Excitation transfer through thick barriers in asymmetric double-quantum-well structures

The transfer of carriers between two adjacent InxGa1-xAs wells with different thicknesses and separated by a thick GaAs barrier has been measured as a function of temperature for different indium concentrations x. The efficiency of the carrier transfer has been determined by photoluminescence excitation measurements. It is roughly constant at low temperature, and increases for increasing temperatures, going through a maximum between 50 and 70 K. At higher temperatures, where the photoluminescence shows a drastic quenching, also the carrier transfer efficiency decreases rapidly. The thermal escape of carriers out of the narrow well mediates the transfer, quenched at higher temperatures by an increased role of nonradiative recombination centres in the well and/or in the barrier. A comparison with a simple rate equation model supports an ambipolar escape of carriers in cw measurements, a unipolar escape of carriers in time-resolved measurements. This is explained in terms of the different excitation conditions in two types of experiments.