Activation of Mg acceptors in GaN:Mg monitored by electron paramagnetic resonance spectroscopy.
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Activation of Mg acceptors in GaN:Mg monitored by electron paramagnetic resonance spectroscopy. D. Matlock, M.E. Zvanut, Jeffrey R. DiMaio1, R.F. Davis1, R. L. Henry2, Daniel Koleske3 and Alma Wickenden4 Department of Physics, University of Alabama at Birmingham, 310 CH, Birmingham, AL 35294-1170 1 Department of Materials Science and Engineering, North Carolina State University 2 Naval Research Laboratory, Code 6861 Washington D.C 20375 3 Sandia National Laboratories, P.O. Box 5800-0601, Albuquerque, NM 87185-0601 4 U.S. Army Research Laboratory, Adelphi MD, 20783-1197 ABSTRACT Hydrogen removal from Mg-doped GaN is necessary to activate p-type conductivity, but the exact chemical process is not yet clear. We have investigated this issue by monitoring the intensity of an electron paramagnetic resonance (EPR) signal attributed to Mg through a series of isochronal and isothermal anneals between 600 and 1000 oC. Measurements made on GaN:Mg epitaxial layers deposited on SiC and annealed between 700 and 850 oC indicate that the Mgrelated EPR signal increases with temperature as expected for depassivation of a Mg complex by removal of hydrogen. However, data obtained outside this temperature range suggest that additional processes may occur. For example, as-deposited films contain a signal resembling the Mg acceptor that is quenched by a 650 oC N2 anneal. Also, for all samples, N2 annealing at T>850 oC irreversibly decreases the signal thought to be due to Mg. Although the presence of the signal in the as-grown films is not fully understood, the effects observed at T>850 oC may be attributed to preferential N-desorption from Mg-N-H complexes.
INTRODUCTION Several experiments have shown that as-deposited CVD GaN doped with Mg is highly resistive. However, the conductivity may be increased to about 1(ohm-cm)-1 by e-beam irradiation or N2 annealing [1]. Today, p-type CVD GaN is produced by a post-growth activation anneal at about 800 oC using either a rapid thermal anneal oven or a conventional furnace. The results of many experiments suggest that the observed behavior is related to the passivation of Mg by hydrogen that is moved from the sample during annealing [1-3]. For example, comparisons between conductivity data and secondary ion mass spectroscopy (SIMS) measurements show that hydrogen is released from the film as the conductivity increases during post-deposition annealing [2]. Infrared spectroscopy measurements are consistent with the SIMS results and suggest that the acceptor is a complex of hydrogen, magnesium and nitrogen [4,5]. In addition to these studies, electron paramagnetic magnetic resonance (EPR) spectroscopy reveals a signal in p-type activated GaN:Mg that correlates with the SIMS measured Mg concentration [6]. Studying a variety of activated films grown by molecular beam epitaxy or chemical vapor deposition, Carlos concludes that the EPR signal is related to the acceptor.
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EPR spectroscopy detects changes in the magnetic dipole orientation of a dangling bond at a defect site (ie. a ‘
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