Effects of Sapphire Substrate Configurations on MBE Growth of ZnO
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Effects of sapphire substrate configurations on MBE growth of ZnO Keiichiro Sakurai, Ken Nakahara1, Tetsuhiro Tanabe1, Shizuo Fujita and Shigeo Fujita Dept. of Electronic Sci. and Eng., Kyoto Univ, Sakyo, Kyoto, 606-8501 Japan 1 Optical Device Research and Development Div., Rohm Co., Ltd., Kyoto, 6158585 Japan ABSTRACT MBE growth of ZnO films for optical semiconductor devices was investigated on off-angled c-plane sapphire substrates. Twin crystal RHEED patterns and surface facetting observed with c-plane just-oriented substrates were suppressed by enlarging the offset angles from near-zero to 2.87 degrees. Though no significant changes were seen in optical characteristics, FWHM of XRC narrowed and surface morphology improved with larger offset angles, indicating that the offset angle is also a sensitive factor for ZnO film growth. INTRODUCTION Considerable attention has been paid to ZnO as a base material of new semiconductor devices, such as short-wavelength light emitting / receiving semiconductor devices utilizing its high exciton binding energy of 60meV. For this purpose, molecular beam epitaxy (MBE) or similar methods on sapphire substrate have been applied by several groups [1-3]. To match the hexagonal structure of ZnO, sapphire is a handy choice, but its surface configuration greatly affects the quality of MBE grown ZnO films, though its mechanism is still not known. In our previous work, we made investigations on a certain kind of surface facetting originating from the substrate-film interface region, and the zinc/oxygen ratio was shown to be a key factor on obtaining smooth surfaces [4]. Still, the configuration of the sapphire substrate itself remains unobserved in many points. In this paper, we have examined the effects of substrate offset angles on ZnO growth using c-plane sapphire. EXPERIMENTAL DETAILS A schematic diagram of our MBE system is shown in figure 1. Oxygen plasma from a radio-frequency (RF; 13.56MHz) radical cell and metal zinc were used as source materials. To distract the ionized species, high voltage of 600V was applied to the charged particles eliminator. Sapphire substrates with offset angles of = 0.04 - 2.87 degrees, tilted from c-plane toward a-axis direction, were used. Substrates were cleaned in vacuum at 700°C for 10 min, followed by ZnO epitaxy for 2 hours. The growth temperature was Tg = 600°C, the flow rate of oxygen was 0.3 sccm, the pressure of Zn was 8x10-7 Torr, and the RF power applied to the oxygen plasma cell was 450W for all samples. The growth rate was about 150nm/ hour. After the growth, the samples were cooled to RT with irradiation of oxygen. Samples were evaluated by reflection highO6.2.1
energy electron diffraction (RHEED), X-ray rocking curve (XRC), scanning electron microscopy (SEM), atomic force microscopy (AFM), and photoluminescence (PL) at room temperature (RT).
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Figure 1. Schematic diagram of the MBE system DISCUSSION c
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