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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
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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.
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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.
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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.
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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.
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TABLE OF CONTENTS
CHAPTER TITLE
DECLARATION
DEDICATION
ACKNOWLEDGEMENT
ABSTRACT
ABSTRAK
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF SYMBOLS
PAGE
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1 INTRODUCTION
1.1 Introduction
1.2 Objectives
1.3 Scope of Study
1.4 Scope of Report
1.5 Literature Survey
1
1
2
2
3
4
2 THEORY
2.1 Introduction
2.2 Tin (IV) Oxide, SnO2
2.3 Sputtering
2.3.1 Radiofrequency Sputtering
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7
8
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2.4 Energy Band Gap
2.4.1 Definition of Energy Band
2.4.2 Charge Carriers in Energy Bands of Intrinsic
Semiconductor
2.5 Optical Properties
2.5.1 Direct Optical Transition
2.5.2 Indirect Optical Transition
2.6 Optical Characterization
2.6.1 Determining Absorption Coefficient, (α)
2.6.2 Optical Characteristics of Semiconductors
2.7 Photoluminescence
2.7.1 Photo Excitation and Emission Processes
2.7.2 Photoluminescence To Determine
Bandgap
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3 METHODOLOGY
3.1 Introduction
3.2 Preparation of Substrates
3.2.1 Substrate Cutting
3.2.2 Substrate Cleaning
3.3 Radiofrequency Sputtering
3.3.1 Procedure Of Operating RF Sputtering
Coating Machine
3.4 Thickness Measurement
3.4.1 The Ellipsometer
3.5 Optical Measurements
3.5.1 Introduction
3.5.2 UV-3101-PC Spectrophotometer
3.6 Photoluminescence
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4 RESULTS AND DISCUSSION
4.1 Introduction
4.2 The Thickness Measurement
4.3 Optical Properties
4.3.1 Transmission Spectrum
4.3.2 Absorption Coefficient
4.3.3 Energy gap of SnO2 Thin Films
4.3.4 Photoluminescence Results
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5 CONCLUSION AND COMENT
5.1 Conclusion
5.1.1 Inaccurate Energy Gap Value
5.2 Suggestion
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REFERENCES
Appendix
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LIST OF TABLES
TABLE NO.
2.1
4.1
4.2
4.3
TITLE
SnO2 Reference Data The Thickness of The Samples Obtained energy gap value Emission Energy for SnO2 Thin Film
PAGE 6
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44
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LIST OF FIGURES
FIGURE
2.1
2.2
2.3
2.4
2.5
2.6
3.1
3.2
3.3
3.4
3.5
4.1
4.2
TITLE
Energy band diagram for a semiconductor (a) Energy band diagram at absolute zero (b) Energy band diagram at T > 0K (a) Direct optical transition (b) Indirect optical transition Schematic diagram showing transmission of photons through a semiconductor slab via propagation of polarities inside the sample. Step ladder model of a large neutral molecule Schematic band diagrams for the photoluminescence processes in a direct gap material (left) and an indirect gap material (right). Glass Substrate Radio Frequency System Ellipsometer Structure UV-Spectrophotometer Diagram Schematic Of Spectroflourometer Graph Thickness Versus Deposition (target-to-substrate) Distance at Different Ambience SnO2 Thin Film Transmission Spectrum for Sample Prepared in Pure Argon Gas Content (Group A)
PAGE
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4.3
4.4
4.5
4.6
4.7
4.8
4.9
4.10
4.11
SnO2 Thin Film Transmission Spectrum for Sample Prepared in 20% Oxygen Mixed Argon Gas Content (Group B) SnO2 Thin Film Transmission Spectrum for Sample Prepared in 10% Oxygen Mixed Argon Gas Content (Group C) Absorption Coefficient, α Versus Photon Energy, hν for Sample Group A Absorption Coefficient, α Versus Photon Energy, hν for Sample Group B Absorption Coefficient, α Versus Photon Energy, hν for Sample Group C Graph of Allowed Direct Transition of SnO2 for Sample Prepared in Pure Argon Gas Content Graph of Forbidden Direct Transition of SnO2 for Sample Prepared in Pure Argon Gas Content Graph of Allowed Indirect Transition of SnO2 for Sample Prepared in Pure Argon Gas Content Graph of Forbidden Indirect Transition of SnO2 for Prepared in Pure Argon Gas Content
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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.
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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:
to calculate the thickness of the thin films
to abtain the transmittance spectrums
to calculate the energy gap
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
