Real space information from fluctuation electron microscopy: applications to amorphous silicon

Ideal models of complex materials must satisfy all available information about the system. Generally, this information consists of experimental data, information implicit to sophisticated interatomic interactions and potentially other a priori information. By jointly imposing first-principles or tightbinding information in conjunction with experimental data, we have developed a method: experimentally constrained molecular relaxation (ECMR) that uses all of the information available. We apply the method to model medium range order in amorphous silicon using fluctuation electron microscopy (FEM) data as experimental information. The paracrystalline model of medium range order is examined, and a new model based on voids in amorphous silicon is proposed. Our work suggests that films of amorphous silicon showing medium range order (in FEM experiments) can be accurately represented by a continuous random network model with inhomogeneities consisting of ordered grains and voids dispersed in the network. (Some figures in this article are in colour only in the electronic version)