1.4 Gantt chart for semester 1

8

Figure 1.5 Gantt chart for semester 2

CHAPTER 2

LITERATURE REVIEW

2.1 Mechanism of dislocation

What is dislocation? Dislocation is crystallographic defect, or irregularity,

within a crystal structure. Dislocation will tend the structure to cause plastic

deformation by shear and also will weaken the crystal structure. There are two type

of dislocation which are edge dislocation and screw dislocation.

2.1.1 Edge Dislocation

Edge dislocation is a distortion exists along an extra half-plane of atoms.

These atoms also define the dislocation line. Also edge dislocation move in response

to shear stress applied perpendicular to the dislocation line. Figure 2.1 will show the

movement of edge dislocation.

Figure 2.1 Movement of edge dislocation

10

2.1.2 Screw Dislocation

The movement of a screw dislocation is also effected from shear stress.

Motion of screw dislocation is perpendicular to force exerted. However, the net

plastic deformation of both edge and screw dislocations is the same. Figure 2.2 show

the mechanism of screw dislocation.

Figure 2.2 The movement of screw dislocation

11

Table 2: Comparison between edge and screw dislocation

Dislocation Property

Type of dislocation

Edge Screw

Relation between dislocation line (t)

and Burgers Vector (b) ||

Slip direction || to b || to b

Direction of dislocation line

movement relative to b ||

Process by which dislocation may

leave slip plane climb Cross-slip

According to a paper written by Guan Ting Chen, who wrote a paper about,

‘Growth of High Quality Aluminium Nitride on Sapphire by Using a Low-

Temperature Aluminium Nitride Interlayer’. He state that, an astounding AlN can be

become on sapphire by utilizing a low-temperature grown 10 nm-thick AlN interlayer

as prove by EPD and XRD estimations. With this low-temperature AlN interlayer, the

thickness of dislocation is essentially reduced. He also make a test on a diode which is

based from Aluminium nitride and the result is , AlGaN/GaN Schottky diodes was

fabricated on the Aluminium nitride template has low buffer leakage current and high

breakdown voltage over 2000 V, confirming the quality of Aluminium nitride layer

prepared by this technique.

12

Figure 2.3 Schematics of the sample structure for :-

(a) Sample A without an Aluminium nitride interlayer,

(b) Sample B with a 10 nm-thick Aluminium nitride interlayer, and

(c) Sample C with a 30 nm-thick Aluminium nitride interlayer[11].

2.2 Effect of AIN buffer layer on crystallographic structure

GaN and gallium aluminium nitride has becoming a popular choice of

electronic devices in today’s world. The main reason for this phenomena are, III-

nitride has wider band gap value which is higher than 3.4eV and many more. However,

this type of semiconductor has its own weakness which limit it to perform at full

potential such as defect that lower it performance and lifetime. There are many

research that had been done related to these materials to increase its performance.

However, it almost impossible to produce defect free because existence the large of

lattice mismatch or dislocation and also difference in thermal expansion coefficient

between the substrate and the buffer layer which is the nitride films. The Table 3 below

shows the lattice and mismatch between the nitride and sapphire layer.

In latest research, AIN layer is grown on GaN. In brief, by depositing a thin

AIN layer as a buffer layer MOVPE, the lattice mismatch is reduced and the crystalline

quality is increase. The Figure 2.4(a) and (b) shows the difference between surface that

has AIN buffer layer(a) and not(b). In Figure 2.4(b), its clearly shows that there GaN

13

column and height was produced to form a rough surface. While, Figure 2.4(a) shows

a smooth surface and crack free surface resulting from the AIN buffer layer. Basically,

the function of AIN buffer layer is to standardize the orientation of the nucleation

center and to promote the lateral growth of the film due to decreasing in interfacial free

energy between the film and substrate.

Figure 2.4(a) GaN deposited with AlN buffer layer[12]

Figure 2.4(b) GaN deposited without AlN buffer layer[12]

14

2.3 Structural and Optical Properties of AIN Thins Films Deposited By

Pulsed DC Magnetron Sputtering

AIN has been used widely in various field due to its properties and performance

that make it very suitable especially in sensor for optical devices. Properties of AIN

such as wide band gap (~6.2eV), high refractive index(~2.0) and low absorption

coefficient(<10-3) make it very suitable and match for development in this field. DC

magnetron sputtering is one of the method that has been tested for developing of AIN

films. This method is used because can produce a higher deposition rate compared to

other method such as RF magnetron sputtering. Characteristic of AIN is influenced by

crystal structure, crystal orientation, microstructure and chemical composition whereas

depend on the variables of the experiment.

As can be seen in Figure 2.5, the graph show deposition rate against flow ration

of Nitrogen. From the graph can be said that, the deposition rate is decreasing as the

flow ratio in increase. In simplicity, rate of deposition is inversely proportional to the

flow ration of Nitrogen. Deposition is calculated by dividing thickness measured in

ellipsometry and deposition time.

Figure 2.5 Graph of deposition rate against flow ration of Nitrogen[13]

15

Different ratio of Nitrogen flow will give a differ colour due to variation in

stoichiometry. Table 3 will give a clear picture of the colour variations. Higher flow

ration will result in violet for the film colour.

Table 3: Colour variation with different ratio on Nitrogen[14]

2.4 Electron Channelling

Electron channelling is a technique used to determine the orientation of a

crystal with SEM equipped with Electron Channelling Contrast Imaging (ECCI).

Crystal orientation mean, the atomic structure of a crystal. Specifically, electron

channelling can be used to determine the dislocation and burger vector of the

dislocation. In Figure 2.6 below will show in detail how electron channelling work. In

electron channelling, the structure is considered as perfect structure when the path of

electron is not blocked by any lattice. If there is any lattice the blocked the electron

path, a defect can be detected.

16

Figure 2.6 An electron beam is project to the lattice of a material[15]

In Figure 2.6(a) shows the electron is blocked by the atom and scatter back to

the detector. So it resulted to a ‘closed channel’. In Figure 2.6(b), an ‘open channel is

produced because the electron can passes through the structure while in 2.6(c), an

‘open channel’ can turn in to ‘closed channel’ due to existence of the half plane

structure. An edge dislocation is seen on the image.

2.5 Determination of Burgers Vector,b Dislocated Crystal Structure

Burgers vector in dislocated crystal is the magnitude and the direction

produce by the dislocation. Also Burgers vector is used to determine the strength

along the dislocation line.

𝐸 ≅1

2𝐺𝑏2 2.1

Where E = energy of dislocation

17

G = Shear modulus

b = Burgers Vector

Burgers Vector can be determine by using Electro Channelling Contrast

Imaging (ECCI). The explanation regarding ECCI is explained in Section 2.4. Then

the image taken by SEM. In Figure 2.7(a) and (b) will show the result from ECCI.

Figure 2.7(a) and (b) The correlation of step spiral geometry with

the direction of Burgers Vector.

18

In Figure 2.7(a) show a clockwise spiral with Burgers Vector go into the

paper while for Figure 2.7(b) the spiral is counter-clockwise and the Burgers Vector

is going out of paper. With reference with Figure 2.7, the direction of Burgers Vector

is determined by using Right Hand Finish to Start Rule (RHFS). To be clear, Figure

2.8 will show how Burgers Vector can be determine using Burgers Circuit.

Figure 2.8(a) Burgers Circuit in perfect crystal

Figure 2.8(b) Burgers circuit in dislocated crystal (edge dislocation)

1. For the perfect crystal, draw a line connecting the atom to form a close

loop (8 atom to the right, 7 down, 8left and 7up).

2. For the dislocated crystal, dram the same loop. There will be an ‘open

loop’. The missing link is the Burgers Vector.

19

For the screw dislocation, the Burgers Circuit is as Figure 2.9.

Figure 2.9 Burgers Circuit for screw dislocation

From the Burgers Vector also can determine the relationship between slip

direction and dislocation line to Burgers Vector. Slip directions for edge dislocation

is parallel to Burgers Vector but perpendicular for dislocation line while for screw

dislocation is both parallel to Burgers Vector.

CHAPTER 3

METHODOLOGY

Figure 3.1 Flow chart of the overall process that take place during research.

The experimental flow is shown in Figure 3. In order to obtain a high quality

of AlN thin films, two stages of analysis and characterization is carried out.

21

First stage: The defect is identified through analysis of the cross sectional

nitride semiconductor thin films using CBED, TEM and SEM.

Second stage: development of high quality thin films using AlN buffer layer

for application in semiconductor devices.

The methodology of this research is to observe the changes in dislocation

behaviour and its pattern by using Transmission Electron Microscope (TEM) and

Scanning Electron Microscope (SEM). For the sample preparation Focused Ion

Beam (FIB) and Metal Organic Vapour Epitaxy (MOVPE) was used.

3.1 Convergent Beam Electron Diffraction

CBED or Convergent Beam of Electron Diffraction is a technique to obtain a

diffraction pattern by focusing a series of electron on the specimen. The diffraction

pattern is then can be analyse. The Figure 3.2 below shows how CBED work.

Figure 3.2 CBED mechanism[16]

A converge electron will passes through the specimen. Then the electron will

reflected to the objectives lens. From the objective lens, a disc of intensity will

22

formed on the diffraction plane. From the image formed, it will give a detail about

the microstructure of the specimen. Ten analysis can be made.

3.2 Metal Organic Vapour Phase Epitaxy (MOVPE)

In order to grown an AlN thin films, a horizontal MOVPE was used. The source

material are Trimethylgallium (TMG), Trimethylaluminium (TMA) and Ammonia

(NH3) and for the carrier gas are Hydrogen (H2) and Nitrogen (N2).

H2 carrier gas is mixed with metal organic (MO) and NH3 in order to reduce

parasitic reaction between MO and NH3. All of it is fed to a slanted substrate through

delivery tube with velocity around 100cm/s. Thus the needed mixture composition can

be obtain by controlling the concentration of TMG and TMA. All this information is

given by the research partner which is Mie University as they involved in thin film

growth process. Therefore the feedback from the characterization and analysis of the

cross sectional TEM observation is very important in order to improve the quality of

nitride thin films. Thus, the parameter used in growth process can used to develop a

high quality of thin film with less defects.

3.3 Focused Ion Beam (FIB)

FIB is used as a tool for microcircuit editing. It has become the preferred tool

in order to make a sample preparation for microscopy specific application. One of

FIB is able to create and modify the sample. Other than this are:

Remove material

Deposit material

Provide localized ion implantations

The other advantage of FIB is, it manage to get the image of the sample

during, before or after the micro milling via secondary electron ion. This is important

because we want to control the process. The main reason this machine is used is to

23

get the cross sectional of the sample so that it is suitable for TEM and SEM that will

be used in this research.

Figure 3.3 Focused Ion Beam (FIB)

3.4 Transmission electron microscope (TEM)

TEM is a series of a high energy electron beam that is transmitted through a

very thin sample, then the image of the microstructure of material will appear. So

that the image can be analyse and observe. The image formed is in atomic resolution.

Electromagnetic lenses is used to focus the beam and the image is appeared in a

fluorescent screen and recorded by negative film or by CCD camera. Acceleration of

electron is a several hundred kV and wavelength smaller than light:

200kV electron have a wavelength 0.0025Å.

24

Figure 3.4 Sample of image taken by TEM

3.5 Scanning Electron Microscope (SEM)

SEM is an electron microscope that produces images of a sample by scanning

it with a focused beam of electron. SEM focuses on the sample’s surface and its

composition. Also SEM is use for examine and analyse the microstructural

characteristic of solid objects.

Figure 3.5 Sample image taken by SEM

3.6 Bright Field Image (BF)

BF is one of the common operation mode for TEM. In this mode, contrast

formation is formed directly by occlusion and absorption of electron in the sample.

100nm

100nm

25

The region with high electron number will appear while for lower number of electron

will appear bright. This is why it is call bright field.

3.7 Dark Field Image (DF)

In DF, the electron beam was blocked by the aperture. While one or more

diffraction beam is allowed to pass the objective aperture. Diffracted bean is strongly

linked with the specimen, an image that give information will appear such as planar

defects, stacking fault or particle size.

3.8 Sample preparation for FIB

In order to produce a high quality of AlN thin films, the step by step technique

or method to get the desire result is very important to identify. First of all, identifying

the defect by using TEM will give some information about the defect especially

dislocation. Then, the formation, behaviour of the defect is studied and analysed. So

that we can get a parameter and condition in growing a high quality of AlN thin films.

Finally, after all the information needed is obtained, the defect can be reduced.

Before using FIB, there are a few step need to be done to the sample. It is a

must follow procedure. The procedure for the sample preparation of FIB are as

follows:

1. The AlN on the sapphire substrate thin films is attached to the on a

thicker glass using a wax. This is because the thin films is too thin

to cut directly using the cutter.

2. After that, the sample was cut into two sizes which are 2x4 mm and

2x3 mm respectively.

3. Then, carbon coating process is take place. The carbon rod was

heated for 30 seconds at 5V.

26

4. The specimen is coated with carbon for 200 seconds (10s x 20

times). So that the thickness of the coating is between 20 to 30 mm.

5. The holder is set in the sub chamber, after that the sample is loaded

using rod.

6. Make an adjustment for certain setting. Such as beam adjustment,

Z height adjustment, focus or stigma adjustment, beam position,

degas and SEM beam adjustment. All this setting should be done

properly to avoid error.

7. When all the setup is finished, Slope, Etch and deposition process

is take place in the processing area. Time taken for the process

undergo is depend on the width and height of the processing area.

8. Lastly, a W needle is used to pick up the etched piece. In order to

avoid the needle and the stage is clash each other, it is important to

make sure that the stage and needle be at their home position or

eucentric position.

Figure 3.6 Cutting machine

Figure 3.7 AlN specimen after

carbon coating

27

3.8.1 Sample preparation for annealing temperature 1650℃

In this research, HITACHI ML-4000L FIB machine is used for the specimen

preparation. Figure 3.10 below show the specimen went through etching and carbon

deposition.

Figure 3.10 Etched and carbon deposited specimen

The specimen then is pick up by using W Needle as shown in Figure 3.11. It

is important to make sure that the depth of the etching is deep enough. This is to

(11-20)-Sap

(1-100)-AlN

(1-100)-Sap

(11-20)-AlN

Figure 3.9 Wax used to

attach the specimen to glass Figure 3.8 Specimen that

will be wax

28

make sure that, the needle can pick the etched specimen easily. If not, the specimen

might break into two pieces.

Figure 3.11 Specimen is picked by W Needle.

After picking up the specimen, the specimen is then brought to TEM mesh. If

the carbon deposited is not thick enough, it may result to poor adhesion causing the

specimen is fall off during deposition.

29

Figure 3.12 Specimen is deposited to TEM mesh before thinning.

After that, the specimen will undergo thinning process as shown in Figure

3.13 and onward. This is to make sure that the specimen is thin enough so that

electron beam can passes through the cross section of the specimen.

Figure 3.13 Specimen undergo thinning using U fine

30

The thinning process was done at ±0˚ 30kV. While for 15kV, 10kV, 5kV

and 3kV the thinning process was done at ±0.5˚. The changes on beam current is

depend on the thickness of the specimen. The thinner the specimen, the lower the

amount of current.

3.8.2 Sample preparation for annealing temperature 1500℃

The same procedure is undergo for this specimen. Only that, the beam current

only available at 15kV and 3kV due to power breakdown. After that Argon milling

take place. Argon milling is a technique to mill a thin sample until it become

transparent. In easy word, polishing. It will polish the sample, so that it can be

imaged under TEM.

Figure 3.14 Specimen is picked by using W Needle

In Figure 3.14 shows the images of the sample that is picked using a W

Needle. This step is done very carefully to make sure that the sample is not fall off

and to avoid the specimen is ripped off.

31

Figure 3.15 Specimen that is attached to TEM mesh

In Figure 3.15 show the image of specimen that is attached to the TEM mesh.

Mesh is a copper material that contain grids. In the figure 3.16 below shows the

sample of mesh.

Figure 3.16 Sample mesh

The specimen will undergo thinning process. This is to make sure that the

thickness of the specimen is thin enough to be process by the TEM. Thinning process

consist of a several type condition. There are mid beam condition, fine beam condition

and u-fine beam condition. The use of these three condition is depend on the thickness

of the specimen.

32

Figure 3.17 Thinning after using mid beam condition

Figure 3.18 Plan view of after mid beam condition

Mid beam condition is used at the start of the process of thinning. This is

because the ion beam energy is high. This is because the surface of the specimen is

still thick and rough. So mid beam ion condition is used.

Fine beam condition is the second step after mid beam condition. This

process is more focus on removing the rough surface after mid beam process.

33

Figure 3.19 Thinning using fine beam condition

Figure 3.20 Thinning using u fine condition

U fine condition is the third step before the TEM can be undergo. U fine

beam condition is more like polishing the surface of the specimen to make it

smoother and flat. U fine beam condition consist of low energy ion beam that used to

smoothing the surface of the specimen.

34

Figure 3.21 Plan view of the U fine beam condition

Figure 3.22 Specimen when using 15kV of beam current

Figure 3.22 shows the image of specimen when using 15kV beam current.

The amount of current control the speed of ion beam when cutting the specimen. The

higher the current, the faster the beam. Usually higher current beam is used when

using mid fine beam condition. This is because, the thickness of the specimen. While

for 3kV beam current is for U fine beam condition.

35

Figure 3.23 Specimen when using 3kV beam current

Figure 3.24 Specimen after Argon milling

After undergo the thinning, the last step in the procedure is to undergo Argon

milling. This type of milling is to make the specimen transparent. Or in general word

as the surface finishing. When the specimen is transparent, TEM can image and

characterized the specimen. Finally the result is analysed.

Sample preparation for annealing temperature 1550℃ and 1600℃ has the

same procedure for 1500℃ and 16500℃.

36

3.9 Procedure on Using a Transmission Electron Microscope (TEM)

In this research, the following type of TEM is used:

i) JEM 2000EX

ii) JEM-2100

iii) JEM-3200FSK

iv) TECNAI-20

The key in capturing a good images is depend on how the specimen is prepared.

The specimen should be prepared accordingly following the procedure for example,

the thickness of the thin films should be less than 200nm, so that the electron beam

can passes through the cross section of the specimen. To explain the procedure, JEM-

2100 type of TEM is choose.

Figure 3.25 JEM-2100

37

A) Right control panel

B) Camera chamber

C) Trackhall

D) Left control panel

E) Selected area aperture

F) Condenser aperture

G) ACD

H) Specimen chamber

1. Initial check and set up

The ion pump reading is checked. The reading should be less than 5 x

10-5 Pa.

Liquid Nitrogen with ACD is filled. This is foe cooling purpose.

Object and selected area aperture is inserted

2. Specimen exchange

The position of the stage is checked at neutral or origin.

The specimen holder is removed from goniometer and new specimen is

loaded on specimen holder carefully.

Checked whether there is no contaminant on the tweezers.

The SH guide pin with guide groove is aligned on the microscope

column.

The specimen holder is hold carefully and inserted in the microscope and

turned clockwise.

The black pin is put at it place.

3. Alignment of illumination system

A) Gun tilt alignment and condenser lens astigmation correction

The value of objective lens is checked at 2.63V

Press F2 button. The filament image is adjusted to be symmetric.

The astigmation is adjusted using DEF/STIG X or Y to sharpen the

image.

38

Figure 3.26 Setting up astigmatism

B) Gun Shift Alignment

The SIZE SPOT knob is set to 1 and focus the electron beam to the

crossover with Brightness knob.

The SIZE SPOT knob is set to 5, and repeat above step until the electron

beam stay at the center.

Keep Spot SIZE 2 and if the CLA or GUN is turn on, press it to switch it

off.

4. Bright field imaging

The MAG 1 button is pressed, and the optimum magnification is set. The

current value is checked at 2.63V.

The beam is expanded fully across the phosphor screen. The object is

moved to the center.

The mode is changed to diffraction mode by pressing SA DIFF button and

the camera length is set to 80cm.

The specimen is tilted to obtain desired condition. A small objective area

is inserted to select a direct spot.

Going back to imaging mode by pressing MAG 1 button.

5. Dark field imaging

39

Checked the objective lens current. Make sure at 2.63V. The MAG 1

button is press.

The selected area aperture is inserted. The size should be larger than the

object and performed at the center of aperture.

Mode is changed to diffraction mode and the camera length is 80cm by

MAG/CAM knob.

To change to dark field imaging mode, Dark Tilt button is pressed and the

electron beam is tilted with DEF/STIG X or Y knob.

Small objective aperture is inserted to select diffracted spot.

Going back to imaging mode by pressing MAG 1 button.

Figure 3.27 TEM-2000EX

Setting up TEM-2000ex is apparently the same with TEM-2100. The only

different is, the setting is done manually in the scene of rotating the knob while the

other use button and knob. Also it provided with CCD camera so that the image

picture can be viewed on the spot on the monitor screen.

CHAPTER 4

RESULT AND DISCUSSION

4.1 Annealing temperature: 1500°C

Figure 4.1 (a) and (b) Bright Field (BF) Image and Diffraction Pattern AlN

Cone shaped Inversion domain

(b)

41

Figure 4.1 give a picture of Bright Field (BF) Image and Diffraction

Pattern AlN taken along [1̅21̅0] zone axis shows cone shaped inversion domain.

There are barely any threading dislocation present in the specimen.

Figure 4.2 (a) and (b) Dark Field image cross sectional TEM image and

diffraction pattern of AlN

Figure 4.2 shows a Dark Field image cross sectional from TEM image and

diffraction pattern of AlN at g=(101̅0) direction. Cone shaped inversion domain is

not visible instead columnar domain can be observed in Figure 4.2(a).From the

invisibility criterion g.b=0, the cone shaped inversion domain that appeared in Figure

4.1(a) and disappeared in Figure 4.2(a) had a Burgers vector normal to (0002)

direction.

Columnar domain

(b)

42

4.2 Annealing temperature: 1550°C

Figure 4.3 (a) and (b) Bright Field image and diffraction pattern of AlN

Figure 4.3 shows a Bright Field image and diffraction pattern of AlN on

sapphire substrate. From the figure it can be said that, a wavy and sharp grain

boundaries. Sharp boundary is because it is parallel with incident beam and wavy

boundary is due to incline with incident beam in X and Y.

43

Figure 4.4 shows a dark field image and diffraction pattern of AlN indicating

that the grain boundaries is slowly disappeared when taken at g= 10-10 direction.

Figure 4.4(a) and (b) Dark Field image and diffraction pattern of AlN

Sapphire

(b)

44

Figure 4.4(c) shows the CBED pattern identifies inverted polarity regions as

CBED pattern axis recorded at region A is reversed relative to that recorded at region

B. The thickness is same in both areas. Therefore this proves that the defects type are

inversion domain.

Figure 4.4(c) CBED pattern

4.3 Annealing temperature: 1600°C

The images from Figure 4.5 is taken at the same diffraction point.

From the image, it is clearly shown a wavy like grain boundary is becoming flatter

and smoother is produced compare to Figure 4.4(a).

(c)

45

Figure 4.5 (a) and (b) and (c) Bright Field and Dark Field Image and

diffraction pattern

(c)

46

Figure 4.6 is taken at the same area and diffraction pattern. From the images

formed, there are some folds like defects being observed.

Figure 4.6 (a), (b) and (c) Bright Field and Dark Field Image of AlN

(c)

47

4.4 Annealing temperature: 1650°C

Figure 4.7 shows a smooth grain boundary and fringe contrast also

can be observed. As the annealing temperature increases the defects decreases and

the microstructure is smoother and defects are reduced.

Figure 4.7(a) and (b) Bright Field and Dark Field image of AlN

(b)

48

Figure 4.8 shows that the grain boundary is barely seen anymore as there is a

line defect observed.

Figure 4.8(a) and (b) Dark Field image and diffraction pattern

(b)

49

As can be seen in previous figure, different temperature will give a different

effect to the specimen. Figure 4.9 below show the changes in microstructure of the

specimen from 1500°C to 1650°C. While Figure 4.10 shows the schematic diagram

of microstructure changes.

Figure 4.9 Changes in microstructure for different annealing

temperature

Figure 4.10 Schematic diagram for microstructure changes

50

Table 5: Changes of microstructure and observation

Annealing

temperature

TEM Image Observation

1500℃

Cone shaped Inversion

domain formed.

There are barely any

threading dislocation present

in the specimen

1550℃

Wavy and sharp grain

boundaries is formed.

1600℃

The wavy like grain boundary

is become flat and smooth.

1650℃

The wavy and sharp

boundaries is decreasing.

51

From the result, we can see that the wavy grain boundaries is reducing to

become flatter and smoother. Unfortunately, it cannot be said that the lattice mismatch

is vanish completely. It is impossible to have a perfect lattice structure because there

is a small dislocation exist and hard to remove.

One of the defect that still remain is inversion domain (ID). ID in general word

is two or more object that coincide each other. In material science, inversion domain

is a defect that cross the film to the surface and form a cone shape like structure.

Inversion domain can influence the performance of the semiconductor. As can be seen

in the Figure 4.1(a), there is ID exist in the structure.

Figure 4.11 Sample of ID

Figure 4.12 and Figure 4.13 are the image taken using Atomic Force

Microscope (AFM). The images shows the surface of AlN buffer layer with

annealing and without annealing.

Figure 4.12 Surface of AlN buffer layer without annealing 1.0 µm

52

Figure 4.13 Surface of AlN buffer layer with annealing at 1500℃

Those two figure shows the effect of the annealing treatment on the surface of

the specimen. As can be seen from Figure 4.13, the surface of the specimen without

annealing is rough and coarsely compare to surface that anneal at 1500℃. The

surface is less coarse and less rough.

When the temperature of annealing is higher, the roughness of the surface

decreasing as in Figure 4.14 below.

Figure 4.14 Surface of AlN buffer layer with annealing at 1600℃

The rough grain boundaries looks like expanding and then explode to form a

smooth candy cotton like shape. Smooth and beautiful compare to surface that look

like sand. Table 4 show the comparison between surface that annealed and not.

53

Table 4: Comparison between annealed surface and normal surface

Without annealing With annealing

1500℃ 1550℃

1600℃ 1650℃

1700℃ 1750℃

54

CHAPTER 5

CONCLUSION AND RECOMMENDATIONS

5.1 Conclusion

From the result shown on Chapter 4, can be seen that the defect especially

lattice mismatch or dislocation is reduced as the annealing temperature become higher.

Annealing is a heat treatment that alters a material to increase the ductility of the

material and to make it more workable. It involves heating a material to above its

critical temperature, maintaining a suitable temperature, and then cooling. For this

research, the effect of annealing treatment on the growth process of Al has been

investigated. As the temperature increases from 1500°C to 1650C, it can be observed

the wavy rough grain boundaries becomes smoother and flat. Crystallinity of AlN

depends on the annealing temperature during the growth process on sapphire substrate.

At temperature 1500˚C and lower, inversion domain boundaries and columnar domain

is observed and this was verified with CBED pattern that showed a reversed region.

The result of this research clearly shown that the dislocation density has been

reduced with the increasing of annealing temperature. Even though there is still

inversion domain remain, the surface of the specimen is free from defect. So it is

suitable to be used as a substrate to produce a high quality of thin films. As the

conclusion, annealing process is affective for semiconductor material in reducing

lattice mismatch or dislocation.

56

5.2 Problems

Even though this research achieve the main objective, but the problem is this

method is only focus on reducing the density of lattice mismatch. Supposedly there are

still other defect that exist in the structure as been discussed in discussion section. To

grow a high quality of thin film, the structure itself should be free from defect.

During the milling process using FIB, the depth of the etched specimen is not

enough. This shows that, the depth of etching specimen is important if not it will

resulted to failing to pick up the specimen using the needle.

The thickness of deposited carbon also important. Poor in adhesion may result

in specimen to fall off from the needle during the deposition process. This is what

happen during the research in the first attempt of transporting the etched specimen to

TEM mesh.

5.3 Recommendations

The result of this research is current not sufficient enough in order to grow a

high quality if AlN thin films. This is because, the defect focused in this research is

only lattice mismatch that is dislocation in the structure of AlN. One of the example is

stacking fault. So, there are some recommendation that need to be considered in order

to continue this research.

1. Use other type of experiment other than annealing.

Annealing treatment is used in this research because it is easy and simplest

method. There is other method can be used such as low temperature buffer

layer between high temperature of nitride material, nitridation and epitaxial

lateral overgrowth. If this method is use, the result might be different from

this research.

57

2. Consider the SEM result.

SEM also might help in producing high quality of thin films. In this

research more focus in using TEM. In the future, SEM should be utilize to

get a variety of data.

58

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