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 excit on to form a bound exciton state. The capture process is understood in bulk material as a phonon mediated relaxation of the free exciton thr ough successive excited states to the ground state of the bound excito n. The equivalent mechanism in a confined system has not been specific ally 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 p icosecond time-resolved photoluminescence technique. We demonstrate th at there are significant differences in the capture mechanism for narr ow quantum wells in comparison to the bulk. In particular the capture efficiency is shown at first to increase with temperature. This behavi our is understood in terms of the role of localization of the free exc iton in the potentials caused by the interface roughness. Higher tempe ratures destroy this localization process which otherwise limits the t otal capture rate for the exciton to the impurity. We also conclude th at the distinct difference in the near bandgap low temperature exciton ic spectra between bulk and confined material at low and moderate dopi ng levels can be understood in terms of the corresponding differences in impurity capture efficiency.