Structural Defects related issues of GaN-based Laser Diodes

Abstract

ABSTRACTStructural defects affecting the lifetime of GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers have been investigated. Almost all of the threading dislocations that appeared in the wing regions have edge character, whereas the dislocations at the coalescence boundaries have both edge character and mixed character. The origins of the threading dislocations in the wing regions are the lateral extension of dislocations from the seed regions that contingently bend upwards to the epi-surface. Thus, edge dislocations are most considerable threading dislocations in GaN-based LDs on ELO GaN layers, since the laser stripes are fabricated in the wing regions. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations were observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. Although the threading dislocations in the LD stripes do not multiply during device operation, our degradation experiments revealed that the lifetime of the GaN-based LDs depends on the dislocation density. The degradation rate was almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed.