Fermi-level effect on steady-state and transient photoconductivity in microcrystalline silicon

We show that steady-state and transient photoconductivity in microcrystalline silicon deposited by hot-wire chemical-vapor deposition depend strongly on the position of the Fermi level. The steady-state mobility-lifetime product increases significantly by shifting the Fermi level from around midgap towards the conduction or valence band. This increase corresponds to a slower decay in the transient photocurrent after pulsed excitation compared to the case with the Fermi level at midgap. We thus attribute the enhancement of the mobility-lifetime product to the increase of the lifetime of the majority carriers due to a change in the thermal occupation of defect centers by the shift in the Fermi level. The mobility-lifetime product information, often used as an indicator for material quality, should be complemented by the value of the dark conductivity, which monitors the Fermi level, in order to allow comparison between different samples. From our transient photoresponse data we deduce a ratio of 10:1 for the mobility of electrons to the mobility of holes.