Chemical Bonding of Alloy Atoms in Amorphous Silicon

Abstract

This paper presents a discussion of the local bonding arrangements of H and F in amorphous silicon. The basis for this model of the short range order derives from an interpretation of the local infrared absorption bands in a-Si:H and a-Si:F alloys that are associated with the incorporation of the respective alloy constituents, H and F. The method of analysis draws heavily on chemical bonding considerations and a review of the contributions to the bonding in Si-H and Si-F groups is presented. The approach to the interpretation of the ir-spectra is based on the use of induction relationships to derive realistic bond-stretching force constants for both Si-H and Si-F vibrations. These force constants change due to changes in near-neighbor bonding arrangements. This in turn allows one to interpret changes in bond-stretching frequencies in terms of changes in the local chemistry; i.e., the near neighbor environments. These bond-stretching force constants, along with bond-bending force constants obtained from various types of substituted silane molecules are used to calculate the frequencies and displacement vectors for localized Si-H and Si-F vibrations. In the case of Si-H,-all of the modes of the SiH and SiH₂ groups are localized, and occur at frequencies above 500 cm^-1, the upper limit of Si network vibrations. For Si-F, the only modes that are localized are the bond-stretching vibrations which occur between 800 and 1025 cm”¹.