Characterization of Dark-Block Defects in Cantilever Epitaxial GaN on Sapphire
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Characterization of Dark-Block Defects in Cantilever Epitaxial GaN on Sapphire P. P. Provencio, D. M. Follstaedt, N. A. Missert, D. D. Koleske, C. C. Mitchell, A. A. Allerman and C. I. H. Ashby Sandia National Laboratories, Albuquerque, NM 87185-1056 ABSTRACT Cantilever epitaxy of GaN on sapphire has been augmented by the use of initial facetted GaN growth on narrow sapphire mesas (< 1µm) in order to turn remaining threading dislocations from vertical to horizontal, and thus reduce the overall dislocation density at the surface where devices would be placed. With this modification, isolated non-radiative block-like defect areas have been introduced that hinder optical and electronic material performance. Here we characterize these defects with microscopy, and show that they are arrays of lateral dislocations, with cracks and voids along their centerlines. We deduce that they result when tilted GaN is joined to neighboring oriented material. Their presence is independent of the type of nucleation layer used. INTRODUCTION The favorable semiconductor properties of GaN make it an important material for optical and electronic applications. Hetero-epitaxial growth of hexagonal GaN results in a high density of vertical threading dislocations (VTDs) that limit device performance [1]. Lateral growth methods have been developed to reduce VTDs. Methods such as ELO [2], LEO [3] and FACELO [4] seed vertical GaN growth through openings in SiO2 or Si3N4 masking layers, and then laterally grow GaN with low dislocation densities over the masks. Pendeo-epitaxy [5] uses mesas patterned into GaN previously grown on a substrate to seed lateral growth of material suspended over trenches between the mesas; the suspended GaN also has low dislocation density. Cantilever epitaxy (CE) [6] was developed to reduce VTDs by initiating growth on mesas patterned directly on (0001) sapphire substrate followed by lateral growth of GaN suspended over trenches etched into the sapphire as in Fig. 1. CE has an important advantage over the other methods in that it requires only a single growth sequence in the metallorganic chemical vapor deposition reactor. Details of our growth techniques are given in Ref. [7]. Using an AlN nucleation layer and substrates etched to produce narrow mesas (
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