Characterization of Cubic GaN Films Using An AlN/GaN Ordered Alloy on GaAs (100) by RF-MBE
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Characterization of Cubic GaN Films Using An AlN/GaN Ordered Alloy on GaAs (100) by RF-MBE Junichi Shike, Atsushi Shigemori, Koichi Ishida, Kiyoshi Takahashi and Ryuhei Kimura High-Tech Research Center, Faculty of Science and Engineering, Teikyo University of Science and Technology 2525 Yatsuzawa, Uenohara, Kitatsuru-gun, Yamanashi, 409-0193, Japan ABSTRACT High quality cubic GaN films were successfully grown on an AlN/GaN ordered alloy by RF-MBE. AlN/GaN ordered alloy is here employed instead of a AlGaN nucleation layer formed by nitridation of an AlGaAs buffer layer. Dominant cubic GaN epilayer (1.0 µm) growth was confirmed by insitu RHEED observations, AFM, TEM, PL and X-ray diffraction measurements. INTRODUCTION Cubic GaN (c-GaN) is expected to have many advantages in physical properties over those of the hexagonal phase due to its higher crystallographic symmetry. We earlier reported that using an AlGaN layer formed by nitridation of an Al0.17Ga0.83As buffer was an efficient process for ensuring growth of a highly pure c-GaN film growth [1,2]. However, there still remain some problems regarding AlGaAs growth. The first relates to fluctuations in Al concentration, which in turn significantly affects the purity of the structural phase of the epilayer. The second is the difficultly of maintaining high quality stoichiometric growth for extend periods of time due to the careful control of the beam fluxes (Al, Ga and As) required. To improve the efficiency of our research, an AlN/GaN ordered alloy (OA) was employed instead of an AlGaAs buffer layer for c-GaN films growth. It is expected that uniform effective Al molar content can be obtained by using an OA layer. Precise control of the Al molar content can be achieved by varying the ratio of alternate layer thickness. This process makes it easy to maintain high quality stoichiometric growth for extended periods as all constituent layers are binary materials. In our previous work, evidence for the
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presence of a polycrystalline component was confirmed for a c-GaN film (0.4 µm) by high resolution X-ray diffraction (HRXRD) and photoluminescence (PL) [3,4]. In this work, a significant improvement in the crystallinity of the c-GaN epilayer was achieved by optimizing the nitridation conditions. Crystallographic properties were characterized by reflection high-energy electron diffraction (RHEED), transmission electron microscopy (TEM) and HRXRD. Atomic force microscopy (AFM) and low temperature PL measurements were carried out in order to characterize the surface morphology and optical properties, respectively. EXPERIMENTAL DETAILS C-GaN films were grown on semi-insulating GaAs (100) substrates by plasma assisted molecular beam epitaxy (RF-MBE) using a radio frequency (13.56 MHz) plasma cell (Oxford Applied Research, model MPD21) to produce active nitrogen. After thermal annealing under As over pressure conditions (4.0 x 10-6 Torr), a GaAs buffer layer was deposited at 650oC (0.2 µm). The surface structure was investigated to confirm the presence of the (
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