Hironori Taguchi

Fabrication of GaAs/Si Tandem Solar Cell by Epitaxial Lift-Off Technique

A GaAs solar cell was successfully transplanted from a GaAs substrate to a Si substrate without degrading the conversion efficiency. The conversion efficiency of the GaAs solar cell bonded to the Si substrate is almost the same as that of the cell grown on a GaAs substrate and is much superior to that of the cell grown on a Si substrate by heteroepitaxy. The GaAs/Si tandem solar cell with the conversion efficiency of 19.4% (top cell: 18.2%, bottom cell: 1.2%) has been demonstrated.

Raman Spectroscopic and X-Ray Diffraction Study of Sulfur under High Pressure

Raman spectrum and X-ray powder pattern of orthorhombic sulfur have been examined at pressures up to about 10 GPa. The Raman spectrum shows reversible phase transition at 5.2 GPa, while the X-ray diffraction pattern shows no sign of phase change up to 8.3 GPa. These contradictory results are explained as follows. A high-pressure phase exists above 5.2 GPa. In this pressure region, the metastable orthorhombic phase can also exist. The argon ion laser (λ=514.5 nm) used for Raman measurement induces a structural transformation from the orthorhombic phase to the high-pressure phase. The Raman spectrum suggests that the high-pressure phase is composed of chain molecules. In addition, evidence for interference between inter- and intramolecular bondings has been observed in this phase.

High-Radiation Resistance of GaAs Solar Cell on Si Substrate Following 1 MeV Electron Irradiation

The current-voltage characteristics of GaAs solar cells grown on GaAs and Si substrates after 1 MeV electron irradiation are compared under darkness and light illumination. It is found that the radiation resistance of the GaAs solar cell on the Si substrate is higher than that on the GaAs substrate. The high radiation tolerance of the GaAs solar cell on the Si substrate is due to the slow generation rate of arsenic vacancies compared with those of the GaAs solar cell on the GaAs substrate caused by electron irradiation.

Electrical Characteristics of GaAs Bonded to Si Using SeS2 Technique

We have developed a new technique for bonding GaAs on Si susbtrates; called surface modification by chemical treatment. The treatment of selenium sulphide (SeS2) with GaAs produces a reconstructed surface which helps in strong fusion between GaAs and Si at lower temperatures and without weights. The current–voltage (I–V) characteristics of both n-GaAs/n-Si and p-GaAs/p-Si were measured at room temperature. The I–V curve did not show a rectifying behaviour when GaAs was bonded to Si with SeS2 because of the formation of a high-resistance layer at the interface. The characteristics were greatly improved by small additions of Sn to SeS2 during the bonding process. Sn forms localized islands in addition to the Ga–Se and Se–S, and acts as shunt resistance between GaAs and Si.

Effects of Homoepitaxial GaAs/GaAs and Heteroepitaxial GaAs/Si Solar Cells after 1 MeV Electron Irradiation for Space Application

The high-energy electron irradiation effect of GaAs solar cells on Si substrate has been described and the results were compared with GaAs solar cell on GaAs substrates. The solar cells were irradiated with 1 MeV electron in the range of fluences from 1 ×1013 cm-2 to 1 ×1016 cm-2. The solar cell parameters such as short-circuit current (Isc), open circuit-voltage (Voc) and conversion efficiency were studied under dark and AM 0 conditions. The degradation rate of Voc and Pmax of GaAs/GaAs solar cell is faster than GaAs/Si solar cell after the fluences higher than 1015 cm-2. The slow degradation of GaAs/Si solar cell has been attributed to slow generation of As vacancy.

Low-energy proton irradiation effects on GaAs/Si solar cell

In this article we describe the radiation effects of low-energy proton (100 keV, fluences of 3 ×1010, 1 ×1011, 1 ×1012 and 3 ×1012 cm-2) on GaAs solar cells on Si substrates (GaAs/Si) and compare the results with GaAs solar cells on GaAs substrates (GaAs/GaAs). Solar cell parameters such as short-circuit current (Isc), open circuit-voltage (Voc) and conversion efficiency are studied under the AM0 condition. In low proton fluences, the degradation rate of Voc for GaAs/Si is lower than that of the GaAs/GaAs solar cell, due to the insensitivity of the saturation current. At higher proton fluences the solar cell properties are equally degraded for both types of cells.

Effects of 1 MeV Electron Irradiation on the Schottky Diode Characteristics of n-GaAs/Si

We have reported the irradiation of high-energy electrons (1 MeV, fluences of 1×1013, 1×1014, 1×1015 and 1×1016 cm-2) on the Schottky diode characteristics of GaAs layers on Si and GaAs substrates grown by metal organic chemical vapor deposition (MOCVD). After the irradiation, barrier height decreases in Schottky diodes on the GaAs substrate but there is no considerable change in diodes on the Si substrate. The new trap level increased after irradiation in GaAs/GaAs compared with that in the case of GaAs/Si. These experimental results suggest that GaAs devices on a Si substrate have a higher radiation resistance than those on a GaAs substrate after 1-MeV electron irradiation.