yielded cells with diffusion lengths in excess of 200 um, a length which exceeds the typical cell thickness quoted for use in silicon-based SPS [6]. In addition to silicon a wide variety of other materials can be purified by zone refining. Iron, titanium, aluminum oxide and titanium oxide are materials found in the lunar soil which have been purified using zone refining [19]. Based on the eutectic temperatures found in the phase diagrams of metals [20], the maximum impurity concentration over which zone refining will be effective can be determined, and is given below, Induction, electron beam, and focused radiant heating are the most common techniques currently used to melt materials. It may be possible to used solar concentrators as a heat source in space. While zone refining promises to be a means by which high purity silicon can be produced, the contamination of silicon by incidental contact with transition metal implements must also be avoided. Recent work has shown that common waferhandling equipment is capable of introducing substantial amounts of transition metals to silicon [21, 22]. Transition metals diffuse so fast in silicon that a few seconds at temperatures in excess of 500C can lead to contamination through out a 625 um thick slice [23]. Fortunately these metals can be gettered just as rapidly to inactive areas of
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