Analysis of transport processes during the growth of single crystals by the vertical Bridgman method.

摘要

An indepth analysis of heat, mass, and momentum transport during the growth of single crystals of oxides and semiconductors is carried out. Computationally efficient techniques based on the finite element method are devised to compute enclosure radiation heat transfer, and some implementations based on parallel computing are delineated.;The effects of internal radiative transfer in both, the crystal and melt is investigated during oxide growth in a vertical Bridgman system. Radiation from the melt has great effect on temperature distribution and interface shapes. However, the occurrence of any supercooling in the melt is relatively rare and absent in typical oxide systems. Melt transparency over small distances, as in YAG, actually flattens the interface. For a system with an almost transparent melt and an opaque crystal, the beginning of growth is characterized by very low temperature gradients in the melt and highly curved interfaces. The temperature gradients and interface shapes are sensitive to the length of radiating melt column.;A transient model is developed to explore the effects of heat transfer and segregation phenomena during the growth of cadmium zinc telluride (CdZnTe). The high latent heat release coupled with the poor thermal conductivity of CdZnTe causes highly deflected and concave interface shapes. This results in large radial temperature gradients that drive strong flows near the interface. Significant differences exist between isoconcentration curves of zinc and interface shapes during growth due to large radial segregation and solid-state diffusion in the crystal. The changing geometry of the ampoule results in a greatly stretched transient growth period, and steady state growth does not occur due to the large time scales that characterize diffusion processes. During the growth of the cadmium telluride binary system, buildup of excess cadmium at the interface and decreasing temperature gradients result in increased chance for interface instability. Interrupting growth alleviates this phenomenon though causing solute striations in the crystal, while slower growth rate effectively sidesteps this disadvantage. Ampoule cone angle and support materials greatly influence heat transfer during growth initiation. Composite support materials with conductive core and insulating outer sheath significantly decrease interface concavity by promoting axial heat transfer.