Exciton states and tunneling in annealed semimagnetic Cd(Mn,Mg)Te asymmetric double quantum wells

Exciton level structure and interwell relaxation are studied in Cd(Mn,Mg)Te-based asymmetric double quantum wells (ADQWs) in normal to plane magnetic fields up to B = 10 T by a steady-state optical spectroscopy. As grown structures with nonmagnetic CdTe quantum wells (QWs) were subjected to quick temperature annealing to introduce Mn and Mg atoms from the barriers inside the wells resulting in formation of magnetic (MW) and nonmagnetic (NMW) QWs, respectively, with the concentration of diffused atoms of about 3--5%. A significant change of exciton energies occurs with magnetic field: at low fields exciton, localized in the MW, is higher in energy than that localized in the NMW and efficient exciton relaxation from the MW to the NMW takes place for all ADQWs. In all structures the giant Zeeman effect in the MW changes the energy order of $\sigma^+$-polarized heavy hole (hh) excitons, localized in different wells, at high B. Levels' crossing is accompanied by a reverse of tunneling direction without anticrossing. Calculations of single-particle states and their change with B indicate that the interwell exciton transfer is forbidden in the single-particle picture at B $\gtrsim$ 1 T for all studied structures. Experimentally, nevertheless, a very efficient interwell relaxation of excitons is found in the whole magnetic field range regardless of tunneling direction which evidences about importance of electron-hole Coulomb correlations in tunneling process. Different charge-transfer mechanism are analysed in details and elastic scattering due to strong disorder is suggested as the main tunneling mechanism of excitons with underlying influence of the valence band-mixing effects on the hh-exciton transfer in ADQWs with relaxed stress.