Evidence from simulations for orientational medium range order in fluctuation- electron-microscopy observations of a-Si

Si and a-Ge, both the structure factor and V(k) consistently show two broad peaks at k1 = 3.2 nm -1 and k2 = 5.5 nm . The paracrystalline model (pc-Si) of a-Si consisting of nano-sized, strained crystalline grains embedded in a continuous random network (CRN) matrix is consistent with FEM data. The magnitude of V(k) varies with the paracrystallite size and volume fraction, but the first peak is always higher than the second as observed in hydrogenated a-Si:H and pure a-Si sputtered at low substrate temperatures. However, a-Si films deposited at substrate temperature T > 250o C, or under ionbombardment during growth, and for a-Ge films grown by evaporation, the second peak is higher. No explanation existed for this observation [2].