Distinct Magnesium Incorporation Behavior in Laterally Grown AlGaN
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Distinct Magnesium Incorporation Behavior in Laterally Grown AlGaN R. Liu, A. Bell, F.A. Ponce, D. Cherns *, H. Amano ** and I. Akasaki** Dept. Physics and Astronomy, Arizona State University, Tempe, AZ, 85287-1504 * H. H. Wills Physics Laboratory, Bristol, BS8 1TL, UK ** Dept. of Materials Science and Engineering, Meijo University, Nagoya 468, Japan ABSTRACT Different magnesium incorporation behavior has been observed in heavily Mg-doped AlGaN epitaxial layers. The films were grown by metal-organic vapor phase epitaxy involving a lateral overgrowth technique on patterned sapphire substrates. TEM observations show that direct growth on sapphire exhibits pyramidal defects, while lateral overgrowth is homogeneous and free of structural defects. The orientation of the growth front significantly influences the microstructure, and the {0001} growth facet appears to be essential for the formation of the pyramidal defects. In addition, cylindrical and funnel-shaped nanopipes have been observed at dislocations with an edge component. The relationship between Mg segregation and these defects is discussed, and formation mechanisms are proposed taking into consideration the orientation of the growth front. INTRODUCTION The development of p-type doping of nitride semiconductors has made possible a number of technological advances including high-efficiency visible light emitting devices and blue laser diodes. Magnesium is currently the most commonly used p-type dopant. The activation of magnesium as an acceptor was first achieved by low-energy electron beam irradiation [1] and subsequently by thermal annealing [2]. In order to fabricate high-performance blue and ultraviolet laser diodes, it is necessary to improve p-type layer conductivity in AlxGa1-xN films. The acceptor levels in these alloys are typically more than 230 meV from the valence band edge, with consequent low thermal activation ratios (
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