Gravity application to anisotropic semiconductor materials: From high- to microgravity conditions

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 Te80Si20 alloy s grown under different gravity levels by a modified Bridgman method using a Te crystal seed. We examined the influence of gravity from the microgravi ty level up to 10g (g is earth's gravity) on the distribution of electrical ly active intrinsic defects and dopants. The conductivity and the Hall effe ct, and their variations along the crystal length, were measured over a wid e temperature range, 77-300 K. The Se distribution in the Te-Se alloy was d etermined by X-ray microanalysis. The hole concentration profile in the spa ce-grown crystal of Te corresponded to an almost uniform distribution of im purities. The hole mobility was less than that in the Ig sample due to hole scattering on lattice imperfections produced by crystal growth. In the gro wn crystals and ingots, the hole mobility increased towards the ends of the samples. The higher the gravity, the better the single crystals, the large r the fraction with a constant hole concentration in Te crystals, and the s moother the Se distribution along Te-Se alloys. These effects can be attrib uted 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 addi tional convection when the melt is heated from above. Mixing of the melt ne ar the solid-liquid interface is intensified under high gravity due to this unusual convection. The situation for the Te-Se alloy is qualitatively com parable to the growth process in Te taking into account some decrease of th e melt viscosity with addition of a small amount of Se. The data on solidif ication of the glassy alloy Te80Si40 in space and at normal gravity on eart h indicate that microgravity suppresses cluster nucleation during the solid ification and promotes ideal glass formation. (C) 2001 Published by Elsevie r Science Ltd.