Takeyoshi Sugaya

Photocatalytic generation of hydrogen by core-shell WO 3 /BiVO 4 nanorods with ultimate water splitting efficiency

Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO3/BiVO4+CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO4 with more conductive WO3 nanorods in a form of core-shell heterojunction, where the BiVO4 absorber layer is thinner than the carrier diffusion length while it’s optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72 mA cm−2 under 1 sun illumination at 1.23 VRHE that corresponds to ~90% of the theoretically possible value for BiVO4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56 mA cm−2 that corresponds to the solar to hydrogen generation efficiency of 8.1%.

Low-Temperature Cleaning of GaAs Substrate by Atomic Hydrogen Irradiation

Low-temperature cleaning of GaAs substrate by atomic hydrogen irradiation has been demonstrated. Atomic hydrogen was provided by dissociation of hydrogen gas, which was carried out in a simple cracking cell with a 1500°C tungsten filament. Auger electron spectroscopy showed that carbon was removed at 200°C and oxygen was removed at 400°C by 30-min atomic hydrogen irradiation. The surface cleaning of GaAs was confirmed by the change of RHEED pattern from halo to streak after the hydrogen irradiation.

Self-starting mode-locked femtosecond forsterite laser with a semiconductor saturable-absorber mirror.

We report on a self-starting mode-locked femtosecond Cr:forsterite laser pumped by a diode-pumped Nd:YVO(4) laser. The mode locking is initiated by a semiconductor saturable-absorber mirror (SESAM). We also present the measured group delay of the forsterite crystal and the SESAM.

Femtosecond Cr:forsterite laser with mode locking initiated by a quantum-well saturable absorber

We studied a mode-locked Cr:forsterite laser pumped by a diode pumped Nd:YVO/sub 4/ laser. Both the Kerr lens mode locking and the semiconductor saturable absorber initiated mode locking have been demonstrated. Using our measured dispersion data of the forsterite crystal, together with our dispersion compensation technique, we obtained 20-fs pulses for the pure Kerr lens mode locking and 36-fs pulses for semiconductor saturable absorber initiated mode locking, respectively.

Ultra-high stacks of InGaAs/GaAs quantum dots for high efficiency solar cells

We report ultra-high stacked InGaAs/GaAs quantum dot (QD) solar cells fabricated by the intermittent deposition of In0.4Ga0.6As under an As2 source using molecular beam epitaxy. We obtain a 400-stack In0.4Ga0.6As QD structure without using a strain balancing technique, in which the total number of QDs reaches 2 × 1013 cm−2. Photoluminescence and cross-sectional scanning transmission electron microscope measurements indicate that the In0.4Ga0.6As QD structure exhibits no degradation in crystal quality, no dislocations and no crystal defects even after the stacking of 400 QD layers. The external quantum efficiency and the short-circuit current density of multi-stacked In0.4Ga0.6As QD solar cells increase as the number of stacked layers is increased to 150. Such ultra-high stacks and good cell performance have not been reported for QD solar cells using other material systems. The performance of the ultra-high stacked QD solar cells indicates that InGaAs QDs are suitable for use in high efficiency solar cells requiring thick QD layers for sufficient light absorption.

Highly stacked and well-aligned In0.4Ga0.6As quantum dot solar cells with In0.2Ga0.8As cap layer

We report In0.4Ga0.6As quantum dot (QD) solar cells with In0.2Ga0.8As cap layers, which extends the photoabsorption spectra toward a wavelength longer than those of In0.4Ga0.6As QD solar cells without cap layers. Well-aligned 50-stack In0.4Ga0.6As QD structures with In0.2Ga0.8As cap layers can be grown without using a strain balancing technique. The photoluminescence wavelength of ten-stack In0.4Ga0.6As QDs with an In0.2Ga0.8As cap layer becomes longer, as a result of the reduced strain in the QDs achieved by using the cap layer. The cell characteristics of multistacked In0.4Ga0.6As QD solar cells are improved by employing In0.2Ga0.8As cap layers.

Low Temperature Surface Cleaning of InP by Irradiation of Atomic Hydrogen

The effects of atomic hydrogen irradiation on the surface cleaning of InP substrates have been investigated by reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). Carbon and oxygen-free clean surfaces of InP have been produced for a temperature range of as low as about 350°C for 30 min irradiation of atomic hydrogen. Under this condition, the RHEED patterns have revealed phosphorus-stabilized (2×4) reconstructed surfaces while indium-stabilized (4×2) reconstruction patterns have been observed upon cleaning by using the conventional arsenic irradiation at 530°C.

Improved optical properties of InAs quantum dots grown with an As2 source using molecular beam epitaxy

We demonstrate the effects of using an As2 source to fabricate self-organized InAs∕GaAs quantum dot (QD) structures. QDs grown with an As2 source have narrower photoluminescence (PL) linewidths and higher PL intensities than those grown with an As4 source at high growth rates. The density of QDs grown with an As2 source is smaller, and the dot size larger than those of QDs grown with an As4 source. The coalescence of QDs is reduced under an As2 source, resulting in improved optical properties. These results are thought to result from the difference in the surface migration of In atoms and the surface structures under As2 and As4 sources.

Highest Density 1.3 µm InAs Quantum Dots Covered with Gradient Composition InGaAs Strain Reduced Layer Grown with an As2 Source Using Molecular Beam Epitaxy

We propose a GaAs-based 1.3 µm InAs quantum dot (QD) structure for optical devices that uses dimeric arsenic (As2) and a highly strained GaInAs cover layer. The characteristics of 1.3 µm InAs QDs that employ As2 are different from those of QDs that use As4. Our optimum structure exhibits the first room temperature emission of over 1.3 µm with a linewidth of 22 meV and a high density of over 1 ×1011 cm-2 using only a cover layer. We were also able to achieve a very high density of 3.3 ×1011 cm-2 and a full width at half mazimum of 23 meV for a triple-stack structure within the critical thickness. This result is promising as regards achieving an optical device with QDs of over 1.3 µm on a GaAs substrate for use in fiber communications.

Self-starting mode-locked Cr 4+ :YAG laser with a low-loss broadband semiconductor saturable-absorber mirror

We report a self-starting mode-locked Cr(4+):YAG laser with a low-loss broadband metal reflector-based semiconductor saturable-absorber mirror. The minimum pulse width was 44 fs at a wavelength of 1520 nm.