Nonequilibrium Green Function Research Articles

Theoretical study on the negative differential resistance behavior of N-doped GeSe monolayer

Using a combination of non-equilibrium Green’s function (NEGF) and density functional theory (DFT), the electronic structures and transport characteristics of N-doped GeSe monolayer were investigated systematically. The calculated band structures illustrated that the doping of N atoms changed GeSe monolayer from a semiconductor to a metal. Further, a clear negative differential resistance behavior (NDR) was found in the voltage-current curves of the N-doped GeSe based devices. Moreover, the current peak-to-valley ratio (PVR) varied with the doping concentration of N atoms. As the doping concentration decreased, the peak of NDR shifted toward the millivolt region. Especially, the NDR behavior with a PVR up to 103 entered the mV bias region when the doping concentration was reduced to 0.69%. These results provide a novel approach to design the low power nanodevice based on the GeSe monolayer.

TEMT: A Transient Electronic–Magnetic–Thermal-Coupled Simulation Framework for STT-MTJs

Being a promising candidate for future non-volatile memory and neuromorphic computing, Spin-Transfer-Torque Magnetic Tunnel Junction (STT-MTJ) devices are gaining substantial momentum in industrial adoption in recent years, calling for EDA support for higher modeling capabilities. However, the complex interplay of multiple physical mechanisms featured in MTJ devices impose substantial challenges to their numerical modeling. In this work, we propose a fully coupled, transient electronic-magnetic-thermal (TEMT) modelling approach for STT-MTJ devices. Aiming to be first-principle and physically holistic, TEMT combines the atomistic non-equilibrium Green’s function (NEGF) model for tunneling currents, the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation for magnetic dynamics and the heat conduction equation (HCE) for thermal dynamics in a self-consistent manner. To alleviate computational burden, we also devise an analytical approximation approach to reduce the number of NEGF solutions in the TEMT framework, which leads to over 15X speed-up with a very mild accuracy loss. An in-depth investigation of STT-MTJ devices using the TEMT framework is conducted to demonstrate its benefits and performance.

A Physics-Based Model and Its Solution Methodology for Hysteresis Mobile-Ionic Dielectrics

A physics-based model and its corresponding solution methodology for the mobile-ionic (MI) dielectrics exhibiting Q-V(Charge-Voltage) hysteresis behaviors are proposed based on ion drift-diffusion (IDD) equations coupling with Poisson’s equation. The proposed model captures the dynamic distribution of mobile ions’ concentrations within dielectric along the external electric field. The accuracy of the proposed model is validated by comparing the computed results with the experimental ones. Moreover, to investigate the synaptic characters of mobile-ionic field-effect transistors (MIFETs), numerical model based on coupled non-equilibrium Green’s function (NEGF), IDD and Poisson’s equations is also established. Potentiation and depression properties of MIFET are decently reproduced with the proposed model, which are consistent with the experiment results.

Surface asymmetry induced turn-overed lifetime of acoustic phonons in monolayer MoSSe

Recent successful growth of asymmetric transition metal dichalcogenides via accurate manipulation of different chalcogen atoms in top and bottom surfaces demonstrates exotic electronic and chemical properties in such Janus systems. Within the framework of density functional perturbation theory, anharmonic phonon properties of monolayer Janus MoSSe sheet are explored. By considering three-phonons scattering, out-of-plane flexural acoustic (ZA) mode tends to undergo a stronger phonon scattering than transverse acoustic (TA) mode and the longitudinal acoustic (LA) mode with phonon lifetime of ZA (1.0 ps)<LA (23.8 ps)<TA (25.8 ps). This is sharply different from the symmetric MoS2 where flexural ZA mode has the weakest anharmonicity and is least scattered. Moreover, utilizing non-equilibrium Green function method, ballistic thermal conductance at room temperature is found to be around 0.11nWK-1nm-2, lower than that of MoS2. Our work highlights intriguing phononic properties of such MoSSe Janus layers associated with asymmetric surfaces.

Negative differential resistance effect and current rectification in WS2 nanotubes: A density functional theory study

Electronic properties of (n,0) zigzag tungsten disulfide WS2 nanotubes and transport properties of (7,0) zigzag nanotube heterojunction are investigated by utilizing non-equilibrium Green's function formalism (NEGF). The results reveal that inclusion of the spin-orbit (SO) coupling significantly reduces the value of the band gap about 15.52\\%. Additionally, strong negative differential resistances take place in voltage regions between −0.2 and −0.4 V as well as between −0.5 and −0.6 V. Moreover, temperature dependent transport properties are elaborately investigated in this work. The results show that, as the temperature increases to 600 K the stronger negative differential resistance occurs in both positive and negative bias voltages. Finally, a reasonably large degree of rectification ratio can be established. Our analysis can provide a comprehensive perspective on the NDR effect and current rectification in WS2 nanotubes and can be used as a beneficial guide for future designing of novel logical and nanoelectronic and spintronic devices.

Accelerating nonequilibrium Green function simulations with embedding self-energies

Real-time nonequilibrium Green functions (NEGFs) have been very successfully used to simulate the dynamics of correlated many-particle systems far from equilibrium. However, NEGF simulations are computationally expensive since the effort scales cubically with the simulation duration. Recently, we introduced the G1--G2 scheme that allows for a dramatic reduction to time-linear scaling [N. Schl\"unzen et al., Phys. Rev. Lett. 124, 076601 (2020); J.-P. Joost et al., Phys. Rev. B 101, 245101 (2020)]. Here we tackle another problem: the rapid growth of the computational effort with the system size. In many situations where the system of interest is coupled to a bath, to electric contacts, or to similar macroscopic systems for which a microscopic resolution of the electronic properties is not necessary, efficient simplifications are possible. This is achieved by the introduction of an embedding self-energy---a concept that has been successful in standard NEGF simulations. Here, we demonstrate how the embedding concept can be introduced into the G1--G2 scheme, allowing us to drastically accelerate NEGF embedding simulations. The approach is compatible with all advanced self-energies that can be represented by the G1--G2 scheme [as described in J.-P. Joost et al., Phys. Rev. B 105, 165155 (2022)] and retains the memoryless structure of the equations and their time-linear scaling. As a numerical illustration, we investigate the charge transfer between a Hubbard nanocluster and an additional site which is of relevance for the neutralization of ions in matter.

Sensing of sulfur containing toxic gases with double transition metal carbide MXenes

Sensing of sulfur containing gases, such as hydrogen sulfide (H2S) and sulfur dioxide (SO2), on double transition metal carbide MXenes has been studied by means of density functional theory calculations. It is found that both H2S and SO2 strongly physisorbed on M2TiC2Tx (M = Mo, Cr; Tx = O, S, OH) monolayers. The adsorption of H2S is stronger in the case of oxygen termination, (Mo2TiC2O2, Cr2TiC2O2), whereas SO2 has stronger adsorption on sulfur terminated (Mo2TiC2S2, Cr2TiC2S2) monolayers. Binding characteristics of H2S and SO2 are further confirmed with Bader charge transfer, density of states, and charge density differences. Using a statistical thermodynamic analysis, the sensing mechanism of H2S and SO2 is studied under varied pressures and temperature conditions. By means of non-equilibrium Green's function calculations, the sensitivities of H2S and SO2 on the Mo2TiC2S2 monolayer are found to be 16.3%, 10.3%, respectively, which showed promising sensing characteristics. Our results reveal that double transition metal carbide MXenes can be a potential candidate for the sensing of sulfur containing gases.

Platinum contacts for 9-atom-wide armchair graphene nanoribbons

Creating a good contact between electrodes and graphene nanoribbons (GNRs) has been a long-standing challenge in searching for the next GNR-based nanoelectronics. This quest requires the controlled fabrication of sub-20 nm metallic gaps, a clean GNR transfer minimizing damage and organic contamination during the device fabrication, as well as work function matching to minimize the contact resistance. Here, we transfer 9-atom-wide armchair-edged GNRs (9-AGNRs) grown on Au(111)/mica substrates to pre-patterned platinum electrodes, yielding polymer-free 9-AGNR field-effect transistor devices. Our devices have a resistance in the range of 106–108 Ω in the low-bias regime, which is 2–4 orders of magnitude lower than previous reports. Density functional theory calculations combined with the non-equilibrium Green's function method explain the observed p-type electrical characteristics and further demonstrate that platinum gives strong coupling and higher transmission in comparison to other materials, such as graphene.

Extraction of Mobility from Quantum Transport Calculations of Type-II Superlattices

Type-II superlattices (T2SLs) are being investigated as an alternative to traditional bulk materials in infrared photodetectors due to predicted fundamental advantages. Subject to significant quantum effects, these materials require the use of quantum transport methodologies, such as the nonequilibrium Green's function (NEGF) formalism to fully capture the relevant physics without uncontrolled approximations. Carrier mobility is a useful parameter that affects carrier collection in photodetectors. This work investigates the application of mobility extraction methodologies from quantum transport simulations in the case of T2SLs exemplified using an InAs/GaSb midwave structure. In a resistive region, the average velocity can be used to calculate an apparent mobility that incorporates both diffusive and ballistic effects. However, the validity of this mobility for predicting device properties is limited to cases of diffusive limited transport or when the entire device can be included in the simulation domain. Two methods that have been proposed to extract diffusive limited mobility, one based on approximating the ballistic component of transport and the other which considers the scaling of resistance with simulation size, were also studied. In particular, the resistance scaling approach is demonstrated to be the method most physically relevant to predicting macroscopic transport. We present a method for calculating the mobility from resistance scaling considerations that accounts for carrier density variation between calculations, which is particularly relevant in the case of electrons. Finally, we comment on the implications of applying the different mobility extraction methodologies to device property predictions. The conclusions of this study are not limited to T2SLs, and may be generally relevant to quantum transport mobility studies.

Exceptional points treatment of cavity spectroscopies.

The infrared response of a system of two vibrational modes in a cavity is calculated by an effective non-Hermitian Hamiltonian derived by employing the nonequilibrium Green's function (NEGF) formalism. Degeneracies of the Hamiltonian (exceptional points, EPs) widely employed in theoretical analysis of optical cavity spectroscopies are used in an approximate treatment and compared with the full NEGF. Qualitative limitations of the EP treatment are explained by examining the approximations employed in the calculation.

Effect of ZnO dimers on the thermoelectric performance of armchair graphene nanoribbons.

Enhancing the thermoelectric performance in engineered graphene nanoribbons is used to produce thermoelectric nanodevices, which are important in many applications. By using a chemical doping method, armchair graphene nanoribbons (AGNRs) can have thermoelectric properties that are tunable. We predicted that changing the number and geometrical pattern of zinc oxide (ZnO) dimers in an AGNR can engineer thermoelectric properties, so we used density functional-based tight binding (DFTB) combined with the non-equilibrium Green's function (NEGF) to investigate the geometric, electronic, and thermoelectric properties of the AGNR with and without various dopants of ZnO dimers. With three forms of ZnO dimers, ortho, meta, and para dimers, different concentration ratios of Zn and O atoms are used. Our results indicate that the electronic features of AGNR are influenced not only by the concentrations of ZnO dimers but also by the geometrical pattern of ZnO dimers in the AGNR. These results are helpful in better understanding the effect of chemical doping on the transport properties of AGNRs and in motivating nanodevices to improve their thermoelectric performance.

The influence of single carbon atom impurity on the electronic transport of (6, 3) two side-closed single-walled boron nitride nanotubes

In this study, the electronic transport of (6, 3) two side-closed single-walled boron nitride nanotubes ((6, 3) TSC-SWBNNTs) located between two electrodes of (5, 5) conductive carbon nanotubes is investigated.Introducing carbon impurities instead of nitrogen and boron atoms in different locations of two side-closed (6, 3) SWBNNTs would change the transmission spectrum and reduce the gap. As the bias voltage increases, the peaksof the transmission spectrum become sharper and narrower. Substituting carbon impurities instead of nitrogenatoms leads to larger currents than substituting carbon impurities instead of boron. For the carbon impurity insteadof N atoms, the current in the center is observed to be larger than currents on the left and right sides. In addition,negative resistance can be seen in current-voltage diagrams, which are used in the construction of high-speedelectronic switches in electrical circuits. Due to the larger number of atoms, the study of structures by the common approach is time-consuming and sometimes impossible. Hence, in this research, the Quantum ATK simulation software is used to investigate the characteristics of electronic transport. For this purpose, the Slater-Koster method and the tight-closure approximation are used. In this method, the non-equilibrium Green function (NEGF) approach is employed.

Designed Graphane-based spin filters by tuning the sp2/sp3 configuration

The electronic transport properties of H-vacancy-defected graphane are investigated by using the density functional theory (DFT) and Non-equilibrium Green’s Function (NEGF) methods. Results show that the band gap of graphane depends on the relative positions of the defect bands introduced by H vacancies and the valence or conduction bands, suggesting the band gap controllability. The defect bands are obviously spin polarized and the ratio is almost 100%. And the magnetization of the system can be tuned by defect concentration and location. Moreover, in the two-probe systems, the presence of H-vacancy-defect bands near the Ef plays an important role in controlling spin-dependent transport of graphane. A perfect spin-filtering effect with 100% spin polarization could be realized by H-vacancy defects. The spin properties indicate the potential of graphane as an option for future graphene-based spin filters and spintronic devices.

Correlated Mott insulators in a strong electric field: The effects of phonon renormalization

We characterize the response of a Mott insulating system to a static electric field in terms of its conducting and spectral properties. Dissipation is included by a coupling to fermionic baths and to either optical or acoustic phonons. This paper extends and completes the analysis made in a previous work by the authors [Mazzocchi et al., Phys. Rev. B 106, 125123 (2022)]. In the present work, phonons are included diagrammatically within the Migdal approximation by also including self-consistency from the electronic feedback. The nonequilibrium steady state is addressed by means of the dynamical mean-field theory based on the nonequilibrium Green's function approach, while the so-called auxiliary master equation approach is employed as impurity solver. With optical phonons the self-consistency suppresses the steady-state current for field strengths comparable to the band gap with respect to the non-self-consistent case. This is due to the interaction of phonons with the hot electrons of the lattice which increases their temperature, thus providing a less effective relaxation channel for the current-induced Joule heat. In addition, in the case of optical phonons, the results are essentially independent of the temperature of the fermionic baths, as the latter is sensibly smaller than their characteristic frequency. On the other hand, with acoustic phonons the steady-state current is slightly suppressed by the self-consistent treatment only at field strengths close to half of the gap and especially at very small phonon frequency. Also, in this case, the results seem to slightly depend on the temperature of the fermionic baths.

Sub‐5 nm 2D Semiconductor‐Based Monolayer Field‐Effect Transistor: Status and Prospects

A detailed review of sub‐5 nm 2D semiconductor monolayer (ML) field‐effect transistors (FETs) is presented. A brief introduction to the fundamentals of 2D materials and their properties is provided followed by a critical review to propose an appropriate channel material from the available 2D materials. Various device simulation techniques incorporating density functional theory (DFT) and nonequilibrium Green's function (NEGF) are discussed in order to comprehend the distinctive features of 2D materials and the ballistic performance of sub‐5 nm ML 2D FETs. Relevant, recent experimental progress is also discussed. The review suggests that sub‐5 nm monolayer FETs based on 2D semiconductors like black phosphorene, WSe2N4, silicene, tellurene, arsenene, WSe2, MoTe2, MoS2, InSe, α‐CS, MoSi2N4, WS2, Bi2O2Se, GeSe, SnSe2, SnS2, AsP, GeS, MoSi2N4, BiH, SnSe2 and MoTe2 show excellent device performance meeting the International Technology Roadmap for Semiconductor (ITRS) requirements (2013 Version) for both high‐performance (HP) and low‐power (LP) applications in terms of on‐state current, subthreshold swing (SS), delay time, and power delay product (PDP).

Ohmic Behavior in Metal Contacts to n/p-Type Transition-Metal Dichalcogenides: Schottky versus Tunneling Barrier Trade-off

High contact resistance (RC) between 3D metallic conductors and single-layer 2D semiconductors poses major challenges toward their integration in nanoscale electronic devices. While in experiments the large RC values can be partly due to defects, ab initio simulations suggest that, even in defect-free structures, the interaction between metal and semiconductor orbitals can induce gap states that pin the Fermi level in the semiconductor band gap, increase the Schottky barrier height (SBH), and thus degrade the contact resistance. In this paper, we investigate, by using an in-house-developed ab initio transport methodology that combines density functional theory and nonequilibrium Green’s function (NEGF) transport calculations, the physical properties and electrical resistance of several options for n-type top metal contacts to monolayer MoS2, even in the presence of buffer layers, and for p-type contacts to monolayer WSe2. The delicate interplay between the SBH and tunneling barrier thickness is quantitatively analyzed, confirming the excellent properties of the Bi–MoS2 system as an n-type ohmic contact. Moreover, simulation results supported by literature experiments suggest that the Au–WSe2 system is a promising candidate for p-type ohmic contacts. Finally, our analysis also reveals that a small modulation of a few angstroms of the distance between the (semi)metal and the transition-metal dichalcogenide (TMD) leads to large variations of RC. This could help to explain the scattering of RC values experimentally reported in the literature because different metal deposition techniques can result in small changes of the metal-to-TMD distance besides affecting the density of possible defects.

Potential differentiation of successive SARS-CoV-2 mutations by RNA: DNA hybrid analyses

The constant mutation of SARS-CoV-2 has triggered a new round of public health crises and has had a huge impact on existing vaccines and diagnostic tools. It is essential to develop a new flexible method to distinguish mutations to prevent the spread of the virus. In this work, we used the combination of density functional theory (DFT) and non-equilibrium Green's function formulation with decoherence, to theoretically study the effect of viral mutation on charge transport properties of viral nucleic acid molecules. We found that all mutation of SARS-CoV-2 on spike protein was accompanied by the change of gene sequence conductance, this is attributed to the change of nucleic acid molecular energy level caused by mutation. Among them, the mutations L18F, P26S, and T1027I caused the largest conductance change after mutation. This provides a theoretical possibility for detecting virus mutation based on the change of molecular conductance of virus nucleic acid.

Hydrogenated cove-edge aluminum nitride nanoribbons for ultrascaled resonant tunneling diode applications: a computational DFT study

While synthesizing quasi-one-dimensional nanoribbons, there is a finite probability that edges have cove-edge defects. This paper focuses on the structural, electronic, and transport properties of cove-edge aluminum nitride nanoribbons (AlNNR) using density functional theory and the non-equilibrium Green’s function (NEGF) method. The cove-edge AlNNRs are thermodynamically stable and exhibit metallic behavior. Interestingly, the calculated current–voltage characteristics of the cove-edge AlNNR-based nanodevices show negative differential resistance (NDR). The H-AlN-Cove nanodevice exhibits high peak-to-valley current ratio (PVCR) of the order of 107. The calculated PVCR of the H-AlN-Cove nanodevice is 106 times higher than that of the silicene nanoribbon (SiNR) and graphene nanoribbon (GNR), and 104 times higher than that of the phosphorene nanoribbon (PNR) and arsenene nanoribbons (ANR)-based devices respectively. The NDR feature with high PVCR provides a prospect for the cove-edge AlNNR in nanodevice applications.

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.