The degradation mechanism of blue-violet laser diodes (BV-LDs) fabricated on epitaxial lateral overgrowth (ELO) GaN on sapphire substrates is investigated and the characteristics of recent BV-LDs fabricated on GaN substrates are reported. The lifetime of lasers on ELO-GaN/sapphire is shown to be strongly dependent on the dislocation density of the basal GaN layer, with the degradation rate being roughly proportional to the square root of the operating time. Electron microscopy confirms that the degradation is not due to the multiplication of threading dislocation but rather the diffusion of defects toward dislocations in the active layer, which results in an increase in the capture cross-section for non-radiative recombination centers. BV-LDs on GaN substrates are demonstrated to have a lifetime of over 2000 h under 160 mW pulsed operation at 80 °C and to achieve a high critical power density at catastrophic optical damage (COD) of 40.8 MWcm-2.
400-nm-band GaN-based blue-violet laser diodes (LDs) operating with a high output power of over 100 mW have been successfully fabricated. A new ridge structure, in which the outside of the ridge was covered with a stacked layer of Si on SiO2 and the ridge width was as narrow as 1.4 μm, was applied to realize the stable lateral-mode
operation. A layer structure around the active layer was carefully designed so as to ensure a high COD level. The lasers have been operated stably for more than 500 h under 130-mW pulsed operation at 60°C. From ambient temperature dependence of the device lifetime, the empirical activation energy was estimated as 0.32 eV. These
results indicate that this LD is suitable for next-generation Blu-ray Disc system.
This paper describes an improved laser structure for AlGaInN based blue-violet lasers (BV-LDs). The design realizes a small beam divergence angle perpendicular to the junction plane and high characteristic temperature wihtout significant increase in threshold current density (Jth) by optimizing the position of the Mg-doped layer and introducing an undoped AlGaN layer between the active layer and the Mg-doped electron-blocking layer. The mean time to failure (MTTF) of devices based on this design was found to be closely related to the dislocation density of ELO-GaN basal layer. Under 50 mW CW operation at 70°C, a MTTF of over 5000 h was realized whenthe dark spot density (indicative of dislocation density) is less than ~5×106 cm-2. Power consumption under 50mW CW operation at 70°C was approximately 0.33 W, independent of the dislocation density.
We have successfully developed GaInN-based 400nm lasers for DVR-blue systems and GaInP-based 650nm lasers for DVD+/- RW systems. The high-performance blue-violet laser developed here has low relative intensity noise (RIN) of -128 dB/Hz, low aspect ratio of 2.3, and a nominal lifetime of 15000 h at 60 degree(s)C and 30 mW output power. The 650nm red laser was developed for DVD+/- RW systems, which require red lasers with output power exceeding 90 mW in order to increase the data transfer speed. The high-power red lasers developed here are capable of 90 to 120 mW output power with high reliability at 60 to 70 degree(s)C and have a low aspect ratio of 2.3.
The longer lifetime is desired for high power AlGaInN based violet lasers. We found that lifetime is strongly dependent on both the initial operating consumption power and the dislocation densities in the laser stripe. Pd/Pt/Au as a metal and AlGaN/GaN superlattice as a p-type cladding layer were incorporated to reduce the operating voltage. The optimization of device parameters as well as the stripe width and the RIE etching device depth led to the lower threshold current of 3.4 kA/cm2. We used the Pendeo epitaxy technique to get lower dislocation density approximately 107 cm-2. The LDs with these technologies showed an output power as high as 35 mW under room temperature CW condition without kink. The lifetime is more than 500 hours under CW operation with a constant power of 20 mW at 25 degree(s)C.
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