Previous experiments have demonstrated unambiguously that a small fraction of the hydrogen in hydrogenated amorphous silicon is present as H-2 molecules trapped in the cages of the amorphous network. In the present paper, small angle neutron scattering (SANS) is employed to investigate the size of the cages containing the H-2 molecules and yields a mean radius of similar to 6.5 Angstrom. The use of H-D contrast techniques indicates that each cage contains, on average, about 60 H-2 (D-2) molecules. Data are also reported for a partially fluorinated sample and are consistent with a previously suggested model of network cages predominantly containing molecular SiF4.

A short summary is presented of inelastic neutron scattering studies of hydrogenated and deuterated amorphous silicon with particular emphasis on the structural role played by the hydrogen (deuterium) atoms. It is concluded that the data are consistent with the existence of molecular H-2 (D2) trapped within the cages of the amorphous covalent network.

The inelastic neutron scattering technique provides information on the vibrational spectra of amorphous network glasses which extends that available from optical and resonance spectroscopy. A range of instrumentation is available to cover energies corresponding to relaxation phenomena (< 1 μeV) up to hydrogen stretching frequencies (500 meV). The resolution attainable is now sufficient to give data, the line width of which is dominated by the natural broadening caused by the structural variations in the networks. Additional information on atomic motions within a vibrational mode is given by the dependence of the scattered intensity on the magnitude of the scattering vector, Q, as revealed by the dynamical structure factor, S(Q, E). Low energy excitations (<10 meV) characteristic of the amorphous state have been related to anomalous low temperature thermal properties and also contain information on the range of topological order within the network. Experimental investigations using a variety of neutron inelastic scattering instrumentation to study vitreous silica, vitreous opal, vitreous boron trioxide, the amorphous germanium-selenium system and amorphous silicon containing hydrogen or deuterium are described. The results are used to discuss the possibilities and limitations of the method for testing structural models of network glasses.