Dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots of CdSe coupled with a diluted magnetic semiconductor layer ofZn0.80Mn0.20Se

Abstract

We study the dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ${\mathrm{Zn}}_{0.80}{\mathrm{Mn}}_{0.20}\mathrm{Se}$, where spin-polarized excitons can be injected from the DMS into the QDs. The degree of circular polarization $P$ of excitonic photoluminescence (PL) at $5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the coupled QDs exhibits a rapid increase with increasing delay time, up to $+0.3$ at $25\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ after the pulse excitation of the DMS by a linearly polarized light. This development of a positive $P$ value directly reflects the spin-injection dynamics from the DMS, since the intrinsic polarization of the QD excitons due to Zeeman splitting is $P\ensuremath{\sim}\ensuremath{-}0.1$ when only the QDs are selectively excited. The $P$ value gradually decays with time after reaching its maximum, as a result of the exciton-spin relaxation with a time constant of $800\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ in the QDs. Time-resolved circularly polarized PL spectra immediately after the pulse excitation directly show the exciton-energy dependence of the spin-injection dynamics in the QD ensemble, where two-dimensional-like QDs with higher exciton energies show higher receptivity to the spin-polarized excitons than three-dimensional-like dots with lower exciton energies. A rate equation analysis reveals all time constants responsible for the spin-injection dynamics. We deduce a time constant of $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for the spin injection. The spin-injection efficiency of 0.94 is also obtained, which corresponds to the ratio between the number of the spin-polarized excitons responsible for the rise of the positive $P$ value in the QD emission and the total number of the excitons injected from the DMS. Moreover, we observe that interdot exciton transfer significantly affects the $P$ value within the QD emission band after the fast spin injection, in addition to the spin relaxation within the QDs.