Spin separation in diluted magnetic semiconductor quantum well systems (invited)

Abstract

Heterostructures containing diluted magnetic semiconductor (DMS) layers offer the possibility of magnetically tuning the heterojunction band alignment due to the extraordinarily large spin-splitting of the DMS bands (large effective g-factor). This field-dependent band alignment has significant consequences for spin-dependent carrier confinement as evidenced in magnetooptic or magneto-transport experiments. We have examined two wide-gap DMS quantum well systems in which the band alignment is dominated by the DMS spin-splitting rather than by the more commonly observed effects of differences in bandgap, natural band offset, and strain. Quantum well structures with (Zn,Fe)Se or (Zn,Mn)Se barriers and ZnSe wells have been grown to investigate magnetically tuned, spin-dependent quantum confinement. In these systems, the band offset appears almost entirely in the conduction band, so that the electrons are confined to the ZnSe wells. However, the hole confinement is continuously tunable by an external magnetic field applied normal to the layer plane, since the valence band spin-splitting is approximately an order of magnitude larger at modest fields (1 T) than the zero field VB offset. This produces a field-induced spin dependent type I/type II band alignment and a consequent spatial spin separation of the holes. The quantum well structure exhibits a type I "straddling" alignment for the mj=+3/2 level, with the spin "up" holes localized in the ZnSe wells, and a type II "staggered" alignment for the mj=-3/2 level in which the spin "down" holes are localized in the barriers. The observed exciton splittings, intensities and temperature dependence are consistent with this model. In the Zn0.91Mn0.09Se/ZnSe system, a second bound state is observed in the conduction band in the σ- polarization in magnetoreflectivity measurements. A quantitative fit to the data yields a value of approximately 1 meV for the zero field valence band offset. These systems represent the first DMS-based heterostructures in which such field-dependent confinement has been observed.