Akimichi Hojo

Band discontinuity for GaAs/AlGaAs heterojunction determined by C‐V profiling technique

The band discontinuity has been determined for a GaAs/AlGaAs heterojunction prepared by molecular beam epitaxy. The conduction band‐discontinuity ΔEc and the valence‐band discontinuity ΔEv were independently obtained by the C‐V profiling technique, taking into account a correction for the interface charge density. The simulation was employed to confirm the reliability of the obtained band discontinuity. The ΔEc dependence on both the Al composition of the AlGaAs layer and the heterojunction structure (AlGaAs on GaAs, or GaAs on AlGaAs) was examined. We found that ΔEc and ΔEv were determined to be 62 and 38% of the band‐gap discontinuity ΔEg, being independent of the structure.

Oxygen precipitation in Czochralski‐grown silicon wafers during hydrogen annealing

In studying the effect of the ramping process on oxygen precipitation in Czochralski‐grown silicon wafers in hydrogen annealing, we have found that the oxygen precipitate density in the bulk region depends on the ramping‐up rate at temperatures between 900 and 1200 °C. Few oxygen defects are observed when the ramping‐up rate is 30 °C/min or more. Decreasing the ramping‐up rate exponentially increases the oxygen precipitate density. The nucleation for oxygen precipitates can therefore be controlled by adjusting the ramping‐up rate during hydrogen annealing.

Atomic steps on a silicon (001) surface tilted toward an arbitrary direction

In this letter, we show that the misorientation of a vicinal (001) surface annealed in hydrogen is accommodated by a pair of near‐Sa steps which are evolved in two perpendicular {110} independently and coordinately. We demonstrate that the deviation of surface orientation in a tiny area can be readily evaluated by inspecting the local variation of the near‐Sa step pair. We finally indicate that the atomic steps on a surface annealed in argon exhibit in contrast, no conformity with the misorientation towards an azimuth other than {110}.

Lattice Bending in LEC-Grown Semi-Insulating GaAs Wafers

Lattice bending in LEC-grown semi-insulating GaAs crystal wafers was investigated by X-ray diffraction. Various distributions in the lattice bending were observed in wafers from different boules. Prominent bends, with radii of curvature of about 20 m, were observed at places where the relevant topographs showed the presence of fine cellular dislocation networks. The apparent wafer bending was also examined optically and was found to be caused mainly by back surface damage.

Minority Carrier Diffusion Length in Si Ribbon Solar Cells

Minority carrier diffusion length Lp in Si ribbon solar cells was investigated using a scanning electron microscope. It was found that Lp was lowered at irregular crystal boundaries. The low Lp region around an irregular boundary was only 10~20 µm in width, and occupied less than 5 percent of the total solar cell area in most cases. The value of Lp in regions without crystal boundaries scattered appreciably. Average Lp value had no dependence on carrier density whose range was 1016~1017 cm-3, which means that photocurrent is almost independent of carrier density. As a result, it is expected that ribbon crystals with higher carrier densities are preferable as base material for solar cells to obtain higher efficiency.

500 Gates GaAs Gate Array

High speed GaAs DCFL gate array consisting of 500 3-INPUT NOR gates, in which 2000 FETs are integrated, has been successfully fabricated by a Pt buried gate planar E/D process technology. Eleven different 15-stage ring oscillators were made on this gate array to investigate the dependence of gate performance on various loading conditions. Typical propagation delay times per gate were 0.16 ns and 0.69 ns under unloaded and loaded (fan-out=3, interconnection length=3 mm, cross-over=20) conditions at a power dissipation of 0.50 mW for 1.7 µm gate devices.

A novel configuration of atomic steps observed on vicinal silicon (100) surface annealed in a hydrogen atmosphere

Abstract The vicinal silicon (100) surface with a misorientation of 0.05° towards [011] and [001] has been investigated with atomic force microscopy after annealing at 1200°C in argon and hydrogen. For the samples tilted towards [011], while the surfaces annealed in argon are featured with the alternate smooth and rough steps of monolayer height (S steps), which unambiguously correlate themselves with the so-called (Sa + Sb) structure, the hydrogen annealing, in contrast, generates a configuration of orthogonal S steps. Through an analysis of the surface tilted towards [001], we show that a miscut toward an arbitrary direction can be accommodated by two identical S steps developed independently along two perpendicular {011} axes if the crystal is annealed in hydrogen. On the contrary, a surface annealed in argon shows no conformability with the misorientation towards an axis other than {011}. It is believed that the formation of the orthogonal S steps is a chemically driven process resulting from the rela...

Evaluation of Si Ribbon Crystals for Solar Cells

Defect structures, electrical properties of the Si ribbon crystals were investigated. Performances of Si ribbon solar cells were also measured. It was found that twin boundaries did not degrade the electrical properties. Whereas irregular boundaries deteriorated the minority carrier lifetime appreciably. The solar cells fabricated from the ribbon crystals did not show inferior properties in spite of the existence of the defects in the crystals when the surface contact was kept off the SiC particles embedded in the ribbon surface.