Electrodeposition of compound thin films can result in high quality materials as it was demonstrated by epitaxial electrodeposition of single crystalline thin films. For example, electrodeposition of ZnO onto Au(100) bulky single crystal and GaN(002) single crystal thin film grown by MOCVD on sapphire lead to the growth of ZnO(002) thin films owing to the small lattice mismatch. In this work, we have employed heavily Si-doped conductive bulky GaN(002) single crystal as an electrode to attempt electrodeposition of single crystalline ZnO from O₂ saturated aqueous solutions of ZnCl₂.

The (002) cut and polished GaN was purchased from Furukawa, Co., Ltd. and soaked in 0.1 M HCl for 2 min before use. When O₂ reduction current was measured in a KCl solution, cathodic current gradually increased below ca. -0.5 V vs. Ag/AgCl but never reached a plateau down to -1.5 V. Potentiostatic electrolysis at -1.3 V, however, indicated slow increase of the current that finally reached a constant value. Electrochemical analysis was carried out for such fully activated electrodes. The plateau current was confirmed as O₂ diffusion limited from Levich analysis. Koutecky-Levich analysis in the potential range between -1.15 and -1.3 V revealed the standard charge transfer rate constant (k⁰) of 1.9×10^-9 cm s^-1 and the transfer coefficient (α) of 0.089 for O₂ reduction, indicating that the GaN electrode we used is fairly inactive.

Electrodeposition of ZnO was performed by potentiostatic electrolysis at -1.0 V after addition of ZnCl₂. Formation of single crystal phase was confirmed by ω scan of XRD. In-plane alignment of the hexagonally shaped deposits was also observed by SEM. The SEM observation, however, found presence of randomly orientated domains, so that the present conditions are not well established for epitaxial electrodeposition. The influence of surface condition to the structure of the deposited layer is studied in detail and will be presented.

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