# Fischetti, Massimo V.

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/2310

An expert in how electrons move in solids, Dr. Fischetti is renowned in the field for the development of DAMOCLES, a computer program that was the first to accurately simulate how electrons move in small semiconductors using what is known as the Monte Carlo transport model. The program is used to design transistors for chips in computers, smartphones and advanced video games.

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# Browsing Fischetti, Massimo V. by Author "0000-0001-5926-0200 (Fischetti, MV)"

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Item Ab Initio Study of the Electronic Properties and Thermodynamic Stability of Supported and Functionalized Two-Dimensional Sn Films(Amer Physical Soc) Negreira, Ana Suarez; Vandenberghe, William G.; Fischetti, Massimo V.; 0000-0001-5926-0200 (Fischetti, MV); A-4508-2012 (Fischetti, MV)Using density-functional theory (DFT), we study the growth of pristine and functionalized tin monolayers (Sn-MLs) on three different substrates, CdTe, InSb, and Si(111), and the impact these substrates have on the topological insulating properties of the electronic band structure. The presence of the substrate leads to strain and electronic charge transfer, which cause significant changes in the stability and electronic properties of the supported Sn-ML. Growth of pristine Sn-MLs on Si(111) leads to metallic behavior resembling that of the high-buckled Sn-ML phase; pristine Sn-MLs grown on InSb do not maintain a gap throughout the entire Brillouin zone; and pristine Sn-MLs grown on CdTe are unlikely to exhibit an experimentally observable gap. Provided the charge transfer from the substrate can be compensated, halogen-functionalized Sn-MLs grown on CdTe and InSb are topological insulators, albeit with a reduced band gap compared to their free-standing counterparts (from 0.34 eV for Sn-ML-I to 0.17 eV for InSb-Sn-ML-I). We employ ab initio thermodynamics calculations to study the thermodynamic stability of the halogenated InSb-Sn-MLs and CdTe-Sn-MLs surfaces for a temperature range of 100-1000 K under two extreme environments: ultrahigh vacuum (used in most of the laboratory characterization techniques) and rich-halogen conditions (10% vol. halogen environment). Our results indicate that it is possible to obtain stable topologically insulating Sn-MLs grown epitaxially on lattice-matched substrates.Item Energies of the X- and L-Valleys in In(0.53)Ga(0.47) as from Electronic Structure Calculations(Amer Inst Physics) Greene-Diniz, Gabriel; Fischetti, Massimo V.; Greer, J. C.; 0000-0001-5926-0200 (Fischetti, MV)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 In(x)Ga(1-x)As 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.Item "Hot Electrons in Si Lose Energy Mostly to Optical Phonons:" Truth or Myth?(American Institute of Physics Inc., 2019-06-05) Fischetti, Massimo V.; Yoder, P. D.; Khatami, Mohammad Mahdi; Gaddemane, Gautam; Van De Put, Maarten L.; 0000-0001-5926-0200 (Fischetti, MV); 0000-0003-0067-8674 (Gaddemane, G); 0000-0001-8014-0350 (Khatami, MM); 0000-0001-9179-6443 (Van de Put, ML); Fischetti, Massimo V.; Khatami, Mohammad Mahdi; Gaddemane, Gautam; Van De Put, Maarten L.Theoretical studies of heat generation and diffusion in Si devices generally assume that hot electrons in Si lose their energy mainly to optical phonons. Here, we briefly review the history of this assumption, and using full-band Monte Carlo simulations - with electron-phonon scattering rates calculated using the rigid-ion approximation and both empirical pseudopotentials and Harris potentials - we show that, instead, electrons lose as much as 2/3 of their energy to acoustic phonons. The scattering rates that we have calculated have been used to study hot-electron effects, such as impact ionization and injection into SiO2, and are in rough agreement with those obtained using density functional theory. Moreover, direct subpicosecond pump-probe experimental results, some of them dating back to 1994, are consistent with the predictions of our model. We conclude that the study of heat generation and dissipation in nanometer-scale Si devices may require a substantial revision of the assumptions that have been considered "common wisdom" so far. © 2019 Author(s).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.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 Superconductivity Induced by Flexural Modes in Non-σ_h-Symmetric Dirac-Like Two-Dimensional Materials: A Theoretical Study for Silicene and Germanene(Amer Physical Soc) Fischetti, Massimo V.; Polley, Arup; 0000-0001-5926-0200 (Fischetti, MV); Fischetti, Massimo V.In two-dimensional crystals that lack symmetry under reflections on the horizontal plane of the lattice (non-σ_h-symmetric), electrons can couple to flexural modes (ZA phonons) at first order. We show that in materials of this type that also exhibit a Dirac-like electron dispersion, the strong coupling can result in electron pairing mediated by these phonons, as long as the flexural modes are not damped or suppressed by additional interactions with a supporting substrate or gate insulator. We consider several models: The weak-coupling limit, which is applicable only in the case of gapped and parabolic materials, like stanene and HfSe₂, thanks to the weak coupling; the full gap-equation, solved using the constant-gap approximation and considering statically screened interactions; its extensions to energy-dependent gap and to dynamic screening. We argue that in the case of silicene and germanene superconductivity mediated by this process can exhibit a critical temperature of a few degrees K, or even a few tens of degrees K when accounting for the effect of a high-dielectric- constant environment. We conclude that the electron/flexural-modes coupling should be included in studies of possible superconductivity in non-σ_h-symmetric two-dimensional crystals, even if alternative forms of coupling are considered.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 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 Understanding the Average Electron-Hole Pair-Creation Energy in Silicon and Germanium Based on Full-Band Monte Carlo Simulations(Institute of Electrical Electronics Engineers Inc, 2019-01) Fang, Jingtian; Reaz, Mahmud; Weeden-Wright, Stephanie L.; Schrimpf, Ronald D.; Reed, Robert A.; Weller, Robert A.; Fischetti, Massimo V.; Pantelides, Sokrates T.; 0000-0001-5926-0200 (Fischetti, MV); Fischetti, Massimo V.The thermalization process of sub-10-eV charge carriers is examined with treating carrier transport with full-band Monte Carlo simulations. The average energy loss (3.69 eV in Si and 2.62 eV in Ge) required to create a thermalized electron-hole pair, obtained from the simulations, is very close to the experimentally measured radiation-ionization energies of Si and Ge irradiated with high-energy particles. These results suggest that only interactions that occur after the radiation-generated charge carriers decay to energies of similar to 10 eV or less determine the fundamental property of the radiation-ionization energies. In addition to an energy loss equal to the band gap energy via impact ionization, acoustic-phonon emission, which has been omitted in prior work, contributes 30% of the remaining carrier energy loss, while optical-phonon emission contributes the other 70%.