Polarity of Hexagonal GaN Grown on GaAs (111)A and (111)B Surfaces by HVPE and Movpe
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POLARITY of HEXAGONAL GaN GROWN on GaAs (111)A and (111)B SURFACES by HVPE and MOVPE O.Takahashi, M.Namerikawa, H.Tanaka*, R.Souda**, T.Suemasu and F.Hasegawa University of Tsukuba, Institute of Applied Physics, Tsukuba. Japan *NTT Cyber Space Laboratories, Musashino-shi, Japan **National Institute for Research in Inorganic Materials, Tsukuba,. Japan Polarity of hexagonal GaN grown by HVPE and MOVPE on GaAs (111) substrates was investigated by CAICISS (Coaxial Impact Collision Ion Scattering Spectroscopy). Both layers of HVPE GaN grown on GaAs (111) A –Ga surface --- and (111)B –As surface — were Ga polarity. On the other hand, MOVPE GaN grown on the (111)B showed N polarity whereas the layer grown on the (111)A showed Ga polarity, inheriting the substrate polarity. Crystal quality was better for Ga polarity layers than for N polarity layers for MOVPE. However, Ga polar GaN grown on GaAs (111)B by HVPE had better crystal quality than that on the (111)A. These differences may be attributed to difference of the buffer layers and difference of growth mode between HVPE and MOVPE. INTRODUCTION It is well known that the polarity of hexagonal GaN influences greatly on the crystal quality of the grown layer, but it is not understood well how the polarity of the grown layer is determinded. GaN grown by MOVPE on a sapphire substrate is known to have usually Ga polarity[1]. Polarity of GaN grown by GSMBE is much more complicated than the MOVPE case and depend on the reactive gas and the buffer layer. GaN grown by RF plasma-assisted MBE is usually mixture of Ga and N polarities, and generally the N polarity is dominant [2,3]. Layers grown with NH3 as the N source is reported to have Ga polarity [4]. Ga polarity layers can be obtained by AlN buffer layer or AlN/GaN super lattice [5,6]. Quality of GaN grown by RF plasma-assisted MBE is improved very much by controlling the polarity to the Ga one [5,6]. When GaN is grown on a substrate having its own polarity, it should inherit the substrate polarity. In this work, GaN was grown on GaAs (111)A & B surfaces by HVPE and MOVPE, and the polarity and crystal quality were investigated on the layers grown on both surfaces. EXPERIMENTAL Halide VPE was performed by a conventional HVPE system using H2 as a carrier gas. Metal Ga and NH3 were used as the gallium and nitrogen sources, respectively. The thermal cleaning was performed at 600 oC for 10 min. in H2 ambient. Slight gas etching of GaAs substrates with HCl and surface treatment with GaCl for several ten minutes were also performed for some samples. A buffer layer was grown at 550oC, and the thickness was changed from70 to 500nm depending on the situation. The layers measured by CAICISS were grown at 850 oC. The V/III ratio for the buffer layer growth was from 25 to 400, and that for the growth at 850 oC was 50 to 200. Details of the growth are described in our previous paper [7]. MOVPE GaN was grown at NTT. The thermal cleaning was performed at 820 oC for 5 min. in H2 + AsH3 ambient. A buffer layer was grown at 590oC, and the thicknes
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