The Interaction of Photoexcited e-h Pairs with a two Dimensional Electron Gas Studied by Intersubband Spectroscopy
We present a study of the e1-e2 intersubband absorption induced by interband excitation (PIA) in two types of quantum-wells: a. Modulation doped GaAs/AlGaAs MQW’s with a density of the two dimensional electron gas (2DEG) in the range of n□ = 1010 — 1012cm-2. b. Mixed type I-type II GaAs/AlAs superlattices (MTSL). For the modulation doped MQW’s, the PIA is studied as a function of n□, laser intensity and its chopping frequency. It is observed that when no 2DEG is present, the e1-e2 PIA is due to short lived excitons. For n□ ≥ 1 × 1010cm-2, the PIA decreases with increasing n□, and for n□ ≥ 7 × 1010cm-2 it vanishes. We interpret the PIA in this density range as due to long lived photoexcited electrons, that in turn result from holes localized at interface fluctuations and have a reduced radiative recombination rate.
In the case of the MTSL’s, excitons or separately confined 2DEG and 2DHG are created, depending on the interband excitation energy. This allows us to directly compare the PIA due to excitons and that due to electrons in the same quantumwell. We thus estimate the ratio of the oscillator strengths to be fexciton/ felectronC10.
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