Decay Measurements of Free and Bound Exciton Recombination in Doped GaAs/GaAIAs Quantum Wells
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DECAY MEASUREMENTS OF FREE AND BOUND EXCITON RECOMBINATION IN DOPED GaAs/GaAIAs QUANTUM WELLS J.P. BERGMAN*, P.O. HOLTZ*, B. MONEMAR*, M. SUNDARAM**, J.L. MERZ** AND A.C. GOSSARD**. *Department of Physics and Measurement Technology Link6ping University, S-581 83 Link6ping, SWEDEN **Department of Electrical and Computer Engineering, and Materials Department University of California, Santa Barbara, California 93106, USA. ABSTRACT In this study we present photoluminescence decay measurements of free exciton (FE) and bound exciton (BE) recombination in doped GaAs/AIGaAs quantum wells (QW). It is found that the FE decay time is reduced in the doped QW's compared to similar undoped samples. The low temperature decay time of the BE is slightly longer than for the FE, with BE decay times from 300 to 600 ps for well widths of 50 to 150 A, respectively. It is also found that the observed decay of the BE is strongly influenced by the decay time of the FE. This is especially observed in the similarity of the temperature dependence for the FE and BE decay time. INTRODUCTION The radiative kinetics of the free exciton (FE) in GaAs/GaAIAs MQW has in recent years been studied both experimentally [1-7] and theoretically [8,9] by a number of authors. It is concluded that potential fluctuations due to variations in the QW width play an important role in the capture and kinetic behaviour of the exciton [4-7]. Some controversy exist, however, whether the observed FE recombination originates from laterally localized excitons or from free excitons with mobility in areas larger than the exciton Bohr radius [4,7]. A linear decrease of the exciton decay time with decreasing QW width is predicted by theoretical calculations [3]. This predicted linear dependence has recently been experimentally verified [1,3]. FE decay times in high quality samples in the range of 300 to 500 ps were reported for QW widths close to 5.0 nm. Furthermore, the FE decay times are found to rapidly increase with increasing temperature. This is explained in terms of a thermal redistribution of the exciton momentum, decreasing the number of excitons obeying the K=0 conservation rule for radiative recombination [3]. The same effect is observed in bulk GaAs [10], although the effect is
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more pronounced in the QW due to less competing non radiative recombination channels. The defect energy levels in QW's are mainly observed through the recombination of an exciton bound to the neutral defect. The pioneer study of the BE associated with the acceptor level caused by Be doping was performed by Miller et.al. [11]. After this initial work, a number of reports have followed showing that e.g. the binding energy of this acceptor BE (ABE) is in the range of 4 to 6 meV, for QW widths appropriate in this study. To our knowledge no studies have been performed on the kinetics of these acceptor BE's (ABE), and its influence
on the kinetics of the FE.
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