Toshimasa Kobayashi

Band lineup for a GaInP/GaAs heterojunction measured by a high‐gain Npn heterojunction bipolar transistor grown by metalorganic chemical vapor deposition

A GaInP(N)/GaAs( p) heterojunction bipolar transistor was fabricated by metalorganic chemical vapor deposition (MOCVD) for the first time. The common‐emitter current gain exceeded 200 at a current density around 10 A/cm2 and the offset voltage was as small as 50 mV. Thermionic emission theory indicates that the conduction‐band discontinuity (ΔEc) at GaInP/GaAs heterointerface is as small as 30 meV at room temperature and this value was more than 160 meV smaller than 0.19–0.22 eV obtained by the C‐V profile method. The band‐gap energy for MOCVD‐grown GaInP was 60 meV smaller than the intrinsic band‐gap energy (1.91 eV), but this value is too small to explain the difference between the present ΔEc value and the previously reported ΔEc value.

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.

Structure Analysis of InN Film Using Extended X-Ray Absorption Fine Structure Method

We investigated the local atomic structure around In atoms of MBE-grown InN which has a direct bandgap energy of 0.8 eV, using extended X-ray absorption fine structure (EXAFS) oscillation of In K-edge. The signals from the first-nearest neighbor atoms (N) and second-nearest atoms (In) from In atoms were clearly observed and the atomic bond length of In-N and In-In was estimated to be d In-In = 0.215 nm and d In-In = 0.353 nm, respectively. The In-N bond length of d In-In = 0.353 nm was closed to the a-axis lattice constant of a = 0.3536 nm, which was determined using X-ray diffraction measurements. The obtained local atomic structure agreed with the calculated ideal structure. We conclude, therefore, that the InN film with a bandgap energy of 0.8 eV has a high structural symmetry in the range of a few A around In atoms.

Threading Dislocations and Optical Properties of GaN and GaInN

We categorized threading dislocations in GaN and GaInN multiple quantum wells and epitaxially lateral overgrown GaN into three types of line defects (edge, screw and mixed dislocations), and investigated the optical properties. It was confirmed by cathodoluminescence measurements that not only screw and mixed dislocations but also edge dislocations act as non-radiative centers in GaN. Epitaxial lateral overgrowth (ELO) technique can reduce the densities of all line-defects in a several μm wide wing region. Growth steps in the wing region were disturbed by the defects which were left in a seed region, and a complicated structure was formed at the surface of GaN and GaInN layers grown on ELO-GaN at low temperature. We believe that this surface structure formed by high supersaturation is a cause of In compositional spatial fluctuation and phase separation of GaInN alloy.

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.

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.

An AlN/GaN insulated gate heterostructure field effect transistor with regrown n+GaN source and drain contact

A nobel insulated gate heterostructure field effect transistor (IG-HFET) was demonstrated. The layer structure under the gate consisted of a thick AIGaN barrier. an n + GaN channel 15 nm thick, and an AIN insulator 4 nm thick, grown sequentially by MOCVD. The device operated with gate voltage up to + 3 V. The pinch-off voltage was about 0 V, which resulted from the very high Schottky barrier height and the very thin AIN layer. The measured transconductance g m was 235 mS/mm for L g = 1.4 μm, which is the highest so far reported.

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.

Read/write characteristics and noise properties of a thin film VCR head

The read/write characteristics and noise properties of the Embedded Thin Film (ETF) head, in which the size of the magnetic core is minimized by using a thin film coil are investigated and compared with those of a Metal in Gap (MIG) head. The small core size results in a smaller increase of impedance noise and it is clearly indicated that reducing the impedance noise is important in improving the Signal-to-Noise Ratio (SNR) over a wide frequency range, because the impedance noise has a greater effect over 30 MHz. In the writing process, a ghost pattern is found to be recorded by a fringing field from the spacer of the multiple-layered magnetic core, which results in a broadening of the isolated pulse. In spite of the ghost, however, the recording properties at a high frequency are not degraded, probably because the influence of the fringing field is small at a high frequency. As a result, the SNR of the ETF head is improved by 2.7 dB over the range of 1-40 MHz compared with that of the MIG head.

Gain‐guided laser diode having a curved‐mirror‐cavity with a low astigmatism

A gain‐guided laser diode having a curved‐mirror‐cavity that has coaxial mirrors and a fan‐shaped stripe structure has been fabricated by dry etching. Although gain guided, the laser has an astigmatism of 4.1 μm, which is very low compared to conventional laser’s. The threshold current of the laser is 35 mA, the longitudinal mode spectrum is multiple, and the transverse optical mode is fundamental. These results are comparable to conventional gain‐guided lasers. Etched mirror laser diodes with flat facets have been stably operated over 1500 h under automatic‐power control at a power of 3 mW/facet at 50 °C, without facet coating