Subpicosecond spin relaxation dynamics of excitons and free carriers in GaAs quantum wells

Abstract

We have obtained a coherent understanding of spin relaxation processes of electrons, holes, and excitons in quantum wells by investigating subpicosecond dynamics of luminescence polarization. We show that the spin behavior for electrons and holes in quasi-two-dimensional systems is distinct from that in bulk semiconductors and that many-body effects and formation processes play an important role in exciton spin relaxation. PACS numbers: 78.47.+p, 71.35.+Z, 7l.70.Ej, 71.70.Gm Investigations of polarization of interband optical transitions provide considerable information about the symmetry of electron, hole, and excitonic wave functions in semiconductors. Such studies have led to the identification of diflferent spin relaxation processes [1,2] in bulk semiconductors. In quantum wells, cw measurements of linear [3] and circular polarization [4,5] of near-bandgap luminescence have been reported and various theories have been proposed to explain these results [6-12]. Time-resolved spectroscopy of luminescence polarization has also been reported recently for very high (lO'^ cm ~^) carrier densities [13], for different samples at intermediate carrier densities with 150-fs time resolution [14,15], and for low densities [16,17].In spite of this intense activity, spin relaxation in quantum wells is poorly understood. The quality of samples is important since spin dynamics can be strongly influenced by localization or defects. On a more fundamental level, the differences in the behavior of electrons, holes, and excitons must be recognized and carefully investigated. The prediction of slower hole spin relaxation in quantum wells compared to bulk semiconductors [10] has not been investigated. Finally, the influence of the formation dynamics of excitons and of many-body effects must be considered.In this Letter, we discuss new insights into many of these unresolved issues obtained through our investigations of spin dynamics of electrons, holes, and excitons in quantum wells. Using «-modulation-doped quantum wells, we obtain the first measurement of hole spin relaxation time in a semiconductor. The measured ^=: 4 ps demonstrates that the usual assumption of instantaneous hole spin relaxation is incorrect. We also show that the electron Fermi sea can be spin polarized under certain excitation conditions. In /?-modulation-doped quantum wells, we measure electron spin relaxation time of ^^^ 150 ps, approximately a factor of 4 shorter than that in comparably /7-doped bulk GaAs. We will show that an electron-hole exchange is responsible for this reduction. For intrinsic quantum wells, we show that polarized and unpolarized spectra exhibit an unusual splitting that depends on excitation density and time delay. This splitting results from many-body exchange interactions between excitons and contributes to an increase in the exciton spin relaxation time at higher density. Finally, we explain why spin relaxation in nonresonantly created excitons is faster than in resonantly created excitons. We believe that our quantitative measurement...

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