FERMI-LEVEL EFFECT ON STEADY-STATE AND TRANSIENT PHOTOCONDUCTIVITY INMICROCRYSTALLINE SILICON

We show that steady-state and transient photoconductivity in microcrys talline silicon deposited by hot-wire chemical-vapor deposition depend strongly on the position of the Fermi level. The steady-state mobilit y-lifetime product increases significantly by shifting the Fermi level from around midgap towards the conduction or valence band. This incre ase 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 in dicator for material quality, should be complemented by the value of t he dark conductivity, which monitors the Fermi level, in order to allo w comparison between different samples. From our transient photorespon se data we deduce a ratio of 10:1 for the mobility of electrons to the mobility of holes.