AMORPHOUS CLUSTERS .3. THE ELECTRONIC-STRUCTURE OF SI CLUSTERS WITH B AND AL IMPURITIES

Amorphous clusters of the type XSi(20)H(28) with X=B and Al have been studied using the pseudopotential SCF Hartree-Fock and the HONDO program. The local electronic density of states and the charge density were obtained for structures determined by energy optimization of the position of the central impurity, and of the position of the central impurity and the four nearest neighbors, using the AM1 method implemented in the MOPAC program. The structures obtained in this manner are such that the impurities are not located at the center of the dusters; instead both the boron and the aluminum are displaced towards three of the four nearest neighbors, a fact that indicates that there is a tendency for the impurity to remain with the same chemical valence in the cluster as when isolated. The gap energy states introduced by both B and Al are near the valence band and are p-like; the state due to aluminum is closer to the valence band than the one introduced by boron. These results are compared with recent experimental findings.