Magnitude of Magnetic Field Dependence of a Possible Selective Spin Filter in ZnSe/Zn_{1-x}Mn_{x}Se Multilayer Heterostructure

Spin-polarized transport through a band-gap-matched ZnSe/Zn_{1-x}Mn_{x} Se/ZnSe/Zn_{1-x}Mn_{x}Se/ZnSe multilayer structure is investigated. The resonant transport is shown to occur at different energies for different spins owing to the split of spin subbands in the paramagnetic layers. It is found that the polarization of current density can be reversed in a certain range of magnetic field, with the peak of polarization moving towards a stronger magnetic field for increasing the width of central ZnSe layer while shifting towards an opposite direction for increasing the width of paramagnetic layer. The reversal is limited in a small-size system. A strong suppression of the spin up component of the current density is present at high magnetic field. It is expected that such a reversal of the polarization could act as a possible mechanism for a selective spin filter device

BINDING ENERGIES OF EXCITONS IN QUANTUM WELL STRUCTURES

Binding energies of excitons in quantum well structures have been studied by solving the time-dependent Schrodinger equation where the potential is made up by the confining quantum well potentials of arbitrary form and the Coulomb interaction between the electron and hole. The problem is solved without the usually assumed variational procedure and the separation ansatz for the confined electron and hole states. The wave functions for electrons and holes can be extracted from the exciton wave function and are used for the interpretation of the charge localization. PACS numbers: 71.35.-y, 73.20.Dx Theory Excitons in dimensional reduced structures have been intensively studied by various theoretical methods. In most approaches variational procedures have been included and a separation ansatz for the electron and hole wave functions has been used In this paper we present an approach which allows us to avoid the use of a large number of variational parameters as well as the separation ansatz for the z-dependence of the exciton wave function. The only fitting parameters are the effective masses of the electron and the hole. Heavy-hole (RH) and light-hole (LH) excitons are treated separately by solving two sets of 2D time dependent Schrödinger equations. In the first step, the coupling between HH and LH bands and the influence of band nonparabolicities are neglected. This procedure is justified because of our intention to study in first respect the dependence of the binding energies on different types of confinement potentials and the interplay with the Coulomb interaction in the localization of electrons and holes. (459

Photoluminescence and photoluminescence excitation studies of lateral size effects in Zn_{1-x}Mn_xSe/ZnSe quantum disc samples of different radii

Quantum disc structures (with diameters of 200 nm and 100 nm) were prepared from a Zn_{0.72}Mn_{0.28}Se/ZnSe single quantum well structure by electron beam lithography followed by an etching procedure which combined dry and wet etching techniques. The quantum disc structures and the parent structure were studied by photoluminescence and photoluminescence excitation spectroscopy. For the light-hole excitons in the quantum well region, shifts of the energy positions are observed following fabrication of the discs, confirming that strain relaxation occurs in the pillars. The light-hole exciton lines also sharpen following disc fabrication: this is due to an interplay between strain effects (related to dislocations) and the lateral size of the discs. A further consequence of the small lateral sizes of the discs is that the intensity of the donor-bound exciton emission from the disc is found to decrease with the disc radius. These size-related effects occur before the disc radius is reduced to dimensions necessary for lateral quantum confinement to occur but will remain important when the discs are made small enough to be considered as quantum dots.Comment: LaTeX2e, 13 pages, 6 figures (epsfig

Mechanisms of enhancement of light emission in nanostructures of II–VI compounds doped with manganese

Intra-shell transitions of transition metal and rare earth ions are parity forbidden processes. For Mn²⁺ ions this is also a spin forbidden process, i.e., light emission should be inefficient. Surprisingly, it was reported that in nanostructures of ZnMnS the ⁴T₁ to ⁶A₁ intra-shell transition of Mn²⁺ results in a bright photoluminescence characterized by a short PL decay time. The model of a quantum confined atom was introduced to explain the observed experimental results. It was later claimed that this model is incorrect. Based on the results of our photoluminescence, photoluminescence kinetics, time-resolved photoluminescence, electron spin resonance and optically detected magnetic resonance investigations we confirm photoluminescence enhancement and decrease of photoluminescence lifetime and relate these effects to spin dependent magnetic interactions between localized spins of Mn²⁺ ions and spins/magnetic moments of free carriers. This mechanism is active in both bulk and in low-dimensional structures, but is significantly enhanced in nanostructure samples

Coherent magnetic semiconductor nanodot arrays

In searching appropriate candidates of magnetic semiconductors compatible with mainstream Si technology for future spintronic devices, extensive attention has been focused on Mn-doped Ge magnetic semiconductors. Up to now, lack of reliable methods to obtain high-quality MnGe nanostructures with a desired shape and a good controllability has been a barrier to make these materials practically applicable for spintronic devices. Here, we report, for the first time, an innovative growth approach to produce self-assembled and coherent magnetic MnGe nanodot arrays with an excellent reproducibility. Magnetotransport experiments reveal that the nanodot arrays possess giant magneto-resistance associated with geometrical effects. The discovery of the MnGe nanodot arrays paves the way towards next-generation high-density magnetic memories and spintronic devices with low-power dissipation

Binding Energies of Excitons in Quantum Well Structures

Binding energies of excitons in quantum well structures have been studied by solving the time-dependent Schrödinger equation where the potential is made up by the confining quantum well potentials of arbitrary form and the Coulomb interaction between the electron and hole. The problem is solved without the usually assumed variational procedure and the separation ansatz for the confined electron and hole states. The wave functions for electrons and holes can be extracted from the exciton wave function and are used for the interpretation of the charge localization