emphasis in semiconductor device technology. The economic growth of a space-based economy centred around the SPS may be limited by the fabrication and maintenance of solar panels. Previous studies [3, 6] have concluded that the large amount of material required for the fabrications of SPS dictates the use of extraterrestrial materials. The primary components of the lunar soil are known to be A1203/10.3-27.2%, CaO/9.7-15.8%, FeO/5.2-11.3%, MgO/5.8-11.3%, SiO2/39.9-48.1%, and TiO2/0.5-9.4% [12, 13], To reduce the complexity and weight of the processing equipment and to become compatible with a zero-gravity environment, open liquids should not be required. The process should also be able to function in vacuum. The human element is one of the most costly in any space programme. To minimize the number of people required to produce the solar arrays, a continuous, automated process which is easily maintained should be employed. Purification of Materials The first step in the production of silicon devices is the purification of the silicon and other materials used in its production (processing gases, liquids and metallization and insulating materials). The two primary types of impurities which affect silicon solar cells are dopant atoms (boron, aluminum, phosphorous) and transition metals which can reduce the minority carrier lifetime and hence reduce the efficiency of the cells. Phinney et al. [14] proposed that slagging and vacuum distillation be used initially to separate silicon from the other components of the lunar soil. Based on similar systems these two techniques could reduce the levels of impurities down to 0.3-2.0%. The aluminothermic reduction of raw silicon dioxide has recently been utilized by Dietl and Holm [15] to obtain high purity silicon for the fabrication of solar cells. The material produced in this manner has a very low boron content (<lppm) which is very significant because the large segregation coefficient of boron in silicon makes it difficult to obtain high resistivity silicon using a non-chlorine-based purification technique. Since aluminum is readily available from lunar soil this purification technique could be adaptable to space processing. Zone refining is another means of obtaining high purity silicon [16]. As a material freezes, it usually rejects impurities into the remaining melt. The ratio of the impurity concentration incorporated into the solid, to the impurity concentration in the melt, is termed the segregation coefficient. The more impurity that is rejected, the smaller the segregation coefficient. Table I gives the segregation coefficients of several impurities in silicon [17, 18], This phenomenon is used in the zone-refining process [19] to purify silicon as well as many other materials. In this process, the silicon ingot is passed slowly through a succession of heating zones, each of which causes local melting of the silicon for a short distance along the axis of the ingot. Commercial units with up to 12 zones are not uncommon [19], As the ingot moves through each zone, the corresponding molten section, in effect, sweeps the entire length of the crystal. The impurities are collected at the end of the ingot, and this section is usually removed. Table I shows that all elements except for boron, phosphorus and arsenic will easily be reduced to very low levels using this technique. The importance of the aluminothermic reduction technique to reduce levels of boron can be seen here, since zone refining does very little to remove boron. Boron is present in the lunar soil in concentrations of 2.0 to 39.0 ppm. Utilizing a 12 zone, zone refiner aluminothermically reduced quartz could yield silicon with resistivities as high as 5-6 ohms-cm. Such silicon resistivities have
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