Darkening of interwell excitons in coupled quantum wells due to a stress-induced direct-to-indirect transition

Abstract

In previous studies, an inhomogeneous strain field was used as a trapping mechanism for interwell excitons in coupled GaAs/${\mathrm{Al}}_{0.45}{\mathrm{Ga}}_{0.55}\mathrm{As}$ quantum wells. Photoluminescence measurements in this system demonstrated the presence of a dark population of excitons at trap center in the low-temperature, high-stress, high-density regime. The dramatic appearance of this effect at low temperature and high density initially suggested that it may be indicative of a Bose-Einstein condensation phase transition. Further experiments revealed that this effect appears more readily in wider quantum wells and occurs at strain values near the heavy-hole/light-hole crossover point. In this paper, it will be shown that this effect occurs in a regime where the heavy-hole valence band maximum shifts away from ${k}_{||}=0$, which is expected to strongly suppress the recombination rate at low temperatures. Simulations based on this assumption will be shown to match the experimentally observed behavior, without requiring the appearance of a Bose-Einstein condensate.