Steady-state photovoltaic effect in asymmetrical graded superlattices.

Abstract

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