Bridgman Growth and Characterization of Bulk Single Crystals of Ga 1-x In x Sb for Thermophotovoltaic Applications
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rnI-V compound semiconductors are important materials for optoelectronic applications, in particular for thermophotovoltaic generation of electricity [1]. By alloying different compounds, the bandgap and the lattice parameter of the resulting combination can be adjusted. Tailoring these properties allows for the fabrication of a wider array of devices than is possible using only
the simple binary materials. One of the many possible combinations currently receiving significant attention is the GaSb-InSb system. Combinations of these compounds can have lattice constants in the range of 0.6096 to 0.6479 nm and bandgaps between 0.17 and 0.726 eV [2]. Ty'pical deposition techniques, such as liquid phase epitaxy, are expensive for fabricating these materials. The ability to grow large, bulk single crystals of these materials is expected to reduce their cost. For this to be effective, however, the composition of the alloy material must be uniform over a significant portion of the resulting boule. This is difficult to achieve for most compound semiconductor materials because of the wide and non-uniform liquidus-solidus separation of the pseudo-binary phase diagram. Additionally, as with the growth of crystals of most materials, solute transport and temperature variation within the melt also have important effects on the composition and quality of the final material produced. Previous work to produce GaSb-InSb alloys via different techniques and conditions is reported in [3] - [7] and [10]. In this paper, results of some experiments conducted to prepare bulk alloys of GalxInxSb (x-0.03 and 0.2) and to explore the effects of growth parameters on the composition of the resulting alloy are presented. A Bridgman technique was used to solidify the melt; single crystal and polycrystalline boules were produced. Seeds of the same diameter as the crucible were used to grow single crystals of the alloy to reduce the likelihood of twinning and other defects. 45 Mat. Res. Soc. Symp. Proc. Vol. 484 0 1998 Materials Research Society
EXPERIMENT Preparation of Binary Starting Compounds The binary compounds, GaSb and InSb, were prepared in the laboratory. The starting materials were elements with 6N purity. In both cases, the Sb-III atom ratio was -1.001 to allow for volatilization of Sb during processing. A fused silica crucible, with an inner diameter of 19 mm and an overall length of approximately 125 mm, was used to contain the mixture. The crucible was cleaned with HF, followed by rinses of deionized water and isopropanol, then dried with filtered argon. The starting materials, without any etching or cleaning, were placed into the cleaned crucible, then inserted into the furnace. A two-zone transparent furnace (Trans-Temp, Chelsea, MA) was used for all experiments (Fig-
ure 1). The crucible was held in a graphite support (Figure 2a). A quartz rod, prepared in the same way as the crucible, was inserted from the top for use in mixing the melt. After the crucible was inserted, the furnace was evacuated to -100 millitorr, then flushed
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