Preparation of Smooth Zinc Oxide Thin Film via Liquid Phase Reaction with Cation Additives
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1113-F03-26
Preparation of Smooth Zinc Oxide Thin Film via Liquid Phase Reaction with Cation Additives Takeyasu Saito, Yoshihisa Hirata, Naoki Okamoto and Kazuo Kondo Department of Chemical Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai 599-8531, Japan ABSTRACT ZnO thin films were deposited in a solution with Zn(NO3)2 and DMAB from 60 to 80°C. The effects of cation additives such as Mg, Ga and Al in a aqueous solution were investigated on surface morphology, crystallographic structure and growth rate. By adding 1E-4 mol/l of Ga or Al, the growth rate was enhanced from 0.13µm/h to 0.35-0.38µm/h. In addition, the surface morphology became flat in the case of Al addition. INTRODUCTION Zinc oxide (ZnO) is a very attractive multifunctional material, which has widely been used for gas sensors, blue/UV light emitters, varistors, surface acoustic wave devices, optical wave guides and other electronics [1-5]. In addition, ZnO is an excellent alternative to replace indium tin oxide (ITO) and tin oxide (SnO) as transparent conducting electrodes in flat panel displays and solar cell devices [6,7]. Fabrication of thin film transistor (TFT) and heterojunction diode have also been investigated where ZnO films are employed as a transparent wide band gap semiconductor [8-10]. Due to a variety of outstanding characteristics including cheap, stable abundance on the earth, ZnO becomes one of the most important materials in near future. Various methods have been proposed to prepare ZnO thin films, such as RF magnetron sputtering [11,12], pulsed laser deposition [13], chemical vapor deposition [14,15], sol-gel method [16,17], spray pyrolysis [18], hydrothermal process [19-23] and direct deposition from aqueous solution [24-30]. Controllable growth of microcrystals and epitaxial growth on GaN were recently reported on the hydrothermal process. However, direct deposition from aqueous solution reported by Izaki et al. [25] still has advantages over other deposition methods because this process can deposit ZnO film on the substrate with large dimensions at low temperatures even if it has complicated features without expensive and energy-consuming equipment. One of the similarities between hydrothermal process and direct deposition from aqueous solution is a zinc source material, however, pH, temperature and pressure are different. Thus there is a different deposition characteristics, for example, main zinc species formed in the solution are considered as Zn(OH)42-, ZnOOH- and ZnO22- [23]. The chemical reactions of direct deposition from aqueous solution can be written as follows [29,31], (CH3)2NHBH3 + OH- → (CH3)2NH + BH3(OH)BH3(OH)- → ·BH2(OH)- + ·H ·BH2(OH)- + OH- → BH2(OH)2- + eBH2(OH)2- + OH- → BH(OH)3- + ·H + eBH(OH)3- + OH- → B(OH)4- + ·H + e·H + ·H → H2 ·H + OH- → H2O + e-
(1) (2) (3) (4) (5) (6) (7)
(CH3)2NH + H2O → (CH3)2NH2+ + OHNO3- + H2O + 2e- → NO2- + 2OHZn2+ + 2OH- → ZnO + H2O
(8) (9) (10)
This direct method is very useful due to its simplicity and low cost. However, the control of the seeding pr
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