Add time:09/26/2019         Source:sciencedirect.com

We have carried out experiments to derive a quantitative understanding of the physical vapor transport (PVT) process and to identify convective effects on the crystal growth process. The experimental growth velocity was several orders of magnitude lower than the theoretically predicted value. The effusion holes were used to disturb the impurity boundary layers. We observed a change of 18% (not an order of magnitude) in growth velocities. The Arrhenius behavior of growth rate with temperature was used to derive the sticking coefficient. Experimental result 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 bifurcation had occured with a resulting change in transport behavior.Growth velocity measurements for the PVT process as a function of orientation of the g-vector were also made. The experimental results clearly showed that the growth velocity varied with g-vector for a particular temperature profile. The effects of convection on crystal quality were studied 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 and strong circulating cells appeared, and also below 10-3 g, where the flow was purely diffusive and no circulating cells were predicted. Therefore 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.

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