Vibrational Spectroscopy of GaN:Mg Under Pressure
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Vibrational Spectroscopy of GaN:Mg Under Pressure M.D. McCluskey, K.K. Zhuravlev, M. Kneissl,1 W. Wong,1 D. Treat,1 S. Limpijumnong,1 C.G. Van de Walle,1 N.M. Johnson1 Department of Physics and Institute for Shock Physics, Washington State University, Pullman, WA 99164-2814, U.S.A. 1 Xerox PARC, 3333 Coyote Hill Rd., Palo Alto, CA 94304, U.S.A. ABSTRACT The microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and experimental investigation. In order to probe the Mg-H structure, we have studied the effect of hydrostatic pressure on the local vibrational mode (LVM) frequency. At ambient pressure, the LVM frequency is 3125 cm-1, which corresponds to a N-H stretching mode. In this study, Fourier-transform spectroscopy was performed on free-standing GaN:Mg,H samples in a diamond-anvil cell, with nitrogen as a pressure-transmitting fluid. The samples had been removed from their sapphire substrate by the laser-liftoff technique. The LVM frequency was measured, at liquid helium temperatures, for pressures ranging from 0 to 5 GPa. The pressure dependence of the frequency is nonlinear: first it decreases with pressure, then it increases. Comparison with first-principles calculations allows us to derive information about the microscopic structure of the Mg-H complex. The calculated stable configuration indeed gives rise to a frequency shift consistent with experiment. Based on the comparison between theory and experiment, we can exclude the bond-center configuration, which would result in a much larger pressure derivative than experimentally observed. INTRODUCTION GaN is a preferred material for optoelectronic devices such as blue-violet and UV lightemitting diodes (LEDs) and lasers [1,2]. Effective p-type doping of GaN has been a significant challenge for the fabrication of efficient devices with long operating lifetime. Metalorganic chemical vapor deposition (MOCVD) is the dominant growth technique for III-V nitride devices, with Mg the most common p-type dopant. As a result of hydrogen passivation during growth, as-grown GaN:Mg is semi-insulating. It was shown empirically that low energy electron beam irradiation [3] or thermal annealing at temperatures above 600oC in an N2 ambient [4] were required to activate the Mg acceptors. Infrared (IR) spectroscopy was used to positively identify the Mg-H complexes [5]. The stretch-mode frequency of hydrogen in the Mg-H complex was calculated to be approximately 3360 cm-1 [6,7], consistent with a N-H stretching mode. The LVM was observed at 3125 cm-1 in 4 µm thick epilayers of MOCVD-grown GaN:Mg [5]. Upon annealing, the peak at 3125 cm-1 decreased by a factor of two and was correlated with an increase in the conductivity. Annealed samples that are exposed to a remote deuterium plasma show a deuterium stretch mode peak at 2321 cm-1. The isotopic frequency ratio is r = νH/νD = 1.346, which is very similar to that of NH3 (r = 1.342), lending further support to the N-H model. The details of the microscopic structure, however, have not been resolved con
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