Magneto-optical investigation of diluted magnetic semiconductor heterostructures: Spin superlattices and single barriers

This dissertation describes magneto-optical absorption measurements on {dollar}rm ZnSe/Znsb{lcub}1-x{rcub}Mnsb{lcub}x{rcub}Se{dollar} superlattices with {dollar}xapprox 0.04,{dollar} as well as on {dollar}rm ZnSe/Znsb{lcub}1-x{rcub}Mnsb{lcub}x{rcub}Se, ZnTe/Znsb{lcub}1-x{rcub}Mnsb{lcub}x{rcub}Te,{dollar} and ZnTe/CdSe single quantum barrier structures. We first give a general presentation of the properties of diluted magnetic semiconductors, with emphasis on their striking magnetic properties. We then discuss the theoretical background of electronic states in semiconductor superlattices within the envelope function approximation. Next, we briefly discuss the experimental aspects of this investigation, including example preparation and the apparatus for the magneto-optical absorption experiments.; We then demonstrate the formation of a spin superlattice in {dollar}rm ZnSe/Znsb{lcub}0.96{rcub}Mnsb{lcub}0.04{rcub}Se{dollar} multilayer structures by both magneto-optical experiments and theoretical calculations. This system has a nearly ideal band alignment in zero field, i.e., the band offsets in both the conduction and the valence bands are close to zero. We show that, when a magnetic field is applied, the spin-up and the spin-down states in this system become spatially separated in both the conduction and the valence bands, forming a periodic sequence of layers with opposite spin states of holes and electrons.; We also report that the existence of small but finite magnetic-field-induced offsets leads to the observation of a series of transitions associated with above-barrier states in such spin-superlattice systems. The observed transitions include 11h{dollar}sb0,{dollar} 11h{dollar}sb1,{dollar} 22h{dollar}sb0,{dollar} 22h{dollar}sb1,{dollar} 33h{dollar}sb0{dollar} for {dollar}Delta n=0,{dollar} 23h{dollar}sb1{dollar} for {dollar}Delta n=1,{dollar} and 13h{dollar}sb0{dollar} for {dollar}Delta n=2.{dollar} The discussion and analysis of these transitions are carried out based on their magneto-optical behavior, with the aid of theoretical calculations. The theoretical calculations make the task of transition identification tractable by predicting the peak intensity, the magnetic-field-dependence of the individual states and of the transition lines, as well as the possible relaxation of selection rules as the offsets become larger.; We also investigate transitions involving quasi-localized states in {dollar}rm ZnSe/Znsb{lcub}1-x{rcub}Mnsb{lcub}x{rcub}Se{dollar} and {dollar}rm ZnTe/Znsb{lcub}1-x{rcub}Mnsb{lcub}x{rcub}Te{dollar} type-I single-barriers, and in the ZeTe/CdSe type-II single layer systems. Quasi-bound exciton transitions appearing at energies above the energy gaps of the barrier materials clearly show that the quasi-localized states lie at energies above the top of the barrier. Magneto-optical experiments confirm that these states are localized in the barrier region. A blue shift of the transition energy, as the barrier width decreases, further signals that the mechanism of localization of tbe quasi-localized state is in many ways similar to the confinement mechanism of states in the well. Finally, we discuss the lifetime of quasibound excitons, based on the behavior of the observed line width in these single barrier systems.