Effect ofsp−dexchange interaction on excitonic states inCdSe∕ZnSe∕Zn1−xMnxSequantum dots

Abstract

Photoluminescence of excitons (X) and biexcitons (XX) from single neutral $\mathrm{CdSe}∕\mathrm{ZnSe}∕\mathrm{ZnMnSe}$ quantum dots (QDs) with various magnitudes of $sp\text{\ensuremath{-}}d$ exchange interaction has been investigated in magnetic fields $B$ up to $10\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ at $1.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The magnitude of the $sp\text{\ensuremath{-}}d$ interaction is varied by changing the penetration of the exciton wave function ${\ensuremath{\psi}}_{X}$ into the diluted magnetic semiconductor (DMS) barrier, $\ensuremath{\eta}$, by variation of the nonmagnetic $\mathrm{ZnSe}$ barrier layer thickness about a value of $1.75\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. The small penetration of ${\ensuremath{\psi}}_{X}$ 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 $\ensuremath{-}1\phantom{\rule{0.3em}{0ex}}\mathrm{X}$ states in low-symmetry QDs by the electron-hole $(e\text{\ensuremath{-}}h)$ exchange interaction. In addition, magnetic fluctuations result in (i) emission depolarization and enhanced splitting of exciton emission lines compared to the $e\text{\ensuremath{-}}h$ 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 $\ensuremath{\eta}\ensuremath{\sim}1%$. The spin relaxation from bright to ``dark'' exciton states is negligible up to $\ensuremath{\eta}\ensuremath{\sim}4%$. Auger recombination of excitons with excitation of Mn ions markedly decreases the quantum efficiency of exciton emission at $B=0$ already at $\ensuremath{\eta}\ensuremath{\sim}4%$.