Technical Insight
ELO-GaN improves laser facets (Research Review)
The performance of violet laser diodes has been improved using epitaxial laterally overgrown (ELO) GaN as a substrate for the subsequent deposition of active layers. ELO-GaN is known to show lower dislocation density than GaN deposited directly onto sapphire substrates, leading to improved device lifetimes. Further increases in lifetime can be achieved if device operating currents are reduced. The operating current of a laser diode is directly related to the quality of the mirror facets. A paper by Tojyo and colleagues of Sony Shiroishi Semiconductor, Japan (Jap. J. Appl. Phys. Pt. One 2001 40(6A) 3206) describes a method of forming much improved laser facets on ELO-GaN. Photoluminescence measurements at 362 nm were made on the ELO-GaN layers. The photoluminescence intensity distribution was taken along a line perpendicular to the ELO stripes. The photoluminescence intensity in the ELO region was three times that of the seed region and 30% greater than the region where adjacent ELO wings coalesce. Since photoluminescence intensity is closely related to defect density, the wing regions should contain a relatively low number of defects. This was confirmed by comparing plan view TEM shots of the ELO wing regions and seed regions. AFM measurements of the cleaved facets of InGaN laser diode structures grown on ELO-GaN and sapphire substrates were also compared. The Ra roughness of the laser facet in the sapphire sample was 10 nm, which would cause mirror losses of between 23%. In the ELO sample the Ra roughness was
