Spin Field Effect Transistor Research Articles

Spin-orbit interactions in semiconductor superlattice

The purpose of this paper is to theoretically investigate the spin-orbit interactions of common semiconductor superlattices. Spin splitting and spin-orbit interaction coefficients are calculated based on interactions between the interface-related-Rashba effect and Dresselhaus effect. Semiconductor superlattice shows a series of specific characteristics in spin splitting as follows. The spin splitting of the superlattice structure is greater than that of a single quantum well, contributing to significant spin polarization, spin filtering, and convenient manipulation of spintronic devices. The spin splitting of some superlattice structures does not change with variation of the size of some constituent quantum wells, reducing the requirements for accuracy in the size of quantum wells. The total spin splitting of lower sub-levels of some superlattice can be designed to be zero, realizing a persistent spin helix effect and long spin relaxation time, however, the total spin splitting of higher sub-levels is still appreciable, contributing to desirable spin polarization. These results demonstrate that one superlattice structure can realize two functions, acting as a spin field effect transistor and a spin filter.

Light-modulated spin precession in a two dimensional spin field effect transistor

Perfect spin precession in a two dimensional electron gas is the basis of the Datta-Das spin field effect transistor. However, in a two dimensional system this ideal spin precession can be washed out by the intersubband mixing effects induced by the Rashba spin–orbit interaction. Here, we propose by applying an off-resonant linearly polarized light a perfect spin modulation of conductance can be demonstrated in a two dimensional spin field effect transistor. This is because in a two dimensional electron gas due to the renormalization of the Rashba spin–orbit coupling strength and the hopping amplitude by the light, the intersubband mixing effects can be weakened. For an appropriate intensity of the light, the intersubband mixing effects are completely suppressed. These phenomena provide a promising way to design an optically-controlled spintronics device.

Spin-orbit interaction and snake states in a graphene p-n junction

We study a model of a $p$-$n$ junction in single-layer graphene in the presence of a perpendicular magnetic field and spin-orbit interactions. By solving the relevant quantum-mechanical problem for a potential step, we determine the exact spectrum of spin-resolved dispersive Landau levels. Close to zero energy, we find a pair of linearly dispersing zero modes, which possess a wave-vector-dependent spin polarization and can be regarded as quantum analogous of spinful snake states. We show that the Rashba spin-orbit interaction, in particular, produces a wave vector shift between the dispersions of these modes with observable interference effects. These effects can in principle provide a way to detect the presence of Rashba spin-orbit interaction and measure its strength. Our results suggest that a graphene $p$-$n$ junction in the presence of strong spin-orbit interaction could be used as a building block in a spin field-effect transistor.

Intriguing p-orbital magnetic semiconductors and carrier induced half-metallicity in surface oxygen-functionalized two-dimensional [formula omitted] ([formula omitted]) crystals

Two-dimensional (2D) ferromagnetic semiconductors with robust magnetism are urgently desired for nanoscale spintronics applications. However, it remains a challenge to realize them experimentally. In this work, we proposed intriguing 2D p-orbital ferromagnetic semiconductors X2NO2 (X=Ca,Sr) monolayer with 3μB magnet per unit under O surface termination using first-principles calculations. The Ca2NO2 monolayer is bipolar magnetic material (BMS) with spin-flip gap 0.24 eV, and the Sr2NO2 is half-semiconductor (HSC) with spin-flip gap 0.31 eV, which is large enough to prevent the spin-flip transition. The Curie temperature can reach to 206 and 239 K, respectively, due to the superexchange interaction between N- ions. The values are much higher than the boiling point of liquid nitrogen (77 K) and comparable to that of the reported ScCl monolayer. Moreover, the half metals are obtained via carrier doping for both the Ca2NO2 and Sr2NO2 monolayers. In addition, the half-metallic completely spin-polarized direction of BMS Ca2NO2 monolayer can be controlled by carrier doping type. Furthermore, the magnetism of X2NO2 monolayer is derived from p orbital, without the involvement of conventional transition metals or rare earth atoms. This is advantageous for high-speed and long-distance spin-polarization transport. These results suggest that the X2NO2 monolayer can develop spin field effect transistor for information processing and storage, and open opportunities for designing new ferromagnetic semiconductors.

Persistent spin helix in Rashba-Dresselhaus ferroelectric CsBiNb2O7

Ferroelectric Rashba semiconductors (FERSC) are a novel class of multifunctional materials showing a giant Rashba spin splitting which can be reversed by switching the electric polarization. Although they are excellent candidates as channels in spin field effect transistors, the experimental research has been limited so far to semiconducting GeTe, in which ferroelectric switching is often prevented by heavy doping and/or large leakage currents. Here, we report that ${\mathrm{CsBiNb}}_{2}{\mathrm{O}}_{7}$, a layered perovskite of Dion-Jacobson type, is a robust ferroelectric with sufficiently strong spin-orbit coupling and spin texture reversible by electric field. Moreover, we reveal that its topmost valence band's spin texture is quasi-independent from the momentum, as a result of the low symmetry of its ferroelectric phase. The peculiar spin-polarization pattern in the momentum space may yield the so-called ``persistent spin helix,'' a specific spin-wave mode which protects the spin from decoherence in the diffusive transport regime, potentially ensuring a very long spin lifetime in this material.

Valley dynamics of different trion species in monolayer WSe2

We develop a model, taking into account optically inactive excitonic dark states, of both spin-forbidden and momentum-forbidden types, to explore valley dynamics of different trion species in monolayer WSe2. We observe two distinct regimes, determined by two typical bright-dark scattering processes, i.e., the spin-flip and spin-conserving ones. When the spin-flip scattering is comparable to the spin-conserving one, we find that the valley polarizations (VPs) of all excitonic states including excitons and singlet and triplet trions are copolarized with respect to the excitation laser, consistent with available experimental measurements. Interestingly, when the spin-conserving scattering dominates over the spin-flip one, we observe the reversal of VP of the singlet trion. This fascinating valley control opens the door to an envisaged valley field-effect transistor (FET) (an analog of spin FET). Furthermore, we reveal that the reversed VP strengthens until it becomes stabilized, offering a promising platform for coherent control of valley pseudospin. Our findings should stimulate experiments probing the VP reversal in samples with strong spin-conserving scattering.

Detection of Rashba spin splitting in 2D organic-inorganic perovskite via precessional carrier spin relaxation

The strong spin-orbit interaction in the organic-inorganic perovskites tied to the incorporation of heavy elements (e.g., Pb and I) makes these materials interesting for applications in spintronics. In conjunction with a lack of inversion symmetry associated with distortions of the metal-halide octahedra, surfaces and interfaces, or the application of a bias, the Rashba effect (used in spin field-effect transistors and spin filters) has been predicted to be much larger in these materials than in traditional III-V semiconductors such as GaAs. Evidence of strong Rashba coupling has been observed in both 3D (bulk) and 2D perovskites, with the relative role of bulk and surface Rashba contributions in the former case under active debate. The varying size of the reported spin splittings points to the need for more experimental studies of Rashba effects in the organic-inorganic perovskite family of materials. Here, we apply time-resolved circular dichroism techniques to the study of carrier spin dynamics in a 2D perovskite thin film [(BA)2MAPb2I7; BA = CH3(CH2)3NH3, MA = CH3NH3]. Our findings confirm the presence of a Rashba spin splitting via the dominance of precessional spin relaxation induced by the Rashba effective magnetic field (also known as D’yakonov Perel spin relaxation). The size of the Rashba spin splitting in our system was extracted from simulations of the measured spin dynamics incorporating LO-phonon and electron-electron scattering, yielding a value of 10 meV at an electron energy of 50 meV above the band gap, representing a 20 times larger value than in GaAs quantum wells.

Electrically reconfigurable logic design using multi-gate spin Field Effect Transistors

In this paper, triple- and quadruple-gate spin-FETs are proposed and later Verilog-A model files of the modelled devices have been created and included in HSPICE tool to obtain various 2-input and 3-input logic functions. Different logic functions are obtained from a single multi-gate spin-FET by changing control voltage at one of the gate terminal, while applying inputs at the other gate terminals. Only one triple-gate and quadruple-gate spin-FETs are employed to implement various 2-input and 3-input logic functions respectively. The implemented functions also include 3-input XOR and majority gate functions which form the sum and carry of full-adder circuit. Besides, the achievement of higher-order logic functions has been demonstrated by cascading the multi-gate spin FETs. In addition, the effect of channel length for obtaining different logic functions has been considered in the paper. The operations of the logic circuits have been verified using HSPICE simulation software.

Reversible Mapping and Sorting the Spin of Photons on the Nanoscale: A Spin-Optical Nanodevice.

The photon spin is an important resource for quantum information processing as is the electron spin in spintronics. However, for subwavelength confined optical excitations, polarization as a global property of a mode cannot be defined. Here, we show that any polarization state of a plane-wave photon can reversibly be mapped to a pseudospin embodied by the two fundamental modes of a subwavelength plasmonic two-wire transmission line. We design a device in which this pseudospin evolves in a well-defined fashion throughout the device reminiscent of the evolution of photon polarization in a birefringent medium and the behavior of electron spins in the channel of a spin field-effect transistor. The significance of this pseudospin is enriched by the fact that it is subject to spin-orbit locking. Combined with optically active materials to exert external control over the pseudospin precession, our findings could enable spin-optical transistors, that is, the routing and processing of quantum information with light on a subwavelength scale.

Effects of Different Lead Magnetizations on the Datta–Das Spin Field-Effect Transistor

A Datta-Das spin field effect transistor is built of a one-dimensional weak link, with Rashba spin orbit interactions (SOI), which connects two magnetized reservoirs. The particle and spin currents between the two reservoirs are calculated to lowest order in the tunneling through the weak link and in the wide-band approximation, with emphasis on their dependence on the origins of the `bare' magnetizations in the reservoirs. The SOI is found to generate magnetization components in each reservoir, which rotate in the plane of the electric field (generating the SOI) and the weak link, only if the `bare' magnetization of the other reservoir has a non-zero component in that plane. The SOI affects the charge current only if both reservoirs are polarized. The charge current is conserved, but the transverse rotating magnetization current is not conserved since the SOI in the weak link generates extra spin polarizations which are injected into the reservoirs.

Spin-dependent carrier mobility and its gate-voltage modifying effects for functionalized single walled black phosphorus tubes

Phosphorene and its derivatives so far have attracted substantial research interest due to its promising properties for developing nanoscale electronic devices. Here, we present a theoretical investigation on the functionalized features, such as the improved electronic structure and carrier mobility, for armchair-edged single walled black phosphorus nanotubes (PNTs) with the substitutional doping of low-concentration transition-metal atoms (Ti, Mn, Fe, and Ni). They are predicted to be exceptional magnetic semiconductors (MSCs), such as half-semiconductor or bipolar MSC. Their spin-resolved carrier mobility at room temperature holds doping element- dependence as well as carrier and spin polarity. Particularly, the difference by two orders of magnitude for carrier mobility emerges due to different TM doping. More interestingly, the carrier mobility in armchair PNTs serving as the channel material of a spin field effect transistor is predicted to be modified strongly by a gate voltage. The enhanced carrier mobility and its gate voltage direction-dependent behavior, as well as the more obvious carrier and spin polarity of mobility, can be observed clearly under gate voltage, which further facilitates the separation of different carriers and spin states and also suggests that realistic carrier mobility is gate voltage-dependent in a field effect transistor.

Cr2TiC2-based double MXenes: novel 2D bipolar antiferromagnetic semiconductor with gate-controllable spin orientation toward antiferromagnetic spintronics.

Antiferromagnetic (AF) spin devices could be one of the representative components for applications of spintronics thanks to the numerous advantages such as resistance to magnetic field perturbation, stray field-free operation, and ultrahigh device operation speed. However, detecting and manipulating the spin of AF materials is still a major challenge due to the absence of a net magnetic moment and spin degeneracy in the band structure. Bipolar antiferromagnetic semiconductors are promising solutions to these problems. Herein, using density functional theory calculations, we present asymmetrical functionalized double MXenes (Cr2TiC2FCl) that behave as a novel bipolar antiferromagnetic semiconductor (BAFS) with vanishing magnetism, in which the valence band and conduction band around the Fermi level exhibit opposite spin directions. Remarkably, gate voltage can manipulate the spin orientation of the AF Cr2TiC2FCl and lead to a transition from BAFS to half-metal antiferromagnets (HMAF). Moreover, the mixed functionalized double MXenes with various F/Cl concentrations show the BAFS feature due to the different chemical environment for the Cr atom. Our results presented herein open a new strategy towards AF spintronics and the realization of the AF spin field effect transistor.

Vertical ferroelectric switching by in-plane sliding of two-dimensional bilayer WTe2.

Based on first-principles calculations, we studied the ferroelectric properties of bilayer 1T'-WTe2. In this work, we discovered that the polarization stems from uncompensated out-of-plane interlayer charge transfer, which can be switched upon interlayer sliding of an in-plane translation. Our differential charge density results also confirmed that such ferroelectricity in the bilayer WTe2 is derived from interlayer charge transfer. The ferroelectric polarization directions further control the spin texture of the bilayer WTe2, which may have important applications in spintronics. Therefore, we propose a spin field effect transistor (spin-FET) design that may effectively improve the spin-polarized injection rate. In addition, the lattice strain has been found to have an important influence on the ferroelectric properties of the bilayer WTe2. One can effectively increase the polarization with a maximum at 3% tensile strain, whereas a 3% compressive strain can transform the bilayer WTe2 from the ferroelectric to paraelectric phase.

Switching domain wall motion on and off using a gate voltage for domain wall transistor applications

Spintronic devices such as magnetic random access memory and domain wall (DW) memory are attracting significant attention. Spin-field effect transistor devices have been proposed and researched for logic applications. In domain wall memory, the information is stored in magnetic domain states, which can be moved with a current above a certain threshold value. So far, the domain wall motion is only determined by the current density for most of the DW devices. Here, we demonstrate experimentally that a significant change in domain wall mobility can be achieved by applying a gate voltage. By applying a positive gate voltage, we show that the threshold current density for DW motion can be reduced by more than 10%. By choosing a suitable operating current, the domain wall motion can be switched on or off by the use of a gate voltage. These results are promising for designing high performance domain wall based transistor devices with faster operation speed and lower power consumption.

First-principle study on honeycomb fluorated-InTe monolayer with large Rashba spin splitting and direct bandgap

Rashba effect is much related to next-generation spintronic devices. It is highly desirable to search for Rashba materials with large Rashba spin splitting, which is considered as the key factor for the application of spin field-effect transistor. Here, we design a two-dimensional monolayer of fluorated-InTe (InTeF) with large Rashba spin splitting and direct bandgap on the basis of first-principles calculations. InTeF monolayer is energetically and dynamically stable based on the calculations of cohesive energy and phonon dispersion. Remarkably, the Rashba parameter αR is about 1.08 eV·Å, comparable to that of the BiTeI monolayer (1.86 eV·Å). The direct bandgap is estimated to be 2.48 eV by HSE06 hybrid functional, which shows good prospects in light-emitting devices and photodetectors. To further explore the effect of substrates on the electronic structure of InTeF monolayer, we build two heterostructures, and the results show that the strength of Rashba effect and the direct bandgap nature in InTeF monolayer can be well preserved under the influence of substrates. Based on the above findings in our work, InTeF monolayer is considered to be one of the promising 2D materials for the application of spintronics as well as optoelectronics.

Ferromagnet-Free All-Electric Spin Hall Transistors.

The spin field-effect transistor, an essential building block for spin information processing, shows promise for energy-efficient computing. Despite steady progress, it suffers from a low-output signal because of low spin injection and detection efficiencies. We demonstrate that this low-output obstacle can be overcome by utilizing direct and inverse spin Hall effects for spin injection and detection, respectively, without a ferromagnetic component. The output voltage of our all-electric spin Hall transistor is about two orders of magnitude larger than that of previously reported spin transistors based on ferromagnets or quantum point contacts. Moreover, the symmetry of the spin Hall effect allows all-electric spin Hall transistors to effectively mimic n-type and p-type devices, opening a way of realizing the complementary functionality.

Proposal for Multi-Gate Spin Field-Effect Transistor

The conductivity of spin field-effect transistor (spin-FET) can be modulated by controlling the spin degree of freedom; therefore, emerging low-power applications can be explored. In this paper, we propose a novel device called multi-gate spin-FET where multi-gate voltages can successively modulate the spin of the electrons. Various logic functions can be implemented with multi-gate spin-FET by setting the appropriate channel lengths. First, we present an electrical model of the multi-gate spin-FET which considers the physical mechanisms and the fitting of experimental results. The availability of the electrical model is validated by the dc simulation. Afterward, typical logic circuits are designed with the multi-gate spin-FET, including NAND gate, XOR gate, full adder, and a representative three-input logic gate. Transient simulation results validate the functional behaviors of these logic circuits. Finally, based on an analysis of power consumption, we conclude that the multi-gate spin-FET is a more energy-efficient device compared with the conventional CMOS FET.

Electrically induced 2D half-metallic antiferromagnets and spin field effect transistors

Engineering the electronic band structure of material systems enables the unprecedented exploration of new physical properties that are absent in natural or as-synthetic materials. Half metallicity, an intriguing physical property arising from the metallic nature of electrons with singular spin polarization and insulating for oppositely polarized electrons, holds a great potential for a 100% spin-polarized current for high-efficiency spintronics. Conventionally synthesized thin films hardly sustain half metallicity inherited from their 3D counterparts. A fundamental challenge, in systems of reduced dimensions, is the almost inevitable spin-mixed edge or surface states in proximity to the Fermi level. Here, we predict electric field-induced half metallicity in bilayer A-type antiferromagnetic van der Waals crystals (i.e., intralayer ferromagnetism and interlayer antiferromagnetism), by employing density functional theory calculations on vanadium diselenide. Electric fields lift energy levels of the constituent layers in opposite directions, leading to the gradual closure of the gap of singular spin-polarized states and the opening of the gap of the others. We show that a vertical electrical field is a generic and effective way to achieve half metallicity in A-type antiferromagnetic bilayers and realize the spin field effect transistor. The electric field-induced half metallicity represents an appealing route to realize 2D half metals and opens opportunities for nanoscale highly efficient antiferromagnetic spintronics for information processing and storage.

Towards a Strong Spin–Orbit Coupling Magnetoelectric Transistor

Here, we outline magnetoelectric (ME) device concepts based on the voltage control of the interface magnetism of an ME antiferromagnet gate dielectric formed on a very thin semiconductor channel with large spin–orbit coupling (SOC). The emphasis of the ME spin field-effect transistors (ME spin FET) is on an antiferromagnet spin–orbit read logic device and a ME spin-FET multiplexer. Both spin-FET schemes exploit the strong SOC in the semiconducting channel materials but remain dependent on the voltage-induced switching of an ME, so that the switching time is limited only by the switching dynamics of the ME. The induced exchange field spin polarizes the channel material, breaks time-reversal symmetry, and results in the preferential charge transport direction, due to the spin–orbit-driven spin-momentum locking. These devices could provide reliable room temperature operation with large on/off ratios, well beyond what can be achieved using magnetic tunnel junctions. All of the proposed device spintronic functionalities without the need to switch a ferromagnet, yielding a faster writing speed (~10 ps) at a lower cost in energy (~10 aJ), excellent temperature stability (operational up to 400 K or above), and requiring far fewer device elements (transistor equivalents) than CMOS.

Large current modulation and tunneling magnetoresistance change by a side-gate electric field in a GaMnAs-based vertical spin metal-oxide-semiconductor field-effect transistor

A vertical spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) is a promising low-power device for the post scaling era. Here, using a ferromagnetic-semiconductor GaMnAs-based vertical spin MOSFET with a GaAs channel layer, we demonstrate a large drain-source current IDS modulation by a gate-source voltage VGS with a modulation ratio up to 130%, which is the largest value that has ever been reported for vertical spin field-effect transistors thus far. We find that the electric field effect on indirect tunneling via defect states in the GaAs channel layer is responsible for the large IDS modulation. This device shows a tunneling magnetoresistance (TMR) ratio up to ~7%, which is larger than that of the planar-type spin MOSFETs, indicating that IDS can be controlled by the magnetization configuration. Furthermore, we find that the TMR ratio can be modulated by VGS. This result mainly originates from the electric field modulation of the magnetic anisotropy of the GaMnAs ferromagnetic electrodes as well as the potential modulation of the nonmagnetic semiconductor GaAs channel layer. Our findings provide important progress towards high-performance vertical spin MOSFETs.