EXCITON-CAPTURE MECHANISM AT IMPURITIES IN GAAS/ALXGA(1-X)AS QUANTUM-WELLS

At low temperatures a neutral impurity is able to capture a free exciton to form a bound exciton state. The capture process is understood in bulk material as a phonon mediated relaxation of the free exciton through successive excited states to the ground state of the bound exciton. The equivalent mechanism in a confined system has not been specifically studied to date. In this paper we present an investigation of the exciton capture process in GaAs/Al0.3Ga0.7 As quantum wells using a picosecond time-resolved photoluminescence technique. We demonstrate that there are significant differences in the capture mechanism for narrow quantum wells in comparison to the bulk. In particular the capture efficiency is shown at first to increase with temperature. This behaviour is understood in terms of the role of localization of the free exciton in the potentials caused by the interface roughness. Higher temperatures destroy this localization process which otherwise limits the total capture rate for the exciton to the impurity. We also conclude that the distinct difference in the near bandgap low temperature excitonic spectra between bulk and confined material at low and moderate doping levels can be understood in terms of the corresponding differences in impurity capture efficiency.