S.D. Cobb

Bridgman Growth of Detached GeSi Crystals

Ge1xSix (0oxo0:12) has been grown by the vertical Bridgman technique using adjustments in the applied temperature profile to control the pressure difference between the bottom and top of the melt. Using this technique, a pressure difference is created by decreasing the temperature in the gas volume above the melt while the sample is molten but prior to growth. A maximum pressure difference approximately equal to the hydrostatic pressure of the molten sample can thus be obtained. Several GeSi crystals were grown in pyrolitic boron nitride ampoules. When a pressure difference was applied, samples were reproducibly grown mostly detached. For comparison, samples were also grown in a configuration in which gas could flow freely between the gap below the melt and the volume above the melt and no pressure difference could be established. These samples were initially attached. Existence of detachment was determined both by measuring the surface roughness of the samples with a profilometer and by observations of the sample surfaces with optical and electron microscopy. r 2002 Elsevier Science B.V. All rights reserved.

Stability of detached-grown germanium single crystals

Several undoped and Ga-doped germanium single crystals were grown by the vertical Bridgman method using a translating furnace and a multizone furnace, respectively. In both cases it was possible to exert influence on the contact between the growing crystal and the wall of the container. This allows growing nearly completely detached crystals as well as attached crystals in pyrolytic boron nitride containers. In detached-grown crystals the gap thickness between the container wall and the crystal, determined by profilometer measurements, varies from 5 to 50 μm. Observed fluctuations of the detachment gap up to 8 μm along the crystal axis in one of the crystals can be explained by a kind of stiction of the melt/crucible interface, which causes a variation of the meniscus shape.

Characterization of directionally solidified Hg1-xZnxSe semiconducting alloys

Hg(1-x)Zn(x)Se alloys with composition between x = 0.08 and 0.115 were synthesized from elemental constituents and were resolidified using a Bridgman-Stockbarger growth technique. By performing precision mass-density measurements on selected wafers cut perpendicular to the growth axis, it was shown that the axial compositional profiles fit a numerical solution to the 1D diffusion equation which takes into account the variation of interface velocity with time. Infrared transmission-edge measurements performed on selected transverse slices from each ingot showed that the relative radial variations in composition decreased with decreasing growth rate. Van der Pauw measurements on selected wafers showed that the 10 exp 18/cu cm electron concentration, typical of as-grown crystals, could be reduced by approximately an order of magnitude by annealing in Se vapor.

Crystal growth of selected II-VI semiconducting alloys by directional solidification in microgravity

A Hg 0.84 Zn 0.16 Te alloy crystal was back-melted and partially solidified during the first United States Microgravity Laboratory (USML-1) mission in the Marshall Space Flight Centers Crystal Growth Furnace. The experiment was inadvertently terminated at about 30% of planned completion. Nonetheless, it was successfully demonstrated that HgZnTe alloy ingots partially grown and quenched on the ground can be back-melted and regrown in space under nearly steady state growth conditions. An identical “ground-truth” experiment was performed following the mission. Preliminary results are presented for both crystals, as well as for a series of other crystals grown prior to the mission for the purposes of optimizing in-flight growth conditions.