Neutron scattering studies of hydrogenated, deuterated and fluorinated amorphous silicon

A comprehensive neutron scattering study has been performed of hydrogenated (Si 0.78 H 0.22 ), deuterated (Si 0.77 D 0.23 ) and partially fluorinated deuterated (Si 0.725 D 0.120 F 0.155 ) amorphous silicon, prepared by the glow-discharge technique. The measurements performed include diffraction, small-angle neutron scattering (SANS) and inelastic neutron scattering, and the data obtained are discussed in terms of various structural models in the literature. The real-space correlation function for Si 0.77 D 0.23 exhibits sharp peaks at 1.49 and 2.36 A, due to Si-D and Si-Si covalent bonds, respectively, while peaks centred at 3.2 and 3.8 A are due to Si-D and Si-Si second-neighbour distances. High-energy resolution inelastic scattering measurements for Si 0.78 H 0.22 show that there are approximately equal numbers of ≡SiH and ≡SiH 2 groupings, there being no indication of excitations corresponding to -SiH 3 groupings. The presence of molecular hydrogen is demonstrated unambiguously by the observation of the ortho-to-para conversion, via molecular rotation modes at 14.5 and 29.4 meV. The shift in the Si-H stretch modes introduced by deuteration is slightly less than the value of √2 expected for free hydrogen, indicating a small but observable influence of the amorphous silicon matrix. The size of the cages containing the H 2 molecules has been investigated via SANS, which yields a mean Guinier radius of ∼5-6 A. In addition, the use of the H-D SANS contrast technique indicates that each cage contains on average about 60 H 2 (D 2 ) molecules. The data for the Si 0.725 D 0.120 F 0.155 sample are consistent with a previously suggested model of network cages predominantly containing molecular SiF 4 .