Possible configurational model for hydrogen in amorphous Si:H. An exodiffusion study

The kinetic hydrogen exodiffusion and its temperature dependence in amorphous silicon prepared by glow discharge of silane has been studied using conductivity, electron paramagnetic resonance, $^{11}\mathrm{B}\ensuremath{\rightarrow}\ensuremath{\alpha}$ nuclear reaction, and infrared absorption measurements. Comparison of the results obtained with these techniques shows the existence of two principal stages in the H exodiffusion. The hydrogen evolution for $T\ensuremath{\le}500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is controlled by a diffusion process with a diffusion coefficient $D$. $D$ is thermally activated; $D={D}_{0}{e}^{\frac{\ensuremath{-}{E}_{D}}{{k}_{B}T}}$ with ${D}_{0}=4.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/${\mathrm{s}}^{\ensuremath{-}1}$ and ${E}_{D}=1.5$ eV (where ${k}_{B}$ is the Boltzmann constant). The hydrogen evolution above 500\ifmmode^\circ\else\textdegree\fi{}C is controlled by a first-order process. The activation enthalpy and entropy are, respectively, $\ensuremath{\Delta}{H}_{2}=3.4$ eV and $\ensuremath{\Delta}{S}_{2}=7.8{k}_{B}$. The fact that the EPR signal appears during the second stage and that $\ensuremath{\Delta}{H}_{2}$ is equal to the Si-H bound energy is a direct evidence that EPR signal is associated with the breaking of this bond. We then deduce an exodiffusion model assuming that hydrogen atoms can be bound in two sorts of centers. A possible configuration of H occupying such sites in the amorphous network is proposed.