Synchrotron white beam x-ray topography characterization of structural defects in microgravity and ground-based CdZnTe crystals

In a microgravity environment, gravity-dependent effects such as buoyancy, convection and hydrostatic pressure are minimized, providing an ideal environment for investigating diffusion-controlled, nonwetting crystal growth processes. To evaluate the influence of microgravity on the resultant crystal quality, Synchrotron white beam x-ray topography is applied to characterize defect structures in both flight and ground-based CdZnTe single crystals. Transmission x-ray topographs recorded from one flight sample revealed regions of very low dislocation density with individual dislocations clearly resolved. Dislocations of very high density arrayed in a mosaic pattern were observed in all ground-base samples grown under identical growth conditions except for the gravity conditions. This observation indicates that the flight samples have much higher structural perfection than the ground-based samples. On the other hand, studies of defect configurations in a different flight sample revealed that structural defects and distributions can be strongly influenced by rapid cooling, thermal gradients, and constrained growth. Large thermal stresses induced by rapid cooling can be multiplied by wall contact leading to the formation of extensive slip bands and small angle tilt boundaries starting at the crystal periphery and propagating into the interior of the sample. It is concluded that an optimization of post solidification cooling rate is important to minimize the occurrence of slip.