Time-resolved measurements of the radiative recombination in GaAs/AlxGa1-xAs heterostructures.

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The radiative recombination of two-dimensional (2D) carriers in an n-type-channel modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As heterojunction has been studied with time-resolved photoluminescence (PL). Two emission bands related to the recombination of 2D electrons, the so-called H band 1 (HB1) and H band 2 (HB2), are observed in PL. Their spectral shape and position are strongly dependent on the sample and the experimental conditions, and are, e.g., found to shift within a large photon-energy range with the excitation intensity. We have in this study measured the decay time of these PL bands as a function of recombination photon energy, under different experimental conditions. We find that the measured decay times of both the HB1 and HB2 emissions are strongly dependent on the detection photon energy. The decay times are found to increase with decreasing photon energy, in the range 1--100 ns for HB1, and 100 ns to 10 \ensuremath{\mu}s for HB2. This increase is explained as due to a spatial separation between the recombining electron and hole. The results are consistent with a recombination process involving 2D electrons, confined in the interface notch, and holes either from the valence band (HB1) or from neutral acceptors (HB2) in the active GaAs layer. We further show that the band bending in the active GaAs region influences the observed decay times of the HB2 emission, while the HB1 emission is almost unaffected.