Structure of amorphous semiconductors: Reverse Monte Carlo studies on a-C, a-Si, and a-Ge

As an attempt to establish a unified treatise of the structure of amorphous germanium, silicon, and (tetrahedral) carbon, a series of reverse Monte Carlo simulations were carried out for all of the three materials. A number of constraints have been applied during the calculations in order to test how many (and how much) different assumptions can be consistent with given experimental data sets. Solely on the basis of diffraction data, the results of the unconstrained calculations seem to be the models of the above systems that are the most consistent with the experiment, but these models are not satisfactory from the electronic structure point of view. With slightly worse agreement with the data, it was possible to generate dominant tetrahedrally coordinated models (with coordination numbers equal to exactly 4) in each case. The most extreme model proposed is the one where no fourfold coordinated atom could be found. However, the proportion of tetrahedral angles in this latter case is still high. It is therefore concluded that the most characteristic common feature of these amorphous semiconductors is not the fourfold coordination but the always apparent presence of closely tetrahedral bond angles.