arXiv subjects

Massimo V. Fischetti

Publications and source records attributed to Massimo V. Fischetti.

At least 19 recordsLinked to original sources

arXiv2026

On the electronic and vibrational dimensionality of nanometer-scale silicon structures

We discuss the problem of assessing the electronic and vibrational dimensionality of a semiconductor nanostructure: How thin and/or wide must a nanostructure be in order to induce electron and phonon confinement? Clarifying the physical justification for common criteria found in the literature, we view the electron coherence length (defined as the electron and phonon inelastic mean free path) as their `field of view' and argue (or, better yet, `speculate') that this sets the important length scale. Considering the example of Si nanosheets at room temperature, and drawing from results found in the literature, we estimate that the critical length below which electrons are subject to quantum confinement is of the order of (or smaller than) 8 nm, when their coherence length is determined by energy losses to phonons and remote phonons in gated structures. On the contrary, no single length-scale can be given for phonons: Taking their coherence length as determined by scattering with electrons and anharmonic three-phonon processes, short wavelength acoustic and optical phonons may be confined only by structures as small as 10 nm. Long-wavelength acoustic phonons, instead, may exhibit a coherence length of the order of 1 micrometer, so that they may be confined over much larger distances.

cond-mat.mes-hall↗

arXiv2026

Electron transport in a 1.6~nm-thick double-gated (100) silicon nanosheet: A theoretical study accounting for phonon confinement and remote-phonon scattering

We study theoretically electron transport in an top-and bottom-gated (100) 1.6 nm-thin silicon nanosheet with SiO2/HfO2 gate stacks, focusing on the intrinsic physical processes that affect transport: the confinement of phonons and the presence of interface hybrid plasmon-phonon excitations (IPPs or `remote phonons'). The band structure is calculated using local empirical pseudopotentials; an approximated elastic continuum model is used to consider the confinement of acoustic phonons; the dielectric continuum limit is used to deal with the IPPs. We find that the electron mobility is affected significantly by the boundary conditions chosen to deal with phonon confinement. The more realistic assumption of phonons clamped at the SiO2/HfO2 interfaces and optical phonons at the Si/SiO2 interfaces results in a room temperature mobility much smaller than what is obtained using the common assumption of bulk phonons in the elastic, high-temperature approximation. We also find that, as a result of the complicated structure of the primed subbands, the high-field saturated velocity is significantly lower than its bulk value, as it had been measured in the past in the case of Si inversion layers but never explained theoretically. Finally, we find that IPP scattering does depress the low-field mobility but to a small extent, thanks to the presence of the interfacial SiO2 layers and to the proximity of the metal gates. Moreover, by keeping electrons `cooler', IPP scattering results in a higher saturated velocity. Therefore, the presence of high-kappa materials in the gate-insulator stacks should not affect negatively the performance of field effect transistors based on Si nanosheets.

cond-mat.mes-hall↗

arXiv2026

On the static dielectric constant of thin dielectrics in extremely scaled silicon nanosheet transistors

We argue that the static dielectric constant of small (thin and/or narrow) semiconductor and insulator nanostructures depends strongly on the their environment. We do so by considering the electronic response simply reviewing, briefly but critically, the existing literature. Regarding the ionic response, in addition to reviewing the literature, we use a simple model to account for the confinement of optical phonons in thin films and show that the reduction of their density of states has a negligible effect on the dielectric constant, in contrast to some claims found in the literature. In general, we argue that in realistic structures, such as double-gated Si nanosheets, the use of the bulk dielectric constants for both the channel and the gate insulators, is justified.

cond-mat.mes-hall↗

arXiv2025

Effects of phonon confinement on electron transport in Si nanowire and armchair-edge graphene nanoribbon transistors: A dissipative quantum-transport study

Electronic transport in low-dimensional structures, such as thin bodies, nanosheets, nanoribbons and nanowires, is strongly affected by electron and phonon confinement, in addition to interface roughness. Here we use a quantum-transport formulation based on empirical pseudopotentials and the Master equation to study the effect of the phonon boundary conditions on the electron transport in field effect transistors (FETs) based on a small cross-section (3$\times$3 cells) Si nanowire (NW) and a 10 armchair graphene nanoribbon (10-aGNR). For the dispersion of the confined phonons we employ a simple empirical model based on the folding of the bulk phonon dispersion that approximates the results of the elastic-continuum model at long wavelengths. We consider two extreme cases for their boundary conditions: clamped boundary conditions (CBCs) or free-standing (FSBCs). We find that phonon confinement affects more severely the Si nanowires than graphene nanoribbons. In particular, for 3$\times$3 SiNW-FETs, CBCs result in a higher room-temperature electron mobility than FSBCs, a result consistent with what previously reported. On the contrary, in the off-equilibrium conditions seen in gate-all-around (GAA) 3$\times$3 SiNW-FETs with 7~nm gate-length, FSBCs yield a higher on-current than what is obtained assuming CBCs. However, for 10-aGNR-FETs, both the electron mobility and the on-current are higher when assuming FSBCs.

cond-mat.mes-hall↗

arXiv2023

Thermalization of radiation-induced electrons in wide-bandgap materials: A first-principles approach

The present study is concerned with simulating the thermalization of high-energy charge carriers (electrons and/or electron-hole pairs), generated by ionizing radiation, in diamond and $β$-Ga$_2$O$_3$. Computational tools developed by the nuclear/particle physics and electronic device communities allow for accurate simulation of charge-carrier transport and thermalization in the high-energy (exceeding $\sim100$ eV) and low-energy (below $\sim10$ eV) regimes, respectively. Between these energy regimes, there is an intermediate energy range of about 10-100 eV, which we call the "10-100 eV gap", in which the energy-loss processes are historically not well-studied or understood. To close this "gap", we use a first-principles approach (density functional theory) to calculate the band structure of diamond and $β$-Ga$_2$O$_3$ up to $\sim100$ eV along with the phonon dispersion, carrier-phonon matrix elements, and dynamic dielectric function. Additionally, using first-order perturbation theory (Fermi's Golden Rule/first Born approximation), we calculate the carrier-phonon scattering rates and the carrier energy-loss rates (impact ionization and plasmon scattering). With these data, we simulate the thermalization of 100-eV electrons and the generated electron-hole pairs by solving the semiclassical Boltzmann transport equation using Monte Carlo techniques. We find that electron thermalization is complete within $\sim0.4$ and $\sim1.0$ ps for diamond and $β$-Ga$_2$O$_3$, respectively, while holes thermalize within $\sim0.5$ ps for both. We also calculate electron-hole pair creation energies of 12.87 and 11.24 eV, respectively.

cond-mat.mtrl-sci↗

arXiv2023

A first-principles approach to closing the "10-100 eV gap" for charge-carrier thermalization in semiconductors

The present work is concerned with studying accurately the energy-loss processes that control the thermalization of hot electrons and holes that are generated by high-energy radiation in wurtzite GaN, using an ab initio approach. Current physical models of the nuclear/particle physics community cover thermalization in the high-energy range (kinetic energies exceeding ~100 eV), and the electronic-device community has studied extensively carrier transport in the low-energy range (below ~10 eV). However, the processes that control the energy losses and thermalization of electrons and holes in the intermediate energy range of about 10-100 eV (the "10-100 eV gap") are poorly known. The aim of this research is to close this gap, by utilizing density functional theory (DFT) to obtain the band structure and dielectric function of GaN for energies up to about 100 eV. We also calculate charge-carrier scattering rates for the major charge-carrier interactions (phonon scattering, impact ionization, and plasmon emission), using the DFT results and first-order perturbation theory. With this information, we study the thermalization of electrons starting at 100 eV using the Monte Carlo method to solve the semiclassical Boltzmann transport equation. Full thermalization of electrons and holes is complete within ~1 and 0.5 ps, respectively. Hot electrons dissipate about 90% of their initial kinetic energy to the electron-hole gas (90 eV) during the first ~0.1 fs, due to rapid plasmon emission and impact ionization at high energies. The remaining energy is lost more slowly as phonon emission dominates at lower energies (below ~10 eV). During the thermalization, hot electrons generate pairs with an average energy of ~8.9 eV/pair (11-12 pairs per hot electron). Additionally, during the thermalization, the maximum electron displacement from its original position is found to be on the order of 100 nm.

cond-mat.mtrl-sci↗

arXiv2022

Theoretical Study of Electronic Transport in Two-Dimensional Transition Metal Dichalcogenides: Effects of the Dielectric Environment

We discuss the effect of the dielectric environment (insulators and metal gates) on electronic transport in two-dimensional (2D) transition metal dichalcogenides (TMD) monolayers. We employ well-known ab initio methods to calculate the low-field carrier mobility in free-standing layers and use the dielectric continuum approximation to extend our study to layers in double-gate structures, including the effects of dielectric screening of the electron-phonon interaction caused by the bottom oxide and the gate insulator, and of scattering with hybrid interface optical-phonon/plasmon excitations (`remote phonon scattering'). We find that the presence of insulators with a high dielectric constant may improve significantly the carrier mobility. However, scattering with the interface hybrid excitations negates this gain and degrades the mobility significantly below its free-standing value. In a double-gate geometry with SiO$_{2}$ as bottom-oxide and various top-gate insulators, we find that the mobility decreases as the top-insulator dielectric constant increases, as expected. However, a high mobility is predicted in the case of the weakly polar hBN, and a mobility much lower than expected is calculated in the case of gate-insulator/TMD/bottom-oxide stacks in which two or more polar materials have optical-phonon with similar resonating frequencies. We also find that the effect of screening by metal gates is noticeable but not particularly strong. Finally, we discuss the effect of the TMD dielectric constant, of the free-carrier density, and of temperature on the transport properties of TMD monolayers.

cond-mat.mes-hall↗

arXiv2020

Monte Carlo analysis of phosphorene nanotransistors

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 non-equilibrium 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.

cond-mat.mes-hall↗

arXiv2020

Real-time ab initio simulation of inelastic electron scattering using the exact, density functional, and alternative approaches

To investigate inelastic electron scattering, which is ubiquitous in various fields of study, we carry out ab initio study of the real-time dynamics of a one-dimensional electron wave packet scattered by a hydrogen atom using different methods: the exact solution, the solution provided by time-dependent density functional theory (TDDFT), and the solutions given by alternative approaches. This research not only sheds light on inelastic scattering processes but also verifies the capability of TDDFT in describing inelastic electron scattering. We revisit the adiabatic local-density approximation (ALDA) in describing the excitation of the target during the scattering process along with a self-interaction correction and spin-polarized calculations. Our results reveal that the ALDA severely underestimates the energy transferred in the regime of low incident energy particularly for a spin-singlet system. After demonstrating alternative approaches, we propose a hybrid ab initio method to deal with the kinetic correlation alongside TDDFT. This hybrid method would facilitate first-principles studies of systems in which the correlation of a few electrons among many others is of interest.

physics.atom-ph↗

arXiv2019

Scalable Atomistic Simulations of Quantum Electron Transport using Empirical Pseudopotentials

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.

cond-mat.mes-hall↗

arXiv2019

Theoretical study of scattering in graphene ribbons in the presence of structural and atomistic edge roughness

We investigate the diffusive electron-transport properties of charge-doped graphene ribbons and nanoribbons with imperfect edges. We consider different regimes of edge scattering, ranging from wide graphene ribbons with (partially) diffusive edge scattering to ribbons with large width variations and nanoribbons with atomistic edge roughness. For the latter, we introduce an approach based on pseudopotentials, allowing for an atomistic treatment of the band structure and the scattering potential, on the self-consistent solution of the Boltzmann transport equation within the relaxation-time approximation and taking into account the edge-roughness properties and statistics. The resulting resistivity depends strongly on the ribbon orientation, with zigzag (armchair) ribbons showing the smallest (largest) resistivity and intermediate ribbon orientations exhibiting intermediate resistivity values. The results also show clear resistivity peaks, corresponding to peaks in the density of states due to the confinement-induced subband quantization, except for armchair-edge ribbons that show a very strong width dependence because of their claromatic behavior. Furthermore, we identify a strong interplay between the relative position of the two valleys of graphene along the transport direction, the correlation profile of the atomistic edge roughness, and the chiral valley modes, leading to a peculiar strongly suppressed resistivity regime, most pronounced for the zigzag orientation.

cond-mat.mes-hall↗

arXiv2018

Superconductivity induced by flexural modes in non $σ_{\rm h}$-symmetric Dirac-like two-dimensional materials: A theoretical study for silicene and germanene

In two-dimensional crystals that lack symmetry under reflections on the horizontal plane of the lattice (non-$σ_{\rm h}$-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 HfSe$_{2}$, 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-$σ_{\rm h}$-symmetric two-dimensional crystals, even if alternative forms of coupling are considered.

cond-mat.supr-con↗

arXiv2018

Theoretical studies of electronic transport in mono- and bi-layer phosphorene: A critical overview

Recent $\textit{ab initio}$ theoretical calculations of the electrical performance of several two-dimensional materials predict a low-field carrier mobility that spans several orders of magnitude (from 26,000 to 35 cm$^{2}$ V$^{-1}$ s$^{-1}$, 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 mono- and bi-layer phosphorene. For thin multi-layer phosphorene we confirm the most disappointing results, with a strongly anisotropic carrier mobility that does not exceed $\sim$ 30 cm$^{2}$ V$^{-1}$ s$^{-1}$ at 300 K for electrons along the armchair direction.

cond-mat.mes-hall↗

arXiv2016

Mermin-Wagner theorem, flexural modes, and degraded carrier mobility in 2D crystals with broken horizontal mirror ($σ_{\rm h}$) symmetry

We show that the electron mobility in ideal, free-standing two-dimensional `buckled' crystals with broken horizontal mirror ($σ_{\rm h}$) 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 $σ_{\rm h}$ symmetry and by the diverging number of long-wavelength ZA phonons, consistent with the Mermin-Wagner theorem. Non-$σ_{\rm h}$-symmetric, `gapped' 2D crystals (such as semiconducting transition-metal dichalcogenides with a tetragonal crystal structure) are affected less severely by the broken $σ_{\rm h}$ 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.

cond-mat.mes-hall↗

arXiv2016

Inter-ribbon tunneling in graphene: an atomistic Bardeen approach

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 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, observed 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 density-of-states.

cond-mat.mes-hall↗

arXiv2014

Mobility enhancement and temperature dependence in top-gated single-layer MoS2

The deposition of a high-$κ$ oxide overlayer is known to significantly enhance the room-temperature electron mobility in single-layer MoS$_{2}$ (SLM) but not in single-layer graphene (SLG). We give a quantitative account of how this mobility enhancement is due to the non-degeneracy of the two-dimensional electron gas system in SLM at accessible temperatures. Using our charged impurity scattering model [Ong and Fischetti, Phys. Rev. B 86, 121409 (2012)] and temperature-dependent polarizability, we calculate the charged impurity-limited mobility ($μ_{\textrm{imp}}$) in SLM with and without a high-$κ$ (HfO$_{2}$) top gate oxide at different electron densities and temperatures. We find that the mobility enhancement is larger at low electron densities and high temperatures because of finite-temperature screening, thus explaining the enhancement of the mobility observed at room temperature. $μ_{\textrm{imp}}$ is shown to decrease significantly with increasing temperature, suggesting that the strong temperature dependence of measured mobilities should not be interpreted as being solely due to inelastic scattering with phonons. We also reproduce the recently seen experimental trend in which the temperature scaling exponent ($γ$) of $μ_{\textrm{imp}}\propto T^{-γ}$ is smaller in top-gated SLM than in bare SLM. Finally, we show that a $\sim37$ percent mobility enhancement can be achieved by reducing the HfO$_{2}$ thickness from 20 to 2 nm.

cond-mat.mtrl-sci↗

arXiv2014

Theoretical analysis of high-field transport in graphene on a substrate

We investigate transport in graphene supported on various dielectrics (SiO2, BN, Al2O3, HfO2) through a hydrodynamic model which includes self-heating and thermal coupling to the substrate, scattering with ionized impurities, graphene phonons and dynamically screened interfacial plasmon-phonon (IPP) modes. We uncover that while low-field transport is largely determined by impurity scattering, high-field transport is defined by scattering with dielectric-induced IPP modes, and a smaller contribution of graphene intrinsic phonons. We also find that lattice heating can lead to negative differential drift velocity (with respect to the electric field), which can be controlled by changing the underlying dielectric thermal properties or thickness. Graphene on BN exhibits the largest high-field drift velocity, while graphene on HfO2 has the lowest one due to strong influence of IPP modes.

cond-mat.mes-hall↗

arXiv2012

Charged Impurity Scattering in Graphene Nanostructures

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 percent in carrier mobility in ultrathin top-gated graphene is predicted.

cond-mat.mes-hall↗