Neutron scattering studies of hydrogenated, deuterated and fluorinated amorphous silicon

A comprehensive neutron scattering study has been performed of hydrogenated (Si_0.78H_0.22), deuterated (Si_0.77D_0.23) and partially fluorinated deuterated (Si_0.725D_0.120F_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_0.77D_0.23 exhibits sharp peaks at 1.49 and 2.36 Å, due to Si-D and Si-Si covalent bonds, respectively, while peaks centred at 3.2 and 3.8 Å are due to Si-D and Si-Si second-neighbour distances. High-energy resolution inelastic scattering measurements for Si_0.78H_0.22 show that there are approximately equal numbers of [Formula: see text] and = SiH₂ groupings, there being no indication of excitations corresponding to -SiH₃ 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 [Formula: see text] expected for free hydrogen, indicating a small but observable influence of the amorphous silicon matrix. The size of the cages containing the H₂ molecules has been investigated via SANS, which yields a mean Guinier radius of ∼5-6 Å. In addition, the use of the H-D SANS contrast technique indicates that each cage contains on average about 60 H₂ (D₂) molecules. The data for the Si_0.725D_0.120F_0.155 sample are consistent with a previously suggested model of network cages predominantly containing molecular SiF₄.