New insights in the study of zinc segregation in CdZnTe crystals

CdZnTe crystals are employed for the preparation of X-ray and gamma-rays detectors. Control of seeding and interface shape must be improved in order to increase the yield of good quality single crystalline crystals. Zinc is used to partially substitute cadmium in the cation site mainly to increase material resistivity. However, zinc segregation in the melt is one of the causes of material non-homogeneity. On the other hand, the segregation of zinc offers a unique chance for studying the growth interface shape after growth. Several CdZnTe crystals were grown by vertical Bridgman. A few crystals were grown in standard closed ampoules. In other cases, the melt was encapsulated by boron oxide during growth, and in others melt and vapour were separated by a quartz disk floating on melt. The zinc distribution and the growth interface shape were studied by photoluminescence mapping on slices cut parallel to the growth axis. Two aspects were mainly studied. The first one concerns with the possibility to obtain a convex interface shape during the whole growth. In fact it is known that in the case of CdZnTe crystals it is unlikely to obtain convex interface shape due to the low thermal conductivity of both the solid and the melt. Results show that this is true in the case of standard vertical Bridgman, but convex interface is actually achieved in the case the vapour phase is separated by the melt (by boron oxide or a quartz disk). In these cases melt flow is modified and so the temperature distribution at the growth interface. The second aspect concerns the zinc distribution in the first to freeze part of the crystal. This is influenced by the effect of supercooling that is unavoidable in the case of unseeded vertical Bridgman growth. However, experimental results could be explained only if solid state thermal migration of zinc and the effects of multi-nucleation sites are taken into account.