Magnetic-field induced type I to type II transition in Zn_1-xMn_xTe/ZnTe multiple quantum well samples

A series of four Zn_1-xMn_xTe/ZnTe multiple quantum well samples with Mn concentrations x of about 7.5% and well widths ranging from 40 Å to 100 Å have been studied by below-bandgap photomodulated reflectivity (BPR) at a temperature of 1.5 K and in magnetic fields up to 6 T in the Faraday geometry. The band alignment in zero magnetic field is type I with the ZnTe being the quantum well material. The giant Zeeman splitting (due to the s, p-d exchange interaction between the Mn²⁺ ions and the free carriers) makes possible a magnetic-field induced transition of the band alignment from type I to type II: for the J_z = -3/2 heavy-hole potential, the well region moves from the ZnTe layer to the Zn_1-xMn_xTe layer when the magnetic field is increased sufficiently. This type I-type II transition can be monitored by the magnetic-field dependence of the excitonic transitions, particularly of those that involve the J_z = -3/2 heavy-hole state, such as the σ⁺e1hh1 excitonic transition in the quantum well. The magnetic field dependences of the heavy-hole excitonic transitions obtained from the BPR spectra were compared with calculations in which the effects of chemical valence band offset (VBO), strain, exciton binding energy, interface roughness and, most importantly, the enhanced paramagnetism at the interfaces were accounted for. It was found that a determination of the chemical VBO is independent of the state of the interface for the samples with wider well width (≥50 Å), but that effects of interface roughness cannot be neglected for smaller well widths (<50 Å). Taking all these factors into account the best agreement between experiment and calculation was obtained for a chemical VBO of 30%±10%.