Browsing by Author "Van de Put, Maarten L."
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Item Carrier Transport in Two-Dimensional Topological Insulator Nanoribbons in the Presence of Vacancy Defects(IOP Publishing Ltd, 2019-02-05) Tiwari, Sabyasachi; Van de Put, Maarten L.; Soreé, B.; Vandenberghe, William G.; 0000-0002-6717-5046 (Vandenberghe, WG); 0000-0002-2216-3893 (Tiwari, S); 0000-0001- 9179-6443 (Van de Put, ML); 21146635654041982414 (Vandenberghe, WG); Tiwari, Sabyasachi; Van de Put, Maarten L.; Vandenberghe, William G.Using the non-equilibrium Green's function formalism, we study carrier transport through imperfect two-dimensional (2D) topological insulator (TI) ribbons. In particular, we investigate the effect of vacancy defects on the carrier transport in 2D TI ribbons with hexagonal lattice structure. To account for the random distribution of the vacancy defects, we present a statistical study of varying defect densities by stochastically sampling different defect configurations. We demonstrate that the topological edge states of TI ribbons are fairly robust against a high concentration (up to 2%) of defects. At very high defect densities, we observe an increased inter-edge interaction, mediated by the localisation of the edge states within the bulk region. This effect causes significant back-scattering of the, otherwise protected, edge-states at very high defect concentrations (>2%), resulting in a loss of conduction through the TI ribbon. We discuss how this coherent vacancy scattering can be used to our advantage for the development of TI-based transistors. We find that there is an optimal concentration of vacancies yielding an ON-OFF current ratio of up to two orders of magnitude. Finally, we investigate the importance of spin-orbit coupling on the robustness of the edge states in the TI ribbon and show that increased spin-orbit coupling could further increase the ON-OFF ratio. ©2019 IOP Publishing Ltd.Item Dielectric Properties of Hexagonal Boron Nitride and Transition Metal Dichalcogenides: From Monolayer to Bulk(Nature Publishing Group) Laturia, Akash; Van de Put, Maarten L.; Vandenberghe, William G.; 0000-0001-9179-6443 (Van de Put, ML); Laturia, Akash; Van de Put, Maarten L.; Vandenberghe, William G.Hexagonal boron nitride (h-BN) and semiconducting transition metal dichalcogenides (TMDs) promise greatly improved electrostatic control in future scaled electronic devices. To quantify the prospects of these materials in devices, we calculate the out-of-plane and in-plane dielectric constant from first principles for TMDs in trigonal prismatic and octahedral coordination, as well as for h-BN, with a thickness ranging from monolayer and bilayer to bulk. Both the ionic and electronic contribution to the dielectric response are computed. Our calculations show that the out-of-plane dielectric response for the transition-metal dichalcogenides is dominated by its electronic component and that the dielectric constant increases with increasing chalcogen atomic number. Overall, the out-of-plane dielectric constant of the TMDs and h-BN increases by around 15% as the number of layers is increased from monolayer to bulk, while the in-plane component remains unchanged. Our computations also reveal that for octahedrally coordinated TMDs the ionic (static) contribution to the dielectric response is very high (4.5 times the electronic contribution) in the in-plane direction. This indicates that semiconducting TMDs in the tetragonal phase will suffer from excessive polar-optical scattering thereby deteriorating their electronic transport properties.Item Phonon-Assisted Tunneling in Direct-Bandgap Semiconductors(Amer Inst Physics, 2019-01-02) Mohammed, Mazharuddin; Verhulst, Anne S.; Verreck, Devin; Van de Put, Maarten L.; Magnus, Wim; Soree, Bart; Groeseneken, Guido; 0000-0001-9179-6443 (Van de Put, ML); Van de Put, Maarten L.In tunnel field-effect transistors, trap-assisted tunneling (TAT) is one of the probable causes for degraded subthreshold swing. The accurate quantum-mechanical (QM) assessment of TAT currents also requires a QM treatment of phonon-assisted tunneling (PAT) currents. Therefore, we present a multi-band PAT current formalism within the framework of the quantum transmitting boundary method. An envelope function approximation is used to construct the electron-phonon coupling terms corresponding to local Frohlich-based phonon-assisted inter-band tunneling in direct-bandgap III-V semiconductors. The PAT current density is studied in up to 100 nm long and 20 nm wide p-n diodes with the 2- and 15-band material description of our formalism. We observe an inefficient electron-phonon coupling across the tunneling junction. We further demonstrate the dependence of PAT currents on the device length, for our non-self-consistent formalism which neglects changes in the electron distribution function caused by the electron-phonon coupling. Finally, we discuss the differences in doping dependence between direct band-to-band tunneling and PAT current. Published under license by AIP Publishing.Item Scalable Atomistic Simulations of Quantum Electron Transport Using Empirical Pseudopotentials(Elsevier B.V., 2019-06-17) Van de Put, Maarten L.; Fischetti, Massimo V.; Vandenberghe, William G.; 0000-0002-6717-5046 (Vandenberghe, WG); 0000-0001-5926-0200 (Fischetti, MV); 21146635654041982414 (Vandenberghe, WG); Van de Put, Maarten L.; Fischetti, Massimo V.; Vandenberghe, William G.The simulation of charge transport in ultra-scaled electronic devices requires the knowledge of the atomic configuration and the associated potential. Such “atomistic” device simulation is most commonly handled using a tight-binding approach based on a basis-set of localized orbitals. Here, in contrast to this widely-used tight-binding approach, we formulate the problem using a highly accurate plane-wave representation of the atomic (pseudo)-potentials. We develop a new approach that separately deals with the intrinsic Hamiltonian, containing the potential due to the atomic configuration, and the extrinsic Hamiltonian, related to the external potential. We realize efficient performance by implementing a finite-element like partition-of-unity approach combining linear shape functions with Bloch-wave enhancement functions. We match the performance of previous tight-binding approaches, while retaining the benefits of a plane wave based model. We present the details of our model and its implementation in a full-fledged self-consistent ballistic quantum transport solver. We demonstrate our implementation by simulating the electronic transport and device characteristics of a graphene nanoribbon transistor containing more than 2000 atoms. We analyze the accuracy, numerical efficiency and scalability of our approach. We are able to speed up calculations by a factor of 100 compared to previous methods based on plane waves and envelope functions. Furthermore, our reduced basis-set results in a significant reduction of the required memory budget, which enables devices with thousands of atoms to be simulated on a personal computer. ©2019 Elsevier B.V.Item Theoretical Studies of Electronic Transport in Monolayer and Bilayer Phosphorene: A Critical Overview(American Physical Society) Gaddemane, Gautam; Vandenberghe, William G.; Van de Put, Maarten L.; Chen, Shanmeng; Tiwari, Sabyasachi; Chen, E.; Fischetti, Massimo V.; 0000-0001-5926-0200 (Fischetti, MV); Gaddemane, Gautam; Vandenberghe, William G.; Van de Put, Maarten L.; Chen, Shanmeng; Tiwari, Sabyasachi; Fischetti, Massimo V.Recent ab initio theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from 26000 to 35 cm²V⁻¹s⁻¹, for example, for the hole mobility in monolayer phosphorene) depending on the physical approximations used. Given this state of uncertainty, we review critically the physical models employed, considering phosphorene, a group-V material, as a specific example. We argue that the use of the most accurate models results in a calculated performance that is at the disappointing lower end of the predicted range. We also employ first-principles methods to study high-field transport characteristics in monolayer and bilayer phosphorene. For thin multilayer phosphorene we confirm the most disappointing results, with a strongly anisotropic carrier mobility that does not exceed ∼30 cm²V⁻¹s⁻¹ at 300 K for electrons along the armchair direction. We also discuss the dependence of low-field carrier mobility on the thickness of multilayer phosphorene. ©2018 American Physical Society.