Semiconductor Spintronic Devices Research Articles

All-electric quantum point contact spin-polarizer

The controlled creation, manipulation and detection of spin-polarized currents by purely electrical means remains a central challenge of spintronics. Efforts to meet this challenge by exploiting the coupling of the electron orbital motion to its spin, in particular Rashba spin-orbit coupling, have so far been unsuccessful. Recently, it has been shown theoretically that the confining potential of a small current-carrying wire with high intrinsic spin-orbit coupling leads to the accumulation of opposite spins at opposite edges of the wire, though not to a spin-polarized current. Here, we present experimental evidence that a quantum point contact -- a short wire -- made from a semiconductor with high intrinsic spin-orbit coupling can generate a completely spin-polarized current when its lateral confinement is made highly asymmetric. By avoiding the use of ferromagnetic contacts or external magnetic fields, such quantum point contacts may make feasible the development of a variety of semiconductor spintronic devices.

Magnetic tunnel contacts to silicon with low-work-function ytterbium nanolayers

Unambiguous proof of spin transport in semiconductor spintronic devices requires a control experiment to exclude spurious signals that arise from the presence of the ferromagnetic contacts. It is shown here that insertion of a low-work-function Yb nanolayer in ferromagnetic tunnel contacts to silicon allows a selective suppression of the tunnel spin polarization for 2 nm of Yb and simultaneous control of the Schottky barrier height. The insertion of a nonmagnetic nanolayer provides a versatile method to exclude artifacts and a solution for nanoscale devices or other geometries in which the frequently employed Hanle effect cannot be applied and a control experiment did not exist.

Spin polarization in an organic semiconductor system: The spin-dependent conductivity effects

In organic semiconductor spintronic devices, the up-spin and down-spin polarons have different density once spin injection happens from ferromagnetic electrodes into organic semiconductors. For the different spin density of two spin channels could induce different conductivity, the different conductivity between the up-spin and down-spin polarons directly dominates the spin polarization. Here the effect of spin-dependent conductivity on the spin polarization is extensively studied applying the spin drift-diffusion equation on the ferromagnetic (FM)/organic (OSE) structure. It is found that the spin dependence of the electrical conductivity is induced by the spin injection and closelyrelatedto the induced spin density (or spin accumulation) μ↓-μ↑. The calculations show that the electrical conductivity induced by up-spin polarons (down-spin polarons) is position-dependentin the OSE just asthe spin accumulation. And the match level of conductivities σ0/σf can affect the spin-dependent conductivity at the interface of the FM/OSE to a great extent. In addition, it is found that the electric-field can affect the current spin polarization α(x) in the FM/OSE system in the low-voltage regime (eV<<kBT).

Enhanced sensitivity in detection of Kerr rotation by double modulation and time averaging based on Allan variance

Experiments in spintronics necessarily involve the detection of spin polarization. The sensitivity of this detection becomes an important factor to consider when extending the low temperature studies on semiconductor spintronic devices to room temperature, where the spin signal is weaker. In pump-probe experiments, which optically inject and detect spins, the sensitivity is often improved by using a photoelastic modulator (PEM) for lock-in detection. However, spurious signals can arise if diode lasers are used as optical sources in such experiments, along with a PEM. In this work, we eliminated the spurious electromagnetic coupling of the PEM onto the probe diode laser, by the double modulation technique. We also developed a test for spurious modulated interference in the pump-probe signal, due to the PEM. Besides, an order of magnitude enhancement in the sensitivity of detection of spin polarization by Kerr rotation, to 3x10(-8) rad was obtained by using the concept of Allan variance to optimally average the time series data over a period of 416 s. With these improvements, we are able to experimentally demonstrate at room temperature, photoinduced steady-state spin polarization in bulk GaAs. Thus, the advances reported here facilitate the use of diode lasers with a PEM for sensitive pump-probe experiments. They also constitute a step toward detection of spin-injection in Si at room temperature.

Built-in electric field assisted spin injection in Cr and Mnδ-layer doped AlN/GaN(0001) heterostructures from first principles

Highly spin-polarized diluted ferromagnetic semiconductors are expected to be widely used as ideal spin injectors. Here, extensive first-principles density-functional theory calculations have been performed to investigate the feasibility of using Cr- and Mn-doped wurtzite polar AlN/GaN0001 heterostructures, with the aim to realize the appealing half-metallic character and, hence, efficient electrical spin injection. To overcome the formation of detrimental embedded clusters, we propose digital -layer doping perpendicular to the growth direction so as to realize enhanced performance at room temperature. The formation energy, electronic and magnetic properties, and the degree of spin polarization for both neutral and charged valence states for various concentrations are studied. Under both metal-rich Al- or Ga-rich and N-rich conditions, Cr and Mn dopants prefer to segregate into the GaN region and reside close to the interface, where dopant incorporation occurs more readily under N-rich conditions. The doped Cr and Mn atoms introduce 3d states in the band gap of the host semiconductor heterostructure. The spin injection channels are constructed via the hybridization between dopant 3d and surrounding host atoms, up to a few monolayers around the interface, where the spin-polarized t2 electrons are injected into AlN without the conductivity mismatch problem. Significantly, for the energetically favorable configurations, the built-in electric field in the AlN/GaN0001 heterointerface serves as a driving force for efficient spin injection through the interface and spin transport in the AlN region. Also importantly, the electronic properties of the heterostructures half metallic, semiconducting, or metallic are found to depend sensitively upon the doping concentration and valence charge states. In general, charged valence states can destroy the ferromagnetic half metallicity for both systems, particularly in n-type materials. These results will be useful with regard to the practical fabrication of desirable heterostructures for optoelectronic and semiconductor spintronic devices.

Operation and Modeling of Semiconductor Spintronics Computing Devices

Semiconductor spintronic devices are considered from the point of view of suitability for digital logic. Functionality of earlier proposed devices is reviewed for cascadability and signal gain. Spin gain transistor, which uses electronic control of ferromagnetism in semiconductors, is treated in more detail via semiconductor Bloch equations for both localized and free-carrier magnetic moments. Dependence of the steady state magnetization and temporal switching characteristics on material parameters is determined. Finally, fundamental physical limits for spintronic devices are determined via use of an idealized model. They are compared with similar limits for electronic devices. It is found that, though spintronic devices switch slower, their switching energy is smaller, therefore they scale with size on a curve with lower power dissipation.

Spin dephasing in drift-dominated semiconductor spintronics devices

A spin-transport model is employed to study the effects of spin dephasing induced by diffusion-driven transit-time uncertainty through semiconductor spintronic devices where drift is the dominant transport mechanism. It is found that in the ohmic regime, dephasing is independent of transit length and determined primarily by voltage drop across the spin-transport region. The effects of voltage and temperature predicted by the model are compared to experimental results from a $350\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-thick silicon spin-transport device using derived mathematical expressions of spin dephasing.

Observation of the anomalous Hall effect in GaAs

Devices for the direct detection of the spin current, based on the anomalous Hall effect (AHE), are fabricated on n-type GaAs bulk semiconductor materials. The AHE is observed in the device when the photoinduced spin-polarized electrons are injected into it, and it is found that the effect depends on the applied electric field. The origin of the field-dependent observed Hall effect is discussed based on the D'yakonov–Perel' (DP) spin relaxation mechanism. The spin-dependent Hall effect is also found to be enhanced with increasing doping concentration. The present experimental results might have potential applications in semiconductor spintronic devices since the effect is closely related to the spin Hall effect.

Electrical spin injection from room-temperature ferromagnetic (Ga, Mn)N in nitride-basedspin-polarized light-emitting diodes

We present the electrical spin injection from room-temperature ferromagnetic (Ga, Mn)Nin nitride-based spin-polarized light-emitting diodes. The electroluminescence spectrafrom the spin LED indicate the existence of the spin polarization via opticalpolarization of emitted light up to room temperature. This demonstrates thatthe spin injection from the (Ga, Mn)N layer into (In, Ga)N quantum wells wasachieved persisting up to room temperature by comparing it with the magneticfield dependence of the Hall resistance, which is proportional to the out-of-planemagnetization. These results support that (Ga, Mn)N is an appropriate material for a spininjection source in room-temperature operating semiconductor spintronic devices.

Tunnel spin polarization ofNi80Fe20∕SiO2probed with a magnetic tunnel transistor

The tunnel spin polarization of ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕{\mathrm{SiO}}_{2}$ interfaces has been investigated using a magnetic tunnel transistor (MTT). The MTT with a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕{\mathrm{SiO}}_{2}$ emitter shows a magnetocurrent of 74% at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, corresponding to a tunnel spin polarization of the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕{\mathrm{SiO}}_{2}$ interface of 27%. This is only slightly lower than the value of 34% for ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}∕{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ interfaces determined in similar MTT structures. This suggests that ${\mathrm{SiO}}_{2}$ can be applied in semiconductor spintronic devices, for example in $\mathrm{ferromagnet}∕{\mathrm{SiO}}_{2}∕\mathrm{Si}$ tunnel contacts for spin injection.

Bias-dependent spin relaxation in a [formula omitted]-InAs/AlSb 2DES

Manipulation of electron spin is a critical component of many proposed semiconductor spintronic devices. One promising approach utilizes the Rashba effect by which an applied electric field can be used to reduce the spin lifetime or rotate spin orientation through spin–orbit interaction. The large spin–orbit interaction needed for this technique to be effective typically leads to fast spin relaxation through precessional decay, which may severely limit device architectures and functionalities. An exception arises in [1 1 0]-oriented heterostructures where the crystal magnetic field associated with bulk inversion asymmetry lies along the growth direction and in which case spins oriented along the growth direction do not precess. These considerations have led to a recent proposal of a spin-FET that incorporates a [1 1 0]-oriented, gate-controlled InAs quantum well channel. We report measurements of the electron spin lifetime as a function of applied electric field in a [1 1 0]-InAs 2DES. Measurements made using an ultrafast, mid-IR pump-probe technique indicate that the spin lifetime can be reduced from its maximum to minimum value over a range of less than 0.2 V per quantum well at room temperature.

상온 강자성 (Ga,Mn)N 박막을 이용한 질화물계 스핀 발광소자의 스핀편극된 빛의 발광

We investigated the fabrication and characteristics of the nitride-based spin-polarized LEDs with room-temperature ferromagnetic (Ga,Mn)N layer as a spin injection source. The (Ga,Mn)N thin films having room-temperature ferromagnetic ordering were found to exhibit the negative MR and anomalous Hall resistance up to room temperature, revealing the existence of spin-polarized electrons in (Ga,Mn)N films at room temperature. The electrical characteristics in the spin LEDs did not degraded in spite of the insertion of the (Ga,Mn)N layer into the LED structure. In EL spectra of the spin LEDs, it is confirmed that the devices produce intense EL emission at 7 K as well as room temperature. These results are expected to open up new opportunities to realize room-temperature operating semiconductor spintronic devices.

Current-Induced Spin Polarization in Strained Semiconductors

The polarization of conduction electron spins due to an electrical current is observed in strained nonmagnetic semiconductors using static and time-resolved Faraday rotation. The density, lifetime, and orientation rate of the electrically polarized spins are characterized by a combination of optical and electrical methods. In addition, the dynamics of the current-induced spins are investigated by utilizing electrical pulses generated from a photoconductive switch. These results demonstrate the possibility of a spin source for semiconductor spintronic devices without the use of magnetic materials.

Electrical Spin Injection and Transport in Semiconductor Spintronic Devices

AbstractSemiconductor heterostructures that utilize carrier spin as a new degree of freedom offer entirely new functionality and enhanced performance over conventional devices. We describe the essential requirements for implementing this technology, focusing on the materials and interface issues relevant to electrical spin injection into a semiconductor. These are discussed and illustrated in the context of several prototype semiconductor spintronic devices, including spin-polarized light-emitting diodes and resonant tunneling structures such as the resonant interband tunneling diode.

Stray fields of iron electrodes for spin-polarized transport

In semiconductor spintronic devices that incorporate ferromagnetic materials the stray-field configuration in the plane of the two-dimensional electron system is of crucial importance. We investigate the stray fields of iron electrodes suitable as injector and detector for spin-polarized transport in hybrid semiconductor/ferromagnet devices. Magnetic-force microscopy images of an electrode pair are derived from simulated magnetization patterns. The calculated averaged stray field is compared to the experimental signal of a ballistic Hall micromagnetometer comprising a two-dimensional electron system in a GaAs/AlGaAs-heterostructure 90 nm below the electrodes. Good agreement between simulation and experiment is obtained.

Electric Field Induced Suppression of the Magnetoresistance of Dilute Magnetic Semiconductor Trilayers

In semiconductor spintronic devices, the semiconductor is usually lightly doped and nondegenerate, and moderate electric fields can dominate the carrier motion. We recently derived a drift-diffusion equation for spin polarization in the semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics and identified a high-field diffusive regime that has no analog in metals. Here high fields are argued to substantially reduce the magnetoresistance observable in a recent experiment on magnetic-semiconductor–nonmagnetic-semiconductor–magnetic-semiconductor trilayers.

Spin diffusion and injection in semiconductor structures: Electric field effects

In semiconductor spintronic devices, the semiconductor is usually lightly doped and nondegenerate, and moderate electric fields can dominate the carrier motion. We recently derived a drift-diffusion equation for spin polarization in the semiconductors by consistently taking into account electric-field effects and nondegenerate electron statistics and identified a high-field diffusive regime which has no analogue in metals. Here spin injection from a ferromagnet (FM) into a nonmagnetic semiconductor (NS) is extensively studied by applying this spin drift-diffusion equation to several typical injection structures such as FM/NS, FM/NS/FM, and FM/NS/NS structures. We find that in the high-field regime spin injection from a ferromagnet into a semiconductor is enhanced by several orders of magnitude. For injection structures with interfacial barriers, the electric field further enhances spin injection considerably. In FM/NS/FM structures high electric fields destroy the symmetry between the two magnets at low fields, where both magnets are equally important for spin injection, and spin injection becomes locally determined by the magnet from which carriers flow into the semiconductor. The field-induced spin injection enhancement should also be insensitive to the presence of a highly doped nonmagnetic semiconductor (NS$^+$) at the FM interface, thus FM/NS$^+$/NS structures should also manifest efficient spin injection at high fields. Furthermore, high fields substantially reduce the magnetoresistance observable in a recent experiment on spin injection from magnetic semiconductors.