Optical Characterization of Bulk GaN Grown from a Na/Ga Flux
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Optical Characterization of Bulk GaN Grown from a Na/Ga Flux K. Palle, L. Chen, H.X. Liu, B.J. Skromme, H. Yamane1, M. Aoki1, C.B. Hoffman2, and F.J. DiSalvo2 Dept of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, AZ, 85287-5706, U.S.A. 1 Institute for Advanced Materials Processing, Tohoku University, Sendai 980-8577, Japan 2 Dept of Chemistry and Chemical Biology, Cornell University, Ithaca, NY, 14853, U.S.A. ABSTRACT Bulk GaN crystals up to several mm in size, grown by a Na/Ga flux method, have been characterized using room and low temperature photoluminescence (PL) and panchromatic cathodoluminescence (CL) imaging. Highly resolved excitonic PL spectra are obtained for material grown in a new, large-scale reactor. The crystal polarity affects the incorporation of residual Zn and Mg or Si acceptors and the deep level luminescence bands in c-oriented platelets. A Zn (Ao,X) triplet structure with unusual thermalization properties and a highly resolved structural defect related PL peak are observed. Striations are found in some of the smaller platelets by CL imaging, but are absent in the prismatic crystals. INTRODUCTION Heteroepitaxial GaN films grown on SiC or sapphire have large lattice and thermal mismatches with their substrates, which leads to the formation of defects. These defects affect the optical and electrical characteristics of the material and of devices made from it. In order to reduce the defect density, true bulk GaN substrates of high quality suitable for homoepitaxy would be desirable. Conventional GaN crystals grown by the high-pressure solution method [12] have attained maximum sizes of ~15 mm, but this method requires very high growth temperature (1500 oC) and N2 pressures (~1 GPa). Recently, bulk GaN platelets grown by the Na/Ga flux method have reached sizes up to ~10 mm [3-7]. This method uses a relatively low growth temperature of 700 oC and a N2 pressure of only 5 MPa. These crystals show X-ray rocking curves with full width at half maxima (FWHM’s) of 45-55 arc-sec for the (0002) reflection, suggesting good structural quality [3]. Previously, we discussed the highly resolved (down to ~0.22 meV FWHM) excitonic photoluminescence (PL) properties and low doping levels (mid 1016-low 1017 cm–3, determined by Raman scattering) of material grown in a medium-scale growth reactor [8,9]. We also showed that different growth conditions produced different crystal habits, with correspondingly different PL spectra. Here, we extend this study to include material grown in a new, large-scale reactor at Cornell, and we perform more detailed studies of material grown in both reactors. Impurity and deep level incorporation is shown to differ significantly on the opposite Ga-polar or N-polar sides of c-axis platelets grown in both reactors. We study fine structure of the Zn neutral acceptor-bound exciton (Ao,X) system and a structural defect-related peak. Cathodoluminescence imaging is used to compare the uniformity of the luminescence emission to
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