Kiyoshi Shirahata

Degradation of Gallium Arsenide Crystals by the Cold-Working Treatment (Abrasion)

The effect of the cold working treatment on the characteristics of GaAs crystals was studied. Abrasion and subsequent heat treatment of the crystals decrease the carrier concentrations and increase the breakdown voltages. After abrading and subsequent heat treatment, the increase in etch-pit density in the crystals was observed. It is supposed that the degradation of the crystals was related to the generation of dislocations during such treatments.

Amorphous SiGe: H for High Performance Solar Cells

A preliminary study has been carried out on electrical and optical properties of amorphous silicon-germanium mixed films (a-SiGe: H) deposited by a glow discharge plasma reaction. As a germanium content in the film increases, an optical absorption coefficient increases and an optical gap energy Egopt. decreases. Growth rate of a-SiGe: H films increases with the germanium content in the film and reaches to the value for pure a-Ge: H films about 3 times larger than that of a pure a-Si: H film. Film properties were examined by dark- and photoconductivity, MOS FET transistor characteristics, and photoluminescence. Photoconductivity and carrier mobilities decreases drastically when the mole ratio GeH4/(GeH4+SiH4) exceeds 25%. Solar cells of various structures using a-SiGe: H and a-Si: H films were fabricated and compared their photovoltaic properties with that of a simple p-i-n type a-Si: H solar cell. Tandem structure with a-Si: H p-i-n and a-SiGe: H p-i-n cells shows a broadened spectral response at long wave length region and an open circuit voltage of 1.13 V, about twice of a single cell.

The Ga–In–Sb Ternary Phase Diagram at Low Growth Temperature

The ternary phase diagram of the Ga–In–Sb system was investigated. The solidus isotherm at 400°C and liquidus isotherms at 400, 500, and 600°C were determined experimentally. The phase diagram of Ga–In–Sb system was calculated on the basis of various thermodynamic parameters. It is found that the calculation with the modified Delta Lattice Parameter (DLP) model best fits the experimental solidus isotherm at 400°C.

A 100 W Static Induction Transistor Operating at 1 GHz

A 1 GHz 100 W in CW Static Induction Transistor (SIT) has been realized. At 1 GHz CW operation, a power gain of 4 dB and a drain efficiency of 55% with an output power of 100 W were obtained. The saturation output power at 1 dB gain compression point was 100 W. To realize above 1 GHz 100 W SIT, two important techniques were improved. One was the fine patterning technique. Very fine patterns with 1 µm width and total length of 19.8 cm were fabricated in a 100 W SIT. Another was the power combination technique. The power was shared equally with 12 transistor blocks by internal matching networks.

Broadening of Spectral Response Width by a-Si/a-SiGe p-i-n type Solar Cells

We report the MIS type a-Si:H solar cells, plasma analysis using quadrupole mass analyzer and tandem type solar cell which improved spectral response in the long wavelength region. MIS type a-Si:H solar cells inserting TiOx layer as an insulator show the conversion efficiency of 4.4%. The improvement of the collection efficiency in the short wavelength region were accomplished by decrease in interface state density with addition of TiOx film. Plasma polymerized species, (SiHm+)2, increased with the rf power up to 40–50 W and decreased as a result of the decomposition to (SiHm+) species, but in the case of GeH4 reaction, (GeHm+)2 species decreased monotonously with rf power. Suitable condition of a-SiGe:H films deposition are related to the behavior of the polymerization and decomposition of SiH4 and GeH4 gases, and high quality films are obtained at relatively low rf power condition before (GeHm+)2 decomposition. Tandem type amorphous solar cell shows 3.2% coversion efficiency and the collection efficiency in the long wavelength region are clearly improved.

Performance of 1 kWp AIGaAs/GaAs Terrestrial Concentrator Solar Cell Array

A 1 kWp (electrical output power at 100 mw/cm2 incident power density) concentrating photovoltaic array with 180 square Fresnel lenses and AIGaAs/GaAs concentrator solar cells has been constructed. The AIGaAs/GaAs concentrator solar cells consist of p-AlGaAs/p-GaAs/n-GaAs with an area of 2×2cm2. The p-GaAs layer thickness is set at 3–5 µm so as to keep the spreading resistance low. The concentrator cells are passively cooled by natural convection. The maximum output power from the best concentrator cell is 9.1 W with the efficiency of 22.6% at about 125 suns. The array output power of 977 W (Isc=8.4 A, Voc= 184 V, FF=0.632) has been obtained at 125 suns.

GaxIn1-xSb Gunn Diodes

GaxIn1-x Sb single crystalline layers grown on GaSb and InSb substrates are prepared in a whole composition range by a liquid phase epitaxial technique. Ga0.82In0.18Sb Gunn diodes are fabricated and preliminary measurements of the oscillator performance in a pulsed operation are obtained. The highest values of maximum efficiency and peak output power are 9.2% at 2.6 GHz and 950 mW at 1.9 GHz, respectively.

A New Structure for High Efficiency and Humidity Resistant AlGaAs/GaAs Solar Cells

For p-AlGaAs/p-GaAs/n-GaAs heteroface solar cells, the results of humidity resistance test under the condition of 65°C and ≥90% relative humidity are described. Humidity resistant cells with high efficiency over 20% have been attained by (1) covering the side walls of the p-AlGaAs layer as well as the top surface with CVD Si3N4 antireflective coating, (2) covering CVD Si3N4 with relatively thick CVD SiO2, and (3) forming p-contact on p-GaAs.