Efficient and accurate defect level modelling in monolayer MoS2_2 via GW+DFT with open boundary conditions

Within the framework of many-body perturbation theory (MBPT) integrated with density functional theory (DFT), a novel defect-subspace projection GW method, the so-called p-GW, is proposed. By avoiding the periodic defect interference through open boundary self-energies, we show that the p-GW can efficiently and accurately describe quasi-particle correlated defect levels in two-dimensional (2D) monolayer MoS$_2$. By comparing two different defect states originating from sulfur vacancy and adatom to existing theoretical and experimental works, we show that our GW correction to the DFT defect levels is precisely modelled. Based on these findings, we expect that our method can provide genuine trap states for various 2D transition-metal dichalcogenide (TMD) monolayers, thus enabling the study of defect-induced effects on the device characteristics of these materials via realistic simulations

Field-Effect Transistors based on 2-D Materials: a Modeling Perspective

Two-dimensional (2D) materials are particularly attractive to build the channel of next-generation field-effect transistors (FETs) with gate lengths below 10-15 nm. Because the 2D technology has not yet reached the same level of maturity as its Silicon counterpart, device simulation can be of great help to predict the ultimate performance of 2D FETs and provide experimentalists with reliable design guidelines. In this paper, an ab initio modelling approach dedicated to well-known and exotic 2D materials is presented and applied to the simulation of various components, from thermionic to tunnelling transistors based on mono- and multi-layer channels. Moreover, the physics of metal - 2D semiconductor contacts is revealed and the importance of different scattering sources on the mobility of selected 2D materials is discussed. It is expected that modeling frameworks similar to the one described here will not only accompany future developments of 2D devices, but will also enable them

Original quantum treatment of inelastic interactions for modeling of atomistic transport in three-dimensional nanostructures

Le formalisme des fonctions de Green hors-équilibre (NEGF pour « Non-equilibrium Green’s function) a suscité au cours des dernières décennies un engouement fort pour étudier les propriétés du transport quantique des nanostructures et des nano-dispositifs dans lesquels les interactions inélastiques, comme la diffusion des électrons-phonons, jouent un rôle significatif. L'incorporation d'interactions inélastiques dans le cadre du NEGF s’effectue généralement dans l'approximation auto-cohérente de Born (SCBA pour « Self-consistent Born approximation) qui représente une approche itérative plus exigeante en ressources numériques. Nous proposons dans ce travail de thèse une méthode efficace alternative dite LOA pour (« Lowest Order Approximation. Son principal avantage est de réduire considérablement le temps de calcul et de décrire physiquement la diffusion électron-phonon. Cette approche devrait considérablement étendre l'accessibilité de l'utilisation de codes atomistiques de transport quantique pour étudier des systèmes 3D réalistes sans faire à des ressources numériques importantes.Non-equilibrium Green’s function (NEGF) formalism during recent decades has attracted numerous interests for studying quantum transport properties of nanostructures and nano-devices in which inelastic interactions like electron-phonon scattering have a significant impact. Incorporation of inelastic interactions in NEGF framework is usually performed within the self-consistent Born approximation (SCBA) which induces a numerically demanding iterative scheme. As an alternative technique, we propose an efficient method, the so-called Lowest Order Approximation (LOA) coupled with the Pade approximants. Its main advantage is to significantly reduce the computational time, and to describe the electron-phonon scattering physically. This approach should then considerably extend the accessibility of using atomistic quantum transport codes to study three-dimensional (3D) realistic systems without requiring numerous numerical resources

Electron-phonon calculations using a Wannier-based supercell approach: Applications to the monolayer MoS2

We present a first-principles method to calculate electron-phonon coupling elements in atomic systems, and showcase its application to the evaluation of the phonon-limited mobility of n-type single-layer MoS2. The method combines a density functional theory (DFT) plane-wave supercell approach with a real-space maximally localized Wannier basis. It enables the calculation of electronic structure, phonon displacements with their corresponding frequencies, and real-space electron-phonon coupling elements on the same footing, without the need for density functional perturbation theory (DFPT) or Wannier interpolation. We report a low-field, intrinsic mobility of 274 cm²/V s at room temperature for MoS2, and highlight its dependence on carrier density and temperature. In addition, we compare our findings to the latest available modeling data and put them in perspective with the experimentally measured values. Based on these observations, the mobilities presented in this work appear to be compatible with experimental results, when taking into account other scattering sources. Hence, the proposed approach provides a reliable framework for mobility calculations that can be extended towards large-scale device simulations.ISSN:0038-110

Ab initio mobility of single-layer MoS2 and WS2: comparison to experiments and impact on the device characteristics

We combine the linearized Boltzmann Transport Equation (LBTE) and quantum transport by means of the Non-equilibrium Green's Functions (NEGF) to simulate monolayer MoS 2 and WS 2 ultra-scaled transistors with carrier mobilities extracted from experiments. Electron-phonon, charged impurity, and surface optical phonon scattering are taken into account with all necessary parameters derived from ab initio calculations or measurements, except for the impurity concentration. The LBTE method is used to scale the scattering self-energies of NEGF, which only include local interactions. This ensures an accurate reproduction of the measured mobilities by NEGF. We then perform device simulations and demonstrate that the considered transistors operate far from their performance limit (from 50% for MoS 2 to 60% for WS 2 ). Higher quality materials and substrate engineering will be needed to improve the situation. © 2019 IEEE

Atomistic Simulation of Nanoscale Devices

Device simulation is nowa-days fully integrated into the production tool chain of transistors. The geometry of the latter can be carefully optimized, possible design pitfalls can be identified early on, and the obtained experimental data can be analyzed in detail thanks to state-of-the-art technology computer aided design tools. However, on the one hand, the dimensions of transistors are reaching the atomic scale. On the other hand, novel functionalities (e.g., light emission/detection) and materials, for example III-V semiconductors, are being added to silicon-based chips. To cope with these challenges it is crucial that device simulators go beyond classical theories, pure electronic transport, and continuum models. The inclusion of quantum mechanical phenomena, electro-thermal effects, and light-matter interactions in systems made of thousands of atoms and of various materials has become critical. In this paper, we review one approach that satisfies all these requirements, the Non-equilibrium Green's Function (NEGF) formalism, focusing on its combination with ab initio bandstructure models. The NEGF method allows to treat electrical, thermal, and optical transport at the quantum mechanical level in multi-material, multi-functional devices, without any empirical parameters. Besides advanced logic switches, it can be used to simulate e.g., photo-detectors, thermoelectric generators, or memory cells composed of almost any materials, in the ballistic limit of transport and in the presence of scattering. The key features of NEGF are summarized first, then selected applications are presented, finally challenges and opportunities are discussed.ISSN:1932-4510ISSN:1932-431

Ab initio modeling framework for Majorana transport in 2D materials: Towards topological quantum computing

We present an ab initio modeling framework to simulate Majorana transport in 2D semiconducting materials, paving the way for topological qubits based on 2D nanoribbons. By combining density-functional-theory and quantum transport calculations, we show that the signature of Majorana bound states (MBSs) can be found in 2D material systems as zero-energy modes with peaks in the local density-of-states. The influence of spin-orbit coupling and external magnetic fields on Majorana transport is studied for two relevant 2D materials, WSe 2 and PbI 2 . To illustrate the capabilities of the proposed ab initio platform, a device structure capable of hosting MBSs is created from a PbI 2 nanoribbon and carefully investigated. These results are compared to InSb nanowires and used to provide design guidelines for 2D topological qubits

Efficient and accurate defect level modeling in monolayer MoS via GW+DFT with open boundary conditions

Within the framework of many-body perturbation theory integrated with density functional theory (DFT), a novel defect-subspace projection GW method, the so-called p-GW, is proposed. By avoiding the periodic defect interference through open boundary self-energies, we show that the p-GW can efficiently and accurately describe quasi-particle correlated defect levels in two-dimensional (2D) monolayer MoS2. By comparing two different defect states originating from sulfur vacancy and adatom to existing theoretical and experimental works, we show that our GW correction to the DFT defect levels is precisely modeled. Based on these findings, we expect that our method can provide genuine trap states for various 2D transition-metal dichalcogenide (TMD) monolayers, thus enabling the study of defect-induced effects on the device characteristics of these materials via realistic simulations.ISSN:0038-110

Impact of Orientation Misalignments on Black Phosphorus Ultrascaled Field-Effect Transistors

Two-dimensional materials with strong bandstructure anisotropy such as black phosphorus (BP) have been identified as attractive candidates for logic application due to their potential high carrier velocity and large density-of-states. However, perfectly aligning the source-to-drain axis with the desired crystal orientation remains an experimental challenge. In this letter, we use an advanced quantum transport approach from first-principle to shed light on the influence of orientation misalignments on the performance of BP-based field-effect transistors. Both n -and p -type configurations are investigated for six alignment angles, in the ballistic limit of transport and in the presence of electron-phonon and charged impurity scattering. It is found that up to deviations of 50° from the optimal angle, the ON-state current only decreases by 30%. This behavior is explained by considering a single bandstructure parameter, the effective mass along transport direction.ISSN:0741-3106ISSN:1558-056

Dynamics of van der Waals charge qubit in two-dimensional bilayer materials: Ab initio quantum transport and qubit measurement

A van der Waals (vdW) charge qubit, electrostatically confined within two-dimensional (2D) vdW materials, is proposed as a building block of future quantum computers. Its characteristics are systematically evaluated with respect to its two-level anticrossing energy difference (Δ). Bilayer graphene (Δ≈ 0) and a vdW heterostructure (Δ≫ 0) are used as representative examples. Their tunable electronic properties with an external electric field define the state of the charge qubit. By combining density functional theory and quantum transport calculations, we highlight the optimal qubit operation conditions based on charge stability and energy-level diagrams. Moreover, a single-electron transistor design based on trilayer vdW heterostructures capacitively coupled to the charge qubit is introduced as a measurement setup with low decoherence and improved measurement properties. It is found that a Δ greater than 20 meV results in a rapid mixing of the qubit states, which leads to a lower measurement quantity, i.e., contrast and conductance. With properly optimized designs, qubit architectures relying on 2D vdW structures could be integrated into an all-electronic quantum computing platform.ISSN:2643-156