Study of the Origin of Misorientation in GaN Grown by Pendeo-Epitaxy
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Study of the Origin of Misorientation in GaN Grown by Pendeo-Epitaxy D.N. Zakharov1, Z. Liliental-Weber1, A. M. Roskowski2, S. Einfeldt3, and R.F. Davis2 1
Lawrence Berkeley National Laboratory, MS 62-203, Berkeley, CA 94720 North Carolina State University, Department of Materials Science and Engineering, Campus Box 7907, Raleigh, NC 27695 3 University of Bremen, Institute of Solid State Physics, P.O. Box 330440, 28334 Bremen, Germany 2
ABSTRACT Growth of pendeo-epitaxial (PE) layers introduces misorientation between the seed layers and the overgrown wing layers. The origin of this misorientation has been studied by Transmission Electron Microscopy (TEM) using a set of samples in which subsequent procedures utilized in PE were applied, i.e. growth of GaN template, stripe etching, annealing at the growth temperature of the PE layers and final PE growth. It was shown that etching of seed-stripes did not change the type of defects or their distribution. However, heating to the PE growth temperature drastically modified the surface and V-shaped pits were formed. The surface became smooth again after the PE growth took place. Overgrowth of the V-shaped pits resulted in formation of edge threading dislocations over a seed-stripe region with a dislocation density of 8.0x108 cm-2. Formation of new edge dislocations over the seed can have an influence on the misorientation between the PE grown regions. INTRODUCTION GaN layers are usually grown on sapphire or SiC substrates with either an AlN or a GaN buffer layer. The large misfit in lattice parameters and in thermal expansion coefficients results in a high density of misfit and threading dislocations in the GaN layers (109-1010 cm-2). Although light emitting diodes with high efficiency have been produced despite the high dislocation density in this material, other applications such as lasers require dislocation reduction. Methods to reduce dislocation density include lateral epitaxial overgrowth (LEO) [1-3] or pendeo-epitaxy (PE) [4-6]. Both these methods result in a much lower dislocation density in the overgrown areas, but at coalescence fronts (CF) the dislocation density often exceeds or is comparable to the dislocation density observed using conventional growth on SiC or Al2O3 substrates. Misorientation (such as tilt/twist) between the seed and overgrown wings has been reported [3, 6-8]. Typical values of misorientation are 1-2° for LEO or PE with mask applied on the seedstripes and 0.05-0.3° for mask-free PE. In order to further reduce dislocations in material grown by PE and understand what leads to the wing misorientation, structural studies were performed for each step of the PE process using TEM. EXPERIMENTAL 1 µm GaN layers were grown by metalorganic vapor phase epitaxy on 6H-SiC (0001) substrates with a 0.1 µm thick AlN buffer layer. GaN stripes along [1100] were fabricated by etching through the epitaxial layers into the substrate with an inductively coupled plasma. The stripe width and the stripe period were 3-5 µm and 5-12 µm, respectively. Subsequ
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