Microstructure and optical properties of GaN films grown on porous SiC substrate by MBE
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Microstructure and optical properties of GaN films grown on porous SiC substrate by MBE F. Yun, M. A. Reshchikov, L. He, T. King, D. Huang, H. Morkoç, C. K. Inoki1, and T. S. Kuan1 Virginia Commonwealth University, Dept. of Electrical Engineering, Richmond, VA23284 1 Univ at Albany, SUNY, Dept of Physics, Albany, NY 12222 ABSTRACT GaN thin films were grown on porous SiC substrates using reactive molecular beam epitaxy with ammonia as the nitrogen source. Microstructure analysis and optical characterization were performed to assess the quality of the effect of pores on the growth and the quality of the GaN films. Results indicate that the GaN films on porous SiC are slightly less defective and more strain-relaxed (some completely relaxed) when grown on porous SiC substrate, as compared to growth on standard 6H-SiC substrates. Rocking curve FWHMs of 3.3 arcmin for (0002) diffraction and 13.7 arcmin for (10 1 2) diffraction were obtained for sub-micron thick GaN films. Excitonic transition with FWHM as narrow as 9.5 meV was observed at 15K on the GaN layer grown on porous SiC without a skin layer. INTRODUCTION High thermal conductivity of SiC substrates has provided the impetus for exploring GaN growth on SiC.1,2,3,4 In addition to the much better thermal conductivity of SiC as compared to the most commonly used substrate sapphire, the lattice mismatch between SiC and AlN is very small, and that between SiC and GaN is much smaller compared to sapphire. As alluded to earlier, the high thermal conductivity of the SiC substrate allows the fabrication of high power/temperature electronics devices. It also exhibits robust mechanical and chemical properties.5 Beside heat removal, high density of threading dislocations and point defects, and to some extent strain, in GaN films represent a bottleneck for improving device performance. To relieve the strain, researchers have opted to introduce multiple buffer layers,6 or grow GaN on GaN nano-columns formed on sapphire substrate.7 As for threading dislocations, approaches such as growth of GaN epilayers on top of porous materials, such as porous SiC (PSC) substrates, are being explored. PSC is of practical interest because it can be readily prepared by electrochemical anodization of conducting SiC substrates and in particular 6H-SiC (0001) substrates which were exploited in this work. There have been reports of SiC growth on PSC substrates8,9 by CVD, and GaN growth on porous GaN layer on top of SiC substrates.10,11 In all of these cases, growth on PSC was accompanied with improvements in defect reduction and stress alleviation. It is therefore interesting to study the growth of GaN directly on PSC substrate. With the formation of nanometer-scale pores, it is expected that the molecular beam epitaxial (MBE) growth of GaN on the PSC surface using ammonia nitrogen source might lead to lateral epitaxial overgrowth (LEO) as in metalorganic chemical vapor deposition (MOCVD), which leads to extended defect reduction. Ammonia based growth exhibits lateral growth, albeit to a smaller exte
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