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

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 μsec for doping levels corresponding to a two-dimensional electron density 1×${10}^{10}$<${\mathit{n}}_{\mathrm{\square }}$7×${10}^{10}$ ${\mathrm{cm}}^{\mathrm{-}2}$. For nominally undoped samples (${\mathit{n}}_{\mathrm{\square }}$<1×${10}^{10}$ ${\mathrm{cm}}^{\mathrm{-}2}$), 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.