The Residual Stress Effect on Microstructure and Optical Property of ZnO Films produced by RF Sputtering
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U8.18.1
The Residual Stress Effect on Microstructure and Optical Property of ZnO Films produced by RF Sputtering Sang Ryu and Youngman Kim Department of Materials Science and Engineering Chonnam National University, Gwangju, 500-757, KOREA. ABSTRACT ZnO films were produced on the Si(100) and sapphire(0001) wafers by RF magnetron sputtering in terms of processing variables such as substrate temperature and RF power. The stress in films was obtained from the Stoney`s formula using a laser scanning device. The stress levels in the films showed the range from ~40
㎫ depending on processing variables.
㎫ to
~-1100
SEM was employed to characterize the microstructure of the films. As the substrate temperature increased, the film surface became rougher and the films showed coarser grains. The optical property of the films was studied by PL measurements. At the highest substrate temperature 800
℃ the film exhibited sharper UV peaks unlike other
conditions. Keyword: Stress, ZnO thin films, R.F. magnetron sputtering
INTRODUCTION ZnO films deposited stoichiometrically have 3.36eV band gap and are direct band gap semiconductors with an emission property of UV range. But most ZnO films produced practically have the characteristics of n-type semiconductor due to an excess of Zn atoms, or lack of O atoms. In this case, ZnO emits visual light such as green and yellow. In bonding structure of ZnO oxygen vacancies remove 6 electrons among 8 electrons around O atoms thus the 2 remaining electrons behave like donor states, in turn ZnO films become n-type semiconductors due to 2 electron defects. Actually various impurity levels may be formed by the defects, as a result various visible range emissions can be obtained [1-2]. These types of defects depend on various factors such as lattice mismatch, residual stress and film’s morphology [2]. Therefore, it is reasonable to investigate the effect of stress and microstructure on the optical properties of ZnO films
U8.18.2
in terms of processing conditions [3-5]. In this study, ZnO films were produced on the Si(100) and Al2O3(0001) wafers by R.F. magnetron sputtering in terms of processing variables such as substrate temperature and RF power. The microstructures, residual stress and optical properties of the deposited films were characterized as a function of processing variables using SEM, laser scanning device (LSD), photo luminescence (PL).
EXPERIMENTAL PROCEDURE ZnO films were deposited by R.F. magnetron sputtering system using a ZnO target (99.99%) with the diameter of 2 in. and the thickness of 6mm. The 200 Si(100) and 430
㎛
㎛ thickness of
thickness of Al2O3(0001) were used for the substrates. The
dimensions of both substrates were 1.5
㎝×0.5㎝. In the case of Si substrates, the surface
oxide film was removed using HF subsequently cleaned with acetone and alcohol for 5 minutes each, and with deionized water for 10 minutes before blow-drying using N2 gas. The Al2O3 substrates were cleaned with acetone, alcohol and deionized water before N2 blow-drying. ZnO films were deposit
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