Exciton states and tunneling in semimagnetic asymmetric double quantum wells

Abstract

Exciton level structure and interwell relaxation are studied in Cd(Mn,Mg)Te-based asymmetric double quantum wells (ADQWs) by a steady-state optical spectroscopy in magnetic fields up to B = 10 T. The as grown heterostructures with CdTe QWs and nonmagnetic interwell CdMgTe barrier were subjected to a rapid temperature annealing to introduce Mn and Mg atoms from opposite barriers inside the QWs which results in a formation of the ADQW with completely different magnetic field behavior of the intrawell excitons. The giant Zeeman effect in the QW with magnetic Mn ions gives rise to a crossing of the ground exciton levels in two QWs at BC ∼ 3–6 T which is accompanied by a reverse of the interwell tunneling direction. In a single-particle picture the exciton tunneling is forbidden at B < 1 T as supported by calculations. Experimentally, nevertheless, a very efficient interwell relaxation of excitons is found at resonant excitation in the whole magnetic field range, regardless of the tunneling direction, emphasizing importance of excitonic correlations in the interwell tunneling. At nonresonant excitation an unexpectedly slow relaxation of the σ−-polarized excitons from the nonmagnetic QW to the σ+-polarized ground state in the semimagnetic QW is observed at B > BC, giving rise to a nonequilibrium distribution of excitons in ADQW. A strong dependence of the total circular polarization degree on the hh–lh splitting Δhh–lh in the nonmagnetic QW is found and attributed to the spin dependent interwell tunneling controlled by an exciton spin relaxation. Different charge-transfer mechanisms are analyzed in details and an elastic scattering due to a strong disorder is suggested as the main tunneling mechanism with the underlying influence of the valence band-mixing.