Amorphous clusters. II. Electronic structure of Ge clusters with N, P, and As impurities

Abstract

The electronic structure of random clusters has been used in the literature to qualitatively understand the properties of amorphous solids. Here impurity clusters of the type XGe20H28 with X = N, P, and As are studied using the pseudopotential SCF Hartree-Fock Method and the HONDO Program. The charge distribution and local density of states are calculated both for pure germanium and for clusters with each of the three impurities mentioned above. It is found that even though the covalent nature of the bonding of pure germanium is still present, its relevance is greatly diminished due to the presence of ionic components and shielding effects caused by the impurities. The charge distribution of the half-filled 57th orbitals clearly shows these effects. The LDOS exhibits the s- and p-components expected for the molecular orbitals of the clusters as well as two types of localized states; one is due to states localized at the bottom of the conduction band whose energies decrease as the mass of the dopants increases, and the other is due to deep states whose energies increase as the mass of the dopants increases. The top of the valence band is practically unaltered by the introduction of the impurities. A comparison is made of these results with experiment, and with those previously reported for amorphous Si clusters.