Optically induced intersubband absorption in the presence of a two-dimensional electron gas in quantum wells.

Abstract

We identify long-lived photoexcited electrons in modulation-doped GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As quantum wells by studying the effect of interband excitation on the electron intersubband absorption. The lifetime of these electrons is of the order of 1--10 \ensuremath{\mu}sec for doping levels corresponding to a two-dimensional electron density 1\ifmmode\times\else\texttimes\fi{}${10}^{10}$${\mathit{n}}_{\mathrm{\ensuremath{\square}}}$7\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. No photoinduced intersubband absorption is observed for ${\mathit{n}}_{\mathrm{\ensuremath{\square}}}$>7\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. For nominally undoped samples (${\mathit{n}}_{\mathrm{\ensuremath{\square}}}$1\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$), the photoinduced absorption is mainly due to short-lived free excitons. These results are consistent with the existence of localized holes that in lightly doped quantum wells greatly reduce the e-h recombination rate, leaving the electrons free for a long time.