Daiji Yamashita

Effect of ion species for the surface activated bonding of GaAs wafers on the characteristics of the bonded interfaces

Effect of ion species for fast atom beam (FAB) irradiation of surface activated bonding (SAB) of GaAs wafers was investigated by current-voltage (I-V) measurements and transmission electron microscopy (TEM) observations. Ne,Ar,Kr and Xe gases were employed for FAB source. We confirm that it is possible to reduce the interfacial damage and improve the conductivity of GaAs/GaAs bonded interface by changing the ion species for FAB irradiation.

Surface activated Ge/GaAs wafer bonding for multi-junction solar cells

In this research, Ge/GaAs wafers were successfully bonded at room temperature by means of surface activated bonding (SAB) using fast atom beam (FAB) in high vacuum condition. Scanning acoustic microscope (SAM) observation shows wafers were bonded over almost the entire area. After 250°C annealing in N2 atomosphere, interfacial voids were reduced. Bonding strength of the interface archived 4.87MPa and most of fractured surface was GaAs. Transmission electron microscope (TEM) observation of bonding interface shows that Ge/GaAs were bonded at atomic level and amorphous layer with thickness of about 5nm was formed. Current-voltage (I-V) characteristic of directly bonded p-Ge/p-GaAs shows diode-like properties. The electrical resistance of bonded interface achieved 0.54Ωcm2 at 0V and 0.16Ωcm2 at 0.1V.

Highly efficient 470 W solar-to-hydrogen conversion system based on concentrator photovoltaic modules with dynamic control of operating point

Using an optimized system comprising concentrator photovoltaic (CPV) modules, electrolyzers, and DC/DC converters for dynamic control, we achieved a one-day solar-to-hydrogen (STH) efficiency of 18.78% — the highest value for a sub-kilowatt-scale photovoltaic (PV) system under outdoor operation. We clarified that incorporating high-efficiency CPV modules and DC/DC converters in PV-driven solar hydrogen production systems is essential for obtaining the highest STH efficiency. Furthermore, we noted that the overpotential of electrochemical (EC) cells is a bottleneck for the improvement in STH efficiency; therefore, future work should focus on reducing such an overpotential.

Development of GaAs//Si current-balanced dual junction solar cell integrated by surface-activated bonding

A thickness controlled dual junction GaAs//Si solar cell for current matching was fabricated and demonstrated. The optically thin GaAs top cell grown by metal-organic vapor phase epitaxy (MOVPE) was directly integrated on the Si bottom cell by surface-activated bonding (SAB) method. Owing to the optically thin (∼300 nm) GaAs top sub-cell, the operation of current-matched dual junction cell was observed under the standard 1 SUN illumination.

Admittance spectroscopy analysis on the interfacial defect levels in the surface-activated bonding of GaAs

Room-temperature surface-activated wafer bonding between bare III-V semiconductor surfaces has become a key technology for high-efficiency multi-junction solar cells, where the reduction of interfacial electrical resistance is of crucial importance for achieving highest efficiency. In the bonding process, surface cleaning using fast atom beam (FAB) of noble gas elements is vital for successful bonding but it damages the surface, resulting in numerous crystal defects at the bonded interface and increases electrical resistance. We here developed quantitative evaluation of such defects introduced by FAB treatment. The surface of n-GaAs was treated with the FAB using Ne, Ar and Kr, and Au Schottky electrodes were formed on the surfaces. Capacitance of a Schottky diode as a function of both probe frequency and DC bias allowed us to characterize both energy depth of the defects and their density profile along the physical depth from the GaAs surface. The results indicated that atoms with the smaller diameter generate high-density defects to the deeper region from the surface. When the defect density exceeding the doping level of GaAs spreads to wider than 5 nm, significant Schottky characteristics appears in the interfacial current-voltage characteristics, as suggested by simulations. Such a tendency was semi-quantitatively in good agreement with the measured current-voltage characteristics of the n-GaAs/n-GaAs bonded interfaces treated with the FAB of Ne, Ar and Kr, suggesting that the capacitance analysis of the FAB-treated surface provides us a direction for optimizing the surface- activated bonding process using FAB.

Directly bonded Ge/GaAs by surface activated bonding for high efficiency III–V multi-junction solar cells

Ge/GaAs wafers have been bonded by surface activated bonding. TEM observation of the bonded interface shows that amorphous layer with thickness of about 5 nm has been formed. I-V characteristic of directly bonded p-Ge/p-GaAs shows diode-like properties. The electrical resistance of the bonded interface has achieved about 0.16 Ωcm2 at 0.1 V.