Vibrational spectra and topological structure of tetrahedrally bonded amorphous semiconductors

Calculations of the vibrational spectra of continuous random network models of tetrahedrally bonded amorphous semiconductors are presented. Nearest neighbour central and non-central force constants are used. The results show that the shape of the TO-like peak of the spectrum depends on both the topology and bondangle distribution of the network. Comparison with experimental data suggests that a-Ge contains slightly fewer odd-membered rings than the Polk model and an average bond-angle distortion of 10° or more. On the other hand the shape of the TA-like peak depends more directly on the topology of the network, giving a clearer distinction between the results for structures containing both odd and evenmembered rings and those containing even rings only. Tentative comparison with experimental data for a-GaAs suggests a structure containing very few oddmembered rings, while the data for a-Ge indicate a significant number of oddmembered rings.