Photoluminescence from freestanding GaN with (1010) orientation
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Photoluminescence from freestanding GaN with ( 10 1 0 ) orientation M. A. Reshchikov,1 A. Teke,1,2 H. P. Maruska,3 D. W. Hill,3 and H. Morkoç1 1 Department of Electrical Engineering and Physics Department, Virginia Commonwealth University, Richmond, VA 23284 2 Department of Physics, Balikesir University, 10100 Balikesir, Turkey 3 Crystal Photonics Inc., Sanford, FL 32773 ABSTRACT Freestanding GaN templates with (10 1 0 ) orientation (M-plane) were obtained by halide vapor phase epitaxy (HVPE) on nearly lattice-matched LiAlO2 and subsequent removal of the substrate by wet chemical etching. Photoluminescence (PL) spectrum from both sides of the GaN template investigated is dominated by peaks at 3.47, 3.42 and 3.36 eV, tentatively attributed to an exciton bound to the neutral shallow donor and two unidentified structural defects, respectively. The quantum efficiency of the exciton-related emission exceeds 10%, whereas that of the combined emission from the defect-related bands (red, yellow and blue) is below 0.1%. The evolution of the PL spectrum with temperature and excitation intensity is analyzed in detail. Effects of polishing and etching on the PL properties are also discussed. INTRODUCTION Large freestanding GaN wafers are valuable for homoepitaxial growth of advanced nitride-based devices. Growth of the M-plane GaN on nearly lattice-matched LiAlO2 was originally proposed by Hellman et al. [1]. The advantage of the ( 10 1 0 ) oriented GaN substrate (M-plane) over the traditional (0001) orientation (C-plane) is due to significant reduction of the undesirable strain induced electric field when heterojunctions are produced [2]. A comprehensive study of thin M-plane GaN layers grown by molecular beam epitaxy (MBE) on LiAlO2 revealed high concentration of stacking faults that apparently correlated with sharp PL line at 3.356 eV [3]. A significant number of structural defects, primarily stacking faults, were observed [4] also in the M-plane freestanding templates studied in this work. While the stacking faults in Ref. [3] were identified as being of the I2 type, the experimental results of Ref. [4] indicated predominant I1 type of stacking faults that have the lowest stacking fault energy. In this paper we report results of a PL study of freestanding GaN templates with the ( 10 1 0 ) orientation. EXPERIMENTAL DETAILS GaN films with 50 mm diameter, each about 300 microns thick, were grown on nearly lattice-matched LiAlO2 substrates by HVPE. GaCl and NH3 were used as precursors, and the growth was performed at 875 oC in a three-zone tube furnace. After removal of the LiAlO2 substrate by wet chemical etching, the freestanding GaN wafers were found to have the nonpolar ( 10 1 0 ) orientation. Front side of as-grown wafer looked smooth but with large-scale triangular-shaped features on it. The backside was featureless and rather smooth. The wafers were cut, the front side of some pieces was mechanically polished and cleaned in hot HCl. Selected samples were dipped in hot (160°C) H3PO4 for 1 minute to remove a surfa
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