Hydrogen dynamics in a-Si:H: Multiple trapping, structural relaxation, and the Meyer-Neldel relation

The power-law time-dependent diffusion constant D(t)=${\mathrm{D}}_{00}$(ωt${)}^{\mathrm{-}\mathrm{a}}$ was measured in undoped hydrogenated amorphous silicon of varying H content [${\mathrm{H}}_{\mathrm{t}}$], diffusion length L, and microvoid content at temperatures T⩽400 °C. α generally deviates from a 1-TR/${\mathrm{T}}_0$ dependence on the temperature T. The temperature dependence of D(${\mathrm{t}}_{\mathrm{L}}$), for constant L, thus deviates from an Arrhenius behavior. The “apparent” activation energy ${\mathrm{E}}_{\mathrm{a}}$ and prefactor ${\mathrm{D}}_0$, defined by the linear best fit of lnD(${\mathrm{t}}_{\mathrm{L}}$) versus 1/T, strongly increase with L at low [${\mathrm{H}}_{\mathrm{t}}$]. The Meyer-Neldel relation ${\mathrm{D}}_0$=${\mathrm{A}}_{00}$exp(${\mathrm{E}}_{\mathrm{a}}$/${\mathrm{kT}}_0^{\prime }$) where ${\mathrm{A}}_{00}$≃3.1×${10}^{\mathrm{-}14}$ ${\mathrm{cm}}^2$/s and ${\mathrm{T}}_0^{\prime }$≃30 K, holds for all 1.3⩽${\mathrm{E}}_{\mathrm{a}}$⩽2.4 eV and 2.5×${10}^{\mathrm{-}5}$⩽${\mathrm{D}}_0$⩽3100 ${\mathrm{cm}}^2$/s. Structural relaxation processes dependent on H content are believed to affect α. The nature of the microvoid-related deep H-trapping sites is also discussed.