Optical imaging of trion diffusion and drift in GaAs quantum wells

Abstract

The in-plane motion of negatively charged excitons ${(X}^{\ensuremath{-}})$ in modulation-doped GaAs quantum wells (QW's) was studied by means of spatially resolved photoluminescence spectroscopy. In the highest-quality QW's, resonantly excited ${X}^{\ensuremath{-}}$ were observed diffusing from the excitation region, characterized by a mobility as high as $6.5\ifmmode\times\else\texttimes\fi{}{10}^{4}{\mathrm{cm}}^{2}{\mathrm{V}}^{\ensuremath{-}1}{\mathrm{s}}^{\ensuremath{-}1},$ independent of temperature and electron density. Under the application of an electric field in the plane of the QW, ${X}^{\ensuremath{-}}$ were found to drift in the direction opposite to the field, whereas neutral excitons (X) do not drift under similar conditions. The results demonstrate that ${X}^{\ensuremath{-}}$ can exist as a free quasiparticle in the best-quality samples. The simultaneous motion of nonresonantly excited X and ${X}^{\ensuremath{-}}$ was studied as a function of the electron density in the QW. The data reveal that the spatial distribution of X and ${X}^{\ensuremath{-}}$ is mainly determined by the motion of X that locally form ${X}^{\ensuremath{-}}.$