Deep centers in GaN layers grown on epitaxial lateral overgrowth templates by metalorganic chemical vapor deposition
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Deep centers in GaN layers grown on epitaxial lateral overgrowth templates by metalorganic chemical vapor deposition S. A. Chevtchenko1, J. Xie1, Y. Fu1, X. Ni1, and H. Morkoç1,2 1 Electrical Engineering Department, Virginia Commonwealth University, Richmond, VA, 23284-3072 2 Physics Department, Virginia Commonwealth University, Richmond, VA, 23284-3072 ABSTRACT The dependence of traps and their concentration in GaN on the quality of templates, on which the layers are grown, has been studied by deep-level transient spectroscopy (DLTS). Thin GaN layers studied were grown on GaN templates employing conventional epitaxial lateral overgrowth (ELO) and nano-ELO with SiNx nanonetwork. The concentrations of traps found in these layers were compared with a reference sample grown on a standard GaN template not utilizing ELO. Two traps A (0.55 eV – 0.58 eV) and B (0.20 eV – 0.23 eV) were detected in the temperature range from 80 K to 400 K. A reduction of traps in layers grown on the ELO and nano-ELO templates was noted. We attribute this reduction to the reduction of threading dislocation density and as a result reduced capture of point defects and complexes as part of dislocation core structure and/or reduced formation of defects and complexes in the vicinity of line defects where the formation can be energetically favorable. INTRODUCTION Owing to the lack of native substrates GaN is typically grown on foreign substrates such as SiC and sapphire. Due to the ensuing lattice mismatch and stacking mismatch the resulting layers contain threading dislocations (TDs) with a density in the range 109–1011 cm-2 unless specific measures, which so far have been effective to some degree, are taken. It is well known that the extended defects can deteriorate optical and transport properties in GaN as they may be directly or indirectly involved in causing point defect formation.1 While the exact mechanism of this link is not yet clear, it is commonly believed that these defects and threading dislocations in particular may induce electronic states in the band gap and/or trap point defects in the stress field around dislocations and whereby accumulate charge in their vicinity.2,3,4 Therefore the reduction of dislocation density is required to reveal the full potential of GaN as a wide band gap material for a variety of applications. Use of either in situ or ex situ patterned templates or a porous dielectric film in template structure paves the way for epitaxial lateral overgrowth (ELO).5 A very attractive approach, which has been shown to reduce the dislocation density to the range 107 – 108 cm-2, is to use a porous SiNx nano-network as ELO mask.6,7,8 Deep levels are expected to be reduced in layers grown on the templates with lower dislocation densities but such reports are lacking in detail in the literature. Several methods have been attempted and developed to prevent defect propagation into the upper portions of the layers analyzed mainly by transmission electron microscopy (TEM). Deep-level transient spectroscopy (DLTS) has
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