Spin injection in self-assembled quantum dots coupled with a diluted magnetic quantum well

Spin injection is studied in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor quantum well (DMS-QW) of Zn1−x−yCdxMnySe, by means of time-resolved circularly polarized photoluminescence (PL). Excitonic PL from the CdSe QDs shows σ−-circular polarization in magnetic fields, mainly due to negative g-values of individual dots, when the energy difference of excitons between the QDs and DMS-QW is large as 300 meV. However, when such energy difference is comparable with LO-phonon energy in the QD, we observe an additional PL peak with the long lifetime as 3.5 ns and σ+-polarization in magnetic fields. It can be attributed to a type-II transition between the down-spin electron injected from the DMS-QW into the QDs, via LO-phonon-assisted resonant tunneling, and the down-spin heavy hole in the DMS-QW. In addition, the electron spin-injection is also evidenced by σ+-polarized PL with the fast rise-time of 20 ps in the QDs.