Structural Defects related issues of GaN-based Laser Diodes
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Structural Defects related issues of GaN-based Laser Diodes Shigetaka Tomiya1, Motonobu Takeya2, Shu Goto2, and Masao Ikeda2 Material Analysis Dept. Sony EMCS Corporation, Atsugi, Kanagawa, 243-0021, Japan 2 Shiroishi Laser Center, MSNC DSC Semiconductor Laser Div., Sony Corporation Shiroishi, Miyagi, 989-0734, Japan 1
ABSTRACT Structural 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. INTRODUCTION GaN-based laser diodes (LDs) are important short-wavelength coherent light sources in the blue and violet spectral region and are already being applied for large-capacity optical data storage systems. Although GaN-based material systems are poorly lattice-matched to all commonly-used substrates, they have been successfully grown on sapphire and SiC substrates by inserting low-temperature grown buffer layers [1]. Due to the large lattice mismatch and the difference between the thermal-expansion coefficients of GaN-based materials and the available substrates, a high density of dislocations (typically 108-1010/cm2) was inevitable. Despite such a high density of threading dislocations, these LEDs had a relatively long lifetime. However, it was rather difficult to fabricate reliable LDs, due to their high threshold current density of 5-6 kA/cm2, high operating voltage and high density of dislocations. A reduction in the dislocation density down to the order of 106/cm2 by using the epitaxial lateral overgrowth (ELO) technique has extended the lifetimes of such LDs [2, 3], since dislocations act as non-radiative centers [4]. It is known that the rapid degradation observed in conventional LDs can be attributed to dislocation multiplication by non-radiative recombination at thr
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