Spin separation and type I/type II behavior in ZnSe/(Zn,Fe)Se quantum wells (abstract)

Quantum well structures with 100-Å Zn_0.9Fe_0.1Se barriers and 100-200-Å ZnSe wells have been grown by molecular-beam epitaxy. The magnetic field splitting of the heavy-hole exciton was studied by magnetoreflectivity at 4.2 K and B≤8 T, and is observed to be strikingly asymmetric, in contrast to the behavior observed in thick single epilayers. This behavior may be quantitatively understood through a model which incorporates strain effects, quantum confinement in the conduction and valence bands, and the strong spin exchange interaction in the (Zn,Fe)Se layers. In this system the band offset appears almost entirely in the conduction band, so that the electrons are confined to the ZnSe wells. The valence-band (VB) lineup is thus determined primarily by the field-induced spin splitting of the heavy-hole level, which at modest fields (1 T) is an order of magnitude larger than the zero-field VB offset. This produces a field-induced spin-dependent band alignment, with the quantum well structure exhibiting a type I "straddling" alignment for the m_j=+ (3)/(2) level, and a type II "staggered" alignment for the m_j=- (3)/(2) level. This leads to a spatial spin separation of the holes, with the m_j=+ (3)/(2) spin-"up" holes localized in the wells, and the m _j=- (3)/(2) spin-"down" holes localized in the barriers. The observed exciton intensities and splittings are both consistent with such a model.