Structure of amorphous semiconductors

Abstract

The main emphasis of this paper is on the problem of molecular configuration within a single amorphous phase, although the physical properties of amorphous semiconductors are also strongly dependent on the spatial distribution of the constituent phases. Structural and thermal studies indicate a very high degree of local order in amorphous semiconductors; in particular, the coordination to nearest neighbors (NN) is close to that required by the chemical valences and the NN separations differ little from their ideal crystal values. There is still much disagreement on how the elements of local order are assembled in the overall structure. Recent model studies indicate that the relative smallness of the magnitudes of the configurational entropy of glasses is associated with the sharpness of the local order, however it occurs in the structure. Structural models for amorphous solids may be classified as follows: microcrystallite, amorphous cluster and continuous random (including random networks). The nature of these models and their application to the interpretation of experience on tetrahedrally coordinated systems are surveyed. The radial distribution functions and densities of fused silica, a-Si and a-Ge are in excellent accord with the predictions of the random network models as deduced from studies of actual models generated by hand or computer. An important problem for the further development of these models is, how much energy is associated with the bond distortions inherent in the random network. Neither the microcrystallite nor the amorphous cluster type of model have been developed to the point where they have as much predictive capabilities as do the random network models. Actually, in their present states, none of the ordered cluster models account satisfactorily for the diffraction results on a-Si and a-Ge. When amorphous semiconductors are formed by deposition, the driving free energy is usually so high that any of the amorphous structures considered would be permitted thermodynamically; therefore what structure actually appears will be determined primarily by kinetic factors. Recent model studies indicate that when deposition is controlled completely by short range interactions at the surface a continuous random type structure is always formed. In hard sphere packing the structure is of the dense random packed type; when the units bind directionally a random network structure is formed.