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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High-power blue-violet laser diodes with aspect ratio as low as 2.3 and threshold current down to 33 mA have been realized. The relationship between threshold current and optical confinement factor was investigated in order to minimize the beam divergence angle perpendicular to the junction plane (θ⊥). θ⊥ was found to decrease with reduction of the optical confinement factor, whereas threshold current density increased. A new layer structure, in which a p-typed cladding layer was located next to an AlGaN electron blocking layer, and a GaInN guiding layer was inserted between the active and the AlGaN electron blocking layer, was effective for obtaining small θ⊥ while maintaining low threshold current.

Degradation experiments for AlGaInN-based laser diodes were conducted for the purpose of constructing a possible model of the degradation mechanism. Lasers in this experiment were aged under 30 mW continuous-wave operation at 60 °C, and the lifetime was defined as the time at which the operating current increased by 20%. The lifetime was found to be dependent on the dislocation density of the basal ELOGaN layer, with the degradation rate being almost proportional to the square root of aging time. A model of degradation in which degradation is governed by a diffusion process is proposed based on these results. Although the model describes the experimental findings well, further investigation is considered necessary before the degradation mechanism in AlGaInN lasers can be fully understood.

We report our recent progress on GaN-based high-power laser diodes (LDs), which will be applied as a light source in high-density optical storage systems. We have developed raised-pressure metal-organic chemical vapour deposition (RP-MOCVD), which can reduce the threading-dislocation density in the GaN layer to several times 108 cm-2, and demonstrated continuous-wave (cw) operation of GaN-based LD grown by RP-MOCVD. Furthermore, we found that the epitaxial lateral overgrowth (ELO) technique is useful for further reducing threading-dislocation density to 106 cm-2 and reducing the roughness of the cleaved facet. By using this growth technique and optimizing device parameters, the lifetime of LDs was improved to more than 1000 hours under 30 mW cw operation at 60 °C. Our results proved that reducing both threading-dislocation density and consumption power is a valid approach to realizing a practical GaN-based LD. On the other hand, the practical GaN-based LD was obtained when threading-dislocation density in ELO-GaN was only reduced to 106 cm-2, which is a relatively small reduction as compared with threading-dislocation density in GaAs- and InP-based LDs. We believe that the multiplication of non-radiative centres is very slow in GaN-based LDs, possibly due to the innate character of the GaN-based semiconductor itself.