A Study on Impact of Process Parameters to Metal Organic Chemical Vapor Deposition Grown (002) Zinc Oxide Thin Films, at
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A Study on Impact of Process Par ameter s to Metal Or ganic Chemical Vapor Deposition Grown (002) Zinc Oxide Thin Films, at 320oC Yuneng Chang, Hengchuan Lu, Yumeng Hung, Chunsung Lee, Jianming Chen, Yichang Jian Lunghwa University of Science and Technology, Dept. of Chemical Engineering, No.300, Sec.1, Wanshow Rd., Gueishan, Taoyuan, 333, Taiwan, R.O.C. ABSTRACT This paper reports preparation of highly oriented (002) ZnO films by atmospheric pressure CVD at 320oC, which is far below previous reported values. In this study, a cold wall horizontal system was used to thermally decompose sublimed zinc acetylacetonate (Zn(acac)2, Zn(C5H7O2)2) vapor, and reacted with water vapor to produce ZnO films at temperatures above 320oC. Through experimental data, we discovered that low deposition temperature, using water vapor as co-reactant and substrates with ZnO buffer layer pre-coated by PVD are the key factors to prepare (002) ZnO films. By using Si(100) pre-coated with sputtered ZnO amorphous buffer layer as substrates, the ZnO growth rate is highest. While using copper oxide pre-coated Si substrates gave the lowest growth rate, and deposited ZnO film is amorphous. Considering influence of CVD co-reactant, using Zn(acac)2 and water vapor gives higher growth rate and better crystallinity than CVD using Zn(acac)2 and oxygen. Water vapor may supply hydrogen to react with released acetylacetonyl ligand (C5H7O2), and help the formation of stable acetylaceton (C5H8O2) molecule. DPA shows that film contain 46% O and 54% Zn. XPS of Zn Auger identified the valence of Zn being Zn2+. It seems that excessive Zn might present as discrete Zn2+ dispersed between ZnO lattices. INTRODUCTION Zinc oxide (ZnO) is an n-type semiconductor with a hexagonal wurtzite structure (a=0.32nm, c=0.52nm). ZnO is a valuable compound semiconductor with piezoelectric, optoelectronic, photoconducting, and optical waveguide applications. ZnO has a wide band gap of 3.3 eV, and can be used as ultra short wavelength LED. ZnO is a piezoelectric material with a wide range of applications as acousto-optic, acousto-electric and opto-electronic devices (1). Because of large exciton binding energy (60 meV) and small Bohr radius (18 Å ), efficient UV excitonic lasing can been observed on ZnO at room temperature (2). ZnO usually contains excess amount of zinc atoms, dispersed between the interstices of ZnO unit lattice. These Zn atoms can be easily ionized at room temperature, ZnàZn2+ +2 e-. These free electrons can enter conduction band easily, thus enabling ZnO as an n type semiconductor, with resistivity ranges from 10 to104Ω cm, and can be used as a gas sensor. Due to high transparency and large conductivity, pure and doped ZnO can be used to replace expensive ITO as transparent conductive oxide (TCO) layer in LCD panel (3). Novel processes have been developed to deposit ZnO over plastic substrate to produce flexible TCO circuit board for PDA uses. Moreover, the unit lattices of ZnO and GaN are both HCP. The difference between their lattice constants is 0.7%
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