Reversible and irreversible structural changes in amorphous silicon

Abstract

The paper attempts to provide an overall view of the hierarchy of structural and configurational equilibria that can be supported by hydrogenated random Si networks. In order to identify intrinsic and H-related structural degrees of freedom, experiments on chemically pure amorphous Si (a-Si) and on hydrogenated amorphous Si (a-Si: H) are discussed in parallel. The comparison shows that both kinds of amorphous material are able to support irreversible and relatively longrange relaxation processes in which the bond-angle disorder is fixed and in which the density of stable danglingbond-defects is established. Because of the rigidity of the random Si networks the corresponding equilibria are frozen in at effective temperatures T* > T_d (T_d is the deposition temperature of the a-Si: H films). Hydrogenated random Si networks, in addition, are able to support reversible valence alternation reactions in which the local coordination of the dopant and defect sites is changed and in which their charge states are altered. Kinetically, these latter changes are allowed by the diffusive motion of the bonded hydrogen within the a-Si: H matrix. These local configurational degrees of freedom are able to thermalize until the annealing temperature T has dropped to the equilibration temperature T_c< T_d. As these valence alternation interactions establish a strong coupling between the electronic system and the local configurations of the dopant and defect sites, the dangling-bond density tends to decouple thermally from the bond-angle distortions in the a-Si:H network. In this situation, new equilibria between the electronic system and the local configurational degrees of freedom of the random networks are enabled. At T<;T_e , finally, the structural degrees of freedom are frozen in and essentially normal semiconductor behaviour is observed.