Diluted Magnetic Semiconductor Layer Research Articles

Diode Heterostructures with a Ferromagnetic (Ga,Mn)As Layer

Diode p-(GaMn)As/n-InGaAs/n+-GaAs heterostructures with different thicknesses (from 5 to 50 nm) of the (Ga,Mn)As dilute magnetic semiconductor layer have been fabricated and studied. The negative magnetoresistance effect reaching 6–8% is observed in a magnetic field of 3600 Oe; this effect is retained up to temperatures of 70–80 K and related to a decrease in the charge carrier scattering due to ferromagnetic ordering in the (Ga,Mn)As layer. The dependence of the magnetoresistance on the forward bias voltage is nonmonotonic, and the magnetoresistance maximum and the operating voltage rate are dependent on the (Ga,Mn)As layer thickness. The magnetic-field dependence of the magnetoresistance have a hysteresis shape determined by the influence of the tensile stresses in the (Ga,Mn)As layer grown above the relaxed InGaAs on the magnetization component perpendicular to the structure surface.

Spin dependent transport in diluted magnetic semiconductor/superconductor tunnel junctions

A modification of Blonder–Tinkham–Klapwijk (BTK) model is proposed to describe transport properties of diluted magnetic semiconductor (DMS)/superconductor(SC)/DMS double tunneling junctions. Coherent spin-polarized transport is studied by taking into account the Andreev reflection on spatial variation of SC barrier parameters in the heterostructure. It is shown that the conductance spectrum exhibits an oscillatory behavior with quasi-particle energy, and the oscillation amplitude is reduced with increasing temperature. We also examine the dependence of tunneling magnetoresistance (TMR) on the barrier strength (κ) and spin polarization (P) of two DMS layers. Our results show that TMR decreases with increasing temperature and barrier strength, which may be useful in designing the nano spin-valve devices based on DMS and SC materials.

Angle dependence of the photonic enhancement of the magneto-optical Kerr effect in DMS layers

We investigate theoretically an angle dependence of the enhancement of the polar magneto-optical Kerr effect (MOKE) obtained thanks to a deposition of a paramagnetic Diluted Magnetic Semiconductor (DMS) layer on a one-dimensional photonic crystal layer. Our transfer-matrix-method–based calculations conducted for TE and TM polarizations of the incident light predict up to an order of magnitude stronger MOKE for a (Ga,Fe)N DMS layer when implementing the proposed design. The maximum enhancement for TE and TM polarization occurs for the light incidence at the normal and at the Brewster angle, respectively. This indicates a possibility of tuning of the MOKE enhancement by adjustment of the polarization and of the incidence angle of the light.

Spin-dependent shot noise in diluted magnetic semiconductor/semiconductor heterostructures with a nonmagnetic barrier

Abstract We investigate quantum size effect on the spin-dependent shot noise in the diluted magnetic semiconductor (DMS)/semiconductor heterostructure with a nonmagnetic semiconductor (NMS) barrier in the presence of external magnetic and electric fields. The results demonstrate that the NMS barrier plays a quite different role from the DMS layer in the electron transport process. It is found that spin-down shot noise shows relatively regular oscillations as the width of DMS layer increases, while the spin-up shot noise deceases monotonically. However, as the width of NMS layer increases, the spin-down shot noise displays irregular oscillations at first and then decreases while the spin-up shot noise decreases at a quite different rate. The results indicate that the shot noise can be used as a sensitive probe in detecting material type and its size.

Magneto-optical effects enhancement in DMS layers utilizing 1-D photonic crystal

We propose a theoretical design of a structure combining a diluted magnetic semiconductor (DMS) layer with one-dimensional photonic crystal and show that it allows for a sizable (a few-fold) enhancement of the magneto-optical Kerr effect (MOKE) magnitude. We base our calculations on the transfer matrix method assuming realistic, and optimal with respect to a sample growth, parameters. We investigate representative cases of (Ga, Mn)N or (Cd, Mn)Te DMS layers deposited on the top of a respectively GaN- or CdTe-based distributed Bragg reflector, and highlight the applicability of the proposed design for both: III–V wurtzite and II–VI zinc-blende DMS.

Thermodynamic Behavior of Excitonic Emission Properties in Manganese- and Zinc-Codoped Indium Phosphide Diluted Magnetic Semiconductor Layers

The thermodynamic behavior of excitonic emission properties in manganese- and zinc-codoped indium phosphide (InMnP:Zn) diluted magnetic semiconductor (DMS) layers was investigated. Compared to the InMnP:Zn DMS layer (Mn ≈ 0.06%), the inhomogeneous thermal-broadening of the excitonic-emission line-width in InMnP:Zn DMS layer (Mn ≈ 0.29%) is dominant at lower temperatures. This is attributed to the increase of ionized impurity scattering from Mn ions and results in the increase of exciton–phonon coupling strength. As a consequence, high Mn content can lead to low excitonic emission efficiency, although generally a larger Mn content is favorable to increase the Curie temperature of a DMS material.

Enhanced Curie temperature and nonvolatile switching of ferromagnetism in ultrathin (Ga,Mn)As channels

The integration of ferroelectric polymer gates on a Mn-doped GaAs magnetic channel provides a promising route for the persistent field-effect control of magnetic properties in high-quality diluted magnetic semiconductors (DMSs) that are otherwise incompatible with traditional oxide ferroelectrics. That control demands the thinnest possible DMS layers, for which to date the Curie temperature (${T}_{C}$) is severely depressed. Here we show that reducing the channel thickness from 7 to 3--4 nm by etching, followed by a brief 135 \ifmmode^\circ\else\textdegree\fi{}C anneal, does not degrade the ${T}_{C}$ (\ensuremath{\sim}70 K) of the 7-nm film. The channel thinning results in a dramatic threefold increase of the ${T}_{C}$ shift controlled by the ferroelectric polarization reversal. Furthermore, we obtain the same exponent $(\ensuremath{\partial}\mathrm{ln} {T}_{C}/\ensuremath{\partial}\mathrm{ln} R)\ensuremath{\equiv}\ensuremath{\gamma}\ensuremath{\approx}\ensuremath{-}0.3$ for all channels with different thicknesses, regardless of the technique used for ${T}_{C}$ determination. These results suggest that the ferromagnetic coupling in an ultrathin 3-nm channel is far from the two-dimensional limit and shows a rather bulklike behavior, similar to well-established 7-nm films.

Thermoballistic spin‐polarized electron transport in paramagnetic semiconductors

AbstractSpin‐polarized electron transport in diluted magnetic semiconductors (DMS) in the paramagnetic phase is described within the thermoballistic transport model. In this (semiclassical) model, the ballistic and diffusive transport mechanisms are unified in terms of a thermoballistic current in which electrons move ballistically across intervals enclosed between arbitrarily distributed points of local thermal equilibrium. The contribution of each interval to the current is governed by the momentum relaxation length. Spin relaxation is assumed to take place during the ballistic electron motion. In paramagnetic DMS exposed to an external magnetic field, the conduction band is spin‐split due to the giant Zeeman effect. In order to deal with this situation, we extend our previous formulation of thermoballistic spin‐polarized transport so as to take into account an arbitrary (position‐dependent) spin splitting of the conduction band. The current and density spin polarizations as well as the magnetoresistance are each obtained as the sum of an equilibrium term determined by the spin‐relaxed chemical potential, and an off‐equilibrium contribution expressed in terms of a spin transport function that is related to the splitting of the spin‐resolved chemical potentials. The procedures for the calculation of the spin‐relaxed chemical potential and of the spin transport function are outlined. As an illustrative example, we apply the thermoballistic description to spin‐polarized transport in DMS/NMS/DMS heterostructures formed of a nonmagnetic semiconducting sample (NMS) sandwiched between two DMS layers. We evaluate the current spin polarization and the magnetoresistance for this case and, in the limit of small momentum relaxation length, find our results to agree with those of the standard drift‐diffusion approach to electron transport.

Thermoballistic spin-polarized electron transport in paramagnetic semiconductors

Spin-polarized electron transport in diluted magnetic semiconductors (DMS) in the paramagnetic phase is described within the thermoballistic transport model. In this (semiclassical) model, the ballistic and diffusive transport mechanisms are unified in terms of a thermoballistic current in which electrons move ballistically across intervals enclosed between arbitrarily distributed points of local thermal equilibrium. The contribution of each interval to the current is governed by the momentum relaxation length. Spin relaxation is assumed to take place during the ballistic electron motion. In paramagnetic DMS exposed to an external magnetic field, the conduction band is spin-split due to the giant Zeeman effect. In order to deal with this situation, we extend our previous formulation of thermoballistic spin-polarized transport so as to take into account an arbitrary (position-dependent) spin splitting of the conduction band. The current and density spin polarizations as well as the magnetoresistance are each obtained as the sum of an equilibrium term determined by the spin-relaxed chemical potential, and an off-equilibrium contribution expressed in terms of a spin transport function that is related to the splitting of the spin-resolved chemical potentials. The procedures for the calculation of the spin-relaxed chemical potential and of the spin transport function are outlined. As an illustrative example, we apply the thermoballistic description to spin-polarized transport in DMS/NMS/DMS heterostructures formed of a nonmagnetic semiconducting sample (NMS) sandwiched between two DMS layers. We evaluate the current spin polarization and the magnetoresistance for this case and, in the limit of small momentum relaxation length, find our results to agree with those of the standard drift-diffusion approch to electron transport.

Propagation dynamics of exciton spins in a high‐density semiconductor quantum dot system

AbstractWe study propagation dynamics of exciton spins in a system composed of high‐density self‐assembled quantum dots (QDs) of CdSe and a diluted magnetic semiconductor (DMS) layer of ZnMnSe, where spin‐polarized excitons are generated in the DMS layer and are subsequently injected into the QDs. The degree of circular polarization, P, of excitonic photoluminescence at 5 T in the coupled QDs shows a rapid increase with increasing delay time after a linearly polarized pulsed excitation, indicating the exciton‐spin injection from the ZnMnSe to CdSe‐QDs. The P value tends to decay gradually because of the exciton‐spin relaxation in the QDs after the injection. The spin‐polarized excitons in the QD ensemble migrate simultaneously from the QDs with higher exciton energies to those with lower exciton energies, where the exciton‐spins relax inside each dot during the migration. Therefore, the observed P values in the QD emission band are affected by this feeding of the relaxed spins. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-temperature ferromagnetism in laser-deposited layers of silicon and germanium doped with manganese or iron impurities

The paper reports on the results of a study of the synthesis conditions effects on magnetic and transport properties of nanosized layers of high- T c diluted magnetic semiconductors (DMS), such as Ge:Mn, Si:Mn and Si:Fe, fabricated by laser-plasma deposition over a wide range of the growth temperature, T g=(20–550) °C on single-crystal GaAs or Al 2O 3 substrates. Ferromagnetism of the layers was detected by measurement data of the magneto-optical Kerr effect, anomalous Hall effect, negative magnetoresistance and ferromagnetic resonance (FMR) at 5–500 K. The optimum growth temperature, T g, for Si:Mn/GaAs layers with T c≈400 K is shown to be about 400 °C. The Si:Mn/Al 2O 3 layers with 35% of Mn have the metal-type of conductivity with manifestation of magnetization up to room temperature. Different types of uniformly doped structures and digital alloys have been investigated. In contrast to GaSb:Mn films, Si-based ferromagnetic layers have strongly different magnetic and electric properties in case of uniformly doped structures and digital alloys. Positive results of the Fermi level variation effect on the improvement of Si- and Ge-based DMS layers have been gained on the use of additional doping with shallow acceptor Al impurity which contributes to the increase of the hole concentration and the RKKY exchange interaction of 3d-ions. The Ge:(Mn, Al)/GaAs or Ge (Mn, Al)/Si layers grown at 20 °C feature surprising extraordinary angular dependence of FMR.

Magneto-optical and tunable laser excitation spectroscopy of spin-injection and spin loss from Zn(Cd)MnSe diluted magnetic quantum well to CdSe non-magnetic quantum dots

Magneto-optical spectroscopy in combination with tunable laser spectroscopy is employed to study the effect of sample design on optical spin-injection efficiency from a Zn(Cd)MnSe diluted magnetic semiconductor (DMS) into non-magnetic CdSe quantum dots (QDs). Two samples with inverted growth sequence of the DMS and QD layers were grown for this purpose. Despite observation of clear evidence for exciton/carrier-injection from the DMS to the QDs in both structures, their spin-injection efficiency is markedly different. The structure grown with the QD layer topmost exhibits rather efficient spin-injection, while the structure grown with the DMS layer topmost does not show any sign of spin-injection. Even in the former case, the spin-polarization of the injected excitons/carriers is deduced to be about 32%, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin-injection across the heterointerfaces.

Structural and magnetic properties of GeMn diluted magnetic semiconductor

Research on diluted magnetic semiconductors (DMS) has generated great interest for their potential application of semiconductors-based magnetic elements. Ferromagnetic (FM) semiconductors simultaneously exhibit semiconducting properties and spontaneous long-range FM order. The coexistence of these properties in a single material provides fertile ground for both fundamental studies and industrial applications. However, device applications have languished because of low magnetic ordering (Curie) temperature and the inability to incorporate these materials in conventional semiconductor material device. In this context, we have focused our study on structural and magnetic properties of Mn x Ge 1− x DMS layers grown by molecular beam epitaxy on semiconducting Ge(1 1 1) substrate. Mn has been incorporated with increasing concentration from x = 0.02 to 0.1. Their structure has been characterized by transmission electron microscope using both conventional imaging and electronic diffraction. Ge–Mn alloy clusters have been clearly observed for the highest Mn concentration, and alloy phase identified. These observations were completed by classic SQUID magnetometry measurements. We will evidence a critical concentration of Ge above which precipitates of Ge 3Mn 5 form, enhancing the FM signal in the layer.

SPIN DEPENDENT TRANSPORT IN QUANTUM AND CLASSICALLY CONFIGURED DEVICES

This paper presents of development of quantum transport equations for barrier devices with both electron and hole transport in dilute magnetic semiconductor (DMS) structures. The equations are developed from the time dependent equation of motion of the density matrix equation in the coordinate representation, from which both the spin drift and diffusion and transient Wigner equations are obtained, for a system in which high 'g' factor materials result in significant spin-splitting of the valence and conduction bands. Then for a structure in which the DMS layer is confined to the first barrier solutions to the coupled Poisson's and spin dependent Wigner equations yield the IV and carrier distributions. Negative differential conductance as well as the significant unequal spinup and spin down charge distributions are obtained.

Angular dependence of tunneling magnetoresistance in magnetic semiconductor heterostructures

Theoretical studies on spin-dependent transport in magnetic tunnel heterostructures consisting of two diluted magnetic semiconductors (DMS) separated by a nonmagnetic semiconductor (NMS) barrier, are carried in the limit of coherent regime by including the effect of angular dependence of the magnetizations in DMS. Based on parabolic valence band effective mass approximation and spontaneous magnetization of DMS electrodes, we obtain an analytical expression of angular dependence of transmission for DMS/NMS/DMS junctions. We also examine the dependence of spin polarization and tunneling magnetoresistance (TMR) on barrier thickness, temperature, applied voltage and the relative angle between the magnetizations of two DMS layers in GaMnAs/GaAs/GaMnAs heterostructures. We discuss the theoretical interpretation of this variation. Our results show that TMR of more than 65% are obtained at zero temperature, when one GaAs monolayer is used as a tunnel barrier. It is also shown that the TMR decreases rapidly with increasing barrier width and applied voltage; however at high voltages and low thicknesses, the TMR first increases and then decreases. Our calculations explain the main features of the recent experimental observations and the application of the predicted results may prove useful in designing nano spin-valve devices.

Negatively charged excitons as an optical probe of spin injection from Cd0.73Mg0.24Mn0.03Te spin aligner into a graded bandgap Cd1−xMgxTe/CdTe quantum well structure

The diffusion of spin-polarized electrons through a graded-bandgap Cd1−xMgxTe layer has been studied. The quaternary compound Cd0.73Mg0.24Mn0.03Te diluted magnetic semiconductor (DMS), with a large energy gap (Eg = ∼2 eV), was used as a spin-polarized source in the presence of an external magnetic field. We exploit the spin-dependent feature of trion formation (X−s), i.e. negatively charged excitons in a spin-singlet ground state, as a sensitive probe of electron spin injection into a CdTe quantum well (QW), separated from the injector by a thick non-magnetic graded gap Cd1−xMgxTe layer (which bends the band edges of the structure similar to electrical biasing). Continuous wave and time-resolved magneto-photoluminescence and reflectivity measurements have been performed. Our results show that at small magnetic fields the ratio of the density of X−s and that of neutral excitons, X0, is large when the light is absorbed only in the QW; it becomes small when the absorption occurs in the non-magnetic graded barrier. It is re-enhanced significantly again when the absorption occurs also in the magnetic layer. The latter finding indicates that electrons do not lose completely their initial spin polarization acquired in the DMS layer (due to the giant Zeeman splitting effect in this layer) when they pass through the non-magnetic region into the QW where they enhance the X−s populations. This re-enhancement occurs in spite of a large energy relaxation (∼300 meV) that the carriers must undergo.

Control of spin functionality in ZnMnSe-based structures: Spin switching versus spin alignment

The ability of attaining desired spin functionality by adjusting structural design is demonstrated in diluted magnetic semiconductor (DMS) quantum structures based on II–VI semiconductors. The following spin enabling functions are achieved by tuning the ratio between the rates of exciton spin relaxation within the DMS and exciton escape from it to an adjacent nonmagnetic spin detector. Spin switching is realized when using a thin layer of Zn0.95Mn0.05Se as a spin manipulator and is attributed to a fast exciton escape from the DMS preceding the spin relaxation. Spin alignment is accomplished in tunneling structures where the presence of an energy barrier inserted between a spin manipulator (a DMS-based superlattice) and a spin detector ensures a slow escape rate from the DMS layer.

Exciton Spin Manipulation in ZnMnSe-Based Structures

Strong effect of structural design on spin functionality is observed in quantum structures based on II–VI semiconductors. Spin switching is realized when using a thin layer of Zn0.95Mn0.05Se diluted magnetic semiconductor (DMS) as a spin manipulator. This is evident from the polarization of photoluminescence related to a spin detector (an adjacent nonmagnetic quantum well (QW)) measured under the resonant excitation of the spin-up and spin-down states of the DMS, which is identical in value but opposite in sign. The achieved spin switching is suggested to reflect fast carrier diffusion from the DMS due to the absence of an energy barrier between the upper spin state of the DMS layer and the QW. On the other hand, the spin alignment is accomplished in the tunneling structures where the presence of the energy barrier inserted between a spin manipulator (i.e., a ZnMnSe/CdSe DMS superlattice) and a spin detector ensures a slow escape rate from the DMS layer.

Enhancement of direct (type-I) excitonic transitions in ZnTe/CdSe-based type-II Bragg confining structures

We have used distributed Bragg reflectors (DBRs) designed for electron de Broglie wavelengths to achieve enhanced localization of above-barrier states in the conduction band of a ZnTe/CdSe-based type-II heterostructure. We studied the effect of such DBRs by observing excitonic transitions between above-barrier electron states which are Bragg confined in the barrier layer of the conduction band and heavy holes localized in the quantum well of the valence band. In a type-II system, both carriers are thus confined in the same layer, leading to a direct (type-I-like) excitonic transition. In order to identify the physical region where this transition actually occurs, we have used a diluted magnetic semiconductor (DMS) alloy Zn0.94Mn0.06Te as the confining layer. In such a structure, the giant Zeeman splitting of the band edges in the DMS layer serves to identify the location where the states participating in the transition are confined. To enhance the intensity of such direct transitions, we have sandwiched the central DMS layer between two DBR stacks, each consisting of 10 alternating quarter-wave layers of ZnTe and CdSe tailored to the de Broglie wavelength of the above-barrier state in the conduction band of the central layer. As expected, the transition of interest was observed at an energy slightly higher than the band gap of the DMS layer. We have observed this direct transition in absorption and, more importantly, also in photoluminescence, attesting to the very significant confinement of the above-barrier layer in such DBR-based structures.

Enhancement of Zeeman splitting in double quantum wells containing ultrathin magnetic semiconductor layers

We have studied a series of ZnSe/Zn 1− y Cd y Se double quantum wells (DQWs) separated by ultra-thin diluted magnetic semiconductor (DMS) layers. Large Zeeman shifts of interband transition energies were observed as the magnetic field was applied, and were compared with magnetic splittings calculated using the 8-band k · p model, including the effect of strain. The Zeeman splitting observed in these DQW structures was found to be significantly enhanced compared to standard calculation. The enhancement of the Zeeman splitting is discussed in terms of effects of reduced dimensionality on the magnetic properties of thin DMS layers.