Takeharu Asano

Dislocations in GaN‐Based Laser Diodes on Epitaxial Lateral Overgrown GaN Layers

We have investigated dislocations in GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy and cathodoluminescence microscopy and found a correlation between dislocations and device reliability. Dislocation density in the seed regions of ELO GaN layers is of the order of 10 8 cm -2 , while that in the wing regions is less than mid-10 6 cm -2 . The origin of dislocations in the wing regions is the extension of defects in highly defective regions near the GaN layer/substrate interface in the seed regions. The lifetime of LDs has a strong correlation with consumption power. However, some LDs have a shorter lifetime although their consumption power is almost the same. In the LDs with short lifetimes, dislocations lying in the c-plane were formed below the active regions, bent towards the c-axis and threaded upwards to active regions. These newly created dislocations can become detrimental to the device lifetime.

High Density Near-Field Optical Disc System

Key technologies for near-field recording/readout systems using a solid immersion lens (SIL) are summarized in this paper. Our system employing a SIL of 1.84 numerical aperture (NA) and a laser diode (LD) of 405 nm wavelength has realized the capacity of a 112 GB in a 12 cm diameter phase-change disc along with a write-power margin of ±11.2% and a data transfer rate of 36 Mbps. Such a larger-capacity data storage system should provide a high data transfer rate. A preliminary result of 1.84 NA dual-channel near-field recording/readout using a monolithic dual-beam LD with 412 nm wavelength is presented.

Over 100-mW blue-violet laser diodes for Blu-ray Disc system

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.

Epitaxial growth of ZnMgTe and double heterostructure of ZnTeZnMgTe on GaAs substrate by metalorganic chemical vapor deposition

Abstract We have epitaxially grown ZnMgTe ternary alloys, whose Mg content is between 0% and 50%, on (100) GaAs substrates by metalorganic chemical vapor deposition. The band-gap energy estimated from the emission wavelength at room temperature can be varied from 2.26 to 2.57 eV. In the photoluminescence (PL) measurements at 4.2 K, band-edge emission was clearly observed up to Mg content of 25%. The refractive index decreases as Mg content increases. In the PL spectra of double heterostructures of ZnTe ZnMgTe at 4.2 K, strong emission and quantum size effect were observed. These results suggest that effective carrier and optical confinement are possible.

CW Operation of AlGaInN–GaN Laser Diodes

We report the advantage of raised-pressure metalorganic chemical vapor deposition (RP-MOCVD) and the room-temperature (RT) continuous-wave (CW) operation of AlGaInN–GaN laser diodes grown at 1.6 atm. Improvement of the crystalline quality by increasing the growth pressure was verified by measuring the optical pumping threshold-power density and etch-pit density. Furthermore, we succeeded in the first RT-CW operation of buried-ridge laser diodes where the ridge region was buried by a partially crystallized layer of Al0.6Ga0.4°C, cracks in the AlGaN burying layer can be prevented. The 4 μm wide buried-ridge laser operated under CW conditions, with an output power up to 30 mW. The threshold current was 165 mA, corresponding to a threshold current density of 4.1 kA/cm2, and the operating voltage at the threshold was 9.0 V.

Epitaxial growth of ZnMgSSe on GaAs substrate by metalorganic chemical vapor deposition

Room temperature blue laser action in an optically pumped metalorganic chemical vapor deposition (MOCVD)-grown ZnSe/ZnMgSSe double heterostructure is demonstrated. In the atmospheric pressure metalorganic chemical vapor deposition process, dimethylzinc (DMZn), bismethyl-cyclopentadienyl-magnesium ((MeCp) 2 Mg), diethylsulfide (DES), and dimethylselenium (DMSe) are used as sources. Mirror surface morphology is achieved up to a Mg content of 13% in the ZnMgSSe. Good crystal quality of the ZnMgSSe layer, lattice-matched to GaAs, is indicated by photoluminescence measurement at 4.2 K as well as by X-ray diffraction. The spectrum of photoluminescence is dominated by a sharp near-band-edge emission. The full width at half maximum of the (400) X-ray diffraction peak of Zn 0.95 Mg 0.05 S 0.15 Se 0.85 layer is 170 arc sec

Estimation of Device Properties in AlGaInN‐Based Laser Diodes by Time‐Resolved Photoluminescence

We estimated the optical characteristics of AlGaInN-based laser structures by time-resolved and standard photoluminescence. It was clarified that the microscopic distribution of the peak wavelength of GaInN active layers has a strong correlation with the threshold current and the slope efficiency. As an index representing the extent of this distribution, we used the depth in the tail states of the active layer, E 0 , for which we found an excellent correlation with the device properties. From an estimation of the external quantum efficiency as a function of equivalent cavity length, it was found that the sample with smaller E 0 has higher internal quantum efficiency and smaller internal absorption coefficient than those with larger E 0 .

High-Power AlGaInN Lasers

We have successfully fabricated high-power AlGaInN laser diodes with long lifetime, high kink level, wide beam divergence angle parallel to the junction plane (θ || ), and low relative intensity noise (RIN). We have also adopted a 3Φ sapphire substrate to improve productivity and have achieved uniform fabrication using a carefully designed quartz reactor and heating system. The introduction of a new laser structure successfully provided a dramatic improvement in device properties. In particular, the characteristic temperature (To) was improved to 235 K, which is the highest ever reported. In addition, we fabricated a super high power (SHP) AlGaInN blue-violet laser diode array, achieving a maximum light output power of 4.2 W under cw-operation at 25 °C with a lifetime under 1 W cw-operation at 25 °C of more than 1000 h.

Novel techniques for stabilizing transverse mode in AlGaInN-based laser diodes

We report on a novel laser structure for stabilizing transverse mode in AlGaInN-based high power laser diodes (LDs). AlGaInN-based laser structures were grown on epitaxially laterally overgrown GaN (ELO-GaN) by MOCVD. We have developed a novel ridge structure with a new current-blocking layer consisting of spin-on-glass, which shows absorption of the emitted light to some extent. With this structure, there was no kink in the light output versus current curve up to 150 mW under cw operation, and the full width at half-maximum angle of far-field pattern in the direction parallel to the epitaxial layers was 9.1°. This large beam divergence was kept stable up to 150 mW, which indicates that our novel structure realized a stable lateral optical confinement.