THERMAL CONVECTIVE EFFECTS ON PHYSICAL VAPOR TRANSPORT GROWTH OF MERCUROUS CHLORIDE (HG2CL2) CRYSTALS FOR AXISYMMETRICAL 2D CYLINDRICAL ENCLOSURE

The effects of thermal convection are investigated for the physical va por transport (PVT) crystal growth process of Hg2Cl2 crystals. These e ffects may lead to increased defect densities and reduced optical homo geneity of the resultant crystals. A two-dimensional axisymmetrical mo del has been developed and applied to simulate the fluid flow, tempera ture, and mass concentration distribution in closed ampoules during PV T crystal growth. The numerical parametric studies show that the growt h rate increases with the one-quarter power of the Rayleigh number, Ra -0.25, for 0 less than or equal to Delta T less than or equal to 30 K (0 less than or equal to Ra less than or equal to 2.62 x 10(3)), and w ith Ra-1.5 in the temperature range of 30 less than or equal to Delta T less than or equal to 50 K (2.62 x 10(3) less than or equal to Ra le ss than or equal to 4.73 x 10(3)). The effect of thermal convection is shown to be negligible for gravitational accelerations less than the order of 10(-1)g(0), for low-aspect-ratio enclosures. The growth rate of Hg2Cl2 decreases exponentially with an increase in the partial pres sure of an impurity B, as well as with a decrease in the ampoule aspec t ratio AR (radius to transport length) for 0.05 less than or equal to AR less than or equal to 0.20.