Intralayer and interlayer energy transfer from excitonic states into the Mn ${3d}^5$ shell in diluted magnetic semiconductor structures

We show that the Dexter-Förster-like radiationless resonance energy-transfer process from excitonic states into the Mn ${3d}^5$ shell in wide-gap (II,Mn)VI semiconductors is strongly enhanced if the total spin of the overall process is conserved. This requirement cannot be fulfilled in processes involving a bright exciton, i.e., other excitonic complexes or processes need to be involved. Of these we discuss dark excitons, donor-bound excitons $D^{0}X,$ negatively charged excitons $X^{-},$ and an Auger-like process. A careful analysis of the magnetic field and time dependence of the excitonic and Mn luminescence in the (Zn,Cd,Mn)Se samples under study gives evidence that the $D^{0}X$ complex plays a dominant role in the intralayer energy transfer. In asymmetric double quantum well (ADQW) structures consisting of a (Zn,Cd)Se well and a (Zn,Cd,Mn)Se well embedded in ZnSe barriers, there is a competition between the intralayer and interlayer energy-transfer processes from excitonic states into the Mn system. These interlayer processes take place between the Mn ions situated in the (Zn,Cd,Mn)Se well and spatially indirect excitons [where the hole is confined in the (Zn,Cd,Mn)Se and the electron confined in the (Zn,Cd)Se] as well as spatially direct excitons of the (Zn,Cd)Se well. The continuous magnetic-field tuning demonstrates convincingly the subtle interplay of excitonic band structure of the ADQW and spin effects in the energy-transfer processes. A spin-dependent energy transfer should be a general feature of rare-earth or transition-metal doped semiconductors such as II-VI or III-V semiconductors or even Er-doped Si or ${\mathrm{SiO}}_2.$