High-symmetry triple junctions in polycrystalline silicon

Triple junctions in polycrystalline silicon for solar cell applications have been studied by TEM (transmission electron microscopy). The typical characteristic of these junctions is the existence of high-symmetry interfaces, i.e. twins of different order. Whenever the triple junction contains two such interfaces, the intersection axis is a CSL (coincidence site lattice) symmetry axis. In this case the mutual relation of the grain boundaries of the triple junctions is revealed by symmetry rules, since the boundaries are described by symmetry elements of the corresponding CSL which do not belong to the symmetry group of the parent lattice. Therefore, the different variant orientations of these CSL symmetry elements, implied by the symmetry of the lattice, are used for the study of triple junctions. A junction is geometrically characterized by the point-group-symmetry properties. The observations concern symmetrically different triple junctions of the same type of macroscopically stable grain boundaries and faceted and microfaceted interfaces. It is shown that in silicon the thermodynamically favored twins are combined by specific symmetry rules. The results are discussed in accordance with the possible growth mechanisms.