Nonequilibrium Green Function Research Articles

Giant tunnel electroresistance in two-dimensional ferroelectric tunnel junctions constructed with a Sc2CO2/In2Se3 van der Waals ferroelectric heterostructure

Two-dimensional (2D) ferroelectric materials have attracted great attention in recent years due to their thin thickness, high stability, and switchable polarization states. In particular, ferroelectric tunnel junctions (FTJs) constructed from 2D ferroelectric materials have been shown to have very high tunnel electroresistance (TER) ratios. In this work, we design a ferroelectric tunnel junction composed of ${\mathrm{Sc}}_{2}{\mathrm{CO}}_{2}/{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ vertical van der Waals (vdW) heterostructure based on two different 2D ferroelectric materials with out-of-plane polarization. Through density functional calculations combined with a nonequilibrium Green's function technique, it is found that the TER ratio as high as ${10}^{7}%$ can be achieved. Analysis shows that it originates from the difference in the work functions of the contact surfaces of the two ferroelectric materials, which makes charge transfer occur or not occur between them and further leads to a metalinsulator switching of the ferroelectric vdW heterostructure upon the reversion of the applied electrical field. The results suggest the importance of ferroelectric vdW heterostructure in the design of FTJs and a feasible design scheme characterized by proper choice of the work functions and band gaps of the component materials.

Using ensemble Monte Carlo methods to evaluate non-equilibrium Green’s functions

The use of ensemble Monte Carlo methods for the simulation of transport in semiconductor devices has become extensive over the past few decades. This method allows for simulation utilizing particles while addressing the full physics within the device, leaving the computational difficulties to the computer. More recently, the study of quantum mechanical effects within the devices, effects which also strongly affect the carrier transport itself, have become important. While particles have continued to be useful in quantum simulations using Wigner functions, interest in analytical solutions based upon the non-equilibrium Green’s functions (NEGF) have become of greater interest in device simulation. While NEGF has been adopted by many commercial semiconductor, there remains considerable computational difficulty in this approach. Here, a particle approach to NEGF is discussed, and preliminary results presented illustrating the computational efficiency that remains with the use of particles. This approach adopts the natural basis functions for use in a high electric field and the preliminary results are obtained for quantum transport in Si at 300 K. This approach appears to offer significant advantages for the use of NEGF.

A Strongly Correlated Quantum Dot Heat Engine with Optimal Performance: A Nonequilibrium Green's Function Approach

Herein, an analytical study of a strongly correlated quantum dot‐based thermoelectric particle‐exchange heat engine for both finite and infinite on‐dot Coulomb interaction is presented. Employing Keldysh's nonequilibrium Green's function formalism for different decoupling schemes in the equation of motion, the thermoelectric properties within the nonlinear transport regime have been analyzed. Initially, Hubbard‐I approximation has been used to study the quantum dot level position (), thermal gradient (), and on‐dot Coulomb interaction (U) dependence of the thermoelectric properties. Furthermore, as a natural extension, a decoupling beyond Hubbard‐I (Lacroix approximation) with infinite‐U limit (strong on‐dot Coulomb repulsion) has been used to provide additional insight into the operation of a more practical quantum dot heat engine. Within this infinite‐U limit, the role of the symmetric dot‐reservoir tunneling (Γ) and external serial load resistance (R) in optimizing the performance of the strongly correlated quantum dot heat engine is examined. The infinite‐U results show a good quantitative agreement with recent experimental data for a quantum dot coupled to two metallic reservoirs.

DFT investigation of hydrogenated cove-edged boron nitride nanoribbons for resonant tunneling diodes application

The cove-defect is found to improve the stability of the nanoribbons through bond length reconstructions at the cove-edge. Density functional theory (DFT) and non-equilibrium Green’s function (NEGF)-based first principles calculations are used to examine the structural, electronic, and quantum transport aspects of the hydrogenated cove-edge defective zigzag boron nitride nanoribbons (ZBNNRs). The additional electronic states, originated by dangling bonds at the cove-edges, exist across the Fermi level making these structures purely metallic. Additionally, with high peak-to-valley current ratios (PVCR), the quantum transport properties’ negative differential resistance (NDR) features are obtained. The highest PVCR is calculated to be 3.60 × 1016. The presented cove-edge defects can have a lot of potential for ultra-scaled devices including resonant tunnel diodes (RTDs), memory, and nanoswitches, according to the NDR phenomenon discovered in cove-edge ZBNNRs.

Design and Analysis of Gallium Arsenide-Based Nanowire Using Coupled Non-Equilibrium Green Function for RF Hybrid Applications

This research work uses sp3d5s* tight-binding models to design and analyze the structural properties of group IV and III-V oriented, rectangular Silicon (Si) and Gallium Arsenide (GaAs) Nanowires (NWs). The electrical characteristics of the NWs, which are shielded with Lanthanum Oxide (La2O3) material and the orientation with z [001] using the Non-Equilibrium Green Function (NEGF) method, have been analyzed. The electrical characteristics and the parameters for the multi-gate nanowires have been realized. A nanowire comprises a heavily doped n+ donor source and drains doping and n-donor doping at the channel. The specified nanowire has a gate length and channel length of 15 nm each, a source-drain device length LSD = 35 nm, with La2O3 as 1 nm (gate dielectric oxide) each on the top and bottom of the core material (Si/GaAs). The Gate-All-Around (GAA) Si NW is superior with a high (ION/IOFF ratio) of 1.06 × 109, and a low leakage current, or OFF current (IOFF), of 3.84 × 10-14 A. The measured values of the mid-channel conduction band energy (Ec) and charge carrier density (ρ) at VG = VD = 0.5 V are -0.309 eV and 6.24 × 1023 C/cm3, respectively. The nanowires with hydrostatic strain have been determined by electrostatic integrity and increased mobility, making them a leading solution for upcoming technological nodes. The transverse dimensions of the rectangular nanowires with similar energy levels are realized and comparisons between Si and GaAs NWs have been performed.

Phase and Thermal-Driven Transport Across T-Shaped Double Quantum Dot Josephson Junction

The phase and thermal driven transport properties of the T-shaped uncorrelated double quantum dot Josephson junction are analyzed by using Keldysh non-equilibrium Green's function equation of motion technique. In this setup, we have shown that the side-attached quantum dot provides an additional route for electron transmission which is affecting the transport properties by adjusting the interdot hopping between the main dot and the side dot. We began with investigating the impact of interdot hopping on Andreev bound states and Josephson supercurrent. When a small thermal bias is applied across the superconducting leads, the system exhibits a finite thermal response which is primarily due to the, thermally induced, quasi-particle current. The behavior of the Josephson supercurrent and the quasi-particle current flowing through the quantum dots is examined for various interdot hopping and thermal biasing. Finally, the system is considered in an open circuit configuration where the thermally driven quasi-particle current is compensated by the phase-driven Josephson supercurrent and the thermophase effect is observed. The effect of interdot hopping and the position of quantum dot energy level on the thermophase Seebeck coefficient is investigated

Switching modulation of spin transport in ferromagnetic tetragonal silicene

We study the band structure and transport properties of ferromagnetic tetragonal silicene nanoribbons by using the non-equilibrium Green's function method. The band structure and spin-dependent conductance are discussed under the combined effect of the external electric field, potential energy, exchange field and the spin-orbit coupling. One can easily realize a phase transition from a semimetallic to a semiconducting state by changing the transverse width of the nanoribbon. Separation of spin-dependent conductances arises from the effect of exchange field and the spin-orbit coupling, while zero-conductance behaviors exhibit spin-dependent band gaps induced by the electric field. We propose a device configuration of four-terminal tetragonal silicene nanoribbon with two central channels. It is found that spin current can be controlled by utilizing two switches. The switch with a high potential barrier can block electrons flowing from the central scattering region into other terminals. Interestingly, applying only one switch can realize spin-dependent zero conductance and large spin polarization. Two switches can provide multiple operations for controlling spin-dependent transport properties. The two-channel ferromagnetic tetragonal silicene nanoribbon can realize an effective separation of spin current, which may be a potential candidate for spintronic devices.

Stacking Order, Perfect Spin Polarization, and Giant Magnetoresistance in Zigzag Graphene/ h - BN Heterobilayer Nanoribbons

In a van der Waals heterostructure of two-dimensional materials, electronic properties are tunable by means of stacking orders. Here, we study the spin-dependent quantum transport in the bilayer of ferromagnetic zigzag-edged graphene/hexagonal-$\mathrm{BN}$ nanoribbons (ZGr-BNNRs) using density-functional theory combined with the Keldysh nonequilibrium Green's-function method. We reveal a strong odd-even effect of transport across the ZGr-BNNRs and a giant magnetoresistance value observed only in even-width ZGr-BNNRs. More interestingly, this value can be optimized by engineering stacking orders, yielding the perfect spin polarization efficiency of 100% and the magnetoresistance value of over ${10}^{4}$ in even-width ZGr-BNNRs. Our results provide a route to design and fabricate high-performance spin filters and magnetic storage devices.

Large magnetoresistance and spin-polarized photocurrent in Mn2.25Co0.75Ga0.5Sn0.5/MgO/Mn2.25Co0.75Ga0.5Sn0.5 magnetic tunnel junctions

With the development of information storage devices with high density, low power consumption and multi-means regulation, magnetic tunnel junctions (MTJs) with completely spin polarized materials as electrodes have attracted much attention. Heusler alloy Mn2.25Co0.75Ga0.5Sn0.5(MCGS) is a recently fabricated spin gapless semiconductor with 100 % spin polarization and high Curie temperature of 725 K. Here, the spin-dependent electronic transport and photoelectric properties of MCGS/MgO/MCGS MTJs are investigated systematically by density functional theory and non-equilibrium Green's function method. It is found that the MCGS/MgO/MCGS MTJs with four different terminations all present high tunnel magnetoresistance (TMR) and the maximum TMR is up to 6.51 × 106 %. The TMR value can be reduced and switched between positive and negative values by applying bias voltage, which can be used to designed magnetic logic devices. Furthermore, perfect spin injection efficiency can be obtained in the MCGS/MgO/MCGS MTJs and the switch of the spin channel can be controlled by the magnetization configuration of the ferromagnetic electrodes. Moreover, nearly 100 % spin-polarized photocurrent can be obtained by tailoring the polarization angle and photon energy of polarized light. The linearly polarized light is more favorable than circularly polarized light to generate high spin-polarized photocurrent. These results provide a theoretical guidance for designing the spin and optical tunable spintronic devices.

Switching limits of top-gated carbon nanotube field-effect transistors

The performance limits of carbon nanotube field-effect transistors (CNFETs) based on a recently reported process are studied by computational techniques at temperatures between 300 K and 4 K. The impact of band-to-band tunneling (BTBT) and source-to-drain tunneling (SDT) is examined for devices with varying gate length through the use of simulations based on the non-equilibrium Green’s function (NEGF) formalism, and calibrated to measurements. Additionally, the case of junctionless chemical doping profiles is analyzed in contrast to electrostatic doping recently reported for test structures. The switching limits of CNFETs are further explored for devices based on carbon nanotubes (CNTs) with more favorable electronic structures.

Thermoresponsive Conductivity of Graphene-Based Fibers.

Smart materials are versatile material systems which exhibit a measurable response to external stimuli. Recently, smart material systems have been developed which incorporate graphene in order to share on its various advantageous properties, such as mechanical strength, electrical conductivity, and thermal conductivity as well as to achieve unique stimuli-dependent responses. Here, a graphene fiber-based smart material that exhibits reversible electrical conductivity switching at a relatively low temperature (60°C), is reported. Using molecular dynamics (MD) simulation and density functional theory-based non-equilibrium Green's function (DFT-NEGF) approach, it is revealed that this thermo-response behavior is due to the change in configuration of amphiphilic triblock dispersant molecules occurring in the graphene fiber during heating or cooling. These conformational changes alter the total number of graphene-graphene contacts within the composite material system, and thus the electrical conductivity as well. Additionally, this graphene fiber fabrication approach uses a scalable, facile, water-based method, that makes it easy to modify material composition ratios. In all, this work represents an important step forward to enable complete functional tuning of graphene-basedsmart materials at the nanoscale while increasing commercialization viability.

Interband Tunneling in a Type-II Broken-Gap Superlattice

Due to its unique features, like tunable gap and high absorption coefficient, type-II superlattices receive growing interest. Substantial progress has been made in the technology of these materials as well as in the processing of superlattice-based devices. On the theoretical side, whereas the methods for superlattice analysis are well developed, for the superlattice-based devices they are well behind. Usually, such devices are modeled with semiclassical methods, in which the superlattice is treated as a bulk material having effective parameters extracted from its analysis with full quantum methods. As there is little theoretical justification for such a substitution, attempts have been made to model a whole superlattice-based device on a fully quantum level. In this paper, such modeling is presented for a broken-gap type-II superlattice diode: the nonequilibrium Green's function method is applied to the two-band model of an $\mathrm{In}\mathrm{As}$/$\mathrm{Ga}\mathrm{Sb}$ superlattice $p$-$i$-$n$ diode. The focus is paid on the band-to-band (BTB) tunneling with the aim of an assessment of equations used for its semiclassical description. The results of calculations presented in the paper demonstrate that, in the superlattice diode, the BTB tunneling occurs only for certain values of the in-plane momentum $k$, for which electronic and hole subbands cross. This is in contrast to the bulk materials, for which there is a range of such $k$ values. The simulations reveal much more differences. Accordingly, care must be taken when applying semiclassical models to describe the interband tunneling in superlattice devices.

Theoretical Prediction of Semiconductor-Free Negative Differential Resistance Tunnel Diodes with High Peak-to-Valley Current Ratios Based on Two-Dimensional Cold Metals

The negative differential resistance (NDR) tunnel diodes are promising alternative devices for beyond-CMOS (complementary metal oxide semiconductor) computing because they offer several potential applications when integrated with transistors. We propose a semiconductor-free NDR tunnel diode concept that exhibits an ultrahigh peak-to-valley current ratio (PVCR) value. Our proposed NDR diode consists of two cold metal electrodes separated by a thin insulating tunnel barrier. The NDR effect stems from the unique electronic band structure of the cold metal electrodes; i.e., the width of the isolated metallic bands around the Fermi level as well as the energy gaps separating higher- and lower-lying bands determine the current–voltage (I–V) characteristics and the PVCR value of the tunnel diode. By proper choice of the cold metal electrode materials, either a conventional N-type or Λ-type NDR effect can be obtained. Two-dimensional (2D) nanomaterials offer a unique platform for the realization of proposed NDR tunnel diodes. To demonstrate the proof of concept, we employ the nonequilibrium Green function method combined with density functional theory to calculate the I–V characteristic of the lateral (AlI2/MgI2/AlI2) and vertical (NbS2/h-BN/NbS2) heterojunction tunnel diodes based on 2D cold metals. For the lateral tunnel diode, we obtain a Λ-type NDR effect with an ultrahigh PVCR value of 1016 at room temperature, while the vertical tunnel diode exhibits a conventional N-type NDR effect with a smaller PVCR value of about 104. The proposed concept provides a semiconductor-free solution for NDR devices to achieve the desired I–V characteristics with ultrahigh PVCR values for memory and logic applications.

Sugar Molecules Detection via C2N Transistor-Based Sensor: First Principles Modeling.

Real-time detection of sugar molecules is critical for preventing and monitoring diabetes and for food quality evaluation. In this article, a field effect transistor (FET) based on two-dimensional nitrogenated holey graphene (C2N) was designed, developed, and tested to identify the sugar molecules including xylose, fructose, and glucose. Both density functional theory and non-equilibrium Green's function (DFT + NEGF) were used to study the designed device. Several electronic characteristics were studied, including work function, density of states, electrical current, and transmission spectrum. The proposed sensor is made of a pair of gold electrodes joint through a channel of C2N and a gate was placed underneath the channel. The C2N monolayer distinctive characteristics are promising for glucose sensors to detect blood sugar and for sugar molecules sensors to evaluate food quality. The electronic transport characteristics of the sensor resulted in a unique signature for each of the sugar molecules. This proposed work suggests that the developed C2N transistor-based sensor could detect sugar molecules with high accuracy.

Dual‐Gate GaAs‐Nanowire FET for Room Temperature Charge‐Qubit Operation: A NEGF Approach

AbstractThe performance of dual‐gate GaAs‐nanowire field‐effect‐transistor (FET) is investigated as a charge‐qubit device operating at room temperature. In compatibility with the state‐of‐the‐art classical bit technology, it is shown that the single gate of a nanowire FET can be replaced by two localized gates to achieve such charge‐qubit operation. On application of relevant biases to the localized gates, two voltage tunable quantum dots are created within the nanowire channel with electrostatically controlled single‐state‐occupancy and interdot coupling leading to charge‐qubit operation at room temperature. The associated electron transport is theoretically modeled on the basis of nonequilibrium Green's function formalism. The “initialization” and “manipulation” for qubit operation are performed by applying suitable gate voltages, whereas the “measurement” is executed by applying a small drain bias to obtain a pulse current of ≈pA order. A ≈25 MHz frequency of coherent oscillation is observed for the qubit and a characteristic decay time of ≈ 70 ns is achieved. The results suggest that such dual gate nanowire FET is a promising architecture for charge‐qubit operation at room temperature.

Tuning transport properties of deformed carbon nanocages by electric field, electrode material, and type of coupling

In this paper, we study the structural, electronic, and quantum transport properties in the smallest fullerene, C20, and the most famous fullerene, C60, and their corresponding fulleranes, C20H20 and C60H60. The molecules are attached to cumulene and graphene nanoribbon (GNR) electrodes. We examine the factors affecting electron transport in carbon molecular bridges, such as the application of an electric field, and changes in the type of and position electrodes. We first perform structural and electronic calculations with density functional theory (DFT), then we introduce the tight-binding parameters of the system for studying the transport properties of it, with the help of the non-equilibrium green's function (NEGF) method. Our findings show that fullerane molecular bridges are always electrically insulator, but fullerene systems can be metal, semiconductor, or even insulator, depending on the position of connection points to the electrodes. Larger fullerene molecular bridges, such as C60, are more chemically stable and can be used as molecular nanoelectronic devices. Besides, by applying an electric field to the C20H20 cage, we observe a phase transition from the semiconductor to the metal.

Chiral-Induced Spin Selectivity in Photon-Coupled Achiral Matters.

Chiral-induced spin selectivity is a phenomenon in which electron spins are polarized as they are transported through chiral molecules, and the spin polarization depends on the handedness of the chiral molecule. In this study, we show that spin selectivity can be realized in achiral materials by strongly coupling electrons to a circularly polarized mode of an optical cavity or waveguide. Through the investigation of spin-dependent electron transport in a two-terminal setup using the nonequilibrium Green's function approach, it is found that a large spin polarization can be obtained if the rate of dephasing is sufficiently small and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the optical mode. To obtain a wider range of energies for a large spin polarization, chiral molecules can be combined with light-matter interactions. To demonstrate this, the spin polarization of electrons transported through a helical molecule strongly coupled to a circularly polarized optical mode is evaluated.

Dynamic mode decomposition for extrapolating nonequilibrium Green's-function dynamics

The HF-GKBA offers an approximate numerical procedure for propagating the two-time non-equilibrium Green's function(NEGF). Here we compare the HF-GKBA to exact results for a variety of systems with long and short-range interactions, different two-body interaction strengths and various non-equilibrium preparations. We find excellent agreement between the HF-GKBA and exact time evolution in models when more realistic long-range exponentially decaying interactions are considered. This agreement persists for long times and for intermediate to strong interaction strengths. In large systems, HF-GKBA becomes prohibitively expensive for long-time evolutions. For this reason, look at the use of dynamical mode decomposition(DMD) to reconstruct long-time NEGF trajectories from a sample of the initial trajectory. Using no more than 16\% of the total time evolution we reconstruct the total trajectory with high fidelity. Our results show the potential for DMD to be used in conjunction with HF-GKBA to calculate long time trajectories in large-scale systems.

Cobalt doping of Mg3Sb2 monolayer: Improved thermoelectric performance

Mg3Sb2 is a layered thermoelectric (TE) material with excellent TE performance. In this work, focus on Mg3Sb2 monolayer, we employed virtual crystal approximation (VCA) to simulate the structure of Co doping Mg3Sb2 monolayer, the doping atoms ratio is 0.1%, namely Mg2.999Co0.001Sb2. The first-principles calculation within the framework of density functional theory (DFT) combined with non-equilibrium Green's Function (NEFG) method and Landauer-Buttiker theory have been used to calculate the phonon spectrum, the electronic structures, the TE parameters, the electron and phonon transmission spectra. From the results we find that doping enhanced the TE transformation efficiency. The maximum ZT of Mg3Sb2 monolayer is 1.38 at 200 K, which appears at the chemical potential of 0.04 eV. Relative to the Mg3Sb2 monolayer, the maximum ZT of Mg2.999Co0.001Sb2 is improved, which can reach 3.22 at 200 K for n-type doping. Our research demonstrates that a little amount of Co doping can improve the TE performance. It provides a new idea for the related experiments.

One-dimensional harmonic chain model of vibration-mode matching in solid-liquid interfacial thermal transport.

Understanding the atomistic mechanism of interfacial thermal transport at solid-liquid interfaces is a key challenge in thermal management at the nanoscale. A recent molecular-dynamics study demonstrated that interfacial thermal resistance (ITR) at the interface between a solid and a surfactant solution can be minimized by adjusting the molecular mass of the surfactant. In the present study, we explain the mechanism of this ITR minimization in view of vibration-mode matching using a one-dimensional (1D) harmonic chain model of a solid-liquid interface having an interfacial adsorption layer of surfactant molecules. The equation of motion for the 1D chain is described by a classical Langevin equation and is analytically solved by the nonequilibrium Green's function (NEGF) method. The resultant ITR is expressed in a form of vibrational matching, and its relationship to the overlap of the vibrational density of states is also discussed. The analysis leads to a conclusion that the damping coefficient η in the Langevin equation should be a finite and sufficiently large value to represent the rapid damping of vibration modes at solid-liquid interfaces. This conclusion provides a clue to seamlessly extend the conventional NEGF-phonon transmission picture of solid-solid interfacial thermal transport, which assumes η to be infinitesimal, to solid-liquid interfaces.