Katsushi Fujii

The Fabrication of Vertical Light-Emitting Diodes Using Chemical Lift-Off Process

Vertical light-emitting diodes (LEDs) were successfully fabricated by a chemical lift-off process using a selectively etchable CrN buffer layer. The novel CrN metallic layer worked well as a buffer layer for growth of the GaN LED and was etched out clearly during selective chemical etching. The vertical LED by chemical lift-off showed very good current-voltage performance with low series resistance of 0.65 Omega and low operated voltage of 3.11 V at 350 mA. Also, this device could be operated at a much higher injection forward current (1118 mA at 3.70 V) by thermally conductive metal substrate which enabled the high current operation with excellent heat dissipation.

Hydrogen Gas Generation by Splitting Aqueous Water Using n-Type GaN Photoelectrode with Anodic Oxidation

Hydrogen gas generation from a counterelectrode was clearly observed for the first time using light-illuminated n-type GaN as a working photoelectrode in an electrolyte. The application of extra bias to a working electrode was required to obtain a sufficient volume of generated gas. The reactions at the GaN photoelectrode were both GaN decomposition and water oxidization, simultaneously.

Photoelectrochemical Properties of p-Type GaN in Comparison with n-Type GaN

We studied the photoelectrochemical properties of p-type GaN compared with those of n-type GaN. Band-edge potentials of p-type GaN were determined from the Mott–Schottky plot, and we clarified the potentials to be identical to those of n-type GaN. Photocurrent in p-type GaN shows its potential as an electrode for photoelectrolysis of water.

Photoelectrochemical reaction and H2 generation at zero bias optimized by carrier concentration of n-type GaN.

The authors studied the photoelectrochemical properties dependent on carrier concentration of n-type GaN. The photocurrent at zero bias became the maximum value at the carrier concentration of 1.7x10(17) cm-3. Using the sample optimized carrier concentration, the authors achieved H2 gas generation at a Pt counterelectrode without extra bias for the first time. The authors also discussed the mechanism of the dependence of photocurrent on the carrier concentration of GaN.

Lattice strain in bulk GaN epilayers grown on CrN/sapphire template

Microphotoluminescence spectroscopy is used to investigate local strain in GaN films grown on c-sapphire with CrN buffer, where the CrN buffer is partly etched. Biaxial compressive strain dominates GaN films grown on CrN buffer/c-plane sapphire. The emission energies of D0X, FXA, and FXA-1LO emission lines shift gradually from a high value to a low one, as the excitation laser beam scans from the unetched side of the sample to the etched side, while the emission intensities show only a slight change. No cracking occurs in the GaN film during etching except a change in bending of the detached part of the GaN film from convex to concave as determined by surface profiler. Both the lattice parameter and the energy position of the bound exciton emission peaks from a completely detached GaN are almost the same as those reported for strain-free GaN. The line width of the (0002) ω scan of a GaN film is narrowed from 352 to 331 arc sec through detaching presumably due to decrease in bending. Those properties sugge...

A 24.4% solar to hydrogen energy conversion efficiency by combining concentrator photovoltaic modules and electrochemical cells

The highest efficiency of 24.4% for the solar-to-hydrogen (STH) energy conversion was obtained in an outdoor field test by combining concentrator photovoltaic (CPV) modules with InGaP/GaAs/Ge three-junction cells and polymer-electrolyte electrochemical (EC) cells. The high efficiency was obtained by using the high-efficiency CPV modules (~31% under the present operation conditions) and the direct connection between the CPV modules and the EC cells with an almost optimized number of elements in series. The STH efficiency bottleneck was clarified to be the efficiency of the CPV modules, the over-potential of the EC cells, and matching of the operation point to the maximal-power point of the CPV modules.

Bias-Assisted H2 Gas Generation in HCl and KOH Solutions Using n-Type GaN Photoelectrode

We studied H 2 gas generation using a GaN photoelectrode in both HCl and KOH solutions. The photocurrent in KOH was larger than that in HCl under a small extra bias, and caused a more extensive H 2 generation at the beginning. GaN corrosion in HCl was less extended than that in KOH during photoelectrochemical reactions. The stability of the GaN photoelectrode in HCl was a result of H 2 O reduction and Cl - oxidation instead of water photoelectrolysis.

Novel Selective Area Growth of AIGaAs and AIAs with HCI Gas by MOVPE

Abstract Selective area growth of AlGaAs and AlAs was successfully demonstrated in the conventional MOVPE (TMG/TMA/ AsH 3 /H 2 ) system by introducing a small quantity of HCl gas during the epitaxial growth. Highly selective epitaxy (HSE) for Al x Ga 1−x As with high Al composition, even for AlAs, was achieved using wide passivation masks (∼ 200 μm). The effective role of HCl in the HSE of AlAs can be explained by an suppression effect of reaction between the Al containing species and mask material. The growth of AlGaAs with or without HCl gas was studied over the temperature range of 500–800°C on unmasked substrates. The new method called HSE has provided good surface morphology and excellent uniformity of thickness and Al composition. The electrical and optical characteristics of epitaxially grown layers by the HSE method were essentially the same as those of the conventionally grown layers.

Optical properties of dislocations in wurtzite ZnO single crystals introduced at elevated temperatures

Optical properties of wurtzite ZnO bulk single crystals in which an arbitrary number (typically 109–1010 cm−2) of fresh dislocations were introduced intentionally by the plastic deformation at elevated temperatures (923–1073 K) were examined. Deformed specimens showed excitonic light emission with photon energies of 3.100 and 3.345 eV, as well as their LO phonon replicas at 11 K. The light intensities increased with increasing dislocation density. The activation energy for a thermal quenching of the 3.100 or 3.345 eV emission band, which corresponds to the depth of the localized energy level associated with the emission band, was estimated to be 0.3±0.1 or 0.05±0.01 eV, respectively. The origin of the energy levels was proposed as point defect complexes involving dislocations. The introduction of the dislocations at the elevated temperatures above 923 K did not influence the intensities of the emission bands except the dislocation-related emission bands.

Band-Edge Energies and Photoelectrochemical Properties of n-Type Al x Ga1 − x N and In y Ga1 − y N Alloys

We studied photoelectrochemical properties of Al x Ga 1-x N for the first time, and compared with those of In y Ga 1-y N. The conduction band-edge energy of n-type Al x Ga 1-x N decreased with Al composition, however the valence band-edge energy did not significantly change. Saturated photocurrent obtained from dynamic photocurrent-voltage measurements under illumination also decreased with increasing Al composition. Greater Al composition also shifted the onset voltage to more negative direction. These phenomena can be explained by the changes of bandgap and band-edge energies with Al composition.