Yoshihiro Kokubun

Room Temperature Pulsed Operation of Nitride Based Multi-Quantum-Well Laser Diodes with Cleaved Facets on Conventional C-Face Sapphire Substrates

We demonstrate room temperature pulsed operation of nitride based multi-quantum-well (MQW) laser diodes with cleaved mirror facets grown on a conventional C-face sapphire substrate. Cleavage was performed along the direction of the sapphire substrate, and the resultant facet was analyzed using an atomic force microscope (AFM) and theoretical calculation. A single peak emisson, at a wavelength of 417.5 nm, with a full width at half-maximum of 0.15 nm, was obtained. The threshold current density of the laser was 50 kA/cm2 and a voltage for the threshold current was 20 V.

Effects of thermal treatment of low-temperature GaN buffer layers on the quality of subsequent GaN layers

The surface morphology and crystalline quality of GaN layers grown by metalorganic chemical vapor deposition on sapphire (0001) substrates were investigated for various thermal treatment conditions of low-temperature (LT)-grown GaN buffer layers. The surface morphology and crystalline quality of subsequently grown high-temperature (HT) GaN layers strongly depended on thermal effects during the temperature ramping process after LT growth of the buffer layers. We have found that the defect density and structure are affected by this temperature ramping process, and that the generation of growth pits is closely related to defects in the HT-GaN layers. High-quality HT-GaN layers with specular surface morphology were obtained with optimum growth and ramping conditions for the LT-GaN buffer layers. Furthermore, the role of thermal treatment during the temperature ramping process was identified, and mechanisms of nucleus formation, HT-growth initiation on the LT-GaN buffer layers, and defect formation are propose...

Deep ultraviolet photodiodes based on β-Ga2O3/SiC heterojunction

A deep Ultraviolet (UV) photodiode was fabricated using a heterojunction between β-Ga2O3 with a band gap of 4.9 eV, and 6H-SiC with a band gap of 3.02 eV, and investigated its UV sensitivity. A thin β-Ga2O3 layer (200 nm) was prepared on a p-type 6H-SiC substrate through gallium evaporation in oxygen plasma. The device showed good rectifying properties. Under reverse bias, the current increased linearly with increasing deep-UV light intensity. The responsivity of the photodiode was highest to deep-UV light below a wavelength of 260 nm. The photodiodes response time to deep-UV light was in the order of milliseconds.

Effects of well number, cavity length, and facet reflectivity on the reduction of threshold current of GaAs/AlGaAs multiquantum well lasers

The optimum design for reducing the threshold current of GaAs/AlGaAs multiquantum well lasers is determined experimentally. The lowest threshold current density is realized by using single and multiquantum wells at long and short cavity lengths, respectively. The threshold current has a minimum at the optimum cavity length: the minimum threshold current is smaller for a larger number of quantum wells, and the optimum cavity length is inversely proportional to the number of wells. Experiments are compared to the theory developed by P.W.A. McIlroy, et al. (ibid., vol.21, no.12, p.1958-63, 1985) and limiting performances of quantum well lasers with various numbers of wells are presented. The reduction of the threshold current by high reflectivity coatings is also demonstrated, and a threshold current as low as 1.86 mA at 15 degrees C is reported. >

Zn doping characteristics for InGaAlP grown by low‐pressure metalorganic chemical vapor deposition

Zn doping characteristics have been studied for In0.5(Ga1−xAlx)0.5P grown by low‐pressure metalorganic chemical vapor deposition, using dimethylzinc (DMZ) as a Zn dopant source. Hole concentration decreased with increasing Al composition x. For low DMZ introduction, the Zn incorporation efficiency decreased with increasing x. For high DMZ introduction, hole concentration saturation occurred as a result of the fall in Zn electrical activity and the Zn electrical activity also decreased with increasing x.

Highly reliable InGaP/InGaAlP visible light emitting inner stripe lasers with 667 nm lasing wavelength

To obtain highly reliable InGaP/InGaAlP inner stripe (IS) lasers, the relation between the maximum CW operation temperature and other laser characteristics has been determined. The Al composition of the cladding layer, the carrier concentration of the p-cladding layer, and the thicknesses of the active layer and cladding layer have been optimized. It was found that an Al composition of 0.7 was the most suitable for the cladding layer, and the optimized carrier concentration was 4*10/sup 17/ cm/sup -3/. A maximum temperature of 90 degrees C was obtained for a 0.1- mu m active layer thickness and a 0.6- mu m cladding layer thickness. This is the highest value for InGaP/InGaAlP IS lasers. In the case of a 0.06- mu m active layer thickness and a 0.8- mu m cladding layer thickness, a maximum temperature of 75 degrees C was obtained. IS lasers with facet coating have been operating stably for more than 8000 h at 40 degrees C and 3 mW and for more than 4000 h at 50 degrees C and 3 mW. >

Effects of substrate misorientation on doping characteristics and band gap energy for InGaAlP crystals grown by metalorganic chemical vapor deposition

Abstract Effects of substrate misorientation on Zn and Si doping characteristics and band gap energy have been investigated for InGaAlP grown by low-pressure metalorganic chemical vapor deposition. The Zn concentration and the net acceptor concentration for In 0.5 (Ga 0.3 Al 0.7 ) 0.5 P monotonically increased with increasing tilt angle, using (100) substrates tilted towards [011], but they at fir st decreased and next increased using (100) substrates tilted towards [011]. A similar behavior was found for the tilt angle dependence of the band gap energy of undoped In 0.5 Ga 0.5 P. The Si concentration and the net donor concentration of In 0.5 (Ga 0.3 Al 0.7 ) 0.5 P gradually increased with increasing the substrate tilt angle, independent of the tilt direction. Neither the Zn nor the Si electrical activity depended on the substrate orientation.

Preparation of ZnO Thin Films on Sapphire Substrates by Sol-Gel Method

ZnO films have been prepared on (0001) sapphire substrates by the sol-gel method from a 2-methoxyethanol solution of zinc acetate dihydrate stabilized by monoethanolamine. The structural properties of the films have been characterized by X-ray diffraction, compared with the films on silica glass substrates. The crystallinity was enhanced in the ZnO films on sapphire substrates by increasing heat-treatment temperature, whereas it was degraded in the films on silica glass substrates at heat-treatment temperatures above 700°C. Completely (0001)-oriented ZnO films were successfully prepared by heat-treatment at temperatures above 800°C on (0001) sapphire substrates annealed at 1000°C in air. The c-axis lattice constants of the ZnO films on sapphire substrates were larger than those of bulk ZnO and ZnO films on silica glass substrates.

Characterization of InGaN multiquantum well structures for blue semiconductor laser diodes

In0.15Ga0.85N/GaN and In0.15Ga0.85N/In0.05Ga0.95N multi quantum well (MQW) structures grown on (0001) sapphire substrates were investigated by high-resolution x-ray diffraction and cross-sectional transmission electron microscopy. The results show that ultrathin MQWs with fairly good crystallinity and precisely controlled clear parallel interfaces were grown. Laser diode structures with MQWs were also studied, and the results suggest that these superlattice structures retain their high quality even after being subjected to high temperatures during the subsequent growth of p-type GaN as the optical guiding layer, p-type GaAlN as the cladding layer, and p-type GaN as the contact layer.

Electrical Properties of CuI Thin Films

Hole concentration of CuI thin film decreased but Hall mobility increased with the rise of reaction temperature of copper films with iodine vapor. The heat-treatment of CuI film in vacuum brings out the decrease of hole concentration because of the escape of excess iodine from the film. Oxygen diffuses into the bulk of Cut film at high temperatures and raises the conductance of the film. The activation energy for diffusion was determined as 0.82 eV. Water vapor is chemisorbed on the surface of CuI and lowers the conductance at room temperature.