amorphous silicon (a-si) is the non-crystalline allotropic form of silicon. it can be deposited in thin films at low temperatures onto a variety of substrates. it offers some unique capabilities for a variety of electronics. ==description== silicon is a fourfold coordinated atom that is normally tetrahedrally bonded to four neighboring silicon atoms. in crystalline silicon (c-si) this tetrahedral structure continues over a large range, thus forming a well-ordered crystal lattice. in amorphous silicon this long range order is not present. rather, the atoms form a continuous random network. moreover, not all the atoms within amorphous silicon are fourfold coordinated. due to the disordered nature of the material some atoms have a dangling bond. physically, these dangling bonds represent defects in the continuous random network and may cause anomalous electrical behavior. likewise, the material can be passivated by hydrogen, which bonds to the dangling bonds and can reduce the dangling bond density by several orders of magnitude. hydrogenated amorphous silicon (a-si:h) has a sufficiently low amount of defects to be used within devices such as solar photovoltaic cells, particularly in the protocrystalline growth regime.r.w. collins, et al., "evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry", solar energy materials and solar cells, 78(1-4), pp. 143-180, 2003.s. y. myong, s. w. kwon, j. h. kwak, k. s. lim, j. m. pearce, and m. konagai, "good stability of protocrystalline silicon multilayer solar cells against light irradiation originating from vertically regular distribution of isolated nano-sized silicon grains", 4th world conference on photovoltaic energy conversion proceedings, p. 492, 2006. however, hydrogenation is unfortunately associated with light-induced degradation of the material, termed the staeblerâwronski effect.c. r. wronski, et al., "intrinsic and light induced gap states in a-si:h materials and solar cells - effects of microstructure", thin solid films, 451-452 pp. 470-475, 2004. == amorphous silicon and carbon == amorphous alloys of silicon and carbon (amorphous silicon carbide, also hydrogenated, a-si1-xcx:h) are an interesting variant. introduction of carbon atoms adds extra degrees of freedom for control of the properties of the material. the film could also be made transparent to visible light. increasing concentrations of carbon in the alloy widen the electronic gap between conduction and valence bands (also called "optical gap" and bandgap). this can potentially increase the light efficiency of solar cells made with amorphous silicon carbide layers. on the other hand, the electronic properties as a semiconductor (mainly electron mobility), 
Amorphous_solids Semiconductor_materials Silicon_forms Thin-film_cells