Shiro Uchida

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.

High-efficiency GaAs and GaInP solar cells grown by all solid-state molecular-beam-epitaxy.

We report the initial results of GaAs and GaInP solar cells grown by all solid-state molecular-beam-epitaxy (MBE) technique. For GaAs single-junction solar cell, with the application of AlInP as the window layer and GaInP as the back surface field layer, the photovoltaic conversion efficiency of 26% at one sun concentration and air mass 1.5 global (AM1.5G) is realized. The efficiency of 16.4% is also reached for GaInP solar cell. Our results demonstrate that the MBE-grown phosphide-contained III-V compound semiconductor solar cell can be quite comparable to the metal-organic-chemical-vapor-deposition-grown high-efficiency solar cell.

AlGaInN high-power lasers grown on an ELO-GaN layer

Abstract Epitaxially laterally overgrown (ELO)-GaN substrates with dislocation-free regions can be prepared with a relatively small thickness using atmospheric pressure metal-organic chemical vapor deposition (MOCVD). In order to determine the effect of ELO-GaN layer on the reliability of blue–violet laser diodes (BV-LDs), we fabricated BV-LDs on the lateral-growth region of the ELO-GaN layer. The thickness of the ELO-GaN layer was maintained at 5xa0μm to inhibit wafer bending, so the total thickness of the BV-LDs was approximately 7xa0μm. The lifetime of the BV-LDs with a constant output power of 20xa0mW under CW operation at 25°C was more than 500xa0h. This lifetime is several times that of our conventional BV-LDs on sapphire substrates. The ELO-GaN layer was very effective in raising the lifetime of BV-LDs.

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.

Room-temperature GaAs/InP wafer bonding with extremely low resistance

Low-temperature direct wafer bonding is a promising technique for fabricating multijunction solar cells with more than four junctions in order to obtain high conversion efficiencies. However, it has been difficult to reduce the bond interface resistance between a GaAs-based subcell wafer and an InP-based subcell wafer. We found that a novel bonding structure comprising heavily Zn-doped (1 x 10(19) cm(-3)) p(+)-GaAs and S-doped (3 x 10(18)cm(-3)) n-InP had an interface resistance of 2.5 x 10(-5) Omega.cm(2), which is the lowest value ever reported. This result suggests that the newly developed room-temperature wafer bonding technique has high potential to realize high-efficiency multijunction solar cells

Monolithic dual-wavelength high-power lasers for CD-R/DVD /spl plusmn/ R/RW/RAM

The monolithic integration of a GaInP-based high-power red laser for DVD /spl plusmn/ R/RW/RAM and an AlGaAs-based high-power infrared laser for CD-R was successfully achieved for the first time. The monolithic device provides reliable output power of 200 mW for red lasers and 240 mW for infrared lasers, allowing writing speeds of 12/spl times/ for DVD-R and 48/spl times/ for CD-R. The kink power of AlGaInP DVD lasers is dramatically improved up to 300 mW from 220 mW even at the high temperature of 85/spl deg/C, due mainly to the reduction of the lateral leakage current. The beam separation is precisely controlled to be within 110/spl plusmn/1 /spl mu/m despite forming separately two ridge stripes.