Thermal spin-wave scattering in hot-electron magnetotransport across a spin valve

The role of thermal scattering in spin-dependent transport of hot electrons at 0.9 eV is studied using a spin-valve transistor with a soft Ni(80)Fe(20)/Au/Co base. Spin-dependent scattering makes the collected electron current depend sensitively on the magnetic state of the base. The magnetocurrent reaches 560% at 100 K, decays with increasing temperature, and a huge effect of 350% still remains at room temperature. The results demonstrate that thermal spin waves produce quasielastic spin-flip scattering of hot electrons, resulting in mixing of the two spin channels.     

Significant magnetoresistance enhancement due to a cotunneling process in a double tunnel junction with single discontinuous ferromagnetic layer insertion

We fabricate CoFe/AlOx/CoFe/AlOx/CoFe ferromagnetic double tunnel junctions and observe spin-dependent tunneling phenomena. A middle CoFe layer becomes discontinuous by forming CoFe particles two dimensionally, of which the average diameter is evaluated to be 2.0-4.5 nm from cross-sectional transmission electron microscopy images. Below 50 K, a Coulomb gap is observed in current-voltage curves, and both magnetoresistance ratios and resistances are found to increase significantly with decreasing temperature. This indicates that a cotunneling process is dominant within the gap, which agrees very well with theoretical prediction [Phys. Rev. Lett. 80, 1758 (1998)].     

Silicon doping effects on optical properties of InAs ultrathin layer embedded in GaAs/AlGaAs:δSi high electron mobility transistors structures

Micro-Raman scattering measurements were used to study the silicon delta-doped layer density variation effect on InAs ultrathin layer embedded in silicon-delta-doped GaAs/AlGaAs high electron mobility transistors (HEMTs) structures properties. These structures were grown by molecular beam epitaxy on GaAs substrates with different silicon (Si) delta-doped layer densities. Two coupled plasmon–longitudinal optical (LO) phonon modes (L− and L+) were observed in the micro-Raman spectra of the Si-delta-doped samples, and both their wave numbers and intensities were dependent on the silicon delta-doped layer density. There is evidence to suggest that the increase of the Si doping level results in the increase of exciton–phonon scattering which is mainly due to the incorporation of Si and the increase of the two-dimensional electron gas (2DEG) in the InAs/GaAs interface. From fitting the temperature-dependence of full width at half maximum (FWHM) of quantum well’s photoluminescence peak (P₁) by the exciton–photon coupling model, it was found that the interaction between exciton and phonon in Si-delta-doped quantum wells was higher than that in the undoped sample. This result was confirmed as resulting from the increase of plasmon–phonon scattering which is attributed to the increase of free carriers donated from implanted Si dopant. The self-consistent Poisson–Schrödinger model calculation results are in good agreement with the experimental results, where the 2DEG densities increase linearly with increasing the Si-delta-doped layer density.     

300% magnetocurrent in a room temperature operating spin-valve transistor

Here we present the realization of a room temperature operating spin-valve transistor with huge magnetocurrent (MC=300%) at low fields. This spin-valve transistor employs hot-electron transport across a Ni₈₁Fe₁₉/Au/Co spin valve. Hot electrons are injected into the spin valve across a Si–Pt Schottky barrier. After traversing the spin valve, these hot electrons are collected using a second Schottky barrier (Si–Au), which provides energy and momentum selection. The collector current is found to be extremely sensitive to the spin-dependent scattering of hot electrons in the spin valve, and therefore on the applied magnetic field. We also illustrate the role of the collector diode characteristics in determining the magnetocurrent under collector bias.     

Electric-field-induced changes in magnetization in PZT/Insulator/CoFe nanometer multilayer

The PZT/Insulator/CoFe nanometer multilayer has been proposed for applications of the spin-dependent devices in this paper. With the strains as a response of PZT layer to an applied electric field in the structure, the magnetization in CoFe layer can be significantly altered, leading to a significance change in magnetoresistance. Our numerical results have demonstrated that the magnetization states in CoFe layer can be tuned by an electric field without a magnetic field being present.     

Reversal of spin polarization in Fe/GaAs (001) driven by resonant surface states: first-principles calculations

A minority-spin resonant state at the Fe/GaAs(001) interface is predicted to reverse the spin polarization with the voltage bias of electrons transmitted across this interface. Using a Green's function approach within the local spin-density approximation, we calculate the spin-dependent current in a Fe/GaAs/Cu tunnel junction as a function of the applied bias voltage. We find a change in sign of the spin polarization of tunneling electrons with bias voltage due to the interface minority-spin resonance. This result explains recent experimental data on spin injection in Fe/GaAs contacts and on tunneling magnetoresistance in Fe/GaAs/Fe magnetic tunnel junctions.     

Opposite spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into a ferromagnet

The spin asymmetry of elastic and inelastic scattering of nonequilibrium holes injected into Co thin films is examined using a p-type magnetic tunnel transistor. Spin-dependent transmission yields a positive or negative magnetocurrent depending on Co thickness and hole energy. Up to a critical thickness of about 3 nm, (quasi)elastic scattering dominates with a short attenuation length (<1 nm) and preferential attenuation of holes in the majority spin bands, consistent with spin-wave emission. At a larger Co thickness, inelastic scattering dominates with a larger attenuation length ( approximately 4 nm) and opposite spin asymmetry.     

Electrically detected magnetic resonance of neutral donors interacting with a two-dimensional electron gas

We have measured the electrically detected magnetic resonance of donor-doped silicon field-effect transistors in resonant X- (9.7 GHz) and W-band (94 GHz) microwave cavities. The two-dimensional electron gas resonance signal increases by 2 orders of magnitude from X to W band, while the donor resonance signals are enhanced by over 1 order of magnitude. Bolometric effects and spin-dependent scattering are inconsistent with the observations. We propose that polarization transfer from the donor to the two-dimensional electron gas is the main mechanism giving rise to the spin resonance signals.     

Monte Carlo simulation of in-plane spin-polarized transport in GaAs/GaAlAs quantum well in the three-subband approximation

We develop a Monte Carlo (MC) tool incorporated with the three-subband approximation model to investigate the in-plane spin-polarized transport in GaAs/GaAlAs quantum well. Using the tool, the effects of the electron occupation of higher subbands and the intersubband scattering on the spin dephasing have been studied. Compared with the corresponding results of the simple one-subband approximation model, the spin dephasing length is reduced four times under 0.125\,kV/cm of driving electric field at 300K by the MC tool incorporated with the three-subband approximation model, indicating that the three-subband approximation model predicts significantly shorter spin dephasing length with temperature increasing. Our simulation results suggest that the effects of the electron occupation of higher subbands and the intersubband scattering on the spin-dependent transport of GaAs 2-dimensional electron gas need to be considered when the driving electric field exceeds the moderate value and the lattice temperature is above 100K. The simulation by using the MC tool incorporated with the three-subband approximation model also indicates that, under a certain driving electric field and lattice temperature, larger channel widths cause spins to be depolarized faster. Ranges of the three components of the spins are different for three different injected spin polarizations due to the anisotropy of spin--orbit interaction.     

Inverse TMR in a nominally symmetric CoFe/AlOx/CoFe junction induced by interfacial Fe3O4 investigated by STEM-EELS

We have found inverse tunneling magnetoresistance (TMR) with a non-symmetric bias voltage dependence in a nominally symmetric Si (001)/Ag/CoFe/AlO_x/CoFe/IrMn/Ag magnetic tunnel junction after field cooling. The O K edge fine structure extracted from electron energy loss spectroscopy spectrum images taken at the interfaces of junctions with inverse TMR shows a thin, discontinuous Fe₃O₄ layer at the CoFe/AlO_x interfaces. The Fe L_2,3 edge core level shifts are also consistent with those of Fe₃O₄. We find no Fe₃O₄ layer in junctions with normal TMR. We believe this Fe₃O₄ layer is responsible for the inverse TMR.     

Magnetotransport and interdiffusion characteristics of magnetic tunnel junctions comprising nano-oxide layers upon exposure to postdeposition annealing

Magnetic tunnel junction (MTJ) structures based on underlayer (CoNbZr)/bufferlayer (CoFe)/antiferromagnet (IrMn)/pinned layer (CoFe)/tunnel barrier (AlO_x)/free layer (CoFe)/capping (CoNbZr) have been prepared to investigate thermal degradation of magnetoresistive responses. Some junctions possess a nano-oxide layer (NOL) inside either in the underlayer or bufferlayer. The main purpose of the NOL inclusion was to control interdiffusion path of Mn from the antiferromagnet so that improved thermal stability could be achieved. The MTJs with NOLs were found to have reduced interfacial roughness, resulting in improved tunneling magnetoresistance (TMR) and reduced interlayer coupling field. We also confirmed that the NOL effectively suppressed the Mn interdiffusion toward the tunnel barrier by dragging Mn atoms toward NOL during annealing.     

Highly spin-polarized room-temperature tunnel injector for semiconductor spintronics using MgO(100)

The spin polarization of current injected into GaAs from a CoFe/MgO(100) tunnel injector is inferred from the electroluminescence polarization from GaAs/AlGaAs quantum well detectors. The polarization reaches 57% at 100 K and 47% at 290 K in a 5 T perpendicular magnetic field. Taking into account the field dependence of the luminescence polarization, the spin injection efficiency is at least 52% at 100 K, and 32% at 290 K. We find a nonmonotonic temperature dependence of the polarization which can be attributed to spin relaxation in the quantum well detectors.     

Spacer layer thickness fluctuation scattering in a modulation-doped Al_xGa_1-xAs/GaAs/Al_xGa_1-xAs quantum well

We theoretically study the influence of spacer layer thickness fluctuation(SLTF) on the mobility of a twodimensional electron gas(2DEG) in the modulation-doped Al x Ga 1 x As/GaAs/Al x Ga 1 x As quantum well.The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained.The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.     

Investigation of an InGaP/GaAs heterostructure-emitter bipolar transistor (HEBT) with reduced potential spike

In this work, an improved InGaP/GaAs heterostructure-emitter bipolar transistor (HEBT) with an InGaP wide-bandgap collector is investigated. In the emitter–base region, the thin narrow bandgap n-type GaAs layer is sandwiched between a wide-bandgap N-type InGaP confinement layer and a narrow-bandgap p-type GaAs base layer. In the collector–base structure, an undoped 30 Å-thick GaAs spacer and a heavily doped 30 Å-thick GaAs are inserted between the base and collector. Due to the absence of a potential spike both at the base–emitter and base–collector junctions, the studied device shows lower offset and saturation voltages. In addition, not only are the excellent current–voltage characteristics observed, but also the undesired effects, e.g. the electron blocking effect, are completely eliminated.     

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.     

Spin-polarized electronic reflections at metal–oxide interfaces

The ultra-thin oxide tunnel barrier employed in magnetic tunnel junctions stack has to be of very high quality in terms of large scale homogeneity of its thickness and height parameters. For controlling precisely oxidation kinetic, we used spin valves as an oxidation progress probe. By measuring the magnetoresistance effect versus the oxidation time we are able to detect under- or over-oxidation of the metallic material. This technique consists of analysing the ability of spin-dependent electron scattering at metal/oxide interfaces.     

Bulk—like contribution of tunnel magnetoresistance in magnetic tunnel junctions

We have demonstrated that the bulk-like contribution to tunnelling magnetoresistance (TMR) exists in the magnetic tunnel junctions, and is determined by the tunnelling characteristic length of the ferromagnetic electrodes. In the experiment, a wedge-shaped CoFe layer is inserted at the interface between the insulating barrier and the reference electrode. It is found that TMR ratio increases from 18% without CoFe layer to a saturation value of 26.5% when the CoFe thickness is about 2.3 nm. The tunnelling characteristic length, l_{tc}, can be obtained to be about 0.8 nm for CoFe materials.     

Influence of Schottky drain contacts on the strained AlGaN barrier layer of AlGaN/AlN/GaN heterostructure field-effect transistors

Rectangular Schottky drain AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with different gate contact areas and conventional AlGaN/AlN/GaN HFETs as control were both fabricated with same size. It was found there is a significant difference between Schottky drain AlGaN/AlN/GaN HFETs and the control group both in drain series resistance and in two-dimensional electron gas (2DEG) electron mobility in the gate-drain channel. We attribute this to the different influence of Ohmic drain contacts and Schottky drain contacts on the strained AlGaN barrier layer. For conventional AlGaN/AlN/GaN HFETs, annealing drain Ohmic contacts gives rise to a strain variation in the AlGaN barrier layer between the gate contacts and the drain contacts, and results in strong polarization Coulomb field scattering in this region. In Schottky drain AlGaN/AlN/GaN HFETs, the strain in the AlGaN barrier layer is distributed more regularly.     

Spectroscopic measurement of spin-dependent resonant tunneling through a 3D disorder: the case of MnAs/GaAs/MnAs junctions

We propose an analytical model of spin-dependent resonant tunneling through a 3D assembly of localized states (spread out in energy and in space) in a barrier. An inhomogeneous distribution of localized states leads to resonant tunneling magnetoresistance inversion and asymmetric bias dependence as evidenced with a set of experiments with MnAs/GaAs(7-10 nm)/MnAs tunnel junctions. One of the key parameters of our theory is a dimensionless critical exponent beta scaling the typical extension of the localized states over the characteristic length scale of the spatial distribution function. Furthermore, we demonstrate, through experiments with localized states introduced preferentially in the middle of the barrier, the influence of an homogeneous distribution on the spin-dependent transport properties.     

Comprehensive studies of InGaP/GaAs heterojunction bipolar transistors with different thickness of setback layers

The performances of InGaP/GaAs heterostructure bipolar transistors (HBTs) with different thickness of setback layers are theoretically studied. The appropriate thickness of the setback layer is an important factor in high-speed HBTs. In this work, it is found that the HBT device with a 60–90 Å setback layer has better DC and RF characteristics due to the absence of potential spike and reduced transit time. In addition, the studied devices with appropriate setback layer thickness have lower offset voltage, reverse saturation voltage, and base and collector current ideality factor.