Saput tipis Timah (IV) Oksida disediakan melalui kaedah percikan ... 950 Å. Sifat-
sifat optik yang dikaji termasuklah sifat penghantaran dalam julat panjang.
OPTICAL PROPERTIES OF TIN OXIDE THIN FILMS
WONG CHENG YEE
A report submitted in partial fulfilment of the requirements for the award of the degree of
Bachelor of Science and Education (Physic)
Faculty of Education Universiti Teknologi Malaysia
May 2006
iii
Especially to my most respected Supervisor, Pn Wan Nurulhuda Wan Shamsuri, my beloved Father and Mother and all my friends. Thanks for all the efforts, guidance, tender support and blessings that shower on me.
iv
ACKNOWLEDGEMENT
First and foremost, I would like to take this opportunity to express my deepest gratitude to my project supervisor, Pn Wan Nurulhuda Wan Shamsuri for her guidance and support through out the whole project. I am greatly indebted to knowledge and the time she had imparted on me. Besides, I would also like to thank my parents for their tender support. They had given me a lot of support in terms of financially and mentally. Also not forgetting to convey my deeply appreciation to Laboratory Assistance, Madam Noor Hayah Bt Jantan, Madam Fadzilah Bt Lasim and Mr. Nazri Bin Kamaruddin, Master students Mr Tan Hang Khume and Miss Yoong Wai Woon who had provided me with ample information and also co-operation during the process of conducting my poject in the thin film and vacuum laboratory. Last but not least, I would like to thanks my coursemates and friends who had given me a lot of mentally support as well as fruitful ideas and comments for the completion of my project. Thank you.
v
ABSTRACT
The objective of this project is to investigate the optical properties of Tin (IV) Oxide thin film. Tin (IV) Oxide thin films were prepared through the radio frequency magnetron sputtering method, onto the surface of glass substrates at different gas contents and at different thickness of the thin films (different target-to-substrate distance). The thicknesses of deposited Tin (IV) Oxide thin film were determined by using Ellipsometer in the range from 400 Å to 950 Å. The optical properties studied include the transmittance in the visible light region of wavelength between 400 nm to 700 nm and the photoluminescence properties. The transmission spectrums in visible light region (400 nm – 700 nm) determined by using the UV Spectrophotometer showed that the energy gap of Tin (IV) Oxide is about 2.50 eV to 3.80 eV. Photoluminescence of the sample has been investigated by using LS55 Photoluminescence Spectrometer, the emission process occurred between wavelength 350 nm - 400 nm in which the energy of the emission is between 3.12 eV to 3.55 eV.
vi
Abstrak
Tujuan projek ini dijalankan adalah untuk mengkaji sifat-sifat optik saput tipis Timah Tin (IV) Oksida. Saput tipis Timah (IV) Oksida disediakan melalui kaedah percikan magnetron frekuensi (RF sputtering) ke atas permukaan substrat kaca pada kandungan gas yang berbeza dan ketebalan saput tipis yang berlainan (jarak antara sasaran dan substrat). Ketebalan saput tipis Timah (IV) oksida yang disediakan adalah di antara julat 400 Å to 950 Å. Sifat-sifat optik yang dikaji termasuklah sifat penghantaran dalam julat panjang gelombang cahaya nampak 400 nm hingga 700 nm dan sifat fotoluminesennya. Sifat penghantaran dalam julat panjang gelombang cahaya nampak (400 nm - 700 nm) yang dikaji dengan menggunakan UV Spektrofotometer menunjukkan bahawa jurang tenaga Timah (IV) Oksida adalah di sekitar 2.50 eV hingga 3.80 eV. Sifat fotoluminesen dikaji melalui LS55 fotoluminesen spektrometer. Proses pancaran fotoluminesen wujud di antara panjang gelombang 350 nm hingga 400 nm yang mana tenaga pancarannya adalah di antara 3.12 eV hingga 3.55 eV.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
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
xii
INTRODUCTION
1
1.1 Introduction
1
1.2 Objectives
2
1.3 Scope of Study
2
1.4 Scope of Report
3
1.5 Literature Survey
4
THEORY
5
2.1 Introduction
5
2.2 Tin (IV) Oxide, SnO2
5
2.3 Sputtering
7
2.3.1 Radiofrequency Sputtering
8
viii
2.4 Energy Band Gap
9
2.4.1 Definition of Energy Band
9
2.4.2 Charge Carriers in Energy Bands of Intrinsic
10
Semiconductor 2.5 Optical Properties
11
2.5.1 Direct Optical Transition
11
2.5.2 Indirect Optical Transition
12
2.6 Optical Characterization
14
2.6.1 Determining Absorption Coefficient, (α)
14
2.6.2 Optical Characteristics of Semiconductors
15
2.7 Photoluminescence
17
2.7.1
Photo Excitation and Emission Processes
17
2.7.2
Photoluminescence To Determine
18
Bandgap
3
METHODOLOGY
20
3.1 Introduction
20
3.2 Preparation of Substrates
21
3.2.1
Substrate Cutting
21
3.2.2
Substrate Cleaning
22
3.3 Radiofrequency Sputtering
23
3.3.1 Procedure Of Operating RF Sputtering Coating Machine 3.4 Thickness Measurement
24 26 26
3.4.1 The Ellipsometer
26
3.5 Optical Measurements
28
3.5.1 Introduction
28
3.5.2 UV-3101-PC Spectrophotometer
28
3.6 Photoluminescence
30
ix
4
5
RESULTS AND DISCUSSION
32
4.1 Introduction
32
4.2 The Thickness Measurement
32
4.3 Optical Properties
34
4.3.1 Transmission Spectrum
34
4.3.2 Absorption Coefficient
36
4.3.3 Energy gap of SnO2 Thin Films
39
4.3.4 Photoluminescence Results
44
CONCLUSION AND COMENT
46
5.1 Conclusion
46
5.1.1 Inaccurate Energy Gap Value 5.2 Suggestion
47 47
REFERENCES
48
Appendix
50
x
LIST OF TABLES
TITLE
TABLE NO.
PAGE
2.1
SnO2 Reference Data
6
4.1
The Thickness of The Samples
33
4.2
Obtained energy gap value
44
4.3
Emission Energy for SnO2 Thin Film
45
xi
LIST OF FIGURES
TITLE
FIGURE
PAGE
2.1
Energy band diagram for a semiconductor
10
2.2
(a) Energy band diagram at absolute zero (b) Energy band diagram at T > 0K
11
2.3
(a) Direct optical transition (b) Indirect optical transition
13
2.4
Schematic diagram showing transmission of photons through a semiconductor slab via propagation of polarities inside the sample.
15
2.5
Step ladder model of a large neutral molecule
18
2.6
Schematic band diagrams for the photoluminescence processes in a direct gap material (left) and an indirect gap material (right).
19
3.1
Glass Substrate
21
3.2
Radio Frequency System
24
3.3
Ellipsometer Structure
27
3.4
UV-Spectrophotometer Diagram
30
3.5
Schematic Of Spectroflourometer
31
4.1
Graph Thickness Versus Deposition (target-to-substrate) Distance at Different Ambience
33
4.2
SnO2 Thin Film Transmission Spectrum for Sample Prepared in Pure Argon Gas Content (Group A)
34
xii
4.3
SnO2 Thin Film Transmission Spectrum for Sample Prepared in 20% Oxygen Mixed Argon Gas Content (Group B)
35
4.4
SnO2 Thin Film Transmission Spectrum for Sample Prepared in 10% Oxygen Mixed Argon Gas Content (Group C)
36
4.5
Absorption Coefficient, α Versus Photon Energy, hν for Sample Group A
37
4.6
Absorption Coefficient, α Versus Photon Energy, hν for Sample Group B
38
4.7
Absorption Coefficient, α Versus Photon Energy, hν for Sample Group C
39
4.8
Graph of Allowed Direct Transition of SnO2 for Sample Prepared in Pure Argon Gas Content
40
4.9
Graph of Forbidden Direct Transition of SnO2 for Sample Prepared in Pure Argon Gas Content
41
4.10
Graph of Allowed Indirect Transition of SnO2 for Sample Prepared in Pure Argon Gas Content
42
4.11
Graph of Forbidden Indirect Transition of SnO2 for Prepared in Pure Argon Gas Content
43
xiii
LIST OF SYMBOLS
CB
-
Conduction band
d
-
Thickness
Ec
-
Conduction band energy
Eg
-
Energy gap
Ev
-
Valance band energy
h
-
Planck’s constant
I
-
Intensity
n
-
Refractive index
R
-
Reflectance
rf
-
Radio frequency
sccm
-
Standard cubic centimeters
T
-
Transmittance
VB
-
Valence band
ν
-
Frequency
α
-
Absorption coefficient
λ
-
Wavelength
CHAPTER 1
INTRODUCTION
1.1 Introduction
What is thin film? Basically, it is the layer of materials with the film thickness less than about one micron (10,000 Angstroms, 1000 nm) on a substrate. If a thin film is not on a substrate, it is a “foil”. The types of materials can be an insulator, a semiconductor or a metal.
In 1852, W.R. Groove discovered the sputter phenomena in which thin film can be generated in a vacuum environment. He found out that the tube wall is polluted when an anode of a vacuum tube is sputtered and lopped. In 1857, M.Faraday examined vacuum epitaxy. Since he intentionally generated thin-film, it was the oldest vacuum thin film generation in history (Tweeny, 2001).
Nowadays, the researches about thin film are widely studied and the applications of thin film have involved so many areas such as electronics and opto-electronics, optics and laser, information technology, advanced materials and etc. The advantages of thin film devices include low power consumption, relatively small and occupying spaces and a high-speed performance.
2
In this project, optical properties in thin transparent films are to be investigated. The optical properties include the photoluminescence properties and the transmittance of the tin oxide to determine its energy gap due to its different thickness in various gas contents. Tin (IV) oxide will be used as a transparent film. The processes that have been used to deposit transparent film are sputtered with the radio frequency (RF) magnetron sputtering.
1.2 Objectives
The main objectives of this study on SnO2 thin films which are prepared in different deposition (target-to-substrate) distance and different oxygen content are as follows: z
to calculate the thickness of the thin films
z
to abtain the transmittance spectrums
z
to calculate the energy gap
z
to obtain the photoluminescence spectrums
1.3 Scope of Study
The scope of this project is to determine the optical characteristics of a tin oxide (SnO2) thin film. A total of twelve samples are used in this research. These twelve samples comprise three groups of samples with different gas contents. There are four samples in each group with different deposition (target-to-substrate) distances. Three different gas content are 100% Argon, 90% Argon + 10%, Oxygen and 80% + 20% Oxygen. The target-to-substrate distance increases by 0.5 cm for each sample from 3.0 cm to 4.5 cm. All twelve samples are deposited in one hour duration with 50W of
