Tatsuya Tanabe

High-radiation-resistant InGaP, InGaAsP, and InGaAs solar cells for multijuction solar cells

The radiation response of 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs solar cells was measured and analyzed in comparison with those of InP and GaAs. The degradation of the minority-carrier diffusion length was estimated from the spectral response data. The damage coefficient KL for the 3 MeV proton-irradiated InGaP, InGaAsP and InGaAs was also determined. The radiation resistance increases with an increase in the fraction of In–P bonds in InGaP, InGaAsP and InGaAs. Differences in the radiation resistance of InGaP, InGaAs and InGaAs materials are discussed. Minority-carrier injection under forward bias is found to cause partial recovery of the degradation on irradiated InGaP and InGaAsP cells.

Metalorganic Vapor Phase Epitaxial Growth of GaNAs Using Tertiarybutylarsine (TBA) and Dimethylhydrazine (DMHy).

GaNAs alloys were successfully grown on GaAs substrates by low-pressure metalorganic vapor phase epitaxy (MOVPE) with all organometallic sources of triethylgallium (TEG), tertiarybutylarsine (TBA), and dimethylhydrazine (DMHy). For nitrogen, the desorption coefficient of 30 kcal/mol was derived from the nitrogen incorporation dependence on growth temperature. Since the nitrogen concentration above 3% was easily achieved by our growth technique, the combination of TBA-DMHy as V precursors is a candidate for the growth of other III-V alloys containing nitrogen. We observed a decrease in PL intensity with enhancing nitrogen incorporation into solids. In order to recover from degradation of optical properties, rapid thermal annealing (RTA) was demonstrated and found to be effective. Therefore MOVPE using TBA-DMHy combined with postgrowth annealing is expected to obtain GaNAs alloys with high nitrogen concentration as well as excellent optical properties.

GaInP/GaAs and mechanically stacked GaInAs solar cells grown by MOCVD using TBAs and TBP as V-precursors

We have applied metallorganic chemical vapor deposition (MOCVD) using less toxic group V-precursors to the fabrication of the monolithic dual-junction GaInP/GaAs and mechanically-stacked GaAs/GaInAs cells, targeting for the super-high-efficiency triple-junction GaInP/GaAs/GaInAs solar cells. The dual-junction GaInP/GaAs cell grown on an n-type GaAs substrate, which is suitable for higher optical transmittance to the bottom cell, showed a conversion efficiency of 25.9% at AM 1.5, 1-sun. Combined with an efficiency of 5.1% for GaInAs bottom cell grown on an InP substrate under the mechanically stacked GaAs top cell, it is possible to attain an efficiency of over 30% by the all organometallic-source MOCVD method.

Si-doping in GaAs grown by metalorganic vapor phase epitaxy using tertiarybutylarsine and tetraethylsilane

Abstract We have developed a much safer metalorganic chemical vapor phase epitaxy (MOVPE) process with all liquid phase organic precursors without using hazardous gases including doping sources. In this study, the silicon doping in GaAs grown using tetraethylsilane with tertiarybutylarsine has been investigated. Excellent controllability and reproducibility of n-type doping level from 1 × 10 17 to 6 × 10 18 cm -3 with low carbon incorporation are confirmed. The carrier concentration increases with increasing growth temperature and decreases with increasing V/III ratio. In addition, we applied the MOVPE process with all liquid sources for the fabrication of the GaAs solar cell, and obtained good cell performance of over the conversion efficiency of 20%.

Over 27% efficiency GaAs/InGaAs mechanically stacked solar cell

Abstract We have applied an InGaAs solar cell (band gap = 0.75 eV) to the bottom cell of the super-high-efficiency tandem solar cell aiming an over 35% conversion efficiency. The InGaAs cell which is lattice-matched to the InP substrate showed the efficiency of 5.5% under the GaAs substrate with low carrier concentration. Combining with the GaAs cell by means of a mechanically stacking technique, we obtained an efficiency of 28.8% at air mass (AM) 1.5, 1-sun. This result suggests the possibility of the cells with the efficiency of over 35% with combining a GalnP/GaAs monolithic tandem cell and the InGaAs cell (or InGaAsP cell).

Raman characterization of lattice-matched GaInAsN layers grown on GaAs (001) substrates

First systematic Raman scattering characterization for the nearly lattice-matched GaInAsN layers has been discussed to investigate the local structural changes as the In and the N compositions increase. It has been found that the formation of the spontaneously ordered clusters in the GaInNAs layers strongly depends on the In incorporation, and the degradation mechanism of the crystal quality of GaInAsN with the high In and N compositions may be completely different from the GaAsN systems.

High-Breakdown-Voltage GaN Vertical Schottky Barrier Diodes with Field Plate Structure

Gallium nitride (GaN) vertical Schottky barrier diodes (SBDs) with a SiNx field plate (FP) structure on low-dislocation-density GaN substrates have been designed and fabricated. We have successfully achieved the SBD breakdown voltage (Vb) of 680V with the FP structure, in contrast to that of 400V without the FP structure. There was no difference in the forward current-voltage characteristics with a specific on-resistance (Ron) of 1.1mcm2. The figure of merit V2b/Ron of the SBD with the FP structure was 420MWcm-2. The FP structure and the high quality drift layers grown on the GaN substrates with low dislocation densities have greatly contributed to the obtained results.

5×5 cm2 GaAs and GaInAs Solar Cells with High Conversion Efficiency

We fabricated GaAs and GaInAs solar cells with a large cell area of 5×5 cm2 as the first step toward their practical use. The 5×5 cm2 cells showed high conversion efficiencies equivalent to those of 1×1 cm2 cells owing to both a good uniformity of epitaxial film characteristics in a 3-inch wafer and the reduction of the series resistance of grid electrodes. Moreover, a higher conversion efficiency of 26.0% under 1-sun air-mass 1.5 global conditions was achieved for the 5×5 cm2 GaAs cell by optimizing the antireflection coating. This is the highest efficiency among GaAs cells ever reported. The 5×5 cm2 GaInAs cell showed an efficiency of 4.2% as the bottom cell under a GaAs top cell. Therefore, a conversion efficiency of more than 30% can be expected for the 5×5 cm2 GaAs/GaInAs mechanically stacked tandem cell.

Mechanically Stacked GaAs/GaInAsP Dual-Junction Solar Cell with High Conversion Efficiency of More than 31%

We successfully fabricated high-performance GaAs and GaInAsP (band gap = 0.95 eV) single-junction solar cells with an area of 1×1 cm2. The conversion efficiencies of the GaAs and GaInAsP cells were 25.0 and 19.3%, respectively, under 1-sun air-mass 1.5 global (AM1.5G) conditions. The GaInAsP cell as the bottom cell under the mechanically stacked GaAs top cell also showed a high efficiency of 6.1%, and a total efficiency of 31.1% was achieved for the GaAs/GaInAsP tandem cell. This is the highest efficiency obtained under 1-sun AM1.5G conditions among the dual-junction cells ever reported.

GaAs solar cell with GaInP window grown by all metalorganic source MOVPE

Tertiarybutylarsine (tBAs) and tertiarybutylphosphine (tBP) are expected as safe alternatives to conventional hazardous hydrides, AsH/sub 3/ and PH/sub 3/. We have applied these safer metalorganic precursors to the GaAs solar cell growth and have obtained a cell efficiency of 23.3% (under AM1.5G/spl middot/100 mW/cm/sup 2/, V/sub OC/=1.025 V, J/sub SC/=26.9 mA/cm/sup 2/, FF=0.843, the conversion efficiency for active area is 25.1%). The cell consists of the GaAs single junction and the lattice-matched GaInP window, and was grown using only metalorganic precursors. The external quantum efficiency of the GaInP window cell in the shorter wavelength range is improved considerably, compared to our previous GaAs cell with AlGaAs window grown with all metalorganic source MOVPE. This improvement is consistent with the results of the interface recombination velocity estimation for GaInP/GaAs interface and AlGaAs/GaAs interface. These results demonstrate that all metalorganic source MOVPE for the GaAs solar cell is a safe and promising alternative to the conventional MOVPE using hydride sources.