EFFECT OF CONSTRAINED GROWTH ON DEFECT STRUCTURES IN MICROGRAVITY GROWN ON CDZNTE BOULES

In a microgravity environment obtainable in an orbiting space shuttle, it is possible to virtually eliminate gravity related effects such as buoyancy driven convection and hydrostatic forces thus providing an i deal environment for diffusion-controlled, containerless crystal growt h processes. Under such conditions, it is possible to investigate the effects of gravity independent growth parameters on crystal growth. St udies of CdZnTe boules grown on space shuttle mission USML-1 revealed that regions of the boules grown with wall contact were associated wit h a higher defect density than regions growth with partial or no wall contact. Defect densities in certain regions grown without wall contac t were as low as 5 x 10(2)/cm(2) to 1.2 x 10(3)/cm(2). More detailed s tudies on the effects of wall contact were sought in the USML-2 missio n. Two CdZnTe boules (GCRC-1 and GCRC-2) were grown by the seeded Brid gman-Stockbarger method. Boule GCRC-1 was grown under constrained cond itions to force full wall contact while boule GCRC-2 had a tapered geo metry designed to minimize wall contact. Defect distributions in the b oules were investigated by synchrotron white beam x-ray topography. Th e sample GCRC-1 was characterized by the presence of large inhomogeneo us strains, numerous grains and twins, all of which are caused by effe cts related to wall contact. On the other hand, a part of the boule GC RC-2 that grew free from wall contact revealed minimum surface strains , the absence of twins and a very high structural uniformity. Results clearly verify that ampoule wall contact plays an important role in de termining the incidence of crystal imperfections.