Ground Based Studies for the Space Processing of Lead-Tin-Telluride
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GROUND BASED STUDIES FOR THE SPACE PROCESSING OF LEAD-TIN-TELLURIDE
ROGER K. CROUCH, A. L. FRIPP, W. J. DEBNAM, I. 0. CLARK NASA Langley Research Center, Hampton, Virginia, 23665, USA and F. M. CARLSON KRAR, Inc., Bagdad Road, Potsdam, New York, 13676, USA
ABSTRACT This paper summarizes the results of a series of studies dealing with crystal growth from the melt and in particular, the growth of the narrow bandgap semiconductor PbSnTe by the Bridgman technique. Theoretical calculations have shown that for low Prandtl number fluids such as semiconductors, convection has no effect on the temperature field in the melt but has a significant effect on the compositional field. They also show that convective flow will exist in a Bridgman growth system on Earth, even in the "thermally stable" configuration. Measurements of the specific heat have been made up to temperatures of 7220 C. The thermal diffusivity has been measured up to 10100 C which includes values for the melt. The interdiffusion coefficients for PbTe and SnTe in the melt have been determined. A technique has been developed and tested which provides a vacuum tight quartz ampoule with electrical feedthroughs for interface demarcation studies during the Bridgman growth. Analysis of crystals grown has indicated that the PbSnTe system is solutally unstable and the distribution of SnTe in the crystal, after an anomalous first to freeze region, follows that predicted by Pfann for convection controlled growth. Studies of the amount of supercooling in PbSnTe indicates that this may explain an anomalous composition distribution in the first to freeze region of an unseeded growth. INTRODUCTION As the applications for remote sensing systems becomes more widespread, the requirements for more sophisticated sensing systems become a major concern for NASA. One of the areas of interest is the development of infrared detectors and tunable diode lasers which operate in the spectral range from 8 to 30 micrometers. One of the materials with the most promise for making large area arrays of detectors which operate in this region is the narrow bandgap semiconductor lead-tin-telluride,, PbxSn(l-x)Te. By changing the ratio of the Pb/Sn, the bandgap of this material can be varied to cover the spectral region from 5 to >30 micrometers and lasers have been built in the material which operate in the region from 8 to 14 micrometers.[l) However, there are presently materials limitations which prevent the development of large area detector arrays and which limit the efficiency and lifetime of the lasers. Consequently, a program at NASA's Langley Research Center has concentrated on understanding the materials parameters of PbSnTe which limit the performance of the detector and laser devices. Part of this program has dealt with improving the understanding of the processes which affect the quality of crystals grown by various techniques. A study of the effects of convection on the
612 quality of crystals grown by the Bridgman technique has been initiated which will lead to utilization
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