The surface morphology and crystalline quality of GaN layers grown by metalorganic chemical vapor deposition on sapphire (0001) substrates were investigated for various thermal treatment conditions of low-temperature (LT)-grown GaN buffer layers. The surface morphology and crystalline quality of subsequently grown high-temperature (HT) GaN layers strongly depended on thermal effects during the temperature ramping process after LT growth of the buffer layers. We have found that the defect density and structure are affected by this temperature ramping process, and that the generation of growth pits is closely related to defects in the HT-GaN layers. High-quality HT-GaN layers with specular surface morphology were obtained with optimum growth and ramping conditions for the LT-GaN buffer layers. Furthermore, the role of thermal treatment during the temperature ramping process was identified, and mechanisms of nucleus formation, HT-growth initiation on the LT-GaN buffer layers, and defect formation are proposed and discussed.

We demonstrate room temperature pulsed operation of nitride based multi-quantum-well (MQW) laser diodes with cleaved mirror facets grown on a conventional C-face sapphire substrate. Cleavage was performed along the <11*BAR*2*BAR*0> direction of the sapphire substrate, and the resultant facet was analyzed using an atomic force microscope (AFM) and theoretical calculation. A single peak emisson, at a wavelength of 417.5 nm, with a full width at half-maximum of 0.15 nm, was obtained. The threshold current density of the laser was 50 kA/cm2 and a voltage for the threshold current was 20 V.

Improvement in temperature characteristics of GaN laser diodes is important for realizing reliable devices operating at high temperatures. Thermal characteristics of GaN lasers have been analyzed by using a three-dimensional thermal conduction model. Maximum operation temperature has also been calculated. It was shown that the reduction in the stripe width as well as the reduction in threshold current density and operation voltage is very important for improving the temperature characteristics of GaN laser diodes.

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Carrier overflow and perpendicular transverse mode in InGaN multi-quantum-well lasers have been analyzed by numerical calculation. A high acceptor concentration for the p-type cladding layer and the reduction of the active layer volume have a large effect on the reduction of carrier overflow. Anti-guide-like behavior of the waveguide mode is formed in InGaN laser structures where the cladding layers have insufficient thickness. The layer structure should be designed so as to reduce the carrier overflow and suppress the anti-guide-like mode.