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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.
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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
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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
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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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