Deep defect states in a-Si:(H,C) alloys

The cluster-Bethe-lattice method is used to study the electronic properties of the a-Si:(H,C) alloys. The sp(3)s* tight-binding-parameters are used in the calculations. The electronic local density of states for several clusters containing various local bonding configurations are calculated. There are no defect states in the gap induced by the ideal bonding of H and C in the cluster whereas there are defect states in the gap due to the Si- and C- dangling bonds. The energy level for the neutral Si dangling bond is at +0.4 eV. For the cluster containing a Si-C bond, we found that the defect energy level associated with the C dangling bond lies at -0.8 eV, and that associated with the Si dangling bond lies at +0.8 eV. The defect states distribution in the gap is wider with the inclusion of C in a-Si:H films.