Vandenberghe, William G.

Permanent URI for this collection

William G. Vandenberghe is an Assistant Professor in Materials Science and Engineering. His research interests include:

  • Tunnel field-effect transistors
  • Band-to-band tunneling
  • Uses of two-dimensional materials and topological insulators for new devices
  • Electron transport at the nanoscale.


Recent Submissions

Now showing 1 - 8 of 8
  • 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
    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
    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.
  • Item
    Modeling of Electron Transport in Nanoribbon Devices Using Bloch Waves
    (Institute of Electrical and Electronics Engineers Inc.) Laturia, Akash A.; Van De Put, Maarten L.; Fischetti, Massimo V.; Vandenberghe, William G.; 0000-0001-5926-0200 (Fischetti, MV); 21146635654041982414 (Vandenberghe, WG); Laturia, Akash A.; Van De Put, Maarten L.; Fischetti, Massimo V.; Vandenberghe, William G.
    One-dimensional (1D) materials present the ultimate limit of extremely scaled devices by virtue of their spatial dimensions and the excellent electrostatic gate control in the transistors based on these materials. Among 1D materials, graphene nanoribbon (a-GNR) prove to be very promising due to high carrier mobility and the prospect of reproducible fabrication process [1]. Two popular approaches to study atomistically the electronic properties expand the wavefunction on either a plane-wave basis set, or through the linear combination of localized atomic orbitals. The use of localized orbitals, especially in the tight-binding (TB) approximation, enables highly scalable numerical implementations. Through continuous improvements in methods and computational capabilities, atomistically describing electronic transport in devices containing more than thousands of atoms has become feasible. Plane waves, while not as scalable, are very popular as the basis of accurate ab-initio software [2]. However, for modeling of transport through larger devices, the computational burden prohibits the direct use of a plane wave basis [3]. Here, we demonstrate a study of the transport characteristics of nanoribbon-based devices using a hybrid approach that combines the benefits of plane waves while retaining the efficiency provided by the TB approximation. © 2018 IEEE.
  • 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
    Minimizing Performance Degradation Induced by Interfacial Recombination in Perovskite Solar Cells through Tailoring of the Transport Layer Electronic Properties
    (Amer Inst Physics) Xu, Liang; Imenabadi, Rouzbeh Molaei; Vandenberghe, William G.; Hsu, Julia W. P.; 0000-0003-2710-5227 (Xu, L); 0000-0002-7821-3001 (Hsu, JWP); Xu, Liang; Imenabadi, Rouzbeh Molaei; Vandenberghe, William G.; Hsu, Julia W. P.
    The performance of hybrid organic-inorganic metal halide perovskite solar cells is investigated using one-dimensional drift-diffusion device simulations. We study the effects of interfacial defect density, doping concentration, and electronic level positions of the charge transport layer (CTL). Choosing CTLs with a favorable band alignment, rather than passivating CTL-perovskite interfacial defects, is shown to be beneficial for maintaining high power-conversion efficiency, due to reduced minority carrier density arising from a favorable local electric field profile. Insights from this study provide theoretical guidance on practical selection of CTL materials for achieving high-performance perovskite solar cells.
  • Item
    Theoretical Simulation of Negative Differential Transconductance in Lateral Quantum Well nMOS Devices
    (American Institute of Physics Inc, 2017-01-23) Vyas, P. B.; Naquin, C.; Edwards, H.; Lee, Mark; Vandenberghe, W. G.; Fischetti, Massimo V.; 0000-0001-5926-0200 (Fischetti, MV); 21146635654041982414 (Vandenberghe, WG); Vyas, P. B.; Lee, Mark; Vandenberghe, William G.; Fischetti, Massimo V.
    We present a theoretical study of the negative differential transconductance (NDT) recently observed in the lateral-quantum-well Si n-channel field-effect transistors J. Appl. Phys. 118, 124505 (2015)]. In these devices, p⁺ doping extensions are introduced at the source-channel and drain-channel junctions, thus creating two potential barriers that define the quantum well across whose quasi-bound states resonant/sequential tunneling may occur. Our study, based on the quantum transmitting boundary method, predicts the presence of a sharp NDT in devices with a nominal gate length of 10-to-20 nm at low temperatures (~10 K). At higher temperatures, the NDT weakens and disappears altogether as a result of increasing thermionic emission over the p⁺ potential barriers. In larger devices (with a gate length of 30 nm or longer), the NDT cannot be observed because of the low transmission probability and small energetic spacing (smaller than k_{B}T) of the quasi-bound states in the quantum well. We speculate that the inability of the model to predict the NDT observed in 40 nm gate-length devices may be due to an insufficiently accurate knowledge of the actual doping profiles. On the other hand, our study shows that NDT suitable for novel logic applications may be obtained at room temperature in devices of the current or near-future generation (sub-10 nm node), provided an optimal design can be found that minimizes the thermionic emission (requiring high p⁺ potential-barriers) and punch-through (that meets the opposite requirement of potential-barriers low enough to favor the tunneling current).
  • Item
    Mermin-Wagner Theorem, Flexural Modes, and Degraded Carrier Mobility in Two-Dimensional Crystals with Broken Horizontal Mirror Symmetry
    (Amer Physical Soc, 2016-04-11) Fischetti, Massimo V.; Vandenberghe, William G.; 0000-0001-5926-0200 (Fischetti, MV); 21146635654041982414 (Vandenberghe, WG); Fischetti, Massimo V.; Vandenberghe, William G.
    We show that the electron mobility in ideal, free-standing two-dimensional "buckled" crystals with broken horizontal mirror (σ_h) symmetry and Dirac-like dispersion (such as silicene and germanene) is dramatically affected by scattering with the acoustic flexural modes (ZA phonons). This is caused both by the broken σ_h symmetry and by the diverging number of long-wavelength ZA phonons, consistent with the Mermin-Wagner theorem. Non-{σ_h}-symmetric, "gapped" 2D crystals (such as semiconducting transition-metal dichalcogenides with a tetragonal crystal structure) are affected less severely by the broken σ_h symmetry, but equally seriously by the large population of the acoustic flexural modes. We speculate that reasonable long-wavelength cutoffs needed to stabilize the structure (finite sample size, grain size, wrinkles, defects) or the anharmonic coupling between flexural and in-plane acoustic modes (shown to be effective in mirror-symmetric crystals, like free-standing graphene) may not be sufficient to raise the electron mobility to satisfactory values. Additional effects (such as clamping and phonon stiffening by the substrate and/or gate insulator) may be required.

Works in Treasures @ UT Dallas are made available exclusively for educational purposes such as research or instruction. Literary rights, including copyright for published works held by the creator(s) or their heirs, or other third parties may apply. All rights are reserved unless otherwise indicated by the copyright owner(s).