FLOATING-ZONE MELTING OF CDTE

To produce superior crystals of CdTe, floating-zone melting in space has been proposed. Techniques required for floating-zone melting of CdTe have been developed. CdTe has been successfully float zoned using square feed rods and resistance heating. The maximum zone length in a 3.5 mm wide square CdTe rod on Earth was about 5 mm. Evaporation of the CdTe melt was controlled by adding excess cadmium to the growth ampoule combined with heating of the ampoule wall. The temperature of the ampoule wall was the primary factor in determining the deposit-free time. Slow rotation of the growth ampoule was necessary to achieve a complete, symmetric molten zone. The maximum zone travel rate, considered from the heat transfer standpoint only, was experimentally determined to be 6 mm h-1. Most of the resultant cylindrical rods were single crystals with twins. Rods grown at a cooling rate of 40-degrees-C h-2 yielded a much lower etch pit density (EPD), ranging from 2.2 x 10(3) to 8.2 x 10(2) cm-2, than those grown by quenching, which had EPD values ranging from 5.4 x 10(4) to 8.7 x 10(4) cm-2. The pits formed in these rods were randomly distributed, which was different from the cellular structure observed in CdTe produced by the vertical Bridgman technique. A lower EPD was obtained at the periphery of our rods. An approximate radiation model was developed to predict the heater power requirement. For CdTe molten zones, the power requirement predicted by the model differed by 5% to 30% from that measured in the experiments. The model predicts that the power requirement for the zone heater to maintain a static zone varies with the rod radius, R1 in the range from R1(1.5)to R1(1.9). This relationship is affected by various factors, such as the ampoule wall temperature and the gap between the rod and the zone heater.