Nonequilibrium Green Function Research Articles

Insights into Electron Transport in a Ferroelectric Tunnel Junction.

The success of a ferroelectric tunnel junction (FTJ) depends on the asymmetry of electron tunneling as given by the tunneling electroresistance (TER) effect. This characteristic is mainly assessed considering three transport mechanisms: direct tunneling, thermionic emission, and Fowler-Nordheim tunneling. Here, by analyzing the effect of temperature on TER, we show that taking into account only these mechanisms may not be enough in order to fully characterize the performance of FTJ devices. We approach the electron tunneling in FTJ with the non-equilibrium Green function (NEGF) method, which is able to overcome the limitations affecting the three mechanisms mentioned above. We bring evidence that the performance of FTJs is also affected by temperature–in a non-trivial way–via resonance (Gamow-Siegert) states, which are present in the electron transmission probability and are usually situated above the barrier. Although the NEGF technique does not provide direct access to the wavefunctions, we show that, for single-band transport, one can find the wavefunction at any given energy and in particular at resonant energies in the system.

Low-Temperature Characteristics of Nanowire Network Demultiplexer for Qubit Biasing.

In current quantum computers, most qubit control electronics are connected to the qubit chip inside the cryostat by cables at room temperature. This poses a challenge when scaling the quantum chip to an increasing number of qubits. We present a lateral nanowire network 1-to-4 demultiplexer design fabricated by selective area grown InGaAs on InP, suitable for on chip routing of DC current for qubit biasing. We have characterized the device at cryogenic temperatures, and at 40 mK the device exhibits a minimum inverse subthreshold slope of 2 mV/dec, which is encouraging for low power operation. At low drain bias, the transmission breaks up into several resonance peaks due to a rough conduction band edge; this is qualitatively explained by a simple model based on a 1D real space tight-binding nonequilibrium Green’s functions model.

Role of Junctionless Mode in Improving the Photosensitivity of Sub-10 nm Carbon Nanotube/Nanoribbon Field-Effect Phototransistors: Quantum Simulation, Performance Assessment, and Comparison.

In this article, ultrascaled junctionless (JL) field-effect phototransistors based on carbon nanotube/nanoribbons with sub-10 nm photogate lengths were computationally assessed using a rigorous quantum simulation. This latter self-consistently solves the Poisson equation with the mode space (MS) non-equilibrium Green’s function (NEGF) formalism in the ballistic limit. The adopted photosensing principle is based on the light-induced photovoltage, which alters the electrostatics of the carbon-based junctionless nano-phototransistors. The investigations included the photovoltage behavior, the I-V characteristics, the potential profile, the energy-position-resolved electron density, and the photosensitivity. In addition, the subthreshold swing–photosensitivity dependence as a function of change in carbon nanotube (graphene nanoribbon) diameter (width) was thoroughly analyzed while considering the electronic proprieties and the quantum physics in carbon nanotube/nanoribbon-based channels. As a result, the junctionless paradigm substantially boosted the photosensitivity and improved the scaling capability of both carbon phototransistors. Moreover, from the point of view of comparison, it was found that the junctionless graphene nanoribbon field-effect phototransistors exhibited higher photosensitivity and better scaling capability than the junctionless carbon nanotube field-effect phototransistors. The obtained results are promising for modern nano-optoelectronic devices, which are in dire need of high-performance ultra-miniature phototransistors.

Influence of phonon-assisted tunneling on photovoltaic properties of BaSi2 and BaGe2 p–n homojunction solar cell devices

The solar cell properties of crystalline BaSi2 and BaGe2 p–n homojunctions are explored using density functional theory combined with a nonequilibrium Green function method. In particular, the quantitative estimates of solar cell parameters such as photocurrent, open-circuit voltage (Voc), short-circuit current (|Jsc|), and efficiency (η) are obtained for LDA and GGA-1/2 functionals. The effect of temperature on solar cell parameters is included through electron–phonon coupling (EPC) using the special thermal displacements method. The magnitudes of Jsc, Voc, and η for BaSi2 (BaGe2) at 300 K are found to be 27.35 mA/cm2 (26.1 mA/cm2), 0.84 V (0.78 V), and 18.0% (16.6%), respectively. Our study strongly suggests that the phonon-assisted photon absorption and thereby EPC significantly affect the photocurrent, and its inclusion is necessary for a proper description of various solar cell parameters. The computed solar cell parameters for BaSi2 (BaGe2) p–n homojunctions can be used as benchmark ab-initio quantum mechanical results and can be used in simulations based on continuum models.

A Current Model for Ballistic/Quasi-Ballistic GNRFETs Considering Line-Edge Roughness

In this article, a physics-based analytical current model for armchair graphene nanoribbon field-effect transistors (A-GNRFETs) is developed, which is valid from ballistic to quasi-ballistic transport regimes. The roles of line-edge roughness (LER) for different roughness parameters and Fermi level of GNRs are numerically investigated. Furthermore, the effect of LER-related tunneling current has a strong impact on the drain–source current for the OFF state, and it is considered in this model. The good agreement between our proposed current model and the nonequilibrium Green function (NEGF) simulations proves its validity. The results indicate that depending on the geometrical and roughness parameters, the transport properties can be ballistic or quasi-ballistic.

Spectral investigation on single molecular optoelectronics of ladder phenylenes

Atomic chains and organic conjugated molecules are of great important research value in molecular optoelectronics, due to their special optoelectronic properties. The fully conjugated nature of ladder phenylenes (LPs) provide some unique properties that have potential applications in the fabrication of molecular electronics devices. Our results reveal optoelectronic properties apply density function theory and non-equilibrium green's function theory, including unit-dependent light absorption, Raman scattering, phonon energy band structure, the chemical potential dependent density of states, electrical conductivity, I-V curve, transmission spectrum, etc. Our research provides theoretical guidance for the regulation of light-harvesting regions based on LPs structures, and theoretical support for the design of nano-scale optoelectronic devices.

Calculated characterisation of a sensitive gas sensor based on PEDOT:PSS

The interactions between poly (3,4-ethylene dioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) and small gas molecules are studied using non-equilibrium Green's function formalism based on the density functional theory. The proposed method is implemented in the Tran SIESTA code to benefit from the potential application of PEDOT:PSS as a gas sensor. The results show that doping with nanoparticles can drastically improve the sensitivity of polymer-based chemical gas sensors. Moreover, among various PEDOT:PSS doping materials, silver nanoparticles have an appropriate response to ammonia, while platinum shows the best response to carbon dioxide. The numerical results can be useful to design PEDOT:PSS-based gas sensors.

Statistical device simulations of III-V nanowire resonant tunneling diodes as physical unclonable functions source

In this paper, utilising the non-equilibrium Green’s function (NEGF) formalism within the new device simulator NESS (Nano-Electronic Software Simulator) developed at the University of Glasgow’s Device Modelling Group, we present quantum mechanical simulations of current flow in double-barrier III-V GaAs-AlGaAs nanowire resonant tunneling diodes (RTDs). NESS is a fast and modular Technology Computer Aided Design (TCAD) tool with flexible architecture which can take into account various sources of statistical variability in nanodevices. The aim of this work is to show that, in the RTD devices with nano-scale dimensions, there is a direct correlation between the position and the numbers of random dopants and the key device parameters, e.g., position of the resonant-peak (VR) variations as well as the shape and number of peaks in the output current–voltage (I-V) characteristics. Such VR variability can be used as a quantum fingerprint which can provide robust security and hence can be used to deliver Physical Unclonable Functions (PUFs).

On-Surface Azide–Alkyne Cycloaddition Reaction: Does It Click with Ruthenium Catalysts?

Owing to its simplicity, selectivity, high yield, and the absence of byproducts, the “click” azide–alkyne reaction is widely used in many areas. The reaction is usually catalyzed by copper(I), which selectively produces the 1,4-disubstituted 1,2,3-triazole regioisomer. Ruthenium-based catalysts were later developed to selectively produce the opposite regioselectivity—the 1,5-disubstituted 1,2,3-triazole isomer. Ruthenium-based catalysis, however, remains only tested for click reactions in solution, and the suitability of ruthenium catalysts for surface-based click reactions remains unknown. Also unknown are the electrical properties of the 1,4- and 1,5-regioisomers, and to measure them, both isomers need to be assembled on the electrode surface. Here, we test whether ruthenium catalysts can be used to catalyze surface azide–alkyne reactions to produce 1,5-disubstituted 1,2,3-triazole, and compare their electrochemical properties, in terms of surface coverages and electron transfer kinetics, to those of the compound formed by copper catalysis, 1,4-disubstituted 1,2,3-triazole isomer. Results show that ruthenium(II) complexes catalyze the click reaction on surfaces yielding the 1,5-disubstituted isomer, but the rate of the reaction is remarkably slower than that of the copper-catalyzed reaction, and this is related to the size of the catalyst involved as an intermediate in the reaction. The electron transfer rate constant (ket) for the ruthenium-catalyzed reaction is 30% of that measured for the copper-catalyzed 1,4-isomer. The lower conductivity of the 1,5-isomer is confirmed by performing nonequilibrium Green’s function computations on relevant model systems. These findings demonstrate the feasibility of ruthenium-based catalysis of surface click reactions and point toward an electrical method for detecting the isomers of click reactions.

Enhancing the sensitivity and selectivity of pyrene-based sensors for detection of small gaseous molecules via destructive quantum interference

Graphene-based sensors are exceptionally sensitive with high carrier mobility and low intrinsic noise, and have been intensively investigated in the past decade. The detection of individual gas molecules has been reported, albeit the underlying sensing mechanism is not yet well understood. We focus on the adsorption of NO$_2$, H$_2$O, and NH$_3$ on a molecular junction with a pyrene core, which can be considered as a minimal graphene-like unit. We systematically investigate the chemiresistive response within the framework of density functional theory and non-equilibrium Green's functions. We highlight the fundamental role of quantum interference (QI) in the sensing process, and we propose it as a paradigmatic mechanism for sensing. Owing to the open-shell character of NO$_2$, its interaction with pyrene gives rise to a Fano resonance thereby triggering the strongest chemiresistive response, while the weaker interactions with H$_2$O and NH$_3$ result in lower sensitivity. We demonstrate that by exploiting destructive QI arising in the meta-substituted pyrene, it is possible to calibrate the sensor to enhance both its sensitivity and chemical selectivity by almost two orders of magnitude so that individual molecules can be detected and distinguished. These results provide a fundamental strategy to design high-performance chemical sensors with graphene functional blocks.

Modeling of molecular ternary logic gates and circuits based on diode structures

Modeling in molecular electronics is of great importance, and the use of semiconductor components for this type of modeling accelerates the development process in this field. In this work, a typical circuit model is proposed for modeling molecular components. Accordingly, an asymmetric oligo-phenylene vinylene (OPV) molecular diode and a bipyridine-biborinine molecular diode are modeled. A good agreement is observed between the current curves from the proposed circuit models and the atomic simulations of the molecules. Additionally, the electron density, the distribution of molecular orbitals, and the potential drop profile at + 1 and - 1V are obtained and analyzed for the bipyridine-biborinine molecular diode using the density functional theory (DFT) in combination with the non-equilibrium Green's function (NEGF). Using different molecular gates and circuits based on the molecular devices, we have modeled the ternary NOT logic gate, ternary NOR logic gate, ternary NAND logic gate, negative ternary inverter (NTI) logic gate, positive ternary inverter (PTI) logic gate, ternary buffer, ternary decoder, and ternary half adder.

NbCX (X = F, Cl, Br, I) with Highly Anisotropic Optical, Mechanical and Electric Transport Properties

Two-dimensional materials with anisotropic properties (for example, mechanical, optical, and electrical transmission anisotropy) have been sought for a long time, especially two-dimensional materials with strong electrical transport anisotropy. Here, we report a class of two-dimensional NbCX(X=F,Cl,Br,I) monolayers with stable performance. Using density functional theory combined with the method of non-equilibrium Green's function, we found that the optical absorption characteristics of two-dimensional NbCX are highly dependent on the crystal lattice direction. The calculated electrical transport properties also show that the NbCX monolayer exhibits strong anisotropy in the bias voltage range of 0.1∼2.0 V. NbCX prefers to transmit current in the a direction, which can be orders of magnitude up to µA. The anisotropy, adjustable band gap and unique transport properties of two-dimensional NbCX can have potential applications in nano-optoelectronics.

Theoretical Study of Quasi One-Well Terahertz Quantum Cascade Laser

Developing a high-temperature terahertz (THz) quantum cascade laser (QCL) has been one of the major challenges in the THz QCL field over recent decades. The maximum lasing temperature of THz QCLs has gradually been increased, arguably by shortening the length of repeating periods of the quantum structure in the device’s active region from 7 wells/14 layers to 2 wells/4 layers per period. The current highest operating temperature of 250 K was achieved in a two-well direct-phonon design. In this paper, we propose a potential and promising novel quantum design scheme named the quasi one-well (Q1W) design, in which each quantum cascade period consists of only three semiconductor layers. This design is the narrowest of all existing THz QCL structures to date. We explore a series of the Q1W designs using the non-equilibrium green function (NEGF) and rate-equation (RE) models. Both models show that the Q1W designs exhibit the potential to achieve sufficient optical gain with low-temperature sensitivity. Our simulation results suggest that this novel Q1W scheme may potentially lead to relatively less temperature-sensitive THz QCLs. The thickness of the Q1W scheme is less than 20 nm per period, which is the narrowest of the reported THz QCL schemes.

Efficient solution strategy to couple micromagnetic simulations with ballistic transport in magnetic tunnel junctions

We present a computationally efficient strategy that allows us to simulate magnetization switching driven by spin-transfer torque in magnetic tunnel junctions within a micromagnetic model coupled with a matrix-based nonequilibrium Green's function algorithm. Exemplary simulations for a realistic set of parameters are carried out and show switching times below $4\phantom{\rule{4.pt}{0ex}}\text{ns}$ for voltages above $300\phantom{\rule{4.pt}{0ex}}\text{mV}$ or around $2\ifmmode\times\else\texttimes\fi{}{10}^{10}\text{A}\phantom{\rule{0.16em}{0ex}}{\text{m}}^{\ensuremath{-}2}$ for the $\text{P}\ensuremath{\rightarrow}\text{AP}$ (parallel-to-antiparallel) direction. For $\text{AP}\ensuremath{\rightarrow}\text{P}$ switching, a trend reversal in the switching time is seen, i.e., the time for magnetization reversal first decreases with increasing bias voltage but then starts to rise again.

Spin transport properties of highly lattice-matched all-Heusler-alloy magnetic tunnel junction

A highly lattice-matched all-Heusler-alloy magnetic tunnel junction (MTJ) is proposed by associating half metallic CoFeTiSi electrode and non-magnetic semiconductor Fe2TiSi barrier. Based on the non-equilibrium Green's function combined with first-principles calculations, spin transport mechanism is studied by analyzing the transmission coefficient, local density of states, and scattering states. The calculated equilibrium tunnel magnetoresistance (TMR) ratio in CoFeTiSi/Fe2TiSi/CoFeTiSi reaches up to about 3.30 × 108%, which is much higher than in CoFeTiSi/MgO/CoFeTiSi. The calculated scattering states reveal that CoFeTiSi/Fe2TiSi/CoFeTiSi MTJ possesses two transport channels, and the contribution of channel I to the total transport is about 80%, which plays a dominate role. The analyses on non-equilibrium spin transport properties reveal that CoFeTiSi/MgO/CoFeTiSi MTJ can output a high intensity transport current with complete spin polarization, and it can output a stable and highly efficient signal. The TMR ratio possesses an extremely large value of about 2.4 × 108% and even the bias voltage increases to 0.2 V, exhibiting a robust current-driven stability.

First-principles scattering with Büttiker probes: The role of self-energies

Understanding electronic transport properties is important for designing devices for applications. Many studies rely on the semiclassical Boltzmann approach within the relaxation time approximation. This method delivers a graphic physical picture of the scattering process, but in some cases it lacks full quantum-mechanical effects. Here, we use a non-equilibrium Green's function Korringa-Kohn-Rostoker (KKR) method with phase-breaking scattering via virtual B\"uttiker terminals as a fully quantum mechanical approach to transport phenomena. With this, we assess the validity of the relation of the self-energy $\mathrm{\ensuremath{\Sigma}}$ to the scattering time $\ensuremath{\tau}$, often used in literature in the case of constant relaxation time approximation. We argue that the scattering time does not affect the thermopower in the Boltzmann approach and thus should take no effect either on the thermopower calculated via the Keldysh approach. We find a nearly linear relation for the transmission function ${T}_{S}({E}_{F},\mathrm{\ensuremath{\Sigma}})$ of free electrons and Cu with respect to $\frac{1}{\mathrm{\ensuremath{\Sigma}}}$. However, we find that this is not the case for Pd. We attribute this to neighboring states contributing due to the additional broadening via the self-energy $\mathrm{\ensuremath{\Sigma}}$. These findings suggest that a simple identification of scattering time and self-energy is not sufficient. Finally, we discuss the benefits and limits of the application of the virtual terminal approach.

Site-dependent spin-polarized tunneling via hybrid interface states on molecule/ferromagnet surface

Based on density functional theory and nonequilibrium Green's function method, we investigate the spin polarization and transport ability of hybrid interface states at benzene/Ni surface by introducing the top probe electrode. The results demonstrate that when the top electrode connects the molecule with different sites, the spin polarization of the projected density of states and the interfacial spin density distribution are disturbed in different ways. A contact site induced inversion of the spin polarization is observed at Fermi energy. The transport calculation indicates that the center contact is better to achieve a large tunneling magnetoresistance at low bias, but the edge and top contacts is useful to get a little larger spin polarization of the current. This work points out that the spin polarization and transport property of the hybrid interface states in a molecular junction depends on the way to introduce the second probe electrode, which can be suitable designed to achieve an excellent performance.

Tuning of the electronic structures and spin-dependent transport properties of phosphorene nanoribbons by vanadium substitutional doping

Based on vanadium-doped zigzag phosphorene nanoribbon (ZPNRs), we investigate the electronic structures and spin-dependent transport properties by the first-principles calculations in combination with the nonequilibrium Green's function approach. We find that the ZPNRs can be tuned from nonmagnetic metals to magnetic metals or half-metals by different doping positions, which are caused by the hybridization among different electron orbitals under the crystal field. Moreover, a robust negative differential resistance effect is observed in all studied devices, and a perfect spin-filter effect with almost 100% spin polarization can be found under low bias. We analyze the transport properties in detail from the viewpoint of the transmission spectrum and molecular energy levels. These results indicate that the vanadium-doped ZPNRs have potential applications as multi-functional spintronic devices. • Zigzag phosphorene nanoribbons can be tuned from nonmagnetic metals to magnetic metals or half-metals. • Robust negative differential resistance effect is observed. • Perfect spin-filter effect with almost 100% spin polarization can be found. • We analyze the transport properties in detail.

Energy transport in Sachdev-Ye-Kitaev networks coupled to thermal baths

We develop a general framework for studying the equilibrium and non-equilibrium properties of arbitrary networks of Sachdev-Ye-Kitaev clusters coupled to thermal baths. We proceed to apply this technique to the problem of energy transport, which is known to be diffusive due to the strange metal behavior of these models. We use the external baths to impose a temperature gradient in the system and study the emerging non-equilibrium steady state using the Schwinger-Keldysh formalism. We consider two different configurations for the baths, implementing either a boundary or bulk driving, and show that the latter leads to a significantly faster convergence to the steady state. This setup allows us to compute both the temperature and frequency dependence of the diffusion constant. At low temperatures, our results agree perfectly with the previously known values for diffusivity in the conformal limit. We also establish a relationship between energy transport and quantum chaos by showing that the diffusion constant is upper bounded by the chaos propagation rate at all temperatures. Moreover, we find a simple analytical form for the non-equilibrium Green's functions in the linear response regime and use it to derive exact closed-form expressions for the diffusion constant in various limits. We mostly focus on uniform one-dimensional chains, but we also discuss higher-dimensional generalizations.

Scaling of GaSb/InAs Vertical Nanowire Esaki Diodes Down to Sub-10-nm Diameter

This work studies the diameter scaling behavior of broken-band GaSb/InAs vertical nanowire (VNW) Esaki diodes. A top-down fabrication process involving precise reactive-ion etching and alcohol-based digital etch has yielded devices with a tunneling junction diameter below 10 nm. Clear Esaki peaks are observed with an average peak current density of 1MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and ideal areal scaling over two orders of magnitude in diameter. In the non-Esaki branch, an average current density of 4 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is demonstrated at 0.3 V. This suggests a great potential for the broken-band GaSb/InAs system for ultralow power VNW tunnel FET (TFET) applications. Toward evaluating the ultimate TFET potential of this material system, we have extracted the series resistance of our diodes and developed a model for it. We find that the main contribution to the series resistance comes from the GaSb body. Comparatively, the contact resistance at the top of the VNW has a minor impact in spite of the tiny Ni/InAs contact area. Self-consistent quantum transport simulations based on non-equilibrium Green’s function (NEGF) formalism are carried out. We simulate two devices with different dimensions, that is, one with strong quantum confinement and one with bulk-like behavior. Inelastic tunneling due to phonon emission is found to both widen the Esaki peak and to suppress tunneling in the non-Esaki branch.