Satoshi Yamauchi*, Hiroto Handa and Takashi Hariu

Department of Systems Engineering, Ibaraki University
4-12-1 Nakanarusawa, Hitachi, Ibaraki 316, Japan
ysatoshi@hcs.ipc.ibaraki.ac.jp

ZnO with a wide direct gap 3.3eV and piezoelectric property is a promising material for ultraviolet light emitting devices and high frequency surface acoustic wave devices. Therefore, various processes have been attempted aiming at the high quality epitaxial growth, for example, magnetron sputtering [1], metalorganic chemical vapor deposition [2], pulsed laser ablation [3] and so on. Recently, a new approach for the low temperature epitaxial growth by molecular beam epitaxy using oxygen plasma source has been reported for Ar and Carbon-free process [4]. The purpose of this paper is to describe a low temperature epitaxial growth of ZnO on sapphire in higher oxygen pressure around 40mtorr with higher growth rate over 400nm/h than that of other process [1, 4] by plasma-assisted epitaxy, which was originally developed in our group.

ZnO epitaxial growth was performed in oxygen plasma excited by 13.56MHz rf-power using inductive coupling. 6-nine purity oxygen and 6-nine purity elemental Zn were used as source gas and material. The elemental Zn was evaporated from quartz crucible with a nozzle by resistive heating around 390~410 C. The crucible temperature was raised in a vacuum before introducing the oxygen gas to prevent oxidation of the source material, and then the rf-power was applied for discharging around 40mtorr just before the growth. C-plane and R-plane sapphire were used as substrates after chemical treatment in hot H2SO4:H3PO4 (3:1) solution and de-ionized water. These substrate were also treated by Ar-plasma cleaning at 450 C in the growth chamber. ZnO layers were grown at substrate temperature ranging from 300 C to 400 C in discharging oxygen plasma at 40mtorr with growth rate around 400nm/h. (001)- and (110)-oriented ZnO layer were grown above 300 C on C-plane and R-plane sapphire substrates, respectively. It was found that optimized condition is different in the growth on C- and R-plane substrates. (001)ZnO growth on C-plane sapphire was obtained above 300 C in oxygen plasma applied with relatively low applied rf-power around 20W, however, higher excitation of plasma with higher applied rf-power around 100W is required for high quality (110)ZnO growth on R-plane sapphire. It is considered that the difference is caused by larger lattice mismatch between (110)ZnO and R-plane sapphire than that of (001)ZnO and C-plane sapphire. XRD measurement showed that the quality of (110)-oriented ZnO film grown at lower temperature around 300 C with optimized RF power is better than that of the film grown at higher temperature around 400 C. It is likely to be resulted from large difference of thermal expansion between ZnO and sapphire. These results show plasma-assisted epitaxy using oxygen plasma is useful for ZnO epitaxial growth and low temperature growth is a key process for fabrication of high quality ZnO films on sapphire.

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