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The epitaxial lateral overgrowth (ELO) technique is an important technology for improving the characteristics of GaN-based laser diodes (LDs). The photoluminescence intensities from GaN and GaInN multiple quantum well active layers in the ELO-GaN wing region were found to be higher than those in the seed region. This indicates that the density of dislocations in the wing region could be reduced significantly. This is evidenced by dislocation densities of less than 106 cm-2 as determined from transmission emission microscopy and etching-pit-density measurements. The cleaved facets of LDs on ELO-GaN and sapphire were observed by atomic forced microscopy. Although the roughness of GaN cleaved facets on sapphire were high (Ra>10 nm), the roughness in the ELO-GaN wing region was found to be as smooth as that of GaAs cleaved facet (Ra<1 nm). The characteristics of LDs on ELO-GaN were found to be superior to those on sapphire as a result of smoother facets and lower dislocation densities.

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PACS 42.55.Px, 42.60.Lh, 78.66.Fd, 81.05.Ea, 81.15Gh GaN-based blue-violet lasers with a kink-free output power of higher than 150 mW have been successfully realized by adopting a new ridge structure and appropriately designing the beam divergence. The new ridge structure is a narrow 1.4 µm ridge covered with a stacked layer of Si on SiO 2 and the beam divergence half-angles parallel and perpendicular to the junction plane are set at 8° and 21°, respectively. These lasers have been operating stably for more than 500 h under 130-mW pulsed operation at 60 °C. The empirical activation energy of device lifetime under 30-mW continuous-wave operation is 0.32 eV.Introduction GaN-based high-power blue-violet laser diodes (BV-LDs) operating in the 400-410 nm wavelength band are promising light sources for large-capacity optical storage systems. The characteristics of BV-LDs have been improved remarkably since Nakamura et al. reported the first roomtemperature continuous-wave (CW) operation in 1996 [1]. Reliable BV-LDs with estimated lifetimes exceeding 10,000 h under 30-mW CW operation at 60 °C have been realized in recent years [2][3][4]. Although these devices can be readily used in high-density digital video recording systems [5], reliable BV-LDs capable of much higher output power will be required in the near future as the technology for optical disk systems extends to multi-layer disks and higher data transfer rates to facilitate higher densities and higher recording speeds.In designing a high-power laser suitable for use in optical disk systems, it is essential to maintain both a high kink level and a high catastrophic optical damage (COD) level of output power. These two levels are closely related to the optical confinement in the stripe of the laser structure, and hence to the beam divergence angles. The lifetime of BV-LDs is also governed by the density of dislocations within the stripe [6]. The stripes of current lasers typically contain significantly less than 100 dislocations, reduced from previously much higher numbers through the use of dislocation-reduction techniques such as epitaxial lateral overgrowth (ELO) [7,8], and lifetimes of over 1,000 h at 60 °C have already been achieved. However, even one dislocation in a stripe causes rapid degradation in other III-V laser diodes such as AlGaAs, GaInAsP and AlGaInP lasers. The remarkably long lifetimes and resilient nature of GaN-based lasers is therefore apparently associated with the inherent rigidity of this material system.In this paper, the kink and COD levels of a laser structure optimized for obtaining high output powers of more than 100 mW are described with reference to the most important device parameters. The dependence of the lifetime of these high-power lasers on the ambient temperature and output power are also reported.

We investigate degraded GaN‐based laser diodes (LDs) fabricated on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy. The dislocation density in the wing region of the ELO is of the order of 106/cm2 and there are approximately ten threading dislocations in the laser stripe. Neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations were observed in the devices, which were degraded within approximately one hundred hours under 30 mW continuous operation at 25 °C. We can, therefore, conclude that degradation in GaN‐based LDs is not responsible for the recombination enhanced dislocation motion that is usually observed in zincblende structure‐based LDs. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)