ABSTRACT
Blue and green LEDs based on Ill-nitride materials have been commercialized with high performance, and many researchers have focused on UV region LED for various applications like indoor, outdoor display and backlighting for liquid crystal display [1] -[5]. However, the suppression of threading dislocation density between epitaxial layer and substrate is prerequisite for the realization of high power and reliable devices. To improve the epitaxially grown GaN film quality, low temperature (LT) buffer layers [2] -[3], epitaxial lateral overgrowth (ELO)[6], and homoepitaxy approaches have been attempted. Especially, low temperature (LT) buffer layer such as AIN, GaN, and ELO growth techniques have been most conventionally used. LT-GaN or -AIN buffer layers critically contributed to the improvement of crystal quality, and electrical and optical properties. However, the reduction of high dislocation density (~108-109/cm2) is still problematic, because there still remains large lattice mismatch of 13-16% between
LT-GaN (or LT-A1N) and AI2 O3 substrate which induces stress during growth. In this study, we propose novel cubic structured CrN as a buffer layer to release this
stress by using intrinsic material properties [7]. CrN films have been commercially applied to surface coating enhancing properties such as 1) mechanical strength, 2) chemical inertness, 3) high temperature stability, and 4) high temperature oxidation resistance so far [8]. We know that cubic structured CrN film have small lattice misfit (approximately 6.6% with sapphire substrate) and 8.9% with GaN film, in compare to LT-GaN buffer layer. Also, CrN film has intermediate thermal expansion coefficient (6xlO'6/K) of sapphire substrate (6.66x10'6/K) and GaN film (5.59x10'6/K). Moreover, conventional GaN films were grown on LT buffer layer with amorphous phase, however, in our experiments, a single crystalline CrN layer was obtained from the very initial stage of growth. This single crystalline buffer layer could be another approach to decrease defects in GaN layer. To my knowledge, there exist nearly no report about the application of CrN layer as a buffer for GaN growth. Therefore, we investigated the feasibility of single crystalline CrN buffer layer for the growth of high quality GaN using MBE.
