Lateral Epitaxial Overgrowth of InAs on (100) GaAs Substrates
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Lateral Epitaxial Overgrowth of InAs on (100) GaAs Substrates Ganesan Suryanarayanan,a Anish A. Khandekar,b Brian E. Hawkins,b Thomas F. Kuecha,b and Susan E. Babcocka,c a Materials Science Program, bDepartment of Chemical Engineering, and cDepartment of Materials Science and Engineering, University of Wisconsin – Madison, Madison, WI 53706. ABSTRACT The microstructure of epitaxial InAs thin films grown by MOCVD on mask-patterned “LEO” (lateral epitaxial overgrowth) GaAs and on unpatterned GaAs substrates was studied using double-crystal x-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. This paper describes the improvement in crystal quality (factor of 20 reduction in x-ray rocking curve width), the order of magnitude reduction in dislocation density, and the rearrangement of the remaining extended defects that were observed in the LEO material when compared to the film grown on the unpatterned wafer. INTRODUCTION InAs is an attractive platform material for high-speed electronic devices because of its small band gap, high carrier mobility and staggered band line-up with GaSb, to which it is latticematched. Wider spread development of InAs-based electronic devices has been limited by the lack of a suitable lattice-matched substrate with semi-insulating properties on which low-defect density films can be grown. Semi-insulating GaAs has been the substrate of choice for most film applications. Although there have been very few direct investigations of the internal microstructure of InAs films on GaAs1,2, it is generally believed that these films contain a high density of mismatch-derived extended defects. The lateral epitaxial overgrowth (LEO) approach has proven to yield defect reduction in other lattice mismatched systems, such as GaSb/GaAs3 and GaN/Al2O34-6, where it can strongly affect the threading dislocation microstructure. In LEO, the initial wafer substrate is coated with an amorphous masking material, into which openings (windows) are patterned. Material that is later grown on the patterned substrate nucleates on the crystalline material exposed in the windows and subsequently grows laterally over the amorphous mask until it coalesces into a continuous film. A reduction in the density of mismatch-derived defects is anticipated because most of the epitaxial film is not in direct contact with a lattice mismatched crystalline substrate. Such defect reduction has, in fact been realized in other heteroepitaxial systems. This paper describes the microstructure of metalorganic chemical vapor deposited (MOCVD) InAs grown on GaAs by LEO and compares it with that of InAs grown on unpatterned GaAs wafers (hereafter, referred to as the “control” samples). EXPERIMENTAL PROCEDURES InAs was grown on LEO substrates with 0.4, 0.6 and 0.8µm wide windows at both 5µm and 10µm pitch and on unpatterned (100) GaAs (hereafter, “conventional”) substrates in a horizontal quartz MOCVD reactor equipped with an rf-heated graphite susceptor. Approximately 120 nm of low-pressure CV
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