Defect-based graphene nanoribbon photodetectors: A numerical study

Herein, a highly practical first‐principles approach for studying phonon transport across various materials, ranging from low‐dimensional systems to three‐dimensional bulk structures, as well as nanostructures, is presented. This method integrates the nonequilibrium Green's function (NEGF) formalism with ab initio calculations of dynamical matrices generated by Quantum Espresso. A detailed technical approach for extracting atomistic force constants and forming dynamical matrices that adhere to the tri‐diagonal technique within the NEGF framework is provided. Additionally, an efficient method for constructing dynamical matrices of conventional cells from those of primary cells is introduced, remarkably reducing the computational costs associated with density‐functional theory calculations. The robustness and applicability of this methodology are validated through several case studies.

This paper describes the point defect treatment in nanowire MOSFETs. The 3D Poisson equation is self-consistently coupled to the Schrodinger equation. We adopt the mode-space representation in which the 3D Schrodinger equation is divided into a 2D equation, including the confinement of the cross-section, and an ID equation describing the ballistic transport based on the nonequilibrium Green's function formalism (NEGF). We use the simplest form of the tight-binding theory, with one orbital per atom, and considering only the first-neighbor-interactions. In this framework, equivalent to the ellipsoidal energy band approximation, influence of point defects on electronic transport is discussed.

We discuss within the nonequilibrium Green's function formalism the self-consistent Born approximation of the electronic self-energy for concurrent coupling of charge carriers to photons and phonons. The expansion of the charge carrier Green's function is shown to provide a description for a range of physical processes with relevance for the photovoltaic device operation, such as indirect optical transitions, thermally assisted photocarrier escape from quantum wells or phonon-mediated interband transfer in tunnel junctions. The nonequilibrium Green's function picture of these processes is reviewed with focus on the evolution of the system towards the steady state in the convergence of the self-consistency iteration process and on the impact of inelastic scattering on device operation.

Exploring potential two-dimensional monolayers with large photogalvanic effect (PGE) has been of great importance for developing self-powered optoelectronic devices. In this paper, we systematically investigate the generation of PGE photocurrent in chalcogen-based Janus XYZ monolayers (X/Y/Z = S, Se, Te; X ≠ Y ≠ Z) based on non-equilibrium Green's function formalism with density functional theory. The optimized Janus SSeTe, SeSTe, and TeSeS monolayers in the rectangular phase are shown stable and, respectively, possess 1.54, 1.49, and 1.74 eV indirect bandgaps. Illuminated by linearly polarized light, the PGE photocurrent without bias voltage can be collected in both armchair and zigzag directions. Unlike common Janus 2D materials with C3v symmetry, the photocurrent peak values of Janus XYZ monolayers do not come up with certain polarization angles, while the relations can be fitted by Iph = α sin(2θ) + β cos(2θ) + γ at each photon energy. Meanwhile, the maximum photoresponses of Janus SSeTe, SeSTe, and TeSeS monolayers are 2.02, 3.33, and 4.42 a20/photon, respectively. The relatively large PGE photocurrents and complicated polarization relations result from the lower symmetry of Janus XYZ monolayers. Moreover, the specific polarization angles for maximum photoresponses at each photon energy and the ratio between two transport directions are demonstrated, reflecting the anisotropy. Our results theoretically predict a potential Janus monolayer family for self-powered optoelectronic applications.

We examine the transport properties of a tunnel-coupled asymmetric double-dot structure in a parallel arrangement between leads in the presence of Aharonov-Bohm magnetic flux and an additional bias voltage applied between the dots. We solve the Schrodinger equation for the energy eigenfunctions and eigenvalues of a biased asymmetric double-dot system. The magnetic flux-induced Peierls phase factors experienced by electrons in tunneling from/to the leads to/from double-dot levels are determined and incorporated. Employing the nonequilibrium Green's function formalism we calculate the lead-to-lead current, taking account of dot-lead tunnel coupling and Coulomb repulsion. We demonstrate that, for appropriate parameter values (such as lead-to-lead bias voltage and equilibrium chemical potential of the leads), the lead-to-lead current oscillates as a function of applied magnetic field, if the dot-to-dot bias compensates the structure asymmetry and only one of the double-dot levels is conductive. We also show that when both levels become conductive, their interference is destructive and the total lead-to-lead current decreases with increasing magnetic field. This decrease is larger for larger dots.

The effects of surface functionalization on the electronic transport properties of the MXene compound Ti3C2 are studied using density-functional theory in combination with the nonequilibrium Green's function formalism. Fluorinated, oxidized and hydroxylated surfaces are considered and the obtained results are compared with the ones for the pristine MXene. It is found that the surface termination has a considerable impact on the electronic transport in MXene. For example, the fluorinated sample shows the largest transmission, whereas surface oxidation results in a considerable reduction of the electronic transmission. The current in the former sample can be up to 4 times larger for a given bias voltage as compared to the case of bare MXene. The increased transmission originates from the extended electronic states and smaller variations of the electrostatic potential profile. Our findings can be useful in designing MXene-based anode materials for energy storage applications, where enhanced electronic transport will be an asset.

Scattering theory of thermocurrent in quantum dots and molecules

The effect of inelastic scattering on quantum electron transport through double-barrier resonant-tunneling structures with large cross-sectional areas is studied numerically using the approach based on the nonequilibrium Green's-function formalism of Keldysh, Kadanoff, and Baym. The Markov assumption is not made, and the energy coordinate is retained. This makes the inclusion of the phonon-energy spectrum straightforward both conceptually and in practice. The electron-phonon interaction is treated in the self-consistent first Born approximation (SCFBA). The Pauli exclusion principle is taken into account exactly within the SCFBA. The retention of the energy coordinate allows the calculation of a number of quantities that give insight into the effect of inelastic scattering on electron transport: The effect of inelastic scattering on the occupation of the energy levels, the density of states, the energy distribution of the current density, and the power density is calculated from a quantum kinetic equation for actual device structures under high bias.

Influence of disorder on the perpendicular magnetoresistance of magnetic multilayers

The molecular spin filter constructed from 1D organic chain

We propose a robust strategy for transforming the Hamiltonian of a metallic structure expressed in a nonorthogonal density-functional-theory (DFT) basis into a low-dimensional space compatible with electron-transport simulations. This mode-space approximation is applied locally to inhomogeneous material stacks including amorphous phases and interfaces. The contacts and periodic parts of the device are transformed into the subspace created, while the active inhomogeneous regions remain represented in real space. The various regions are connected to each other through mode-space--real-space hybrid blocks of the Hamiltonian and overlap matrices. This approach allows harnessing of the full flexibility of DFT combined with the power of the nonequilibrium Green's function formalism at a moderate cost. In particular, for a realistic resistive random-access memory cell composed of 3390 atoms, a performance improvement by a factor of 136 compared with a pure real-space treatment is demonstrated, with an error of less than 2%.

The problem of currents induced in the scanning tunnelling microscope due to incident laser radiation is analysed. The authors uses a light-binding description of the microscope tunnelling junction, and the interaction with the laser field is taken into account by an effective time-dependent coupling between tip and sample. The currents generated at frequencies that are linear combinations of the incident frequencies are then obtained with the help of a nonequilibrium Green's function formalism. Particular attention is paid to the rectification and difference frequency generation effects. In this work they extend previous results for the rectified current to the case where a direct bias is applied in addition to the laser field. It is shown that in the limit of very low frequencies the dynamical response may be deduced from the static characteristic curve. In order to compare with recent experimental data, they perform model calculations for a graphite sample and study the induced photocurrent as a function of bias voltage, tip-sample distance and photon energy. The limitations of the adiabatic approximation are discussed. Finally, they present results for the rectified current in the presence of an adsorbed molecule with a characteristic vibrational mode. The contribution from inelastic processes is obtained to the lowest order in the electron-phonon coupling. It is shown that the onset of inelastic tunnelling should be reflected as a singularity in the rectified current as a function of both bias voltage and photon energy.

The influence of impurity doping distribution in the channel region of p-type silicon nanowire FET has been studied using atomistic three-dimensional quantum simulator based on non-equilibrium Green's function formalism. The valence band has been modeled using a 6-band k.p Hamiltonian. Carrier transport has been treated in the coupled mode-space which considerably reduces the simulation time compare to real-space approach without losing the accuracy. We consider a FET with 10-nm gate length, 5×5 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> cross-section and different doping densities.

Schottky barrier field-effect transistors (SBFETs) based on few and mono layer phosphorene are simulated by the non-equilibrium Green's function formalism. It is shown that scaling down the gate oxide thickness results in pronounced ambipolar I-V characteristics and significant increase of the minimal leakage current. The problem of leakage is especially severe when the gate insulator is thin and the number of layer is large, but can be effectively suppressed by reducing phosphorene to mono or bilayer. Different from two-dimensional graphene and layered dichalcogenide materials, both the ON-current of the phosphorene SBFETs and the metal-semiconductor contact resistance between metal and phosphorene strongly depend on the transport crystalline direction.

A combination of density functional theory and nonequilibrium Green's function formalism has been applied to the atomic scale calculation of the leakage current through SiO2 and SiOxNy dielectrics of MOSFETs. Samples with different dielectric thicknesses and nitrogen concentrations have been taken into account in order to study the dependence of the leakage current on these two parameters. It has been shown that the incorporation of nitrogen atoms into SiO2 increases the density of gap states, which leads to barrier lowering for both electrons and holes. The calculated leakage currents through different SiO2 polymorphs (e.g., s-cristobalite, ?-quartz, and s-quartz) show thickness dependences, which are in very good agreement with measured values for amorphous SiO2. The dependence of the leakage current on the concentration of nitrogen atom in oxynitride gate dielectrics has also been calculated and shown to be in good agreement with the values extracted from measured data.