the device when mechanical damage [23, 24], laser damage [23, 25], ion damage [26], or a heavily doped phosphorus layer or polysilicon layer is applied [27, 28], The formation of oxygen precipitates has also been used to getter metals [29]. This method can only be used when sufficient oxygen is present in the silicon. The oxygen usually comes from the wall of the quartz crucible used to hold the molten silicon during crystal growth. In the gettering process metal atoms become trapped at dislocations and grain boundaries by the reduction in lattice strain associated with such a reaction. Such gettering results in the improvement of the minority carrier lifetime [25], An external gettering step can be located at any step in the process provided a heat treatment is employed to allow the metal atoms to diffuse to the gettering site. Crystal Growth The most common method currently used for the growth of silicon crystal is the Czochralski method, in which a crystal is pulled from an open melt. The individual wafers from which devices are made are obtained by slicing the crystal, then lapping and polishing the slices. This method is not suitable for space applications because it employs an open melt, liquids are required for slicing, lapping and polishing and it is very wasteful since a great deal of the raw silicon is lost during the slicing and lapping operations. Material experiments performed during the Skylab missions revealed that the crystal surface facets produced in zero-gravity appear to be atomically smooth [30]. Carruthers [31] has pointed out that it may be possible to perform direct semiconductor processing on such a flat surface without further lapping and polishing. Almost 30 years of research and development has gone into the fabrication of solar cells on ribbon crystals which stems from even earlier work dating back to the 1930s [32] on the growth of thin monocrystalline plates. Only a few techniques satisfy the space fabrication criteria used in this paper. They include the float zone technique which has already been demonstrated for silicon on the space shuttle [33], the sintering of polycrystalline rods, the pulling or extruding of crystal from an enclosed melt, and combinations of the above. As has been pointed out in an earlier study, Stepanov's method appears to be the best candidate for space applications. In Stepanov's method the molten silicon is held in a piston-like chamber, as shown in Fig. 1 [11], the silicon is forced out from a specially shaped orifice to produce a ribbon of silicon. It is likely that the walls of the chamber will be made of quartz which will introduce oxygen into the silicon and allow for internal gettering later on in the process. In addition to the growth of free silicon crystal, films of silicon can be epitaxially grown on a variety of substrates. Most techniques employ silicon chlorides to deposit the films, but a few methods such as molecular beam epitaxy, ion cluster beam epitaxy, and ion beam epitaxy do not require liquids or gases. In the past few years several large conferences have been held in silicon molecular beam epitaxy [34, 35], and so it is expected that technological advancements will continue in this field. A variety of methods which apply silicon films on substrates made of steel, quartz, graphite and polycrystalline silicon have been developed [36-40]. These methods may find application in space although the efficiencies of solar cells fabricated on such substrates have been relatively low [36, 40],
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