Browsing by Author "Fischetti, Massimo V."

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2D Materials: Theoretical Study of Magnetic and Contact Properties
(December 2023) Reyntjens, Peter Dirk Jan 1994-; Liu, Jin; Vandenberghe, William; Kim, Moon J.; Fischetti, Massimo V.; Sorée, Bart
The integrated circuit is without a doubt one of the most influential inventions in all of human history. While every technological revolution has had massive impacts across human societies, modern electronic circuits have increased the rate of change by orders of magnitude and this process shows no signs of stopping. As society has become accustomed to the rapid pace of technological development, the expectations for further improvements are more and more demanding. The silicon transistor was the ideal vehicle for such a rapid development, as transistors typically become more powerful and less costly to make when their size is decreased. With the added bonus of being able to cram more transistors into the same chip, the electronics revolution started, and a snowball effect of increasingly complexity and performance was unleashed onto the market, leading to the highly interconnected society we live in today. However, the benefits of decreasing transistor dimensions cannot last forever. There are certain extremely fundamental limits, at the nanometer scale, to how far one can go in making smaller and smaller devices. At some point, transistors begin to suffer from all sorts of performance-degrading issues, such as short-channel effects, increased leakage, fabrication difficulties, etc. Even more fundamental issues arise once the device dimensions go down to only a few nanometers, where quantum effects can seriously degrade traditional silicon-based transistors. It is with these scaling limitations in mind that researchers started looking very seriously at a relatively new class of materials: two-dimensional (2D) materials. 2D materials are atomically thin materials, consisting of a single layer not bound covalently in the out-of-plane direction. The 2D nature of these materials is of course in stark contrast with more “normal” materials, such as silicon or iron, which have covalent bonds in three dimensions. It turns out that due to the special structure of 2D materials, the physical properties are also extremely interesting, and worth investigating seriously. At present, various classes of 2D materials have been found, and many 2D materials have corresponding stacked layered versions with their own special properties. Add in, for example, the fact that one can dope these materials of make heterostructures out of several different kinds, then one can start to appreciate the vast parameter space that can be explored in the search for interesting applications. In this work, I focus on the applications of 2D materials in logic and memory devices. More specifically, I discuss the studies done by myself and my collaborators on the magnetic properties of layered WSe2 and PtSe2, and the calculation of the contact resistance between a metal and a 2D semiconductor. In the first part of the thesis, I share our investigation on the nature and stability of magnetic phases of doped intercalated WSe2 and PtSe2. We showed that, depending on the dopant, the stable magnetic phase at low temperature can be drastically different in both stability and type (ferro- or antiferromagnetic). We further showed that the presence of W or Pt vacancies in the lattice can be used to control the thermodynamic stability of the intercalated structures. Finally, we investigated the effect of the Pt vacancies on the magnetism in intercalated PtSe2. We showed that even though the spin polarization around the Pt atoms is very small, the Pt electronic cloud mediates longer magnetic interactions. Therefore, the presence or absence of Pt vacancies has a strong impact on the magnetic phases in the intercalated PtSe2. In the second part of this thesis, transport properties at a metal-2D semiconductor contact are the main topic. More specifically, I, along with my collaborators, have created a flexible model that can be used to efficiently simulate metal-2D semiconductor contacts and extract key parameters, such as the contact resistance. We studied the effects of device parameters, such as backgate bias, but also simulation parameters, such as the size of the simulation domain used to solve the Poisson equation. Crucially, we found that the contact resistance can be underestimated by over an order of magnitude when the Poisson domain is too small. In the final chapter, I provide an overview of the main achievements of the thesis and discuss potential avenues for future research.
A First Principles Approach to Closing the “10-100 eV GAP” for Electron Thermalization in Wurtzite GaN
(May 2023) Nielsen, Dallin O 1993-; Fischetti, Massimo V.; Akin, Bilal; Vandenberghe, William; Cho, Kyeongjae; Gelb, Lev D.
Since the 1960s, when radiation-induced disruption of electronic devices in space was first observed, the study of the effects of ionizing radiation on electronics has grown into an extensive field of its own. The present work is concerned with accurately modelling the energy-loss processes that control the thermalization of hot carriers (electrons and/or electron-hole pairs) 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 the high-energy range (kinetic energies exceeding ~100 eV), and the electronic-device community has done extensive work in the lower-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 are poorly known (the “10-100 eV gap”). The aim of this research is to close this gap. To this end, Density Functional Theory (DFT) is utilized to obtain the band structure of GaN for bands reaching energies above 100 eV. Furthermore, charge-carrier scattering rates for the major charge-carrier interactions (phonon scattering, impact ionization, and plasmon emission) are calculated, using the DFT results and first-order perturbation theory (Fermi’s Golden Rule). With this information, the thermalization of electrons starting at 100 eV is simulated in a Monte Carlo code, allowing the electrons to interact stochastically according to the calculated interaction rates and generate electron-hole pairs as they go, which are also tracked in the simulation. Full thermalization of electrons is complete within 1 ps, and that of holes is complete in approximately half the time. Electrons lose 90% of their energy (90 eV) during the first few ~0.1 fs, due to rapid plasmon emission and impact ionization at high energies. The remainder is lost more slowly as phonon emission dominates at lower energies (below ~10 eV). During the thermalization, hot electrons generate electron-hole pairs with an average energy of ~8.9 eV/pair (11-12 pairs per hot electron). Additionally, upon full thermalization, the average electron displacement from its original position is found to be on the order of 100 nm.
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.
Calculation of Room Temperature Conductivity and Mobility in Tin-Based Topological Insulator Nanoribbons
Vandenberghe, 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.
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.
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.
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.
Figure of Merit for and Identification of Sub-60 mV/Decade Devices
Vandenberghe, 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.
Fundamental Limitations of Hot-Carrier Solar Cells
Kirk, 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.
"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).
Intrinsic Broadening of the Mobility Spectrum of Bulk N-Type GaAs
Jolley, 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.
Mechanism of Fermi Level Pinning for Metal Contacts on Transition Metal Dichalcogenides and Their Interface Thermal Stability
(December 2022) Wang, Xinglu; Wallace, Robert; Frensley , William; Fischetti, Massimo V.; Young, Chadwin D.; Kim, Jiyoung; Lv, Bing
Transition metal dichalcogenides (TMDs) have demonstrated immense potential for application in state-of-the-art electronic, optoelectronic, and spintronic devices because of their outstanding electronic, optical, mechanical, and magnetic properties. However, the failure of tuning the Schottky barrier height by the work function of metal contacts strongly limits the efficiency of carrier injection and hence the electronic performance of TMD-based devices. This dissertation focuses on the interface between covalent and van der Waals metal contacts and TMDs to study the origin and mechanism of Fermi level pinning. Firstly, the interface chemistry and band alignment of Ni and Ag contacts on MoS2 is studied. Then the mechanism of Fermi level pinning of metal contacts on other Mo- and W-based TMDs are uncovered by considering interface chemistry, band alignment, defects and impurities of W-TMDs, contact metal adsorption mechanism and the resultant electronic structure. Also, the thermal stability of Ni/MoS2 systems is investigated in the aspects of interface chemistry, elemental diffusion, and band alignment.
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.
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.
Modelling topological and magnetic materials for charge and spin-based devices
(2022-05-01T05:00:00.000Z) Tiwari, Sabyasachi; Vandenberghe, William; Wong, W. Eric; Fischetti, Massimo V.; Kim, Moon J.; Soree, Bart
The imminent halt of Moore’s law and discontinuation of scaling of transistors based on three-dimensional materials, e.g., silicon, has prompted researchers to look for different ma- terials and device systems apart from the conventional ones to form the backbone of the electronics industry of the future. Topological insulators (TIs) open a vast avenue to realize devices with high ON current and low power consumption. TIs are a class of materials with topologically protected edge states which are spin-polarized and robust against impurity scattering. The possibility of spin-polarization in TIs and efficient transfer of spin-current in soft-layered magnets opens another avenue of research for realizing fast memory devices. In this dissertation, first, we model carrier transport through imperfect two-dimensional (2D) TI ribbons. In particular, we investigate the impact of vacancy defects on the carrier trans- port of 2D TIs. We show that carrier transport through the topologically protected edge states is robust against a high percentage of defects (up to 2%), whereas the carrier trans- port through the bulk state is strongly suppressed due to backscattering. We show that the suppression of bulk transport in 2D TIs can be used to design devices using 2D TI ribbons. Next, we develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature from the first principles, taking into account both the magnetic anisotropy as well as the out-of-plane interactions. We ap- ply our method on Cr-compounds: CrI3, CrBr3, and CrGeTe3, and FeCl2, and show that our predictions match well with experimental values. Using the same model we next inves- tigate the magnetic order in two-dimensional (2D) transition-metal-dichalcogenide (TMD) monolayers: MoS2, MoSe2, MoTe2, WSe2 , and WS2 substitutionally doped with period-four transition-metals (Ti, V, Cr, Mn, Fe, Co, Ni). We show that five distinct magnetically or- dered states can exist among the 35 distinct TMD-dopant combinations including the non- magnetic (NM), the ferromagnetic (FM) with out-of-plane spin polarization (Z FM), the out-of-plane polarized clustered FMs (clustered Z FM), the in-plane polarized FMs (X–Y FM), and the anti-ferromagnetic (AFM) state. Most remarkably, we find from our study that V-doped MoSe2 and WSe2, and Mn-doped MoS2, are the most suitable candidates for realizing a room-temperature FM at a 16–18% atomic substitution. We then compare three first-principles methods (the MC, the Green’s function, and the RNSW) of calculating the Curie temperature in 2D FMs in the presence of exchange anisotropy, modeled using the Heisenberg model. We find that the Curie temperature obtained from the Green’s function in high-anisotropy regimes is higher compared to MC, whereas the Curie temperature cal- culated using the renormalized spin-waves (RNSW) is lower compared to the MC and the Green’s function for all anisotropies. Finally, we present a theoretical model to simulate spin- dynamics and spin-induced switching in a semiconductor-ferromagnet heterostructure. Our theoretical model combines the non-equilibrium Green’s function method for spin-dependent electron transport and time-quantified Monte-Carlo for simulating magnetization dynamics. We use the adiabatic approximation for combining the electron dynamics and the magne- tization dynamics. We study spin-induced switching in a 2D TI-FM interface. Finally, we show that for a certain range of magnetic exchange parameters (or certain materials), it is possible to change magnetic domains in a 2D FM using spin-torque from TIs, which can be used for designing high-speed memories.
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.
TBD
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.
Secrecy and covertness in the presence of multi-casting, channel state information, and cooperative jamming
(2021-12-01T06:00:00.000Z) ZivariFard, Hassan; Nosratinia, Aria; Bloch, Matthieu R.; Fischetti, Massimo V.; Fonseka, John P.; Makris, Yiorgos; Minn, Hlaing
We study secret communication over multi-transmitter multicast problem in the presence of an eavesdropper, wherein weak and strong secrecy regimes are studied. For the weak secrecy regime, the method of Chia and El Gamal is extended to two transmitters. We show that the achievable region calculated for the weak secrecy regime in this channel configuration is no bigger than the one calculated under strong secrecy. Two examples are presented in which the inner and outer bounds of secrecy region meet. In the process, we also characterize the minimum amount of randomness necessary to achieve secrecy in the multiple-access wiretap channel. We consider the problem of covert communication over a state-dependent channel when the Channel State Information (CSI) is available either non-causally, causally, or strictly causally, either at the transmitter alone, or at both transmitter and receiver. In contrast to previous work, we do not assume the availability of a large shared key at the transmitter and legitimate receiver. Instead, we only require a secret key with negligible rate to bootstrap the communication and our scheme extracts shared randomness from the CSI in a manner that keeps it secret from the warden, despite the influence of the CSI on the warden’s output. When CSI is available at the transmitter and receiver, we derive the covert capacity region. When CSI is only available at the transmitter, we derive inner and outer bounds on the covert capacity. We also provide examples for which the covert capacity is positive with knowledge of CSI but is zero without it. We consider the problem of covert communication in the presence of a cooperative jammer. It is known that in general, a transmitter and a receiver can communicate only O( √ n) covert bits over n channel uses, i.e., zero rate. Here, we show that a cooperative jammer can facilitate the communication of positive covert rates, subject to the presence of friendly jammer in the environment. We consider various scenarios in which it is possible to achieve positive rate for covert communication. For these scenarios, we derive inner and outer bounds on the covert capacity region, and also we characterize the covert capacity region for some of these scenarios.
Signatures of Dynamic Screening in Interfacial Thermal Transport of Graphene
Ong, 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.
Structural, Electronic, and Transport Properties of Silicane Nanoribbons
Kim, 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.