AlInGaN based blue and blue-green LDs were investigated with regard to the
characteristics of GaN semiconductor laser diodes. High power, single mode blue LDs with
high COD level (~334mW under CW operation at 25°C, kink-free at 150mW) and long lifetime
(~10000 hours under CW operation, 50mW 25°C) were achieved. No significant characteristic
differences between blue LDs on LEO-GaN/sapphire and GaN substrate were observed. The
blue-green LD which has the wavelength of 485 nm was successfully fabricated and
demonstrated under CW operation 25°C, while it showed poor performances of LD
characteristics compared to those of blue LDs. We believe that the poor performance of blue-green
LDs were caused by the piezo-electric effect by lattice mismatch along C-axis of GaN, In
fluctuation by lattice mismatch and In solubility limit in InGaN QWs and thermal annealing
which was performed during the p-layer growth.
We report on the development of GaN-based violet laser diodes (LDs) for the high-capacity optical storage application and blue LDs for the laser projection display application. InGaN LDs with emission wavelength of ~405 nm are already being adopted for next-generation optical-storage systems. We present results on >400 mW single-mode output power under pulsed operation which can be employed in 100 Gbyte multi-layer BD systems. We designed LD layer structures to exhibit high level of catastrophic optical damage (COD) and small beam divergence. In addition, GaN-based blue LDs with emission wavelength of ~450 nm have also been developed for the application to the blue light sources of laser display systems. We demonstrate single-mode blue InGaN LDs with >100 mW CW output power. Interestingly, we observed anomalous temperature characteristics from the blue InGaN LDs, which has shown highly-stable temperature dependence of output power or even negative characteristic temperature (T<sub>0</sub>) in a certain operation temperature range. This unusual temperature characteristic is attributed to originate from unique carrier transport properties of InGaN QWs with high In composition, which is deduced from the simulation of carrier density and optical gain.
The enhanced output power with improved lifetime is required for the GaN-based blue-violet laser diode (LD) as a light source for Blu-ray Disc or HD-DVD. In this paper, the output power levels and aging behaviors in GaN-based LDs grown on sapphire substrates were compared in epi-up and epi-down bonding. At low current level, the two bondings
show little differences in L-I characteristics. At high current level, however, the epi-up bonding shows a rapidly decreased slope efficiency in L-I characteristics with increasing current injection. On the contrary, the slope efficiency in epi-down bonding is not so much deteriorating as that in epi-up bonding. The differences in junction temperature between epi-up and epi-down bonding are large at higher current levels. The junction temperature of epi-up bonding is
about two times higher than that of epi-down bonding, implying efficient heat dissipation in epi-down bonding. At aging test, the epi-down bonding LD shows lower degradation rate at the aging slope than that of epi-up bonding LD. The degradation rate is accelerated by poor heat dissipation in epi-up bonding. Thus, for the higher power and longer lifetime, it is necessary to employ efficient heat dissipation structures such as epi-down bonding for the GaN-based LD
on sapphire substrate.
High power and high efficiency AlInGaN-based laser diodes with 405 nm were fabricated for the post-DVD applications. Magnesium doped AlGaN/GaN multiple quantum barrier (MQB) layers were introduced into the laser diode structure, which resulted in considerable improvement in lasing performances such as threshold current and slope efficiency. Asymmetric waveguide structure was used in order to improve the characteristics of laser diodes. Aluminum content in the n-cladding layer was varied in connection with the vertical beam divergence angle and COD level. By decreasing Al content in the n-cladding layer, the vertical divergence angle was reduced to 17 degree and the COD level was enhanced to over 300mW. The maximum output power reached as high as 470 mW, the highest value ever reported for the narrow-stripe GaN LDs. In addition, the fundamental transverse-mode operation was clearly demonstrated up to 500 mW-pulsed output power.