# 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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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 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 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%.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 Deformation Potentials for Band-To-Band Tunneling in Silicon and Germanium from First Principles(American Institute of Physics Inc.) Vandenberghe, William G.; Fischetti, Massimo V.The deformation potentials for phonon-assisted band-to-band tunneling (BTBT) in silicon and germanium are calculated using a plane-wave density functional theory code. Using hybrid functionals, we obtain: D(TA) = 4.1 x 10⁸ eV/cm, D(TO) = 1.2 x 10⁹ eV/cm, and D(LO) = 2.2 x 10⁹ eV/cm for BTBT in silicon and D(TA) = 7.8 x 10⁸ eV/cm and D(LO) = 1.3 x 10⁹ eV/cm for BTBT in germanium. These values agree with experimentally measured values and we explain why in diodes, the TA/TO phonon-assisted BTBT dominates over LO phonon-assisted BTBT despite the larger deformation potential for the latter. We also explain why LO phonon-assisted BTBT can nevertheless dominate in many practical applications.Item Calculation of Room Temperature Conductivity and Mobility in Tin-Based Topological Insulator NanoribbonsVandenberghe, William G.; Fischetti, Massimo V.; A-4508-2012 (Fischetti, MV)Monolayers of tin (stannanane) functionalized with halogens have been shown to be topological insulators. Using density functional theory (DFT), we study the electronic properties and room-temperature transport of nanoribbons of iodine-functionalized stannanane showing that the overlap integral between the wavefunctions associated to edge-states at opposite ends of the ribbons decreases with increasing width of the ribbons. Obtaining the phonon spectra and the deformation potentials also from DFT, we calculate the conductivity of the ribbons using the Kubo-Greenwood formalism and show that their mobility is limited by inter-edge phonon backscattering. We show that wide stannanane ribbons have a mobility exceeding 10 6 cm(2)/Vs. Contrary to ordinary semiconductors, two-dimensional topological insulators exhibit a high conductivity at low charge density, decreasing with increasing carrier density. Furthermore, the conductivity of iodine-functionalized stannanane ribbons can be modulated over a range of three orders of magnitude, thus rendering this material extremely interesting for classical computing applications.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 Theory of Interfacial Plasmon-Phonon Scattering in Supported GrapheneOng, Zhun-Yong; Fischetti, Massimo V.One of the factors limiting electron mobility in supported graphene is remote phonon scattering. We formulate the theory of the coupling between graphene plasmon and substrate surface polar phonon (SPP) modes and find that it leads to the formation of interfacial plasmon-phonon (IPP) modes, from which the phenomena of dynamic antiscreening and screening of remote phonons emerge. The remote phonon-limited mobilities for SiO2, HfO2, h-BN, and Al2O3 substrates are computed using our theory. We find that hexagonal boron nitride (h-BN) yields the highest peak mobility, but in the practically useful high-density range, the mobility in HfO2-supported graphene is high, despite the fact that HfO2 is a high-k dielectric with low-frequency modes. Our theory predicts that the strong temperature dependence of the total mobility effectively vanishes at very high carrier concentrations. The effects of polycrystallinity on IPP scattering are also discussed.Item Pseudopotential-Based Electron Quantum Transport: Theoretical Formulation and Application to Nanometer-Scale Silicon Nanowire Transistors(American Institute of Physics Inc) Fang, Jingtain; Vandenberghe, William G.; Fu, Bo; Fischetti, Massimo V.; Fang, Jingtain; Vandenberghe, William G.; Fu, Bo; Fischetti, Massimo V.TBDItem Fundamental Limitations of Hot-Carrier Solar CellsKirk, Alexander P.; Fischetti, Massimo V.Sunlight-generated hot-carrier transport in strongly absorbing direct band-gap GaAs-among the most optimal of semiconductors for high-efficiency solar cells-is simulated with an accurate full-band structure self-consistent Monte Carlo method, including short- and long-range Coulomb interaction, impact ionization, and optical and acoustic phonon scattering. We consider an ultrapure 100-nm-thick intrinsic GaAs absorber layer designed with quasiballistic carrier transport that achieves complete photon absorption down to the band edge by application of careful light trapping and that has a generous hot-carrier retention time of 10 ps prior to the onset of carrier relaxation. We find that hot-carrier solar cells can be severely limited in performance due to the substantially reduced current density caused by insufficient extraction of the widely distributed hot electrons (holes) through the requisite energy selective contacts. © 2012 American Physical Society.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 Charged Impurity Scattering in Top-Gated Graphene NanostructuresOng, Zhun-Yong; Fischetti, Massimo V.We study charged impurity scattering and static screening in a top-gated substrate-supported graphene nanostructure. Our model describes how boundary conditions can be incorporated into scattering, sheds light on the dielectric response of these nanostructures, provides insights into the effect of the top gate on impurity scattering, and predicts that the carrier mobility in such graphene heterostructures decreases with increasing top dielectric thickness and higher carrier density. An increase of up to almost 60% in carrier mobility in ultrathin top-gated graphene is predicted.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 Structural, Electronic, and Transport Properties of Silicane NanoribbonsKim, Jiseok; Fischetti, Massimo V.; Aboud, S.Silicane ribbons do not suffer from aromatic dependence of the band gap making them a more promising candidate for near-term nanoelectronic application compared to armchair graphene nanoribbons. The structural, electronic, and transport properties of free-standing sp3-hybridized armchair- and zigzag-edge silicane nanoribbons have been investigated using ab initio and nonlocal empirical pseudopotential calculations. Under ambient conditions, two-dimensional silicane sheets will spontaneously break into stable one-dimensional ribbons similar to density functional theory studies of graphene ribbons. The calculated low-field electron mobility and ballistic conductance show a strong edge dependence, due to differences in the effective mass and momentum relaxation rates along the two transport directions. The mobility in zigzag-edge ribbons is found to be approximately twenty times higher than in armchair-edge ribbons. © 2012 American Physical Society.Item Figure of Merit for and Identification of Sub-60 mV/Decade DevicesVandenberghe, William G.; Verhulst, Anne S.; Soree, Bart; Magnus, Wim; Groeseneken, Guido; Smets, Quentin; Heyns, Marc; Fischetti, Massimo V.A figure of merit I₆₀ is proposed for sub-60 mV/decade devices as the highest current where the input characteristics exhibit a transition from sub- to super-60 mV/decade behavior. For sub-60 mV/decade devices to be competitive with metal-oxide-semiconductor field-effect devices, I₆₀ has to be in the 1-10 μA/μm range. The best experimental tunnel field-effect transistors (TFETs) in the literature only have an I₆₀ of 6 x 10⁻³ μA/μm but using theoretical simulations, we show that an I₆₀ of up to 10 μA/μm should be attainable. It is proven that the Schottky barrier FET (SBFET) has a 60 mV/decade subthreshold swing limit while combining a SBFET and a TFET does improve performance.Item Intrinsic Broadening of the Mobility Spectrum of Bulk N-Type GaAsJolley, G.; Umana-Membreno, G. A.; Akhavan, N. D.; Antoszewski, J.; Faraone, L.; Fischetti, Massimo V.Modern devices consisting of multiple semiconductor layers often result in the population of numerous distinct carrier species. Conventional Hall measurements at a single-magnetic-field strength provide only a weighted average of the electron mobility and carrier concentration of a semiconductor structure and, therefore, are of limited use for the extraction of carrier transport information. In recent years, mobility spectrum analysis techniques, which have been developed to extract a mobility spectrum from magnetic field-dependent conductivity-tensor measurements, have been applied in the analysis of carrier conductivity mechanisms of numerous semiconductor structures and devices. Currently there is a severe lack of reported studies on theoretical calculations of the mobility distribution of semiconductor structures or devices. In addition, the majority of reports on experimental mobility spectrum analysis are of complex, multi layered structures such as type-II superlattices, and the interpretation of the mobility spectra has been difficult. Therefore, a good understanding of the mobility spectrum has yet to be developed. For example, it is often assumed that distinct peaks of a mobility spectrum result from fundamentally different conduction mechanisms such as the bulk and surface conduction of narrow-bandgap semiconductors. In this article, we present calculations of the electron mobility distribution of bulk GaAs, which predict the existence of multiple mobility spectrum peaks that result from electron conductivity in the Γ conduction band. This report serves as an important and simple test case upon which experimentally measured mobility spectra can be compared. It also presents insight into the general nature of electron mobility distributions.Item Depression of the Normal-Superfluid Transition Temperature in Gated Bilayer Graphene(American Institute of Physics) Fischetti, Massimo V.It is shown that the normal-superfluid transition in bilayer graphene predicted to occur at a high temperature is strongly affected not only by the dielectric constants of the substrate, interlayer, and gate insulators but also by the proximity of ideal metal gates. Even assuming optimistically a completely unscreened interlayer Coulomb interaction-thus bypassing the controversial problems regarding the proper way to screen the interlayer Coulomb interactions-it is shown that employing a gate-insulator thickness smaller than about 2-to-5 nm of equivalent SiO2-thickness pushes the transition temperature significantly below 300K to the 1 K-1mK range, depending on the dielectric constant of the gate insulator and on the dielectric mismatch of the insulators employed. These results imply that thicker and low-dielectric-constant gate insulators should be employed to observe the phase transition, but exploiting the superfluid state of gated graphene-bilayers in room-temperature device applications may be challenging.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 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 Signatures of Dynamic Screening in Interfacial Thermal Transport of GrapheneOng, Zhun-Yong; Fischetti, Massimo V.; Serov, Andrey Y.; Pop, Eric; Fischetti, Massimo V.The interaction between graphene and various substrates plays an important and limiting role on the behavior of graphene films and devices. Here we uncover that dynamic screening of so-called remote substrate phonons (RPs) has a significant effect on the thermal coupling at the graphene-substrate interface. We calculate the thermal conductance hRP between graphene electrons and substrate, and its dependence on carrier density and temperature for SiO₂, HfO₂, h-BN, and Al₂O₃ substrates. The dynamic screening of RPs leads to one order of magnitude or more decrease in hRP and a change in its dependence on carrier density. Dynamic screening predicts a decrease of ~1 MW K⁻¹ m⁻² while static screening predicts a rise of ~10 MW K⁻¹ m⁻² when the carrier density in Al₂O₃-supported graphene is increased from 10¹² to 10¹³ cm⁻².