Hydrogen-Associated Disorder Modes in Amorphous Si:H Films

NMR spin-lattice relaxation rates ${T_1}^{-1\mathrm{}}$ of ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ in $a$-Si:H films exhibit a maximum near 40 K. Both the magnitude, the temperature, and frequency dependences of $T_1$ are well described by a model which assumes a coupling to disorder modes which are associated with some hydrogen atoms. Results suggest that some hydrogen sites may contribute to the electronic states within the gap.