OPTICALLY INDUCED INTERSUBBAND ABSORPTION IN THE PRESENCE OF A 2-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 musec for doping levels corresponding to a two-dimensional electron density 1 X 10(10) < n(square) < 7 X 10(10) cm-2. No photoinduced intersubband absorption is observed for n(square) > 7 X 10(10) cm-2. For nominally undoped samples (n(square) < 1 X 10(10) 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.