Metastable effects in the dc conductivity of undoped glow discharge and sputtered hydrogenated amorphous silicon

Thermal equilibrium processes have been studied in undoped rf glow discharge (GD) and undoped dc sputtered (SP) films by means of dark dc conductivity measurements. Two regimes of behavior separated by the equilibration temperature $T_E$, 180 °C for GD films and 145 °C for SP films, have been observed. Below $T_E$, the nonequilibrium dark dc conductivity induced by thermal quenching is reduced in GD films and increased in SP films compared to its equilibrium value. The time dependence of the small departures from equilibrium dark conductivity is well described by a stretched exponential function. In GD films the relaxation is interpreted in terms of dispersive diffusion of hydrogen, repeatedly trapped and released by excitation up to interstitial level. In SP films it is not clear whether the equilibrium process is due to hopping of hydrogen over barriers or to a phonon assisted tunneling process.