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 onl y 5-atom rings to study their effect on the electronic properties of the co rresponding amorphous materials. Using density functional theory and the lo cal density approximation we calculate the electronic structure of pure and contaminated clusters, symmetric and nonsymmetric, with the impurity in th e 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 va lence band moves to lower energies. Without the hydrogen contribution the e nergy gap for the pentagonal carbon cluster is larger than that for the hex agonal one and the gap for both silicon clusters is practically the same, c ontrary to expectations due only to size. For the impure clusters the carbo n gap values decrease as the atomic number of the impurity increases, where as the effect is opposite for silicon; also, the width of the valence band is larger in all eases than that for the pure clusters.