Absence of carrier separation in ambipolar charge and spin drift in p⁺-GaAs

The electric field-induced modifications of the spatial distribution of photoelectrons, photoholes, and electronic spins in optically pumped p⁺ GaAs are investigated using a polarized luminescence imaging microscopy. At low pump intensity, application of an electric field reveals the tail of charge and spin density of drifting electrons. These tails disappear when the pump intensity is increased since a slight differential drift of photoelectrons and photoholes causes the buildup of a strong internal electric field. Spatial separation of photoholes and photoelectrons is very weak so that photoholes drift in the same direction as photoelectrons, thus exhibiting a negative effective mobility. In contrast, for a zero electric field, no significant ambipolar diffusive effects are found in the same sample.