Non-Equilibrium Acceptor Concentration in GaN:Mg Grown by Metalorganic Chemical Vapor Deposition
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Non-Equilibrium Acceptor Concentration in GaN:Mg Grown by Metalorganic Chemical Vapor Deposition Y. Gong1, Y. Gu1, Igor L. Kuskovsky1, G.F. Neumark1, J. Li2, J.Y. Lin2, H.X. Jiang2, and I. Ferguson3 1 Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, U.S.A. 2 Department of Physics, Kansas State University, Manhattan, KS 66506, U.S.A. 3 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0250 U.S.A. ABSTRACT It is shown that the high p-type conductivity in GaN:Mg, grown by metal-organic chemical vapor deposition followed by post-growth annealing, is due to non-equilibrium acceptor concentrations. A series of samples cut from a single GaN:Mg wafer, which initially had undergone rapid thermal annealing (RTA) after growth, has been investigated. The samples were annealed at various temperatures in nitrogen ambient for over 12 hours, and temperaturedependent Hall effect measurements were performed. For samples annealed at temperatures higher than 850 °C, the hole concentrations decrease by at least an order of magnitude, compared with the original sample. This behavior is explained by an Mg acceptor concentration in excess of its equilibrium solubility limit in the original sample; thus, at high enough temperatures, in the absence of hydrogen, Mg acceptors diffuse either to form electrically inactive precipitates or are eliminated. It is worth noting that the acceptor activation energy remains the same for all samples. INTRODUCTION GaN and its alloys have great potential for various electronic and optoelectronic applications [1-3]. Often good bipolar doping is an essential requirement for optimal device performance; however, it is difficult to obtain highly p-type GaN and many related alloys. Although rapid progress has been made and hole concentrations of ~1018 cm-3 have been achieved by both molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD) (for review see e.g. Ref. [4]), some further investigations on the basic mechanism of p-type doping of GaN appear useful. Regarding MOCVD, it is well known that as-grown p-type (e.g. Mg doped) GaN is highly resistive and post-growth treatments such as low-energy electron-beam irradiation [5] or thermal annealing in non-hydrogen containing atmospheres (e.g. N2) at temperatures above 700°C [6] are required to “activate” acceptors. It is widely accepted (see e.g. Ref 7-9) that for MOCVD-grown GaN:Mg atomic hydrogen acts as a compensating donor, leading to the highly resistive as-grown material. The acceptors, however, are activated (as mentioned above) when the hydrogen is removed. Theoretical studies [10] have shown that, neglecting compensation aspects, the limiting factor in controlling doping in wide bandgap semiconductors, like GaN, is the solubility limit of dopants. To overcome this problem, one has to use growth techniques that result in nonequilibrium dopant concentrations. For MBE the non-equilibrium doping has been discussed
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