Evolution of Subgrain Boundaries in Heteroepitaxial GaN/AlN/6H-SiC Grown by Metalorganic Chemical Vapor Deposition
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Evolution of Subgrain Boundaries in Heteroepitaxial GaN/AlN/6H-SiC Grown by Metalorganic Chemical Vapor Deposition H.X. Liu, G.N. Ali, K.C. Palle, M.K. Mikhov, B.J. Skromme, Z. J. Reitmeyer,1 and R.F. Davis1 Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, AZ, 85287-5706, U.S.A. 1 Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC, 27695-7907, U.S.A. ABSTRACT We have characterized the surface morphology and luminescence properties of GaN/AlN/ SiC layers of various thicknesses using secondary electron imaging (SEI), panchromatic room temperature cathodoluminescence (CL), atomic force microscopy (AFM), optical Nomarski microscopy, and room and low temperature photoluminescence (PL). The nominally undoped GaN layers were grown by MOCVD on 0.1 µm thick AlN buffer layers on commercial 6HSiC(0001) substrates. The GaN layer thicknesses are 0.5, 1.0, 1.6, and 2.6 µm. A second 1.0 µm thick layer was grown by identical procedures on a 6H-SiC substrate that was first etched in H2 to remove scratches and damage due to mechanical polishing. Biaxial compressive lattice mismatch stress is present in all layers and decreases with increasing layer thickness, while PL linewidths decrease. The 1 µm layer on the H-etched substrate is as relaxed as the 2.6 µm layer on a non H-etched substrate, however. Pronounced surface structures, apparently corresponding to columnar subgrain boundaries, are observed on the samples on non H-etched SiC. Their typical sizes increase from about 3 to 10 µm with increasing layer thickness. They are absent in the H-etched sample. These structures are generally nonradiative in CL images, although mottled contrast is also observed inside them. Similar layers doped with 3×1018 cm–3 Si do not show these features, suggesting a different microstructure. INTRODUCTION Gallium nitride and related semiconductors have attracted more and more attention in recent years, mainly due to their promising applications in short-wave light-emitting devices and photodetectors, as well as in radiation-hard, high frequency and high power electronic devices [1]. To date, it is still very hard to grow bulk GaN crystals, so heteroepitaxial growth of high quality GaN thin films is the premise for the development of GaN-based devices. Compared with the more common growth on sapphire, epitaxial growth of GaN on SiC substrates offers advantages such as high electrical conductivity, smaller lattice mismatch, and higher thermal conductivity [2, 3]. In this sense, better material quality and subsequent device performance should be expected for GaN grown on SiC. However, the quality of GaN on SiC reported to date does not very much surpass that of GaN on sapphire [4], and the effect of structural properties on luminescence has not yet been widely studied. In this work, we report characterization of the surface morphology and luminescence properties of GaN/AlN/SiC layers of various thicknesses. The characterization techniqu
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