Effect of sp-d exchange interaction on excitonic states in CdSe/ZnSe/Zn1-xMnxSe quantum dots

Photoluminescence of excitons (X) and biexcitons (XX) from single neutral CdSe/ZnSe/ZnMnSe quantum dots (QDs) with various magnitudes of sp-d exchange interaction has been investigated in magnetic fields B up to 10 T at 1.8 K. The magnitude of the sp-d interaction is varied by changing the penetration of the exciton wave function psi(X) into the diluted magnetic semiconductor (DMS) barrier, eta, by variation of the nonmagnetic ZnSe barrier layer thickness about a value of 1.75 nm. The small penetration of psi(X) into the DMS barrier allowed a decrease of the X and XX emission line broadening caused by magnetic fluctuations and resolution of the fine structure of X states in a single QD. In contrast to bulk DMSs, the exciton line broadening when B is normal to the QD plane was found to be a nonmonotonic function of magnetic field: at high B it is suppressed due to Mn ion spin alignment, whereas at low B it is decreased due to the mixing of J(z)=+1 and -1 X states in low-symmetry QDs by the electron-hole (e-h) exchange interaction. In addition, magnetic fluctuations result in (i) emission depolarization and enhanced splitting of exciton emission lines compared to the e-h exchange interaction at B=0 and (ii) strong enhancement of spin relaxation between "bright" J=1 exciton states in magnetic fields normal to the QD plane already at eta similar to 1%. The spin relaxation from bright to "dark" exciton states is negligible up to eta similar to 4%. Auger recombination of excitons with excitation of Mn ions markedly decreases the quantum efficiency of exciton emission at B=0 already at eta similar to 4%.