Johji Nishio

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...

p-type conduction in as-grown Mg-doped GaN grown by metalorganic chemical vapor deposition

We have clarified the effect of H2 and NH3 on the passivation of Mg acceptor in p-type GaN films grown by metalorganic chemical vapor deposition. It has been found that the small amount of H2 carrier gas strongly influences the electrical property of the Mg-doped GaN films. Low-resistivity p-type GaN has been obtained by H2-free growth without any post-treatments. Its acceptor concentration is as high as that obtained by conventional H2-rich growth with subsequent thermal annealing. It has also been clarified that hydrogen produced by NH3 dissociation does not prevent Mg from electrically activating in H2-free growth.

Magnetic field effect on residual impurity concentrations for LEC GaAs crystal growth

Abstract A horizontal magnetic field applied LEC (MLEC) apparatus, for mass-producing large diameter undoped LEC GaAs, has been newley developed. The detailed magnetic field effect on residual impurity concentrations has been studied. The carbon and boron concentrations have been found to be decreased by the MLEC technique, related to the applied magnetic field strength. Carbon and boron impurity concentrations in MLEC GaAs crystals decreased as the gallium molar ratio in molten GaAs decreased. The carbon concentration in MLEC GaAs decreased by decreasing the argon pressure in a high pressure chamber. The thermochemical mechanism for impurity incorporation into GaAs single crystals has been clarified through this study. The oxidation rate of free gallium and deoxidation rate of carbon oxide are shown to be the rate limiting reactions determining the impurity incorporation rate into GaAs crystals. The obtained results suggest that applying a magnetic field to molten GaAs suppresses the chemical reaction closely related to the impurity incorporation into resultant GaAs single crystals. The experimental results show good agreement with this predicted thermochemical mechanism.

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.

Stoichiometry of undoped LEC GaAs

Abstract Stoichiometry of undoped LEC GaAs has been studied by improved coulometric analysis. The solidus phase boundary shape of GaAs has been predicted, based on experimental results. The congruent point has been found to be on the slightly arsenic-rich side. The solidus boundary shape, extending on the gallium-rich side, has been suggested to be flat, while the other boundary, extending to the arsenic-rich side, has been suggested to be steep. The influence of temperature fluctuations, in the vicinity of the growing crystal-melt interface, on compositional variation along the crystals has been clarified. The magnetic field applied LEC (MLEC) GaAs crystals showed a clear relationship with a predicted solidus phase boundary shape, whereas conventional LEC crystals showed an ambiguous correlation.

Influence of melt preparation on residual impurity concentration in semi-insulating LEC GaAs

Residual impurity concentrations in LEC GaAs crystals have been found to be controllable depending on the time during which the melt is held in the molten state, the water content in the encapsulant and the addition of in-situ melt purification. Carbon, boron and oxygen concentrations varied widely from 1.0x1015, 5.9x1015 and 2.0x1015 to 3.9x1016, 1.5x1017 and 3.3x1016 cm−3, respectively. Through this study, it is suggested that the impurity incorporation and extraction mechanisms are due to reduction-oxidation (redox) reactions at the melt (GaAs)-encapsulant (B2O3) interface.

Ultralow-Loss SiC Floating Junction Schottky Barrier Diodes (Super-SBDs)

We have applied the floating junction (FJ) structure, which has been confirmed to be effective in reducing the on-resistance of Si power devices, to SiC FJ Schottky barrier diodes (SiC Super-SBDs). Optimization of the device parameters, which are derived by making improvements in the device simulator, and development of the fabrication process have enabled realization of Super-SBDs with a breakdown voltage of 2700 V and a specific on-resistance of 2.57 mOmega ldr cm2. These values correspond to the world record of 11.3 GW/cm2 for Baligas figure-of-merit (BFOM = 4Vbd 2/Ron-sp).

Ambient gas constituents and segregation of carbon and boron in LEC GaAs single crystals : the role of water in boric oxide encapsulants

Abstract The effect of water in boric oxide (B 2 O 3 ) encapsulants has been examined based on the results of carbon and boron concentrations and corresponding gas analysis inside the LEC puller during growth. It has been found that the hydrogen concentration in the puller atmosphere increases with increasing water content of the encapsulant while the carbon monoxide concentration remains constant. This suggests that hydrogen in the puller is correlated with both the carbon and boron contents of the crystal more significantly than carbon monoxide in the puller. The segregations of both carbon and boron show significant deviations from the normal freezing relationship. The degree of deviation was found to be dependent on the water content of the B 2 O 3 . This can be explained by the impurity incorporation to the melt and/or extraction from the melt. The analysis of the experimental results has led to an integrated description of the reduction-oxidation reactions at the B 2 O 3 /GaAs melt interface.

Gas phase contribution to carbon incorporation and extraction mechanisms for LEC GaAs

The H 2 and CO concentrations in the atmosphere inside an LEC puller have been found to vary widely according to several melt preparation condition, such as in-situ melt purification, water content of the B 2 O 3 encapsulant, the quartz cap attachment on the crucible, as well as with the progress in the pulling process. Results obtained from measurements of the carbon content of the crystals and from gas analysis in the puller have led to the conclusion that H 2 and CO in the atmosphere play significant roles in carbon incorporation and extraction for LEC GaAs single crystals.