Nonequilibrium Green Function Research Articles

Investigation of the role of defects on channel density profiles and their effect on the output characteristics of a nanowire FET

This work investigates the effect of defects on the electron density profiles of nanowire FETs with a rectangular cross-section. It also presents a framework for the discretization of the nanowire channels with defects. A self-consistent procedure using Schrodinger-Poisson solver with density matrix formalism calculates the local electron density profiles. The local electron density decreases due to defect-induced scattering potentials. The electron density profiles vary according to the nature of the intrinsic defects. The effect of defect-induced potentials on the output characteristics of the nanowire FET device is studied using the non-equilibrium Green’s function (NEGF) methodology. An increase in scattering potential in the nanowire channel causes a considerable decrease in the saturation voltage and current. This results in a faster saturation which changes the overall device performance. Hence, defect-controlled channels can be utilized to fabricate FETs with desired characteristics.

Quantum many-body states and Green's functions of nonequilibrium electron-magnon systems: Localized spin operators versus their mapping to Holstein-Primakoff bosons

The operators of localized spins within a magnetic material commute at different sites of its lattice and anticommute on the same site, so they are neither fermionic nor bosonic operators. Thus, to construct diagrammatic many-body perturbation theory, the spin operators are usually mapped to the bosonic ones with Holstein-Primakoff (HP) transformation being the most widely used in magnonics and spintronics literature. However, to make calculations tractable, the square root of operators in the HP transformation is expanded into a Taylor series truncated to some low order. This poses a question on the range of validity of truncated HP transformation when describing nonequilibrium dynamics of localized spins interacting with each other or with conduction electron spins. Here we apply exact diagonalization techniques to Hamiltonian of fermions (i.e., electrons) interacting with HP bosons vs. Hamiltonian of fermions interacting with the original localized spin operators in order to compare their many-body states and one-particle equilibrium or nonequilibrium Green functions. The Hamiltonian of fermions interacting with HP bosons gives incorrect ground state and electronic spectral function, unless large number of terms are retained in truncated HP transformation. Furthermore, tracking nonequilibrium dynamics of localized spins over longer time intervals requires progressively larger number of terms in truncated HP transformation. Finally, we show that recently proposed [M. Vogl et al., Phys. Rev. Research 2, 043243 (2020); J. K\"{o}nig et al., SciPost Phys. 10, 007 (2021)] resummed HP transformation resolves the trouble with truncated HP transformation, while allowing us to derive an exact (manifestly Hermitian) Hamiltonian consisting of finite and fixed number of boson-boson and electron-boson interacting terms.

Effect of finite temperature and external magnetic field on non-equilibrium transport in a single molecular transistor with quantum dissipation: Anderson-Holstein-Caldeira-Leggett model

A magnetic field and temperature dependent quantum transport is investigated in a single molecular transistor. Our system is modelled by an onsite electron-electron correlation, polaronic interaction in the quantum dot and the coupling between quantum dot local phonon and the substrate phonon which produces a damping effect on the dynamics of the quantum dot. We use the Anderson-Holstein Hamiltonian along with the Caldeira-Leggett model to tackle the dissipation and polaronic effects and mean-field Hartree-Fock approximation to treat the onsite Coulomb correlation. We calculate transport properties such as spectral density function, tunnelling current, differential conductance employing finite-temperature Keldysh non-equilibrium Green function method under the influence of a finite temperature and an external magnetic field. It is shown that temperature has an intervening effect on the spin-resolved transport properties which can be used as a tuneable spin-filter.

Impact of Kondo correlations and spin–orbit coupling on spin-polarized transport in carbon nanotube quantum dot

Spin polarized transport through a quantum dot coupled to ferromagnetic electrodes with noncollinear magnetizations is discussed in terms of nonequilibrium Green functions formalism in the finite-U slave boson mean field approximation. The difference of orientations of the magnetizations of electrodes opens off-diagonal spin–orbital transmission and apart from spin currents of longitudinal polarization also spin-flip currents appear. We also study equilibrium pure spin current at zero bias and discuss its dependence on magnetization orientation, spin–orbit coupling strength and gate voltage. Impact of these factors on tunneling magnetoresistance (TMR) is also undertaken. In general spin–orbit coupling weakens TMR, but it can change its sign.

Two-Dimensional GeC2 with Tunable Electronic and Carrier Transport Properties and a High Current ON/OFF Ratio.

In this study, we present that 2D tetrahex-GeC2 materials possess novel electronic and carrier transport properties based on density functional theory computations combined with the nonequilibrium Green's function method. We show that under the 4% (-4%) in-plane expansion (compression) along the a-direction (b-direction) of the tetrahex-GeC2 monolayer, the bandgap can be enlarged to a desirable 1.26 eV (1.32 eV), close to that of silicon. The carrier transport properties of both the sub-10 nm tetrahex-GeC2 monolayer and the bilayer show strong anisotropy within the bias from -1 to 1 V. The current ON (a-direction)/OFF (b-direction) ratio amounts to 105 for the tetrahex-GeC2 monolayer. A striking negative differential conductance arises with the maximum Ipeak/Ivalley on the order of 104 under the 4% uniaxial expansion along the b-direction of the tetrahex-GeC2 monolayer. Overall, the 2D tetrahex-GeC2 monolayer and bilayer possess highly tunable electronic and carrier transport properties under uniaxial strain, which can be exploited for potential applications in nanoelectronics.

Theoretical design of thermal spin molecular logic gates by using a combinational molecular junction

Based on the density functional theory combined with the nonequilibrium Green function methodology, we have studied the thermally-driven spin-dependent transport properties of a combinational molecular junction consisting of a planar four-coordinate Fe molecule and a 15,16-dinitrile dihydropyrene/cyclophanediene molecule, with single-walled carbon nanotube bridge and electrode. Our results show that the magnetic field and light can effectively regulate the thermally-driven spin-dependent currents. Perfect thermal spin-filtering effect and good thermal switching effect are realized. The results are explained by the Fermi–Dirac distribution function, the spin-resolved transmission spectra, the spatial distribution of molecular projected self-consistent Hamiltonian orbitals, and the spin-resolved current spectra. On the basis of these thermally-driven spin-dependent transport properties, we have further designed three basic thermal spin molecular AND, OR, and NOT gates.

Inorganic gas sensing performance of χ3-borophene and the van der Waals heterostructure

Recently, two-dimensional (2D) material χ3-borophene has been synthesized in experiment. Combining density functional theory with non-equilibrium Green's function method, in the present work we theoretically investigated the adsorption behaviors of small gas molecules NO, NO2, NH3, CO and CO2 on χ3-borophene, and corresponding transport properties and sensing performance. Our results suggest that NO, NO2, NH3 and CO adsorb on this 2D material by chemical adsorption, while CO2 adopts physical adsorption. The transport properties of χ3-borophene are sensitive to the adsorption of NO, NO2, NH3 and CO, thus it can be used to construct gas sensor for these gas molecules. Especially, χ3-borophene is very sensitive to NO and, more interestingly, the sensing performance for NO can be greatly improved by applying in-plane strain. In addition, using semiconductor WSe2 as the substrate, we theoretically predicted a kind of van der Waals heterostructure. It is found that, although the adsorption behaviors of NO, NO2, NH3 and CO on χ3-borophene are hardly affected by the substrate, WSe2 can effectively enhance the stability of χ3-borophene and positively modulate the sensing performance, suggesting an actual way to realize χ3-borophene gas sensor.

Disorder-Induced Degradation of Vertical Carrier Transport in Strain-Balanced Antimony-Based Superlattices

We investigate carrier transport in gallium-free strained-balanced $\mathrm{In}\mathrm{As}/{\mathrm{In}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}\mathrm{Sb}$ type-II superlattices in the presence of positional and compositional disorder. We use a rigorous nonequilibrium Green's function model based on fully nonlocal scattering self-energies computed in the self-consistent Born approximation and a multiband description of the electronic structure. Layer-thickness fluctuations, nonuniform antimony composition, and segregation throughout the superlattice stack lead to as-grown disordered structures that are quite different from the abrupt interface ideal superlattices. We find that regardless of its nature and cause, disorder significantly affects vertical-carrier-transport properties, by impeding the coherent propagation of carriers in the minibands. In particular, the minority-carrier hole mobility is fundamentally limited by the nonideal properties of the superlattice, namely the layer-thickness fluctuation and the nonuniform antimony distribution. Furthermore, upon reducing the temperature, holes become fully localized and transport occurs by hopping, which explains published measured detector data that demonstrates the quantum efficiency, exhibiting a very strong temperature dependence that degrades as the temperature is reduced. As a result, photodetectors that employ holes as minority carriers will be limited in performance, especially for long-wavelength infrared applications at low temperature. However, we find that minority-carrier electron mobility is largely unaffected by disorder, indicating the $p$-type absorbing layer as the preferred option.

Thermal Transport in Disordered Harmonic Chains Revisited: Formulation of Thermal Conductivity and Local Temperatures

In this paper, thermal transport in bond-disordered harmonic chains is revisited in detail using a nonequilibrium Green's function formalism. For strong bond disorder, thermal conductivity is independent of the system size. However, kinetic temperatures described by the local number of states coupling to external heat reservoirs are anomalous since they form a nonlinear profile in the interior of the system. Both results are accounted for in a unified manner in terms of the frequency-dependent localization length. From this argument, we derive a generic formula describing the asymptotic profile of local temperatures in a disordered harmonic chain. A linear temperature profile obeying Fourier's law is recovered by contact with a self-consistent reservoir of Ohmic type even in the limit of weak system-reservoir coupling. This verifies that mechanisms leading to local thermal equilibrium and breaking total momentum conservation are essential for Fourier transport in low dimensions.

Heat transport in a two-level system driven by a time-dependent temperature

The field of thermotronics aims to develop thermal circuits that operate with temperature biases and heat currents just as how electronic circuits are based on voltages and electric currents. Here, we investigate a thermal half-wave rectifier based on a quantum two-level system (a qubit) that is driven by a periodically modulated temperature difference across it. To this end, we present a non-equilibrium Green's function technique, which we extend to the time domain to account for the time-dependent temperature in one of two thermal reservoirs connected to the qubit. We find that the qubit acts a thermal diode in parallel with a thermal capacitor, whose capacitance is controlled by the coupling to the reservoirs. These findings are important for the efforts to design non-linear thermal components such as heat rectifiers and multipliers that operate with more than one diode.

Role of line defect in the bandgap and transport properties of silicene nanoribbons

By using the tight-binding model and non-equilibrium Green's function method (NEGF), we study the band structures and transport properties of a silicene nanoribbon with a line defect where a bulk energy gap is opened due to the sublattice symmetry breaking. The flat subband bends downwards or upwards due to the effect of the line defect. The spin-orbit coupling induces quantum spin Hall states. Especially, the energy band depends on the distance between the line defect and the edge of the nanoribbon. The effects of the on-site energies on the band spectra of the two defect configurations are different. There always exists one band gap for different on-site energies for the defect configuration of case 1. However, a gapless state and a band gap can be modulated by changing the on-site energy, the sublattice potential and spin-orbit couplings for the defect configuration of case 2. Accordingly, the variation trends of the conductance including zero conductance can be well understood in terms of the combined effect of the sublattice potential, the on-site energy and spin-orbit couplings on the band structures. Thus it is easy and effective to modulate the transport property of the silicene nanoribbon with the defect configuration of case 2 by utilizing the sublattice potential, the on-site energy and spin-orbit couplings. This study is of great significance for the fabrication and the modulation of the transport property of silicene-based devices.

Green's function formalism for nonlocal elliptical magnon transport

We develop a non-equilibrium Green's function formalism to study magnonic spin transport through a strongly anisotropic ferromagnetic insulator contacted by metallic leads. We model the ferromagnetic insulator as a finite-sized one-dimensional spin chain, with metallic contacts at the first and last sites that inject and detect spin in the form of magnons. In the presence of anisotropy, these ferromagnetic magnons become elliptically polarized, and spin conservation is broken. We show that this gives rise to a novel parasitic spin conductance, which becomes dominant at high anisotropy. Moreover, the spin state of the ferromagnet becomes squeezed in the high-anisotropy regime. We show that the squeezing may be globally reduced by the application of a local spin bias.

Spin thermoelectric effects on aluminum or phosphorus doped zigzag silicene nanoribbons

Based on the density-functional theory (DFT) combined with nonequilibrium Green’s function (NGF), this paper investigates the effects of either single aluminum (Al) or single phosphorus (P) atom substitutions at different edge positions of zigzag-edged silicene nanoribbons (ZGNRs) in the ferromagnetic state on the spin-dependent transport properties and spin thermoelectric effects. It has been found that the spin polarization at the Fermi level can reach 100% or –100% in the doped ZSiNRs. Meanwhile, the spin-up Seebeck effect (for -100% case) and spin-down Seebeck effect (for 100% case) are also enhanced. Moreover, the spin Seebeck coefficient is much larger than the corresponding charge Seebeck coefficient at a special doping position and electron energy. Therefore, the study shows that the Al or P doped ZSiNRs can be used to prepare the ideal thermospin devices.

Selective Edge-Hydrogenated Zigzag Boron Nitride Nanoribbons for Giant Magnetoresistance and Rectifying Behavior

The structural stability, spin-polarized electronic and transport properties of edge-hydrogenated zigzag boron nitride nanoribbons (ZBNNRs) are investigated using density functional theory (DFT) combined with a non-equilibrium Green’s function (NEGF) approach. The considered structures are observed to be energetically stable in the magnetic ground state. The calculated spin-dependent transport properties indicate that a selective-edge hydrogenated ZBNNR device shows bipolar spin filter characteristics. Interestingly, high spin giant magnetoresistance (GMR) and a rectification ratio (RR) are observed of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> , respectively. The observed GMR and rectification behavior in edge-hydrogenated ZBNNRs have potential applications for designing future spintronic nanodevices.

A Multiscale Simulation Framework for Steep-Slope Si Nanowire Cold Source FET

Source engineering is an emerging technique to achieve steep-slope switching FET. To bridge the new carrier filtering mechanism and device performance, a multiscale simulation framework is presented in this article and is applied in Si nanowire (NW) cold source FET (CSFET). By the fit-parameter-free density functional theory (DFT) method, the key component of cold source (CS) design for broken-gap-like band alignment and high cold carrier injection is demonstrated. The novel device switching mechanism is also verified in the entire device scale with fully quantum atomistic tight-binding (TB) and nonequilibrium Green’s function (NEGF) methods. Although these tools are physics-based and accurate, the device scale is limited, and the computation burden is heavy. Thus, half-empirical TCAD simulation is suitable for device design and path-finding in realistic geometry. Key components of the CS and energy filtering effect can be verified by DFT-NEGF and TB-NEGF methods. Based on TCAD results, we implement a circuit-level benchmark for early stage path-finding. The results show that gate-all-around (GAA) Si NW CSFET is a potential candidate for low-power application, which enables supply voltage scaling.

Electron transport properties of molecular devices based on silicon-1, 6-hexanedithiol-silicon

To reveal how the electronic transport properties of the silicon-sulfur molecular junction depend on its microscopic structure, we studied the configuration and electronic transport properties of a silicon-1, 6-hexanedithiol-silicon molecular device, based on density functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) method. Eight groups of molecular junctions were modeled, each consisting of a 1, 6-hexanedithiol molecule spanning two silicon tips. Following optimisation of the molecular structure, first of the 1, 6-hexanedithiol by itself and then of the junction as a whole, the zero-bias conductance and electronic transmission spectrum were calculated. The results show that silicon-sulfur molecular junctions have characteristics that differ significantly from those of gold-sulfur molecular junctions. The structural details of the silicon-sulfur molecular junction are shown to have a crucial influence on its electronic transport properties such as zero-bias piezoelectric conductance and transmission spectra.

Nitrogen doped armchair ZnO nanoribbons for potential rectification applications: DFT analysis

This work investigates the structural, electronic, and transport properties of pristine and N-doped armchair ZnO nanoribbons (ZnONRs) using density functional theory (DFT) in combination with non-equilibrium Green's function (NEGF). It is reported here that the N atom doping at the O atom site slightly reduces the binding energy (E b ) and it also decreases band gap (E g ). For 9-atom width, the pristine structures are most stable with a binding energy of −4.961 eV and a band gap of 2.012 eV. Additionally, the Fermi energy level in doped ZnONRs lies near the valence band thereby induces the p-type characteristics. The transport properties of the two-terminal devices have also been examined. These devices exhibit asymmetric I–V characteristics leading to the rectification phenomenon. Relative to the pristine device, N-doped devices demonstrated improved I–V characteristics. Interestingly, the N-doped two-terminal devices exhibit a higher rectification ratio (RR) in contrast to the pristine device. Similar characteristics are observed for the devices irrespective of width. For the 9-atom width ZnONR devices with N doped at the edge and center positions demonstrate significantly high RR of about 4.54 × 10 8 and 1.56 × 10 8 at 1.35 V and 1.0 V bias points, respectively. With such enhanced rectification characteristics, ZnONRs can be used as potential candidates for future nanoelectronic switching devices. • The structural, electronic, and transport properties of pristine and N-doped armchair ZnONRs have been investigated here. • The N atom doping at the O atom site slightly reduces the E b and it also decreases E g . • The Fermi energy level in doped ZnONRs lies near the valence band thereby induces the p-type characteristics. • N-doped two-terminal devices exhibit a higher RR. • For 9-ZnONR devices with N doped at the edge and center positions, significantly high RR is of the order of 10 8 .

Impurity Substitution Enhances Thermoelectric Figure of Merit in Zigzag Graphene Nanoribbons

The thermoelectric properties of zigzag graphene nanoribbons (ZGNRs) are sensitive to chemical modification. In this study, we employed density functional theory (DFT) combined with the nonequilibrium green’s function (NEGF) formalism to investigate the thermoelectric properties of a ZGNR system by impurity substitution of single and double nitrogen (N) atoms into the edge of the nanoribbon. N-doping changes the electronic transmission probability near the Fermi energy and suppresses the phononic transmission. This results in a modified electrical conductance, thermal conductance, and thermopower. Ultimately, simultaneous increase of the thermopower and suppression of the electron and phonon contributions to the thermal conductance leads to the significant enhancement of the figure of merit in the perturbed (i.e., doped) system compared to the unperturbed (i.e., nondoped) system. Increasing the number of dopants not only changes the nature of transport and the sign of thermopower but also further suppresses the electron and phonon contributions to the thermal conductance, resulting in an enhanced thermoelectric figure of merit. Our results may be relevant for the development of ZGNR devices with enhanced thermoelectric efficiency.

Spin-triplet superconductor–quantum anomalous Hall insulator–spin-triplet superconductor Josephson junctions: 0−π transition, ϕ0 phase, and switching effects

We study the Josephson effect in spin-triplet superconductor$-$quantum anomalous Hall insulator$-$spin-triplet superconductor junctions using the nonequilibrium Green function method. The current-phase difference relations show strong dependence on the orientations of the $\bf{d}$-vectors in superconductors. We focus on two $\bf{d}$-vector configurations, the parallel one with the left and right ${\bf{d}}$-vectors being in the same direction, and the nonparallel one with the left ${\bf{d}}$-vector fixed at the $z$-axis. For the parallel configuration, the $0$-$\pi$ transition can be realized when one rotates the ${\bf{d}}$-vectors from the parallel to the junction plane to the perpendicular direction. The $\phi_{0}$ phase with nonzero Josephson current at zero phase difference can be obtained as long as ${d_{x}}{d_{z}}\ne0$. For the nonparallel configuration, the $0$-$\pi$ transition and the $\phi_{0}$ phase still exist. The condition for the formation of the $\phi_{0}$ phase becomes $d_{Rx}\ne0$. The switch effects of the Josephson current are found in both configurations when the ${\bf{d}}$-vectors are rotated in the $xy$ plane. Furthermore, the symmetries satisfied by the current-phase difference relations are analysed in details by the operations of the time-reversal, mirror-reflections, the spin-rotation and the gauge transformation, which can well explain the above selection rules for the $\phi_{0}$ phase. Our results reveal the peculiar Josephson effect between spin-triplet superconductors and the quantum anomalous Hall insulator, which provide helpful phases and effects for the device designs. The distinct current-phase difference relations for different orientations may be used to determine the direction of the ${\bf{d}}$-vector in the spin-triplet superconductor.

Ballistic transport in sub-10 nm monolayer planar GaN transistors for high-performance and low-power applications

With the constant scaling down of transistors, “More Moore” has put forward requirements for channel materials. Two-dimensional materials are considered as potential next-generation channel materials due to their unique physical properties. Herein, we study the ballistic transport characteristics of sub-10 nm monolayer (ML) planar GaN metal–oxide–semiconductor field-effect transistors (MOSFETs) based on density functional theory and the non-equilibrium Green's function method. The calculation results indicate that n-type ML planar GaN MOSFETs with gate lengths not less than 5 nm exhibit excellent subthreshold characteristics, fast switching speeds, and low switching energies, and the corresponding parameters (off-current, on-current, delay time, and power-delay product) can simultaneously meet the requirements of the International Roadmap for Devices and Systems for the 2028 horizon for high-performance and low-power applications. Therefore, ML planar GaN is predicted to be a reliable next-generation channel material to extend Moore's law.