No abstract

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.

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.

We have investigated the dependence of crystallographic tilt and defect distribution on mask material in metalorganic chemical vapor deposition grown GaN layers formed utilizing an epitaxial lateral overgrowth (ELO) technique using x-ray diffraction and transmission electron microscopy. Crystallographic tilt in the ELO GaN layer was suppressed by changing the mask material from electron beam (EB)-evaporated SiO2 to plasma enhanced chemical vapor deposition (PECVD) grown SiO2 and PECVD SiNx. Defect distribution also changes in accordance with mask materials. By depositing a thin PECVD SiNx layer on the PECVD SiO2 mask, the crystalline quality of the ELO layer changes from that used with the SiO2 mask to that used with the SiNx mask. These results suggest that the interface between the ELO GaN layer and the mask has a significant effect on crystallographic tilt and defect distribution.

We have investigated dislocations in GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy and cathodoluminescence microscopy and found a correlation between dislocations and device reliability. Dislocation density in the seed regions of ELO GaN layers is of the order of 10 8 cm --2 , while that in the wing regions is less than mid-10 6 cm --2 . The origin of dislocations in the wing regions is the extension of defects in highly defective regions near the GaN layer/substrate interface in the seed regions. The lifetime of LDs has a strong correlation with consumption power. However, some LDs have a shorter lifetime although their consumption power is almost the same. In the LDs with short lifetimes, dislocations lying in the c-plane were formed below the active regions, bent towards the c-axis and threaded upwards to active regions. These newly created dislocations can become detrimental to the device lifetime.