Anomalous conductivity behavior of microcrystalline silicon

In academic and industrial laboratories worldwide, large area microcrystalline silicon is under development for utilization in high efficiency thin film silicon tandem modules. Microcrystalline silicon (nowadays also named nanocrystalline silicon) is a complicated material: it has a multi-phase structure consisting of small nanocrystallites embedded in a matrix of a-Si:H. At fixed growth conditions the structure grows inhomogeneously in the growth direction and the evolution of the nanocrystalline fraction (X_c) is dependent on the nature of the substrate. Formation of mid gap states, in the form of silicon dangling bonds, is hard to control during growth and at increasingly high X_c, the probability of incorporating oxygen and other contaminants increases. If extended voids are present, also after deposition the material can easily become contaminated with moisture or oxygen. Due to the complex growth mechanism and unstable nature of the material in air the characterization and modeling of materials with high X_c with respect to conductivity is ambiguous. Often an electrical conductivity activation energy (E_a) measurement is used to determine the electronic quality, which should show “intrinsic” behavior. However, with increasing crystallinity, E_a drops to values far below that of intrinsic c-Si. This anomalous behavior is explained in this paper with an effective medium approximation for the electron density of states of μc-Si:H. Our model offers an alternative explanation (in contrast with models assuming a change in conduction path, percolation transport, or a highly conductive amorphous network) for the behavior of E_a with increasing crystalline fraction. The model leads to a straightforward distinction into three classes of microcrystalline materials.