H1NMR ina-Si

Results of pulsed NMR studies of hydrogen in $a$-Si: H prepared at several laboratories by glow discharge of silane are presented. The origins of the two ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ NMR lines seen in almost all samples of $a$-Si: H are discussed. Solid-echo measurements are presented which indicate that these two components are due to spatially isolated groups of protons. We attribute the narrow line to protons slightly clustered in the bulk of the material and the broad line to protons distributed on internal surfaces. The spin-lattice relaxation time shows a minimum at $T\approx 30$ K which is interpreted as due to relaxation via spin diffusion to a small number of ${\mathrm{H}}_2$ molecules acting as relaxation centers. Annealing results suggest that all the hydrogen molecules are trapped in very similar sites.