Papers by Massimo Fischetti
Ab initio Methods for Electronic Transport in Semiconductors and Nanostructures
Springer eBooks, Nov 11, 2022
Monte Carlo study of carrier transport in two-dimensional transition metal dichalcogenides: high-field characteristics and MOSFET simulation
Journal of Computational Electronics, Jul 6, 2023
Physical review, Sep 13, 2019
Using first-principles calculations we have previously shown that placing graphene on a (111)-ori... more Using first-principles calculations we have previously shown that placing graphene on a (111)-oriented perovskite SrTiO 3 (STO) surface provides a possible doping mechanism [D. Shin and A. A. Demkov, Phys. Rev. B 97, 075423 (2018)]. Further theoretical analysis presented here suggests that coupling of electrons in graphene to interfacial hybrid plasmon/optical modes via remote-phonon scattering may result in an effective attractive electron-electron interaction that, in turn, could lead to electron pairing and superconductivity. Specifically, we consider top-gated graphene supported by STO. Using the full dynamic polarizability within the random phase approximation for the entire system (including the hybrid modes arising from the coupling of the graphene plasmons to the optical phonons of the STO substrate and gate insulator), we estimate the superconducting transition temperature in the strong-coupling limit.
Effects of the Dielectric Environment on Electronic Transport in Monolayer MoS<sub>2</sub>: Screening and Remote Phonon Scattering
We investigate theoretically the impact of the dielectric environment on electronic transport in ... more We investigate theoretically the impact of the dielectric environment on electronic transport in monolayer MoS2. In particular, we extend our first-principles Monte Carlo method to account for the screening of the electron-phonon interaction by the free carriers in the layer and the dielectric environment. In addition, we include the effect of remote-phonon scattering induced by the surrounding dielectrics. For monolayer MoS2 on various dielectric substrates, we find that screening could improve the mobility significantly, but the inclusion of remote-phonon scattering degrades the mobility below its free-standing value. In our model, the introduction of gates in a dual-gate configuration does not appreciably decrease the remote-phonon interaction as it does in inversion layers or thicker films. However, for a double-gate field-effect transistor, we still obtain reasonable transport characteristics.
Journal of Computational Electronics, Jan 3, 2021
Experimental studies on two-dimensional (2D) materials are still in the early stages, and most of... more Experimental studies on two-dimensional (2D) materials are still in the early stages, and most of the theoretical studies performed to screen these materials are limited to the room-temperature carrier-mobility in the free standing 2D layers. With the dimensions of devices moving towards nanometer-scale lengths, the room-temperature carrier-mobility-an equilibrium concept-may not be the main quantity that controls the performance of devices based on these 2D materials, since electronic transport occurs under strong off-equilibrium conditions. Here we account for these nonequilibrium conditions and, for the case of monolayer phosphorene (monolayer black phosphorus), show the results of device simulations for a short channel n-MOSFET, using the Monte Carlo method coupled with the Poisson equation, including full bands and full electron-phonon matrix elements obtained from density functional theory. Our simulations reveal significant intrinsic limitations to the performance of phosphorene as a channel material in nanotransistors.
arXiv (Cornell University), Sep 30, 2015
The tunneling current between two crossed graphene ribbons is described invoking the empirical ps... more The tunneling current between two crossed graphene ribbons is described invoking the empirical pseudopotential approximation and the Bardeen transfer Hamiltonian method. Results indicate that the density of states is the most important factor determining the tunneling current between small (∼nm) ribbons. The quasi-one dimensional nature of graphene nanoribbons is shown to result in resonant tunneling.
Physical review, Apr 11, 2016
We show that the electron mobility in ideal, free-standing two-dimensional 'buckled' crystals wit... more 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.
Monte Carlo simulation of double-gate silicon-on-insulator inversion layers: The role of volume inversion
Journal of Applied Physics, May 15, 2001
The electron mobility in a double-gate silicon-on-insulator (DGSOI) device is studied as a functi... more The electron mobility in a double-gate silicon-on-insulator (DGSOI) device is studied as a function of the transverse effective field and silicon layer thickness. The contributions of the main scattering mechanisms (phonon scattering, surface roughness scattering due to both Si–SiO2 interfaces, and Coulomb interaction with the interface traps of both interfaces) are taken into account and carefully analyzed. We demonstrate that the contribution of surface scattering mechanisms is by no means negligible; on the contrary, it plays a very important role which must be taken into account when calculating the mobility in these structures. The electron mobility in DGSOI devices as Tw decreases is compared with the mobility in single-gate silicon-on-insulator structures (i) when only phonon scattering is considered, (ii) when the effect of surface-roughness scattering is taken into account, and (iii) when the contribution of Coulomb interaction with charges trapped at both interfaces is taken into consideration (in addition to phonon and surface roughness scattering). From this comparison we determined (in the three cases above) the existence of the following three regions: (i) A first region for thick silicon layers (Tw&gt;20–30 nm), where mobility for both structures tends to coincide, approaching the bulk value. (ii) As Tw decreases we show that volume inversion modifies the electron transport properties by reducing the effect of all scattering mechanisms. Accordingly, the electron mobility in DGSOI inversion layers increases by an important factor which depends on the silicon thickness and the transverse effective field. (iii) Finally, for very small thicknesses, the limitations to electron transport are due to geometrical effects, and therefore the two mobility curves, which again coincide, fall abruptly. We show the existence of a range of thicknesses of a silicon layer (between 5 and 20 nm in which electron mobility is improved by 25% or more.
Modeling Contact Resistivity in Monolayer Molybdenum disulfide Edge contacts
We calculate the resistivity of Schottky edge contacts between a metal and a transition-metal dic... more We calculate the resistivity of Schottky edge contacts between a metal and a transition-metal dichalcogenide (TMD) thin layer. The electrostatic potential is obtained by solving numerically the Poisson equation; the transmission probability is computed using the Wentzel–Kramers–Brillouin (WKB) approximation using the full-band density of states obtained from density functional theory (DFT); the effect of the image force is obtained analytically using the Green’s function for the Poisson equation with boundary conditions appropriate to the geometry we have considered. We find that the dielectric environment surrounding the 2D layer largely controls the electrostatics and image-force barrier lowering. Low-resistance metal-TMD Schottky edge contacts are obtained using low-κ top and bottom insulators.
npj 2D materials and applications, Mar 10, 2023
The performance of transistors based on two-dimensional (2D) materials is affected largely by the... more The performance of transistors based on two-dimensional (2D) materials is affected largely by the contact resistance due to high Schottky barriers at the metal-2D-material interface. In this work, we incorporate the effect of surrounding dielectrics and imageforce barrier-lowering in calculating the resistance of Schottky edge-contacts between a metal and a transition-metal dichalcogenide (TMD) thin layer. The electrostatic potential is computed by solving the Poisson equation numerically. The transmission probability is computed using the Wentzel-Kramers-Brillouin (WKB) approximation using the full-band density of states obtained from density functional theory (DFT). The effect of the image force is obtained analytically using the Coulomb kernel of a point charge with boundary conditions appropriate to the geometry we have considered. We find that the image-force barrier-lowering (IFBL) in edge-contacts is determined mainly by the dielectric permittivity of the surrounding oxide. We find that low-κ surrounding dielectrics are crucial for obtaining low resistance monolayer-TMD edge-contacts. Our results show metal-ton(p)-type MoS 2 (WSe 2) edge-contacts with SiO 2 as top and bottom insulators, a doping concentration > 1 × 10 13 cm −2 and a metal work-function < 5.1 eV (> 4.6 eV) result in a contact resistance as low as 50 Ω ⋅ μm.
Journal of Applied Physics, Jun 7, 2016
A weakly coupled system of two crossed graphene nanoribbons exhibits direct tunneling due to the ... more A weakly coupled system of two crossed graphene nanoribbons exhibits direct tunneling due to the overlap of the wavefunctions of both ribbons. We apply the Bardeen transfer Hamiltonian formalism, using atomistic band structure calculations to account for the effect of the atomic structure on the tunneling process. The strong quantum-size confinement of the nanoribbons is mirrored by the one-dimensional character of the electronic structure, resulting in properties that differ significantly from the case of inter-layer tunneling, where tunneling occurs between bulk two-dimensional graphene sheets. The current-voltage characteristics of the inter-ribbon tunneling structures exhibit resonance, as well as stepwise increases in current. Both features are caused by the energetic alignment of one-dimensional peaks in the density-of-states of the ribbons. Resonant tunneling occurs if the sign of the curvature of the coupled energy bands is equal, whereas a step-like increase of the current occurs if the signs are opposite. Changing the doping modulates the onset-voltage of the effects as well as their magnitude. Doping through electrostatic gating makes these structures promising for application towards steep slope switching devices. Using the atomistic empirical pseudopotentials based Bardeen transfer Hamiltonian method, inter-ribbon tunneling can be studied for the whole range of two-dimensional materials, such as transition metal dichalcogenides. The effects of resonance and of step-like increases of the current we observe in graphene ribbons, are also expected in ribbons made from these alternative two-dimensional materials, because these effects are manifestations of the one-dimensional character of the densityof-states.
Springer Handbook of Semiconductor Devices
Springer Handbooks
Physical Chemistry Chemical Physics, 2020
Inelastic electron scattering phenomena in chemical/physical/materials interests: electron radiat... more Inelastic electron scattering phenomena in chemical/physical/materials interests: electron radiation damage in materials; DNA damaged by electron scattering; electron therapy; electron microscope; electron-beam-induced deposition for nanofabrication.
Computer Physics Communications, 2019
The simulation of charge transport in ultra-scaled electronic devices requires the knowledge of t... more 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 widelyused 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 partitionof-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.
Journal of Applied Physics, 2016
Physical Review B, 2016
We derive a microscopic Poisson equation using the density-density response function. This equati... more We derive a microscopic Poisson equation using the density-density response function. This equation is valid for any realistic potential perturbation and permits the study of dielectric response in nanostructures, especially in one-dimensional nanostructures and quantum dots. We apply this equation to simulate a nanoscale parallel-plate capacitor (nanocapacitor) with graphene as dielectric and two nanocapacitors with a graphene nanoribbon (GNR) as dielectric. The density-density response function is calculated using first-order perturbation theory and empirical pseudopotentials. From the microscopic electric field of the graphene nanocapacitor, we calculate the out-of-plane microscopic dielectric constant of graphene and from the electric field of GNR nanocapacitors, we calculate the full microscopic dielectric tensor of several GNRs with different widths. We find that the out-of-plane microscopic dielectric constants of GNRs and graphene do not depend on their energy band gap. We also study the effect of a surrounding dielectric on the dielectric permittivity of graphene and we conclude that the surrounding dielectric barely affects the dielectric permittivity of graphene.
Dielectric Properties of Semiconductors
Advanced Physics of Electron Transport in Semiconductors and Nanostructures, 2016
We discuss the dielectric response of a crystal. We consider separately the ionic response and th... more We discuss the dielectric response of a crystal. We consider separately the ionic response and the electronic response in the Random Phase Approximation. We then consider explicitly the interesting limits of the response of an electron gas in three and two dimensions at low temperature, and in the static- and long-wavelength limits.
Journal of Applied Physics, 2016
Several theoretical electronic structure methods are applied to study the relative energies of th... more Several theoretical electronic structure methods are applied to study the relative energies of the minima of the X- and L-conduction-band satellite valleys of InxGa1−xAs with x = 0.53. This III-V semiconductor is a contender as a replacement for silicon in high-performance n-type metal-oxide-semiconductor transistors. The energy of the low-lying valleys relative to the conduction-band edge governs the population of channel carriers as the transistor is brought into inversion, hence determining current drive and switching properties at gate voltages above threshold. The calculations indicate that the position of the L- and X-valley minima are ∼1 eV and ∼1.2 eV, respectively, higher in energy with respect to the conduction-band minimum at the Γ-point.
2015 International Workshop on Computational Electronics (IWCE), 2015
2015 International Workshop on Computational Electronics (IWCE), 2015
The tunneling current between two crossed graphene ribbons is described invoking the empirical ps... more The tunneling current between two crossed graphene ribbons is described invoking the empirical pseudopotential approximation and the Bardeen transfer Hamiltonian method. Results indicate that the density of states is the most important factor determining the tunneling current between small (∼nm) ribbons. The quasi-one dimensional nature of graphene nanoribbons is shown to result in resonant tunneling.
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Papers by Massimo Fischetti