Yasuhiko Nomura

Surface diffusion length of Ga adatoms in molecular-beam epitaxy on GaAs(100)–(110) facet structures

Abstract By the molecular-beam epitaxial (MBE) growth of GaAs on [001]-mesa stripes patterned on GaAs(100) substrates, (110) facets were formed on the mesa edges defining (100)–(110) facet structures. The surface diffusion length of Ga adatoms along the [010] direction on the mesa stripes was obtained for a variety of growth conditions by in-situ scanning microprobe reflection high-energy electron diffraction (μ-RHEED). Using these values and the corresponding growth rate on the GaAs(110) facets, the diffusion length on the (110) plane was estimated. We found that the Ga diffusion length on the (110) plane is longer than that on the (100) and (111)B planes. The long diffusion length on the (110) plane is discussed in terms of the particular surface reconstruction on this plane.

Operation at 700°C of 6H-SiC UV Sensor Fabricated Using N+ Implantation

We have realized, for the first time, a UV sensor that operates at temperatures up to 700°C by using N+ implantation into 6H-SiC. The photocurrent of the sensor increased with temperature, and at 400°C and 700°C, the photocurrent is approximately double and triple that at room temperature (RT), respectively. It was clarified that the temperature dependence of the photocurrent reveals the characteristics of absorption particular to indirect transition, and also the minority carrier diffusion length. Dark current increased rapidly at temperatures exceeding 450°C.

Lateral growth of GaAs on patterned {1¯1¯1¯}B substrates for the fabrication of nano wires using metalorganic molecular beam epitaxy

We have grown GaAs nano wire structures (45 × 20 nm2) buried in AIAs layers by lateral metalorganic molecular beam epitaxy on the terraced sidewalls of mesa-grooved (-1-1-1)B substrates. The growth of GaAs occurred primarily on the sidewall of the mesa-grooves and not on the (-1-1-1)B surface for arsenic pressures greater than 2.0 × 10−3 Pa at a substrate temperature of 480°C. An (0-1-1) facet formation during the lateral epitaxy at the intersection region between the bottom (-1-1-1)B surface and the (1-2-2)A sidewall has been directly observed by real-time scanning microprobe reflection high-energy electron diffraction. The growth rate on the (0111) facet was estimated from the variation of its width with growth time.

Surface cleaning of Si-doped/undoped GaAs substrates

It was found that the surface morphology and residual impurities after substrate-cleaning processes are influenced by the existence of a Si dopant in a GaAs (001) substrate. Atomic-force microscopy observations have revealed that the surface of an undoped substrate after conventional thermal cleaning under an As4 flux is smoother than that of a Si-doped substrate. The surface roughness of Si-doped substrates was greatly improved by cleaning using hydrogen radicals ( H•). Furthermore, secondary ion mass spectrometry revealed that the concentrations of residual carbon and oxygen on a H•-cleaned surface were also markedly reduced on undoped substrates, compared to those on Si-doped substrates.

Reduction in Operating Current of High-Power 660 nm Laser Diodes Using a Transparent AlGaAs Cap Layer.

We have introduced a transparent AlGaAs cap layer instead of a conventional GaAs cap layer into high-power 660-nm-band-laser diodes with weaker optical confinement in the perpendicular direction, since this structure enables us to weaken the optical confinement without increasing the internal loss for a real index-guided structure. The fabricated laser diodes have demonstrated reduced operating current of 150 mA at 100 mW and increased maximum light output power of 200 mW under the pulsed condition. An aspect ratio of 1.5, which is the smallest of all 660-nm-band high-power laser diodes reported to date, has also been achieved. These laser diodes have been operating stably under the pulsed condition for nearly 2000 h at 60°C with a light output power of 100 mW, which is also the highest of all real index-guided 660-nm-band laser diodes reported to date.

Real-time observations of III-V growth on patterned substrates by μ-RHEED

Abstract Scanning microprobe reflection high-energy electron diffraction (μ-RHEED) reveals microscopic surface features during MBE growth on a non-planar substrate. The Ga droplet formation and their movement were observed in GaAs growth with an alternating source supply on a vicinal plane. Appearance and development of additional facets on the edge of the mesa stripe were monitored during growth in real time. The growth front of the lateral epitaxy was observed, and it was found that the (311) facet was formed as a sidewall of a mesa stripe along the [0 11 direction on the (111)B substrate. InAs hillock formation which depended on the line spacing of the stripe pattern of the substrate was observed on high-temperature substrates, and the real-time observation showed that the hillocks originated in the droplets on the surface.

High-Power 660-nm-Band AlGaInP Laser Diodes with a Small Aspect Ratio for Beam Divergence

High-power 660-nm-band AlGaInP laser diodes with a small aspect ratio have been successfully fabricated with a window-mirror structure. The relationship between optical confinement in the perpendicular direction and internal loss was investigated, and a real index-guided structure with an AlInP current blocking layer was applied on the basis of this investigation. A high-power laser diode with a small aspect ratio of 1.65 for beam divergences of 16.5° and 10° in the perpendicular and parallel directions, respectively, has shown a high kink level of 160 mW and a high maximum light output power of 180 mW under pulsed condition. These laser diodes have operated stably for more than 1500 h with a light output power of 90 mW at 60°C under pulsed condition. Stable pulsed operation at 60°C with a high power of 90 mW and small aspect ratio of 1.65 have been simultaneously achieved for the first time.

High-Power Blue-Violet Laser Diode Fabricated on a GaN Substrate

We have successfully fabricated blue-violet laser diodes, consisting of nitride-based semiconductors, with both high-power and low-noise characteristics on GaN substrates. These laser diodes have a ridge waveguide structure with a dielectric current blocking layer. By improving the crystal quality of the grown materials and optimizing the optical confinement in the device, a kink level as high as 250 mW has been achieved. Optimized optical confinement is also assumed to result in far field patterns without any additional peaks. In addition to this, since the threading dislocation density at the active layer below the ridge portion is reduced to less than 105cm-2, these laser diodes have been operating reliably for more than 1000 h with a light output power of 100 mW at 60°C under pulsed operation. We have also confirmed that these laser diodes have a noise level as low as -130 dB/Hz, which meets the requirement for practical use, for a light output power of 5 mW. These laser diodes are expected to enable dual layer recording in nextgeneration, large-capacity optical disc systems using blue-violet laser diodes.

High-power blue-violet laser diodes for next-generation optical disc systems

High-power and low-noise characteristics are strongly required for blue-violet laser diodes in practical application to next generation optical disc systems. We have successfully fabricated blue-violet laser diodes meeting these requirements. These laser diodes have a ridge waveguide structure with a dielectric current blocking layer. Improving the crystal quality of the grown materials on a GaN substrate and optimizing the optical confinement in the emission layer have achieved a kink level as high as 250 mW. Optimized optical confinement is also assumed to result in a good beam profile with no fringe. In addition to this, since the threading dislocation density at the emission layer below the ridge portion is reduced to less than 10 5cm-2, these laser diodes have been operating reliably for more than 1000 h with a light output power of 100 mW at 60 °C under pulsed operation. This light output power makes it possible to realize dual layer recording systems. We have also confirmed that these laser diodes have a noise level as low as -130 dB/Hz for a light output power of 5 mW.

Low-Temperature Surface Cleaning of GaAs Using Trisdimethylaminoarsine.

We applied trisdimethylaminoarsine (TDMAAs) to the surface cleaning of GaAs (111)B substrates in a high-vacuum environment in order to lower the treatment temperature. The native oxide formed on the substrate was removed at substrate temperatures as low as 400°C under TDMAAs pressure. Characterizations by atomic force microscopy and secondary ion mass spectrometry showed that TDMAAs cleaning markedly improves the surface smoothness and reduces residual impurities (carbon and oxygen) compared to conventional thermal cleaning using As4. The photoluminescence spectra (77 K) of GaAs/AlGaAs quantum wells grown by molecular beam epitaxy directly (without a GaAs buffer layer) on a TDMAAs-cleaned substrate were comparable to those on a 500-nm-thick GaAs buffer layer after thermal cleaning.