Numerical and experimental investigation of CdZnTe growth by the boron oxide encapsulated vertical Bridgman method

Abstract

Cadmium zinc telluride crystals of 4.8 cm in diameter grown by the boron oxide encapsulated vertical Bridgman technique exhibit good structural quality with large grains and low dislocation density. However, the ingots contain Te inclusions, which are trapped at the growth interface during the crystallization process. Global modeling of the furnace was applied in order to investigate the temperature gradients and the evolution of the growth interface shape in this system. Transient computations, which include the crucible movement, show that the crystal-melt interface is concave toward the melt in the conical part of the ampoule, then becomes convex during the growth in the cylindrical part of the ampoule. Concave shapes of the interface and very homogeneous temperature distribution at the beginning of the solidification process promote the growth of a polycrystalline material. A novel hypothesis explaining the mechanism of grain formation at the ampoule tip was formulated. Our calculations show that the anomalous Zn segregation is due to the poor solute mixing at the beginning of the solidification process. The modeling of the standard growth process reveals very low vertical temperature gradients in the crystal (2-4 K/cm). Such low temperature gradients are not favorable to eliminate the Te inclusions in the as grown crystal. A numerical model was employed to analyze the temperature field influence on the migration/size reduction of Te inclusions.