GRAVITY APPLICATION TO ANISOTROPIC SEMICONDUCTOR MATERIALS: FROM HIGH- TO MICROGRAVITY CONDITIONS

Abstract

Growth of single crystals under different gravity conditions is of special importance for the development of growth technology for semiconductors. We have studied the properties of tellurium crystals, Te–Se and Te 80Si 20 alloys grown under different gravity levels by a modified Bridgman method using a Te crystal seed. We examined the influence of gravity from the microgravity level up to 10 g ( g is earth's gravity) on the distribution of electrically active intrinsic defects and dopants. The conductivity and the Hall effect, and their variations along the crystal length, were measured over a wide temperature range, 77– 300 K . The Se distribution in the Te–Se alloy was determined by X-ray microanalysis. The hole concentration profile in the space-grown crystal of Te corresponded to an almost uniform distribution of impurities. The hole mobility was less than that in the 1 g sample due to hole scattering on lattice imperfections produced by crystal growth. In the grown crystals and ingots, the hole mobility increased towards the ends of the samples. The higher the gravity, the better the single crystals, the larger the fraction with a constant hole concentration in Te crystals, and the smoother the Se distribution along Te–Se alloys. These effects can be attributed to the anomaly of the liquid Te density near the melting point due to the formation of spiral chains of Te atoms. This anomaly gives rise to additional convection when the melt is heated from above. Mixing of the melt near the solid–liquid interface is intensified under high gravity due to this unusual convection. The situation for the Te–Se alloy is qualitatively comparable to the growth process in Te taking into account some decrease of the melt viscosity with addition of a small amount of Se. The data on solidification of the glassy alloy Te 80Si 20 in space and at normal gravity on earth indicate that microgravity suppresses cluster nucleation during the solidification and promotes ideal glass formation.