Computations to model three-center bonds in hydrogenated amorphous silicon

Effective-potential ab initio self-consistent-field molecular-orbital computations of electronic wave functions and total energy have been made on the molecule H₃SiH·{}SiH₃, which was chosen to model the paramagnetic three-center bonded system T⁰ postulated by Fisch and Licciardello to exchange charge and produce the diamagnetic anion T^- and cation T⁺ exothermically (in a process with negative U) with states in the gap of hydrogenated amorphous silicon. Structural computations described here predict a three-center bonded system for T⁺, which is symmetrical. For T⁰ and T^- a much weaker three-center bond is found. It is more appropriate to describe T⁰ and T^- as SiH₄ weakly associated with SiH₃ and SiH₃^-, respectively. We find that stability of the three-center bond of T⁺ provides a driving force of 33 kcal/mole for charge transfer, while the 1-kcal "hydrogen bond" of T^- is unimportant in this regard. We are unable to estimate reliably the sign of U for charge transfer by T⁰ embedded in hydrogenated amorphous silicon because of the large uncertainty in our estimates of the Born cavity energy: Our results do not exclude the possibility that U is negative. We suggest that at low concentrations of hydrogen in hydrogenated amorphous silicon, a more probable process is for a dangling bond of silicon proximate to a Si---H bond to form T⁺ with the transfer of its electron to an isolated dangling bond. We model this process with the reaction T⁰+SiH₃=T⁺+SiH₃^-.