Transport, Growth Mechanisms, and Material Quality in GaN Epitaxial Lateral Overgrowth
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Michael E. Coltrin, Christine C. Willan, Michael E. Bartram, Jung Han, Nancy Missert, Mary H. Crawford, Albert G. Baca Sandia National Laboratories, Albuquerque, NM 87185 Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G6.9(1999) ABSTRACT Growth kinetics, mechanisms, and material quality in GaN epitaxial lateral over-growth (ELO) were examined using a single mask of systematically varied patterns. A 2-D gas phase reaction/diffusion model describes how transport of the Ga precursor to the growth surface enhances the lateral rate in the early stages of growth. In agreement with SEM studies of truncated growth runs, the model also predicts the dramatic decrease in the lateral rate that occurs as GaN over-growth reduces the exposed area of the mask. At the point of convergence, a step-flow coalescence mechanism is observed to fill in the area between lateral growth-fronts. This alternative growth mode in which a secondary growth of GaN is nucleated along a single convergence line, may be responsible for producing smooth films observed to have uniform cathodoluminescence (CL) when using 1µm nucleation zones. Although emission is comprised of both UV (~365nm) and yellow (~550nm) components, the spectra suggest these films have reduced concentrations of threading dislocations normally associated with non-radiative recombination centers and defects known to accompany growth-front convergence lines. INTRODUCTION GaN grown on sapphire or SiC has a high defect density due to a significant lattice mismatch at the substrate-material interface. This high defect density (>109 per cm2 [1]) contributes to the poor electrical and optical materials quality [2-4]. There have been many demonstrations that epitaxial lateral overgrowth (ELO) can reduce the dislocation density in GaN films grown by metal-organic chemical vapor deposition (MOCVD) [5-7]. This reduction is the key to fabricating optoelectronic and electronic devices with high performance and reliability. It has been shown that ELO feature morphology is influenced by several factors that include temperature, V/III ratio, and the mask fill factor [8-11]. The manipulation of these factors results in the cross-section morphology changing systematically from triangles, to fivesided polygons, to rectangles. The manner in which these initial growth features converge and coalescence into continuous films has a direct effect on optical quality and uniformity of the resulting smooth film. EXPERIMENT A 2 µm thick GaN base layer was grown by MOCVD in a rotating disk reactor on 2-in diameter sapphire wafers using a low temperature buffer layer followed by high temperature planar growth. This was then covered with 1000 Å SiO2 or Si3N4 and patterned using standard
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