STEADY-STATE PHOTOVOLTAIC EFFECT IN ASYMMETRICAL GRADED SUPERLATTICES

In a graded InAlGaAs superlattice grown by molecular-beam epitaxy at 500-degrees-C, we have observed a novel steady-state photovoltaic effect which arises from an internal polarization field under above-band-gap (hv > E(g)) illumination. The saturation value is approximately 0.1 V and the response time to the illumination is limited by the oscilloscope resolution to approximately 2 ns. The observations are in quantitative agreement with a theoretical model by which we fit our data and extract a minority-carrier lifetime tau-e of approximately 30 ps. This effect is substantially reduced in a second wafer grown at 550-degrees-C, in which tau-e is expected to be long.