REVERSIBLE AND IRREVERSIBLE STRUCTURAL-CHANGES IN AMORPHOUS-SILICON

The paper attempts to provide an overall view of the hierarchy of stru ctural and configurational equilibria that can be supported by hydroge nated random Si networks. In order to identify intrinsic and H-related structural degrees of freedom, experiments on chemically pure amorpho us Si (a-Si) and on hydrogenated amorphous Si (a-Si:H) are discussed i n parallel. The comparison shows that both kinds of amorphous material are able to support irreversible and relatively long-range relaxation processes in which the bond-angle disorder is fixed and in which the density of stable dangling-bond defects is established. Because of the rigidity of the random Si networks the corresponding equilibria are f rozen in at effective temperatures T > T-d (T-d is the deposition tem perature of the a-Si:H films). Hydrogenated random Si networks, in add ition, are able to support reversible valence alternation reactions in which the local coordination of the dopant and defect sites is change d and in which their charge states are altered. Kinetically, these lat ter changes are allowed by the diffusive motion of the bonded hydrogen within the a-Si:H matrix. These local configurational degrees of free dom are able to thermalize until the annealing temperature T has dropp ed to the equilibration temperature T-e < T-d. As these valence altern ation interactions establish a strong coupling between the electronic system and the local configurations of the dopant and defect sites, th e dangling-bond density tends to decouple thermally from the bond-angl e distortions in the a-Si:H network. In this situation, new equilibria between the electronic system and the local configurational degrees o f freedom of the random networks are enabled. At T < T-e, finally, the structural degrees of freedom are frozen in and essentially normal se miconductor behaviour is observed.