Shigeo Kodama

SiGe bulk crystal as a lattice-matched substrate to GaAs for solar cell applications

SiGe bulk crystal fabricated by a multicomponent zone-melting method was used as a substrate for epitaxial growth of GaAs. Compared with conventional GaAs/Ge heterostructure, the lattice mismatch of GaAs/Si0.022Ge0.978 was confirmed to be reduced by a decrease of the peak separation of (400) x-ray diffraction from the epitaxial GaAs layer and the substrate. Furthermore, the linewidth of the rocking curve of GaAs on SiGe was found to be narrower than that of GaAs on Ge. These results show that SiGe is promising material as an alternative substrate to Ge for realization of exactly lattice-matched GaAs/SiGe solar cells.

Molecular beam epitaxy of GaAs on nearly lattice-matched SiGe substrates grown by the multicomponent zone-melting method

A detailed study of molecular beam epitaxy of GaAs on homemade SiGe substrates has been performed. It was found that the initial migration-enhanced epitaxy process with As prelayer is crucial to obtain high-quality GaAs. By (004) x-ray diffraction, the lattice mismatch between GaAs and SiGe was demonstrated to be reduced compared with the conventional GaAs/Ge heterostructure. Furthermore, narrower halfwidth of the rocking curve and stronger photoluminescence intensity were found for GaAs on SiGe. These results show that SiGe is a promising material as an alternative substrate to Ge to realize exact lattice matching to GaAs for solar cell applications.

GaAs solution growth experiment in microgravity

We ran a GaAs solution growth experiment aboard the German sounding rocket TEXUS as a preliminary study for a Spacelab D-2 experiment. The experiment involved a growth technique that avoids surface-tension induced convection which destroys diffusion controlled growth, even in microgravity. We noted a remarkable difference in surface morphology between the space-grown crystal and an earth-grown reference crystal. Many hillocks and craters were found on the microgravity-grown crystal, but the surface of the earth-grown crystal was relatively flat. We discuss the growth mechanism under microgravity as compared to that under a lg condition.

Compositional variation in AlGaAs crystals grown by LPE under microgravity and terrestrial conditions

Abstract We grew AlGaAs crystals on GaAs substrates from solutions aboard the Japanese free-flying satellite SFU, using low temperature range type isothermal heating furnaces. Six GaAs substrates forming a cube eliminate the free surface from the solution, which causes surface tension-driven convection. The samples were heated to 850°C to make Al–Ga–As solutions by dissolving the GaAs substrate surfaces into the Al–Ga solutions. Then the furnace temperature was reduced gradually to grow crystals from the supersaturated solution. The space experiments were carried out as planned, and AlGaAs crystals were grown under convectionless conditions. The surface morphology of the μ-g sample was much smoother than that of the 1-g sample. The growth thickness difference in the six substrates used in the μ-g experiments was much less than that of those used in the 1-g experiments. These facts prove that convectionless growth was achieved. The Al composition in the crystal decreases as the distance from the growth interface increases. This decrease in the μ-g sample is more gradual than that in the 1-g sample. Our calculational results using a theoretical model maintaining phase-equilibrium together with the constancy of diffusion flux at the growth interface at the same time showed the same tendency, while conventional diffusion-limited model results in opposite tendency. These results prove that our model for calculating compositional variation in ternary LPE layers correctly expresses diffusion-limited growth conditions.

GaAs solution growth experiment in microgravity using Get Away Special program

Sn-doped GaAs crystals were grown from Ga-As solution using a gradual cooling method aboard the space shuttle Columbia and on the ground. The sample, six GaAs substrates assembled to form a cube with Ga solvent placed in the resulting box, was firstly heated to dissolve the substrate surfaces into the solution and then cooled gradually to recrystallize onto the substrates. Type II striation-free growth was achieved in the microgravity experiment while striations occurred in reference samples grown on the ground. Flat interfaces were maintained during both dissolution and crystal growth in microgravity.

LT-SEM Studies of Superconducting Bi-Sr-Ca-Cu-O Film

Electron beam induced voltage images, which directly indicate the distribution of the critical current density, are observed in the superconducting strips of Y1Ba2Cu3O7−δ and Bi-Sr-Ca-Cu-O films by Low Temperature Scanning Electron Microscopy (LT-SEM). Band patterns appear in several places within the strip of Y1B2C3O7−δ whereas the vague homogeneous voltage contrast extends throughout the strip of Bi-Sr-Ca-Cu-O. Voltage images of 90° misoriented grain-boundaries yield dark contrast which is considered to arise from semiconductor-like behavior of the grain boundary resistivity.