Ab initio studies of 5-atom ring carbon and silicon amorphous clusters both pure and with group-V impurities

We have constructed hydrogen saturated carbon and silicon clusters with only 5-atom rings to study their effect on the electronic properties of the corresponding amorphous materials. Using density functional theory and the local density approximation we calculate the electronic structure of pure and contaminated clusters, symmetric and nonsymmetric, with the impurity in the center or in the nearest neighbor position. For the pure cluster we find, by comparison to reference clusters with 6-atom boat-type rings, that the pentagonal clusters have a narrower valence band and that the top of the valence band moves to lower energies. Without the hydrogen contribution the energy gap for the pentagonal carbon cluster is larger than that for the hexagonal one and the gap for both silicon clusters is practically the same, contrary to expectations due only to size. For the impure clusters the carbon gap values decrease as the atomic number of the impurity increases, whereas the effect is opposite for silicon; also, the width of the valence band is larger in all eases than that for the pure clusters.