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We report about optical experiments on electric field tunable AlAs/GaAs coupled quantum well structures in the regime of the electric field induced -X transition. Using the energetically tunable X-point state in the AlAs layer as an internal energy spectrometer and charge reservoir we are able to map out the electronic states in the neighboring GaAs quantum well in great detail. In spatially resolved and bias voltage dependent photoluminescence experiments we find sets of extremely narrow emission lines below the fundamental band gap energy of the GaAs quantum well. The new emission lines are shown to originate from natural quantum dots which are formed by well width fluctuations of the GaAs quantum well.
Zrenner et al. (Mon,) studied this question.