Steady-state photovoltaic effect in asymmetrical graded superlattices

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