Studies of Mg-GaN grown by MBE on GaAs(111)B substrates
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Internet Journal o f
Nitride S emiconductor Research
Volume 2, Article 13
Studies of Mg-GaN grown by MBE on GaAs(111)B substrates T. S. Cheng, C. T. Foxon, N. J. Jeffs Department of Physics, University of Nottingham D. J. Dewsnip, L. Flannery, J. W. Orton Department of Electrical and Electronic Engineering, University of Nottingham S. V. Novikov, B. Ya Ber, Yu A. Kudriavtsev Ioffe Physical-Technical Institute This article was received on June 3, 1997 and accepted on July 22, 1997.
Abstract This paper discusses the growth of Mg-doped GaN samples using a modified Molecular Beam Epitaxy (MBE) method. Our results suggest that the dopant is incorporated from a surface population maintained by the incident Mg flux by a rapid diffusion mechanism. It follows that the chemical concentration will increase with time of growth and that the p-doping level will also increase progressively with film thickness for a given Mg flux. Increasing the Mg flux to the surface results at first in a higher doping density, but this saturates when the Mg surface concentration reaches a finite value.
1. Introduction Mg-doping has been used successfully to produce a variety of group III-Nitride LED and LD structures [1] [2] [3] [4]. The key to this improved technology being the efficient p-doping of the GaN and (AlGa)N films. In growth by MOVPE, post growth annealing in a nitrogen atmosphere is needed to activate the dopant, but in MBE due to the absence of atomic hydrogen no post growth treatment is necessary. However, in MBE grown films, various authors report widely different behaviour for the Mg-doped films [5] [6] [7]. In the work of Lin et al [5] the SIMS data shows a very non uniform Mg profile in the layer with the concentration rising strongly towards the surface. Wang et al [6] do discuss the uniformity of the doping, but they comment that no effect of substrate type is observed. Brandt et al [7] present SIMS data for a film which shows a dip in the Mg concentration close to the substrate epilayer interface with a decreasing concentration towards the surface. They also discuss the effects of hydrogen passivation. The purpose of this paper is try to understand the origin of this variability in the reported behaviour of Mg.
2. Experimental Methods We have grown a series of Mg-doped GaN grown by Molecular Beam Epitaxy. Active nitrogen is produced by an Oxford Applied Research CARS25 plasma source. Further details of the methods used to grow the films have been presented elsewhere [8]. Growth rates are typically 0.3µm/hour. Before growth, to remove the oxide, the GaAs substrates were heated to 620oC. They were then exposed to the nitrogen plasma whilst heating to the growth temperature of approximately 700oC. We have investigated the structural properties of the films using a Philips Xpert diffractometer employing both θ-2θ and w scans. Hall effect measurements have been used to study the electrical properties. It is important to use a true van der Pauw geometry in order to get reliable electrical data. SIMS measurements have been u
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