OPTICAL PROPERTIES OF TIN OXIDE THIN FILMS WONG CHENG YEE. Bachelor of Science and Education (Physic)

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3 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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5 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.

6 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.

7 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 nm in which the energy of the emission is between 3.12 ev to 3.55 ev.

8 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 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.

9 vii TABLE OF CONTENTS CHAPTER TITLE PAGE DECLARATION DEDICATION ACKNOWLEDGEMENT ABSTRACT ABSTRAK TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS ii iii iv v vi vii x xi xii 1 INTRODUCTION 1.1 Introduction 1.2 Objectives 1.3 Scope of Study 1.4 Scope of Report 1.5 Literature Survey THEORY 2.1 Introduction 2.2 Tin (IV) Oxide, SnO2 2.3 Sputtering Radiofrequency Sputtering

10 viii 2.4 Energy Band Gap Definition of Energy Band Charge Carriers in Energy Bands of Intrinsic Semiconductor 2.5 Optical Properties Direct Optical Transition Indirect Optical Transition 2.6 Optical Characterization Determining Absorption Coefficient, (α) Optical Characteristics of Semiconductors 2.7 Photoluminescence Photo Excitation and Emission Processes Photoluminescence To Determine Bandgap METHODOLOGY 3.1 Introduction 3.2 Preparation of Substrates Substrate Cutting Substrate Cleaning 3.3 Radiofrequency Sputtering Procedure Of Operating RF Sputtering Coating Machine 3.4 Thickness Measurement The Ellipsometer 3.5 Optical Measurements Introduction UV-3101-PC Spectrophotometer 3.6 Photoluminescence

11 ix 4 RESULTS AND DISCUSSION 4.1 Introduction 4.2 The Thickness Measurement 4.3 Optical Properties Transmission Spectrum Absorption Coefficient Energy gap of SnO2 Thin Films Photoluminescence Results CONCLUSION AND COMENT 5.1 Conclusion Inaccurate Energy Gap Value 5.2 Suggestion REFERENCES Appendix 48 50

12 x LIST OF TABLES TABLE NO. 2.1 SnO 2 Reference Data TITLE PAGE The Thickness of The Samples Obtained energy gap value Emission Energy for SnO 2 Thin Film 45

13 xi LIST OF FIGURES FIGURE TITLE Energy band diagram for a semiconductor (a) Energy band diagram at absolute zero (b) Energy band diagram at T > 0K PAGE (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 SnO 2 Thin Film Transmission Spectrum for Sample Prepared in Pure Argon Gas Content (Group A)

14 xii SnO 2 Thin Film Transmission Spectrum for Sample Prepared in 20% Oxygen Mixed Argon Gas Content (Group B) SnO 2 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 SnO 2 for Sample Prepared in Pure Argon Gas Content Graph of Forbidden Direct Transition of SnO 2 for Sample Prepared in Pure Argon Gas Content Graph of Allowed Indirect Transition of SnO 2 for Sample Prepared in Pure Argon Gas Content Graph of Forbidden Indirect Transition of SnO 2 for Prepared in Pure Argon Gas Content

15 xiii LIST OF SYMBOLS CB - Conduction band d - Thickness E c - Conduction band energy E g - Energy gap E v - 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

16 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.

17 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 SnO 2 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 (SnO 2 ) 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