Magnetic Semiconductor Layers Research Articles

Spin-Current Oscillations in Diluted Magnetic Semiconductor Multibarrier GaMnAs/GaAs: Role of Temperature and Bias Voltage

This paper has studied the electronic properties of multi-diluted magnetic semiconductor (DMS) layers Ga(1 − x)MnxAs interposed between nonmagnetic GaAs layers. The asymmetry of confining potential on the transmission coefficient by tuning the temperature and the size of the (DMS) layers was discussed. The diluted magnetic layers Ga(1 − x)MnxAs behave as barriers for spin-up holes and quantum wells for spin-down holes. Furthermore, we have addressed the impact of an applied bias voltage and the temperature on the variation of the spin-polarization and spin current densities. Our findings reveal that the transmission coefficients present an oscillating behavior due to the resonant states and strongly depend on the temperature of the system and the number of magnetic layers. Furthermore, the obtained results demonstrated that the number of these states is multiplied by augmenting the magnetic layers. Moreover, we demonstrate that the asymmetric structure presents a completely different transmission of holes than the symmetric structure. Furthermore, the negative differential resistance (NDR) is demonstrated in the current density variations. Especially, this (NDR) was more intense for spin-up holes than spin-down holes. The findings in the present paper can be useful in manufacturing spin-filters by adjusting the values of the temperature and the external voltages.

Magneto-Optics of Excitons Interacting with Magnetic Ions in CdSe/CdMnS Colloidal Nanoplatelets.

Excitons in diluted magnetic semiconductors represent excellent probes for studying the magnetic properties of these materials. Various magneto-optical effects, which depend sensitively on the exchange interaction of the excitons with the localized spins of the magnetic ions can be used for probing. Here, we study core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets hosting diluted magnetic semiconductor layers. The inclusion of the magnetic Mn2+ ions is evidenced by three magneto-optical techniques using high magnetic fields up to 15 T: polarized photoluminescence, optically detected magnetic resonance, and spin-flip Raman scattering. We show that the holes in the excitons play the dominant role in exchange interaction with magnetic ions. We suggest and test an approach for evaluation of the Mn2+ concentration based on the spin-lattice relaxation dynamics of the Mn2+ spin system.

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.

Enhancing spin injection efficiency through half-metallic miniband conduction in a spin-filter superlattice

We theoretically and numerically studied the band structure and spin transport of electrons subject to a superlattice structure where magnetic semiconductor layers lie between normal semiconductor layers to form periodic spin-filter tunnel barriers. In this alternately deposited superlattice structure, due to the induced periodicity of the envelope wavefunctions, there are additional allowed and forbidden energy regions established, i.e. forming minibands that are far narrower than the conventional conduction bands. The number and thickness of the stacked potential profiles can finely tune these minibands. The spin dependent potential barriers also induce spin splitting at the bottom of each miniband, which generates strongly spin-dependent miniband conduction. Most strikingly, the lowest lying miniband is 100% spin-polarized mimicking a half-metallic behavior on this conduction channel. The total transmission electron current carries thus near-perfectly polarized spin currents when the superlattice falls into suitable miniband conduction regime. This half-metallic miniband enhanced spin-filtering capability paves the way to generate highly polarized spin current without incurring exponentially increased device impedance, as usually happens when only a single spin-filter barrier is applied.

Peculiarities of the exciton scattering in double semiconductor quantum wells with disordered layers

Effects of the presence of isolated disordered layers on the exciton scattering by compositional fluctuations in double semiconductor quantum wells have been studied. In the structures containing both ordered and disordered layers, the probability of the scattering depends on the degree of the exciton wavefunction localization in the disordered layers, where it interacts with the fluctuations. For some parameters of the structure the exciton wavefunction can penetrate deeply into the ordered layers of the structure, which leads to a sharp drop of the probability of the scattering and, consequently, to the narrowing of the optical exciton bands. It has been shown that for heterostructures containing diluted magnetic semiconductor layers, the probability of the scattering can be tuned by external magnetic field.

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.

Large and controllable tunneling magnetoresistance in ferromagnetic/magnetic-semiconductor/ferromagnetic trilayers

In this work, we selected a magnetic-semiconductor as an interlayer and investigated the electronic transport properties in the ferromagnetic/ferromagnetic-semiconductor/ferromagnetic (FM/FS/FM) trilayers. The results indicate that the large TMR comparable to that in ferromagnetic/metal oxide/ferromagnetic sandwich can be obtained in the FM/FS/FM multilayers with considering the spin filter effect in the magnetic semiconductor layer. Moreover, the transmission coefficient and TMR can be tuned through thickness, Rashba spin-orbit coupling strength and molecular field of the magnetic semiconductor. Our calculations could provide a way to design the semiconductor spintronic devices with excellent and controllable properties.

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.

Room-temperature ferromagnetic behaviour of InMnAs films grown by laser ablation technique

InMnAs layers were fabricated by pulsed laser ablation of solid targets (Mn and InAs) in flow of hydrogen and arsine. The InMnAs layers with thickness ranging from 130 to 270 nm were deposited on semi-insulating GaAs (100) substrates at 320°C. The Mn quantity was controlled by changing ratio of sputtering time of Mn and InAs targets. The X-ray diffraction measurements identified the InMnAs as mosaic monocrystal with MnAs phase texture inclusions. Room temperature ferromagnetism of these InMnAs layers is evident from magnetometry and magneto-optical measurements. In addition, the InMnAs layers show anomalous Hall effect with the hysteresis loop and saturation magnetic field HS ≈ 2500 Oe at temperatures up to 300K depending on the Mn content. The Curie temperature higher than 300K allows using these magnetic semiconductor layers as a source of spin polarized carriers in room temperature spintronic devices.

Quantum size effects on spin-tunneling time in a magnetic resonant tunneling diode

We study theoretically the quantum size effects of a magnetic resonant tunneling diode (RTD) with a (Zn,Mn)Se dilute magnetic semiconductor layer on the spin-tunneling time and the spin polarization of the electrons. The results show that the spin-tunneling times may oscillate and a great difference between the tunneling time of the electrons with opposite spin directions can be obtained depending on the system parameters. We also study the effect of structural asymmetry which is related to the difference in the thickness of the nonmagnetic layers. It is found that the structural asymmetry can greatly affect the traversal time and the spin polarization of the electrons tunneling through the magnetic RTD. The results indicate that, by choosing suitable values for the thickness of the layers, one can design a high-speed and perfect spin-filter diode.

Transfer dynamics of spin-polarized excitons into semiconductor quantum dots

We study on the transfer dynamics of spin-polarized excitons into self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ZnMnSe through a ZnSe barrier layer. A variation in the degree of circular polarization P of excitonic photoluminescence in the coupled QDs was observed between samples with different thicknesses ( L B) of the non-magnetic barrier. Relatively high values of P (∼0.3) were obtained in the samples with L B=1 or 2 nm, and the time dependences of the P value indicate the spin-transfer dynamics. From the L B dependence of the spin-transfer time, the spin-transfer mechanism is discussed, namely spin-conserving energy transfer.

Electronic transport properties of the multilayer structure double spin-filter tunnel junction

Based on the transfer matrix method and the quantum coherent transport theory of Mireles and Kirczenow, the effects of the Rashba spin-orbit coupling and the spin-filter on the electronic transport properties in the NM/FS1/I/FS2/NM（NM represents the nonmagnetic metal layer, I represents the nonmagnetic insulator layer, and FS represents the magnetic semiconductor layer）double spin-filter tunnel junction are investigated. The influence of thickness of insulator layer and magnetic semiconductor layer on the tunnel magnetoresistance （TMR） and conductance are studied for different Rashba spin-orbit coupling strength and the different angle θ between the two magnetic moments of the left and right magnetic semiconductor layer. The results indicate that: in the presence of the spin-filter effect and the Rashba spin-orbit coupling interaction in the magnetic semiconductor layer, large TMR can be obtained in this double spin-filter junction. With the strength of Rashba spin-orbit coupling increasing, the tunnel magnetoresistance and conductance exhibit rapidly oscillating behavior and the oscillation period decreases gradually.

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)

X-ray photoelectron and auger spectroscopy analysis of Ge:Mn-based magnetic semiconductor layers

Thin (∼60 nm) germanium layers supersaturated with a manganese impurity of 10–16 at % have been studied by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The layers have been fabricated by pulsed laser deposition onto a semi-insulating single-crystal GaAs substrate. The results of XPS analysis of the Ge:Mn layers reveal a change in the line shape of germanium and manganese (2p) in the surface region compared to deeper layers, which indicates a transition from the oxidized form of the base material (Ge2+ and Ge1+) and impurity (Mn2+) near the surface to the unoxidized state of germanium (Ge 0) and manganese (Mn0) in the interior of the layer. The XPS spectra of the valence electrons of the Ge:Mn structure indicate that the density of states in the valence band of the ferromagnetic Ge:Mn structures is caused not only by mechanical mixing of germanium and manganese. The composition of heterogeneous inclusions in Ge:Mn films has been studied using scanning Auger microscopy.

Spin-Injection Dynamics and Effects of Spin Relaxation in Self-Assembled Quantum Dots of CdSe

We study the spin injection dynamics and the effects of spin relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ZnMnSe, where spin-polarized excitons can be injected from the DMS into the QDs because of the potential difference. The degree of circular polarization, $P$, of excitonic photoluminescence (PL) at 5 T in the coupled QDs shows a rapid increase with increasing delay time after a linearly polarized pulse excitation, indicating the spin-injection dynamics. The $P$ value tends to decay gradually because of the exciton-spin relaxation in the QDs after the spin injection. The spin-polarized excitons in the QD ensemble migrate simultaneously from QDs with higher exciton energies to those with lower exciton energies. This inter-dot transfer of excitons also affects the $P$ value in the lower energy region of the QD-emission band because the excitons lose their spin polarizations due to the spin relaxation in the dots during the migration. The detailed analysis for the exciton-spin transfer is presented in a coupled QD system after spin injection.

Spin transport properties in double-barrier systems with diluted magnetic semiconductor doped layers

We have studied spin-polarized currents in resonant tunneling diodes containing diluted magnetic semiconductor layers with different geometries. The spin polarization effects on the resonant current are studied within Green's function formalism following the diagrammatic technique for non-equilibrium processes as proposed by Keldysh, using the one-band tight-binding modeling. The magnetic impurities can be located inside the well or the barrier layers of a diode sample. The resonant I×V curves are analyzed as a function of the magnetic potential strength induced by the magnetic ions.

Magnetic Field Control of THz Relaxation Oscillations in RTDs With Diluted Magnetic Semiconductor Layers

This discussion concentrates on novel semiconductor quantum barrier/well devices that utilize spin-control mechanisms available in diluted magnetic materials for achieving higher-level functionality (e.g., transistor action) at very high switching speeds and frequencies. The potential simplicity in the design of DMS devices compared with standard three terminal transistors with gate controlled I-V characteristics, is that for DMS structures, no more than two terminals are required, as the magnetic field in controlling the output of the DMS device functions as a controlling third contact. Indeed, properly designed, the magnetic field can transform a passive device into an active device, tune the output of a resonant tunneling device (RTD) fabricated with DMS layers and modify the logic state of a device.

SPIN DEPENDENT WIGNER FUNCTION SIMULATIONS OF DILUTED MAGNETIC SEMICONDUCTOR SUPERLATTICES – B FIELD TUNING

Two terminal devices have traditionally provided band-structure based high frequency operation. Third terminal control often involves hybrid design approaches. The presence of diluted magnetic semiconductor layers in device fabrication should permit the magnetic field to function as a pseudothird terminal. This is discussed for single barrier, double barrier and superlattice structures, where control is demonstrated. The limits of high frequency operation are discussed in general terms with application to barrier devices and superlattices containing DMS layers.

Dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots of CdSe coupled with a diluted magnetic semiconductor layer ofZn0.80Mn0.20Se

We study the dynamics of exciton-spin injection, transfer, and relaxation in self-assembled quantum dots (QDs) of CdSe coupled with a diluted magnetic semiconductor (DMS) layer of ${\mathrm{Zn}}_{0.80}{\mathrm{Mn}}_{0.20}\mathrm{Se}$, where spin-polarized excitons can be injected from the DMS into the QDs. The degree of circular polarization $P$ of excitonic photoluminescence (PL) at $5\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ in the coupled QDs exhibits a rapid increase with increasing delay time, up to $+0.3$ at $25\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ after the pulse excitation of the DMS by a linearly polarized light. This development of a positive $P$ value directly reflects the spin-injection dynamics from the DMS, since the intrinsic polarization of the QD excitons due to Zeeman splitting is $P\ensuremath{\sim}\ensuremath{-}0.1$ when only the QDs are selectively excited. The $P$ value gradually decays with time after reaching its maximum, as a result of the exciton-spin relaxation with a time constant of $800\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ in the QDs. Time-resolved circularly polarized PL spectra immediately after the pulse excitation directly show the exciton-energy dependence of the spin-injection dynamics in the QD ensemble, where two-dimensional-like QDs with higher exciton energies show higher receptivity to the spin-polarized excitons than three-dimensional-like dots with lower exciton energies. A rate equation analysis reveals all time constants responsible for the spin-injection dynamics. We deduce a time constant of $10\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for the spin injection. The spin-injection efficiency of 0.94 is also obtained, which corresponds to the ratio between the number of the spin-polarized excitons responsible for the rise of the positive $P$ value in the QD emission and the total number of the excitons injected from the DMS. Moreover, we observe that interdot exciton transfer significantly affects the $P$ value within the QD emission band after the fast spin injection, in addition to the spin relaxation within the QDs.