Effects of Substrate Orientation on the Spontaneous Ordering of GaAsSb Epilayers Grown by Molecular Beam Epitaxy
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T10.6.1
Effects of Substrate Orientation on the Spontaneous Ordering of GaAsSb Epilayers Grown by Molecular Beam Epitaxy Brian P. Gorman1, Andrew G. Norman2, Reiko Lukic-Zrnic1, Terry D. Golding1, Chris L. Littler1 1 Department of Physics, University of North Texas, Denton, TX 76203 U.S.A. 2 National Renewable Energy Laboratory, Golden, CO 80401 U.S.A. ABSTRACT Spontaneous atomic ordering is investigated in a series of GaAs1-xSbx epilayers (0.51 < x < 0.71) grown by molecular beam epitaxy (MBE) on GaAs substrates with surface orientations of (001), (001) - 80 toward (111)A, (001) - 80 toward (111)B, (115)A, (115)B, (113)A, and (113)B. Atomic ordering in these epilayers was observed from a decrease in the energy gap measured by Fourier transform infrared (FTIR) absorption spectroscopy and corroborated by superlattice reflections in transmission electron diffraction and Raman spectroscopy. Contrary to previous investigations of ordering in III-V alloys, a marked energy gap reduction corresponding to CuPtB type ordering is observed in the GaAs1-xSbx grown on (111)A-type orientations. INTRODUCTION GaAs1-xSbx is a promising material for the fabrication of electronic and optoelectronic devices, and also can exhibit strong atomic ordering when grown by molecular beam epitaxy (MBE) [1-5]. Spontaneous atomic ordering causes changes in the bandstructure, and, as such, it may be useful in the engineering of optoelectronic devices where energy gap tailoring is needed [6]. It has been suggested that atomic ordering is a result of the reconstruction of the growing surface and that incorporation of atoms at surface steps plays a critical role in the ordering process [7,8]. A fundamental understanding of the origins of ordering is still needed in order to successfully utilize ordered materials in engineered devices. In this study, Fourier transform infrared (FTIR) absorption spectroscopy is used to determine the bandgap reduction (as compared with standard alloy reduction) in GaAs1-xSbx epilayers grown on different orientations of GaAs. The bandgap reduction is then used to determine the ordering parameter of the GaAs1-xSbx epilayers. This material system has been studied previously by our groups [9], and this study will serve as complementary evidence to the previous work. The band gap reduction results are compared to the observed superlattice spot intensities in transmission electron diffraction (TED) patterns and changes in the polarization – dependent Raman scattering data. EXPERIMENTAL GaAs1-xSbx epilayers (0.51 < x < 0.71) were grown using MBE on GaAs substrates with surface orientations of (001), (001) - 80 toward (111)A, (001) - 80 toward (111)B, (115)A, (115)B, (113)A, and (113)B. The series of substrate orientations was chosen to potentially induce a range of ordering parameters. A VG V80H MBE system used at the Blackett Laboratory, Imperial College, utilized As4, Sb4, and Ga as sources. GaAs buffer layers were initially grown on the cleaned substrate at 580°C. Film compositions and substrate orientations are summarized
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