PHYSICAL VAPOR TRANSPORT GROWTH OF MERCUROUS CHLORIDE CRYSTALS

We have carried out experiments to derive a quantitative understanding of the physical vapor transport (PVT) process and to identify convect ive effects on the crystal growth process. The experimental growth vel ocity was several orders of magnitude lower than the theoretically pre dicted value. The effusion holes were used to disturb the impurity bou ndary layers. We observed a change of 18% (not an order of magnitude) in growth velocities. The Arrhenius behavior of growth rate with tempe rature was used to derive the sticking coefficient. Experimental resul ts on growth velocity as the aspect ratio was varied showed that with increasing convection, the growth rate increased up to a certain value and then dropped to a constant value. This indicated that a bifurcati on had occured with a resulting change in transport behavior. Growth v elocity measurements for the PVT process as a function of orientation of the g-vector were also made. The experimental results clearly showe d that the growth velocity varied with g-vector for a particular tempe rature profile. The effects of convection on crystal quality were stud ied by varying the thermal conditions (source and crystal temperatures ) which affects thermal convection during PVT. The results showed that crystals grown at low Rayleight numbers had better homogeneity. While no microgravity experiments were conducted, computation of mass flux for the horizontal orientation for various gravitational levels showed two distinct regions; above 10(-3) g where the flow was convective an d strong circulating cells appeared, and also below 10(-3) g, where th e flow was purely diffusive and no circulating cells were predicted. T herefore it is postulated that for the conditions of growth considered , space flight experiments with acceleration less than 10(-3) g could yield crystals grown under diffusive transport.