Sun polarised dynamics in quantum wells

Abstract

The ability to preferentially spin-polarise a photoexcited carrier population in a quantum well by optical pumping methods has enabled us to study the fine structure and some of the parameters governing the spin relaxation dynamics of excitons, heavy-holes and electrons in a number of type I GaAs/AlxGa1-xAs and type I InxGa1-xAs/GaAs single and multiple quantum wells.

The electron, hole and excitonic effective Lande g-factors have been measured as a function of quantum well thickness in GaAs/Al0.36Ga0.64As and In0.11Ga0.89As/GaAs. For the GaAs/AlGaAs wells, we observed a change in sign of the exciton g-factor and using k.p perturbation theory to model the form of the electron g-factor, we obtained the hole g-factor as a function of well width. Both the electron and hole g-factors also exhibited a change in sign. In the case of the InGaAs/GaAs wells, the exciton g-factor was small and of positive magnitude for all the wells we studied. Describing the form of the electron g-factor by a strain modified k.p perturbation theory and using the measured exciton g-values, the hole g-factor was calculated to take small, but slightly more positive values than the exciton g-factor.

The electron-hole exchange interactions have been measured in GaAs/Al0.36Ga0.64As and GaAs/A1As samples as a function of well width. In both cases the exchange splitting between the optically active and inactive levels was consistent with theory, falling rapidly with decreasing confinement towards the measured value for bulk GaAs. A finite exchange splitting of the optically active levels at zero field demonstrates the symmetry of the quantum well is less than Dja, possibly due to growth induced imperfections.

We have measured the spin relaxation times of electrons, holes and excitons in GaAs/Al0.3Ga0.7As quantum wells. We observed a much shorter relaxation time for the excitons, lOOps, compared to the free electron and hole relaxation times which were both of the order of Ins. We have attributed the fast exciton spin relaxation to the strong
exchange between the electron and hole forcing the more rapidly relaxing particle to govern the spin relaxation dynamics. Our results suggest that the hole is the more rapidly relaxing particle when confined in an exciton. We have attributed this rapid spin relaxation of the holes to the mixing of the light and heavy-hole bands for wavevectors away from k=0 in a quantum wells. We have also observed a strong temperature dependence of the hole relaxation time which is consistent with a wavevector dependence of the hole relaxation time. Finally we have direct measurements of the wavevector dependence of hole relaxation time and the reduction in this due to the mixing of the valence band states in a quantum well away from k=0.

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