Spin-polarized exciton transfer in Zn-Cd-Mn-Se multi quantum wells

Four-quantum-well structure consisting of a ZnCdMnSe diluted magnetic semiconductor (DMS) well and three ZnCdSe non-magnetic semiconductor (NMS) wells separated by 15 nm-thick ZnSe barrier layers were prepared by using ALE and MBE technique. The sample was excited by CW and pulse lasers with photon energy larger than the barrier layer band gap, and circularly polarized PL spectra due to recombination of excitons localized in each well were measured as a function of external magnetic field in the Faraday geometry up to 8 T at 4 K. The PL from the NMS well next to the DMS well and from the NMS well next-but-one to the DMS well showed positive circular polarization due to LO phonon assisted transfer of positively spin polarized excitons from the DMS well to the NMS wells. The two NMS wells showed different magnetic field dependence of the degree of circular polarization of PL, which suggested that the exciton transfer from the DMS well to the NMS well next-but-one was governed by direct interaction between the two wells, not by cascade transfer process. The degree of circular polarization of PL from the NMS wells increased from zero to 20∼30% after pulse excitation. Raise time of the polarization degree was discussed on the basis of the difference of distance between the NMS well and the DMS well. (