Mass transport studies in the growth of cdTe crystals by multi-tube physical vapour transport

Abstract

This thesis covers some of the development of a novel vapour growth system for the production of large, high quality crystals of cadmium telluride, focusing in particular on ampoule development, flow calibration, and mass transport studies. The Multi Tube Physical Vapour Transport system is an evolution of the Markov design, the unique feature being a U-Tube ampoule configuration which allows direct viewing of the crystal surface, and in-situ, non invasive vapour pressure monitoring. The system incorporates a capillary flow restrictor between source material and the growing crystal which allows the mass transport rate to be controlled almost independently of crystal temperature. The growth tube section of the ampoule has been modified to allow hardened platinum wires of well defined diameter to be inserted between the crystal holder (a conical plug with polished upper surface which fits into a ground glass socket in the growth tube) and ampoule wall, allowing the aimulus gap to be defined by the thickness of the wire. A shielding system for the vapour pressure monitoring system has been developed and tested which prevents cadmium or cadmium telluride (which has escaped from the growth ampoule via non perfect ground glass joints) from condensing on the sapphire windows of the outer vacuum jacket. The capillary flow restrictor has been calibrated for viscous, molecular and mixed flow regimes using several test gasses. This information has been used to model the mass transport of CdTe in 3 growth runs of the MTPVT system by integrating viscous and molecular flow rates with respect to time. The mass of CdTe transported from source to crystal during each run selected for study was in good agreement with the mass transport of CdTe predicted by the flow model.