Spin lifetime from Hanle-effect measurements in samples with InAs quantum dots embedded in different Al_xGa_1−xAs matrices

Excitation of electron hole pairs by circularly polarized light yields an electron spin polarization in InAs self-assembled quantum dots embedded in AlxGa1−xAs. This spin polarization causes circular polarization of the photoluminescence light, its degree decreasing in a magnetic field perpendicular to the initial spin vector of the electrons (Hanle effect). With increasing aluminium content x the width of the Hanle curve is reduced from about 100 mT for x = 0 to 17.5 mT for x = 1. The corresponding electron spin lifetime increases with increasing x. For samples with QDs in the indirect gap matrix material AlAs, the spin lifetimes exceed 1.5 ns. A finite photoluminescence polarization evidences that the exciton spin lifetime inside a quantum dot cannot be much smaller than its recombination time. A single Lorentzian shape of the obtained Hanle curves and the observed field-independent part of the photoluminescence polarization could be explained by a small (zero) exciton g-factor in the dots at moderate magnetic fields, which could stem from the zero hole g-factor due to the electron–hole exchange interaction. The spin lifetime deduced from the Hanle curves could in this case be attributed to the thermalized electron capture time in the quantum dots.