TEM Analysis of Threading Dislocations in ELO-GaN Grown with Controlled Facet Planes
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TEM Analysis of Threading Dislocations in ELO-GaN Grown with Controlled Facet Planes Noriyuki Kuwano1, Kayo Horibuchi2, Hideto Miyake3 and Kazumasa Hiramatsu3 1
Applied Science and Technology Center for Cooperative Research, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2 Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 3 Department of Electrical and Electronic Engineering, Faculty of Engineering, Mie University, Tsu, Mie 514-8507, Japan
ABSTRACT Cross sectional transmission electron microscope (TEM) observation has been performed for specimens of ELO-GaN (ELO: epitaxial-lateral-overgrowth) in order to analyze the behavior of dislocations, with special reference to the effect of facet plane orientation and the size of mask. An ELO-GaN layer was grown overlying on a thick GaN layer with a patterned mask by MOVPE with a carrier gas of hydrogen (H2) under a low-ambient pressure. The growth temperature and the reactor-pressure were controlled in a two-step way during the growth of ELO-GaN layer in order to change the dominant facet-planes and the aspect ratio in growth rate. The experimental results revealed that (a+c)-type threading dislocations (TDs) show a 90-degree-bending in the specimen with slanting facets (2 1 1 2 ) , but not in those with vertical ones (21 1 0) .a-type TDs run upward without bending irrespective of the orientation of the facet planes. Dislocations lying on (0001) planes, or horizontal dislocations (HDs), have been generated in the specimens with wide mask-terraces. It is thought that the formation of HDs relieved stresses in the ELO-GaN and then suppressed the bending ofa-type TDs. In the specimens with narrow terraces, the both type TDs penetrate upward without bending and few HDs are generated. The behavior of dislocations is attributable to the fact that the small size of terrace generates small stresses and promotes a fast meeting of wings of ELO-GaN. INTRODUCTION
Since Amano et al.[1] and Nakamura [2] made a break-through to obtain a thin film of GaN with dislocations of 108 - 109/cm2 by a two-step growth technique adopting a low-temperature (LT) buffer layer of AlN or GaN, practical applications of GaN to light-emitting diodes (LEDs) and laser diodes (LDs) became to be realized. For a long life time of LDs, however, the dislocation density needs to be decreased more by 1/10 - 1/100 at least. Various attempts have been made for growing higher quality crystals of GaN and its alloys. Among these, the epitaxial lateral overgrowth (ELO) technique has been attracting much attention since it was expected that threading dislocations would be terminated at the mask. Nakamura et al. [3] succeeded in fabricating LDs which have a life time longer than 10,000 hrs by applying ELO-GaN as a substrate. However, the threading dislocations actually show complicated behaviors. Usui and his research group in NEC [4-7] investigated in detail the behaviors of threading dislocations
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