Combined MOCVD and MBE growth of GaN on porous SiC
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Combined MOCVD and MBE growth of GaN on porous SiC Ashutosh Sagar (a), R. M. Feenstra (a), C. K. Inoki (b), T. S. Kuan (b), D. D. Koleske (c) (a) Dept. of Physics, Carnegie Mellon University, Pittsburgh, PA 15213 (b) Dept. of Physics, University at Albany, SUNY, Albany, NY 12222 (c) Chemical Processing Science Dept., Sandia National Lab., Albuquerque, NM 87185 ABSTRACT GaN films have been grown homoepitaxially by MOCVD on MBE-grown GaN template layers, using both porous and nonporous SiC substrates. The effect of the porous SiC substrates on dislocations in the MBE and MOCVD GaN layers has been studied using TEM and x-ray characterization. A reduction in dislocation density from ≥1×1010 cm-2 in the MBE template to 2.5×109 cm-2 at the top of the MOCVD film is found, with similar final values in the MOCVD films for both porous and nonporous substrates. We discuss various mechanisms by which dislocation density is reduced in the MOCVD layers. INTRODUCTION Porous SiC has attracted considerable interest in the recent years as an attractive substrate for epitaxy of GaN and SiC [1-6]. Earlier studies have shown improved GaN film quality on porous SiC [2-4]. It was recently shown that the GaN grown by molecular beam epitaxy (MBE) on porous SiC is considerably more strain relaxed than the GaN grown on nonporous SiC [7]. It was found for porous substrates that the MBE GaN films contain open tubes originating from the pores in the SiC substrate and extending up to the surface of the film [7]. These tubes formed in the MBE-grown GaN because of insufficient lateral growth over the substrate pores. Half-loop dislocations were found to punch in from the walls of the tubes in the GaN, providing (at least in part) the source of strain relaxation. Such half-loop dislocations were not present in films grown on nonporous SiC substrates. Additionally, some lateral overgrowth of GaN was found to occur over SiC pores larger than ∼200 nm, and in regions between tubes the density of threading dislocations was found to be relatively low. Nevertheless, there was no overall improvement in the dislocation density of the GaN films on porous compared to nonporous substrates [7]. In the present study, we have grown GaN by metal organic chemical vapor deposition (MOCVD) on top of MBE-grown GaN films prepared as in our prior work [7]. The aim of the experiment was to use the lateral growth in MOCVD to close-up the open tubes observed in the MBE GaN on porous SiC and, in the process of doing so, perhaps to reduce the dislocation density in the overgrown film. Plan view and cross sectional TEM images were acquired, as well as x-ray rocking curve data, to study the dislocations in the MOCVD overgrown layers. EXPERIMENTAL The porous SiC samples used in this study were purchased from TDI, Inc. They were prepared by anodic etching at a current density of 7 mA/cm2 for 3 min under a 250 W Hg lamp illumination. All the SiC substrates used in this study, both porous and nonporous, were on-axis 6H polytypes. Prior to the MBE growth on porous and nonpor
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