NASA Logo

NTRS

NTRS - NASA Technical Reports Server

Back to Results
Growth of Compound Semiconductors in a Low Gravity Environment: Microgravity Growth of PbSnTeThe growth of the alloy compound semiconductor lead tin telluride (PbSnTe) was chosen for a microgravity flight experiment in the Advanced Automated Directional Solidification Furnace (AADSF), on the United States Microgravity Payload-3 (USNP-3) in February, 1996 and on USNW- 4 in November, 1997. The objective of these experiments was to determine the effect of the reduction in convection, during the growth process, brought about by the microgravity environment. The properties of devices made from PbSnTe, an alloy of PbTe and SnTe, are dependent on the ratio of the elemental components in the starting crystal. Compositional uniformity in the crystal is only obtained if there is no significant mixing in the liquid during growth. The technological importance of PbSnTe lies in its band gap versus composition diagram which has a zero energy crossing at approximately 40% SnTe. This facilitates the construction of long wavelength (greater than 6 gm) infrared detectors and lasers. The properties and utilization of PbSnTe are the subject of other papers. 1,2 PbSnTe is also interesting from a purely scientific point of view. It is, potentially, both solutally and thermally unstable due to the temperature and density gradients present during growth. Density gradients, through thermal expansion, are imposed in directional solidification because temperature gradients are required to extract heat. Solutal gradients occur in directional solidification of alloys due to segregation at the interface. Usually the gradients vary with both experiment design and inherent materials properties. In a simplified one dimensional analysis with the growth axis parallel to the gravity vector, only one of the two instabilities work at a time. During growth, the temperature in the liquid increases ahead of the interface. Therefore the density, due to thermal expansion, is decreasing in that direction. However, the phase diagram shows that the lighter SnTe is preferentially rejected at the interface. This causes the liquid density to increase with distance away from the interface.
Document ID
20000010695
Acquisition Source
Langley Research Center
Document Type
Conference Paper
Authors
Fripp, A. L.
(NASA Langley Research Center Hampton, VA United States)
Debnam, W. J.
(NASA Langley Research Center Hampton, VA United States)
Rosch, W. R.
(National Academy of Sciences - National Research Council Hampton, VA United States)
Baker, N. R.
(Lockheed Martin Engineering and Sciences Co. Hampton, VA United States)
Narayanan, R.
(Florida Univ. Gainesville, FL United States)
Date Acquired
August 19, 2013
Publication Date
September 1, 1999
Publication Information
Publication: Fourth United States Microgravity Payload: One Year Report
Subject Category
Solid-State Physics
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.
Document Inquiry

Available Downloads

There are no available downloads for this record.
No Preview Available