Tsuyoshi Tojyo

100-mW kink-free blue-violet laser diodes with low aspect ratio

400-nm-band GaN-based laser diodes (LDs) operating with a kink-free output power of over 100 mW and having a low aspect ratio of 2.3 have been successfully fabricated for the first time. A new ridge structure, in which the outside of the ridge is covered with a stacked layer of Si on SiO/sub 2/ and the ridge width is as narrow as 1.5 /spl mu/m, is applied to realize high kink-free output power with a wide beam divergence angle parallel to the junction plane. A new layer structure around the active layer is demonstrated to be quite effective for obtaining a narrow beam divergence angle perpendicular to the junction plane, maintaining low threshold current. Ten LDs with low aspect ratio have been operated stably for over 1000 h under 30-mW continuous-wave operation at 60/spl deg/C. Relative intensity noise measured under optical feedback with high-frequency modulation is as low as -125 dB/Hz. These results indicate that this LD is suitable for next-generation high-density optical storage systems.

GaN-Based High Power Blue-Violet Laser Diodes

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.

High-Power AlGaInN Laser Diodes with High Kink Level and Low Relative Intensity Noise

High-power AlGaInN laser diodes (LDs) with both high kink level and low relative intensity noise (RIN) are successfully fabricated. A kink level of higher than 100 mW is obtained by using a new ridge structure and by narrowing the ridge width down to 1.5 µm. This structure consists of a thin Si on SiO2 multiple buried layers instead of the conventional thick SiO2 buried layer. The low RIN of these devices is obtained by decreasing the threshold current; under optical feedback with high-frequency modulation, the RIN values are as low as -125 dB/Hz. As the RIN of these devices exhibits a tendency to deteriorate with increasing operating current, the lifetime criterion was redefined as the point at which the operating current has increased by 20%. Even under this stricter lifetime criterion, the estimated lifetime of these LDs exceeds 15,000 h under 30 mW cw operation at 60°C.

GaN-based blue laser diodes

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.

AlGaInN laser diodes grown on an ELO-GaN substrate vs. on a sapphire substrate

The lifetime of AlGaInN-based violet laser diodes is restricted by the high dislocation densities caused by the large difference between the lattice constants and thermal expansion coefficients of GaN and sapphire. In order to reduce the dislocation density, epitaxial lateral overgrowth (ELO) techniques have been proposed, leading to the higher performance of AlGaInN lasers. In this work, we compared the lasing characteristics of AlGaInN lasers grown on ELO-GaN with those grown on sapphire substrates.

Photo-assisted metalorganic vapor-phase epitaxy for nitrogen doping and fabrication of blue-green light emitting devices of ZnSe-based semiconductors

Abstract Photo-assisted metalorganic vapor-phase epitaxy was proposed as a promising technique for acceptor doping in ZnSe-based semiconductors. The growth was carried out using diethyzinc and dimethylselenide as source precursors, and tertiarybutylamine as a dopant material. Photoluminescence, electrical properties of Schottky diodes, and electroluminescence of junction diodes supported p-type behavior of the ZnSe:N layers. As an example, net acceptor concentration was estimated as 2 × 10 17 cm -3 for the ZnSe:N layer with nitrogen atomic concentration of 5 × 10 17 cm -3 . However, it was also pointed out that the doping characteristics were seriously affected by purity of source precursors, and this problem should be overcome, together with optimization of doping conditions, in order to achieve higher acceptor concentrations without heavy compensation.

AlGaInN-based laser diodes

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.

Thermal annealing effects on p-type conductivity of nitrogen-doped ZnSe grown by metalorganic vapor phase epitaxy

Post-growth thermal annealing (e.g., 500°C, 30 min), is proposed as one of the promising techniques to realize and to improve the quality of p-type ZnSe layers grown by metalorganic vapor phase epitaxy (MOVPE). The layers were grown at low temperature (350°C) by photo-assisted MOVPE with doping nitrogen from tertiarybutylamine (t-BuNH2). The flow rate of t-BuNH2 was limited to be relatively low, in order to avoid heavy doping, with which as-grown layers exhibited electrically high-resistivity; but the thermal annealing converted the layers to p-type. As the as-grown layers exhibited the stronger donor-to-acceptor pair recombination lines or the weaker donor-bound excitonic emission (Ix) lines in photoluminescence, the annealed layers resulted in higher net acceptor concentration, which was 1 x 1017 cm−4 at the optimum conditions at present.

Small Integrated Optical Head Device Using a Blue-Violet Laser Diode for Blu-ray Disc System

We report the first integrated optical head device using a blue-violet laser diode (LD), which is a key device for realizing a small and thin Blu-ray Disc drive. While integrating seven optical elements and semiconductor chips into one device by adopting a molded optical element with high transmittance to the blue-violet wavelength, both small aberration and the small device size of 11 mm×6 mm×4.1 mm have been realized. We have also improved heat dissipation efficiency from this device to the base unit by adopting a newly developed package. At this time, we have implemented a performance evaluation of this small head device and have confirmed its good read/write performance as well as its adequate tolerances required for the Blu-ray Disc system.

GaN-based high-power laser diodes

Abstract We report our recent progress on GaN-based laser diodes (LDs) which will be applied as a light source in high-density optical storage systems. Recently we achieved a lifetime of more than 500 hours under continuous-wave operation with a constant power 20 mW at 25°C using GaN-based LDs with a standard ridge structure. We also report the potential of GaN-based LDs with another structure of a buried-ridge. The far-field pattern of the LDs with a buried-ridge structure strongly depended on the Al content of the AlxGa1−xN burying layer. This dependency showed that the device characteristics change from gain-guiding to refractive index-guiding. The critical point was around x=0.30 of an Al content which corresponds to Δn=0.007 of a lateral index step. It was, therefore, found that the optical transverse mode can be controlled by adjusting the Al content of the AlxGa1−xN burying layer.