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/Al0.3Ga0.7As 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 1×1010<n□<7×1010 cm-2. No photoinduced intersubband absorption is observed for n□>7×1010 cm-2. For nominally undoped samples (n□<1×1010 cm-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.