Influence of Annealing Temperature on ZnO Thin Films Prepared by Single and Multi-step Sol-gel Processes
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Influence of Annealing Temperature on ZnO Thin Films Prepared by Single and Multi-Step Sol-Gel Processes Lee Huat Kelly Koh1, Shane O'Brien1, Pierre Lovera2, Gareth Redmond2, and Gabriel M. Crean1,3 1 Mesoscale Materials Science, Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland 2 Nanotechnology Dept., Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland 3 Dept. of Microelectronic Engineering, University College Cork, Cork, Ireland
ABSTRACT ZnO thin films were prepared on borosilicate glass from both single- and multi- step coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C2H3O2)2], monoethanolamine [H2NC2H4OH ] and isopropanol. ZnO films prepared over a range of zinc acetate concentrations, for a fixed annealing temperature, showed that sol-gels prepared with a 0.3M zinc acetate concentration resulted in the formation of films with the greatest degree of caxis orientation. In this study, a detailed investigation of the influence of process annealing temperature over the range 450 – 550oC on film properties around this 0.3M zinc concentration set point is presented. X-ray analysis showed that all single-step deposition thin films were preferentially orientated along the [002] c-axis direction of the crystal. In contrast, only the multi-layer film annealed at 550oC showed similar preferential orientation. INTRODUCTION ZnO is a wide direct band-gap oxide semiconductor that has significant potential application in electronic, optoelectronic and information technology device platforms due to its electrical and optical properties [1-3]. Several thin-film deposition techniques have been demonstrated to produce pure ZnO films, including sputtering [4], molecular beam epitaxy [5], metal-organic chemical vapour deposition [6], pulsed laser deposition [7], spray pyrolysis [8] and the sol-gel process [9]. The sol-gel method has several distinct potential advantages over its counterparts, due to its lower crystallization temperature, ability to tune microstructure via solgel chemistry, conformal deposition ability, compositional control and large surface area coating capability. However, to date, multiple layers of coatings have been generally required to produce thin films of required thickness for most specific technological applications. Typical multi-step deposition processes that have been demonstrated include the application of five consecutive sequential depositions to achieve a 100nm film [10]. Recently, a single step deposition process capable of achieving transparent c-axis orientated ZnO thin films up to 350nm film thickness was demonstrated using an anhydrous zinc acetate [11]. The use of anhydrous zinc acetate eliminated the introduction of water which was shown to improve the control of reactions occurring within the sol-gel. It was further identified that a zinc concentration of 0.3M provided an optimum ZnO thin film for the fixed annealing temperature. In this study, a detailed investigation of the influence of process annealing temperatu
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