Search

In the last decade, several works have focused on exploring the material and electrical properties of $\text{GeTe/Sb}_{2}\text{Te}_{3}$ superlattices (SLs) in particular because of some first device implementations demonstrating interesting performances such as fast switching speed, low energy consumption, and non-volatility. However, the switching mechanism in such SL-based devices remains under debate. In this work, we investigate the prototype $\text{GeTe/Sb}_{2}\text{Te}_{3}$ SLs, to analyze fundamentally their electronic and thermal properties by ab initio methods. We find that the resistive contrast is small among the different phases of $\text{GeTe/Sb}_{2}\text{Te}_{3}$ because of a small electronic gap (about 0.1 eV) and a consequent semi-metallic-like behavior. At the same time the out-of-plane lattice thermal conductivity is rather small, while varying up to four times among the different phases, from 0.11 to 0.45 W/m$^{-1}$K$^{-1}$, intimately related to the number of Van der Waals (VdW) gaps in a unit block. Such findings confirm the importance of the thermal improvement achievable in $\text{GeTe/Sb}_{2}\text{Te}_{3}$ super-lattices devices, highlighting the impact of the material stacking and the role of VdW gaps on the thermal engineering of the Phase-Change Memory cell., Comment: 5 pages, 3 figures

The conductance of bulk metal--insulator--metal junctions is evaluated by the Landauer formula using an \textit{ab initio} electronic structure calculated using a plane-waves basis set within density-functional theory (DFT) and beyond, i.e.\ including exact non-local exchange using hybrid functional (HSE) or many-body $G_0W_0$ and COHSEX quasiparticle (QP) schemes. We consider an Ag/MgO/Ag heterostructure model and we focus on the evolution of the zero-bias conductance as a function of the MgO film thickness. Our study shows that the correction of the electronic structure beyond semi-local density functionals goes in the right direction to improve the agreement with experiments, significantly reducing the zero-bias conductance. This effect becomes more evident at larger MgO thickness, that is in increasing tunneling regime. We also observe that the reduction of the conductance seems more related to the correction of wavefunctions rather than energies, and thus not directly related to the correction of the band-gap of bulk MgO. $G_0W_0$ and HSE both provide a correct band-gap in agreement with experiments, but only HSE gives a significant reduction of the conductance. COHSEX, while overestimating the band-gap, gives a reduction of the conductance very close to HSE., Comment: 11 pages, 12 figures

International audience

International audience; Thanks to their thinness, self-passivated surface and large variety, two-dimensional materials have attracted much interest for their possible application in nanoelectronics. In particular, semiconducting transition metal dichalcogenides and their van der Waals heterostructures are very promising for the realization of low-power tunnel fieldeffect transistors. By means of self-consistent quantum transport simulations, we explore the performances of two alternative architectures for the devices: the planar architecture and the vertical architecture. While for the former, which is based on a p-i-n junction, the tunneling occurs laterally within the same two-dimensional material layer, in the latter the tunneling occurs through the vertical heterostructure between two different materials, which are chosen to have a convenient band alignment. Our results enable a comparison of the performance of two architectures in the ideal case, and can serve as a first guide for the choice of the transistor design based on the desired application.

We present first-principles many-body perturbation theory calculations of the quasiparticle electronic structure and of the optical response of HfO$_2$ polymorphs. We use the $GW$ approximation including core electrons by the projector augmented wave (PAW) method and performing a quasiparticle self-consistency also on wavefunctions (QS$GW$). In addition, we solve the Bethe-Salpeter equation on top of $GW$ to calculate optical properties including excitonic effects. For monoclinic HfO$_2$ we find a fundamental band gap of $E_g = 6.33$ eV (with the direct band gap at $E_g^d = 6.41$ eV), and an exciton binding energy of 0.57 eV, which situates the optical gap at $E^o_g = 5.85$ eV. The latter is in the range of spectroscopic ellipsometry (SE) experimental estimates (5.5-6 eV), whereas our electronic band gap is well beyond experimental photoemission (PE) estimates ($< 6$ eV) and previous $GW$ works. Our calculated density of states and optical absorption spectra compare well to raw PE and SE spectra. This suggests that our predictions of both optical and electronic gaps are close to, or at least lower bounds of, the real values., 7 pages, 8 figures, 4 tables

International audience; This paper presents recent progress on Gate-All-Around (GAA) stacked-NanoWire (NW) / NanoSheet (NS) MOSFETs. Key technological challenges will be discussed and recent research results presented. Width-dependent carrier mobility in Si NW/NS and FinFET will be analyzed, and intrinsic performance and design considerations of GAA structures will be discussed and compared to FinFET devices with a focus on electrostatics, parasitic capacitances and different layout options. The results show that more flexibility can be achieved with stacked-NS transistors in order to manage power-performance optimization.

International audience; GAA nanowires (NW) transistors are promising candidates for sub 10 nm technology nodes. They offer optimal electrostatic control, thereby enabling ultimate CMOS device scaling. Horizontally stacked they are a natural extension of today's mainstream technology. Considering enlarged NWs in Nanosheets (NS) allows to target the best compromise in power and performance for future applications. In this paper we will first briefly introduce the technology and then review what can bring advanced simulation focusing on both mobility and contact resistance. Then we will focus on devoted compact modeling fed by both TCAD capturing electrostatics of the Device and above mentioned advanced simulation for mobility. Finally we will demonstrate the capability of the model to capture actual hardware data as well as to benchmark the different architectures in competition down to 5 nm technology node.

We compute the electron and hole mobilities in ultrathin body and buried oxide, fully depleted silicon on insulator devices with various high-\(\kappa \) metal gate-stacks using nonequilibrium Green's functions (NEGF). We compare our results with experimental data at different back gate biases and temperatures. That way, we are able to deembed the different contributions to the carrier mobility in the films (phonons, front and back interface roughness, and remote Coulomb scattering). We discuss the role played by each mechanism in the front and back interface inversion regimes. We draw attention, in particular, to the clear enhancement of electron- and hole-phonons interactions in the films. These results show that FDSOI devices are a foremost tool to sort out the different scattering mechanisms in Si devices, and that NEGF can provide valuable inputs to technology computer aided design.

We discuss the performances of (001) and (110) oriented gate-all-around silicon nanowire (Si NW) transistors within a nonequilibrium Green's functions framework, taking surface roughness and phonon scatterings into account. We show, in agreement with previous studies, that uniaxial tensile (respectively, compressive) strains can significantly improve the mobility of electrons (respectively, holes) in the channel. This does not, however, necessarily result in a comparable enhancement of the device performances. Indeed, the current in short channels is limited by both the scattering and the number of sub-bands available for carrier transport in quantum confined systems (intrinsic “ballistic” resistance). The dependence of the mobility and ballistic resistance on strains can be different, which calls for a careful design of the devices. We show, in this respect, that (110) Si NWs provide the best opportunities for strain engineering in ultimate short channel transistors.

International audience; The introduction of a high-κ/metal gate stack in metal-oxide-Semiconductor field-effect transistors can cause a significant degradation of the mobility, especially at weak inversion densities. This degradation is commonly ascribed to remote Coulomb scattering (RCS, i.e., charges trapped at the SiO$_2$/HfO$_2$ interface). However, very large densities of RCS charges are usually needed to reproduce the experimental data. In this work, we explore alternative scattering mechanisms by quantum calculations of the carrier mobilities. We consider, in particular, metal grain workfunction fluctuations and local dipoles at the SiO$_2$/HfO$_2$ interface. Similarly to RCS, both scattering mechanisms are found to reduce the carrier mobility significantly at low carrier densities. However, the mobility exhibits a different dependence on the thickness of high-κ layer, which provides a way to identify the dominant mechanism.

We compute the contact resistances at low drain bias in Trigate and FinFET devices with widths and heights in the 4 to 24 nm range, using a Non-Equilibrium Green's Function approach. Electron-phonon, surface roughness and Coulomb scattering are taken into account. The analysis of the quasi-Fermi level profile reveals that the areas under the spacers are major contributors to the contact resistance at low field, due to the poor electrostatic control over the carrier density under the spacers. The impact of design parameters (cross-section and doping profile) on the contact resistance is analyzed and the simulations are compared to experimental data. At high drain bias, the contributions of phonons and impurities scattering in the source and drain are also computed and discussed.

This chapter presents a detailed study of wavepacket propagation in various physical situations. This approach provides an intuitive understanding of quantum transport and its semiclassical limit. The chapter addresses the transmission formalism, which treats transport through a small conductor connected to electrodes. It introduces the Landauer–Buttiker formulation of transport, based on the quantum transmission of wavepackets through a conductor connected to electrodes. This formalism is very well suited to the study of electron transport through a small system connected to macroscopic electrodes. The chapter presents a step-by-step introduction to the Green's function method, which allows calculating the quantum transmission efficiently. It highlights the link between the Green's function and wavepacket propagation. Stationary states built from the Green's functions can be viewed as wavepackets with a spatial extension tending to infinity. Simulating transient regimes and noise is of great importance for technological applications.

International audience; In this work, a predictive and physical compact model for NanoWire/NanoSheet (NW/NS) Gate-All-Around (GAA) MOSFET is presented. Based on a novel methodology for the calculation of the surface potential including quantum confinement, this model is able to handle arbitrary NW/NS cross-section shape of stacked-planar and vertical GAA MOSFETs (circular, square, rectangular). This Nanowire Surface Potential (NSP) based model, validated both by numerical simulations and experimental data, is demonstrated to be very accurate in all operation regimes of GAA MOSFETs.

International audience; Multi-gate transistors have attracted considerable attention as a way to overcome the scaling issues of planar MOSFETs. Although the effects of structural confinement on the carrier mobilities have been discussed extensively, the transition from silicon thin films to silicon nanowires (SiNWs) has been little investigated. In this contribution, we perform quantum calculations of the size-dependent carrier mobilities in gate-all-around rectangular SiNWs with leading dimension up to 50 nm, in the non-equilibrium Green's functions (NEGF) framework. We find that when the smallest width or height falls in the sub-10 nm range, nearest neighbor corner channels tend to merge and form "side channels" with much lower mobilities. On top of the numerical results, we have derived a simple model, which bridges square NW devices with thin film devices, and describes the size dependence of the carrier mobilities in rectangular SiNWs in a wide range of dimensions.

Carbon nanotubes (CNTs) have exceptional physical properties that make them one of the most promising building blocks for future nanotechnologies. They may in particular play an important role in the development of innovative electronic devices in the fields of flexible electronics, ultra-high sensitivity sensors, high frequency electronics, opto-electronics, energy sources and nano-electromechanical systems (NEMS). Proofs of concept of several high performance devices already exist, usually at the single device level, but there remain many serious scientific issues to be solved before the viability of such routes can be evaluated. In particular, the main concern regards the controlled synthesis and positioning of nanotubes. In our opinion, truly innovative use of these nano-objects will come from: (i) the combination of some of their complementary physical properties, such as combining their electrical and mechanical properties; (ii) the combination of their properties with additional benefits coming from other molecules grafted on the nanotubes (this route being particularly relevant for gas- and bio-sensors, opto-electronic devices and energy sources); and (iii) the use of chemically- or bio-directed self-assembly processes to allow the efficient combination of several devices into functional arrays or circuits. In this article, we review our recent results concerning nanotube chemistry and assembly and their use to develop electronic devices. In particular, we present carbon nanotube field effect transistors and their chemical optimization, high frequency nanotube transistors, nanotube-based opto-electronic devices with memory capabilities and nanotube-based nano-electromechanical systems (NEMS). The impact of chemical functionalization on the electronic properties of CNTs is analyzed on the basis of theoretical calculations. To cite this article: V. Derycke et al., C. R. Physique 10 (2009).

In this contribution, we present a numerical study of quantum transport in carbon nanotubes based materials. After a brief presentation of the computational approach used to investigate the transport coefficient (Kubo method), the scaling properties of quantum conductance in ballistic regime as well as in the diffusive regimes are illustrated. The impact of elastic (impurities) and dynamical disorders (phonon vibrations) are analyzed separately, with the extraction of main transport length scales (mean free path and localization length), as well as the temperature dependence of the nanotube resistance. The results are found in very good agreement with both analytical results and experimental data, demonstrating the predictability efficiency of our computational strategy. To cite this article: H. Ishii et al., C. R. Physique 10 (2009).

In recent years, the understanding and accurate simulation of carbon nanotube-based devices has become verychallenging. Conventional simulation tools of microelectronics are necessary to envision the performances and useof nanotube transistors and circuits, but the models need to be refined to properly describe the full complexityof such novel type of devices at the nanoscale. Indeed many issues such as contact resistance, low dimensionalelectrostatics and screening effects, as well as nanotube doping or functionalization, demand for more accuratequantum approaches. In this article, we review our recent progress on multiscale simulations which aim at bridgingfirst principles calculations with compact modelling, including the comparison between semi-classical Monte Carloand quantum transport approaches.R´esum´eSimulation Multi-Echelle des Dispositifs `a Nanotube de CarboneCes derni`eres ann´ees, la compr´ehension et la simulation pr´ecise des dispositifs `a base de nanotubes de car-bone est devenue une tˆache ambitieuse. Les outils de simulation conventionnels de la micro´electronique sontn´ecessaires pour imaginer les performances et l’utilisation des transistors et des circuits `a base de nanotubes,mais les mod`eles doivent ˆetre affin´es pour d´ecrire correctement la complexit´e de ces nouveaux types de dispositifs`a l’´echelle nanom´etrique. En effet, de nombreuses questions comme la r´esistance de contact, l’´electrostatique enbasse dimensionalit´e et les effets d’´ecrantage, et le dopage ou la fonctionnalisation des nanotubes, n´ecessitentdes approches quantiques plus pr´ecises. Dans cet article, nous exposons nos progr`es r´ecents pour des simulationsmulti-´echelles qui visent `a connecter des calculs bas´es sur les premiers principes `a la mod´elisation compacte, enpassant par la comparaison entre les approches Monte Carlo semi-classique et de transport quantique.

Two-dimensional graphene, carbon nanotubes and graphene nanoribbons represent a novel class of low dimensional materials that could serve as building blocks for future carbon-based nanoelectronics. Although these systems share a similar underlying electronic structure, whose exact details depend on confinement effects, crucial differences emerge when disorder comes into play. In this short review, we consider the transport properties of these materials, with particular emphasis to the case of graphene nanoribbons. After summarizing the electronic and transport properties of defect-free systems, we focus on the effects of a model disorder potential (Anderson-type), and illustrate how transport properties are sensitive to the underlying symmetry. We provide analytical expressions for the elastic mean free path of carbon nanotubes and graphene nanoribbons, and discuss the onset of weak and strong localization regimes, which are genuinely dependent on the transport dimensionality. We also consider the effects of edge disorder and roughness for graphene nanoribbons in relation to their armchair or zigzag orientation., Comment: 32 pages, 19 figures, to appear in Nano Research

International audience; We investigate remote surface roughness (RSR) scattering by the SiO2/HfO2 interface in Fully Depleted Silicon-on-Insulator devices using Non-Equilibrium Green's Functions. We show that the RSR mobility is controlled by cross-correlations between the surface roughness profiles at the Si/SiO2 and SiO2/HfO2 interfaces. Therefore, surface roughness and remote surface roughness cannot be modeled as two independent mechanisms. RSR tends to enhance the total mobility when the Si/SiO2 interface and SiO2 thickness profiles are correlated, and to decrease the total mobility when they are anti-correlated. We discuss the implications for the high-j/Metal gate technologies. (C) 2015 AIP Publishing LLC.

We discuss the calculation of the carrier mobility in silicon films within the quantum Non-Equilibrium Green's Functions (NEGF) framework. We introduce a new method for the extraction of the carrier mobility that is free from contact resistance contamination, and provides accurate mobilities at a reasonable cost, with minimal needs for ensemble averages. We then introduce a new paradigm for the definition of the partial mobility $\mu_{M}$ associated with a given elastic scattering mechanism "M", taking phonons (PH) as a reference ($\mu_{M}^{-1}=\mu_{PH+M}^{-1}-\mu_{PH}^{-1}$). We argue that this definition makes better sense in a quantum transport framework as it is free from long range interference effects that can appear in purely ballistic calculations. As a matter of fact, these mobilities satisfy Matthiessen's rule for three mechanisms [surface roughness (SR), remote Coulomb scattering (RCS) and phonons] much better than the usual, single mechanism calculations. We also discuss the problems raised by the long range spatial correlations in the RCS disorder. Finally, we compare semi-classical Kubo-Greenwood (KG) and quantum NEGF calculations. We show that KG and NEGF are in reasonable agreement for phonon and RCS, yet not for SR. We point to possible deficiencies in the treatment of SR scattering in KG, opening the way for further improvements.

International audience; This paper reports a theoretical study of the effect of boron and nitrogen codoping on the transport properties of carbon nanotubes (CNTs) at the mesoscopic scale. A new tight-binding parametrization has been set up, based on density functional theory calculations, that enables a reliable description of the electronic structure of realistic BN-doped CNTs. With this model, we have carried out a deep analysis of the electronic mean free path (MFP) exhibited by these nanostructures. The MFP is highly sensitive to the geometry of the scattering centers. We report that the relative distance between B and N atoms in the network influences drastically the electronic conduction. Moreover, we point out that the scattering induced by small hexagonal BN domains in the carbon network is less important than the BN-pair case.

We report fully quantum simulations of realistic models of boron-doped graphene-based field effect transistors, including atomistic details based on DFT calculations. We show that the self-consistent solution of the three-dimensional (3D) Poisson and Schr\"odinger equations with a representation in terms of a tight-binding Hamiltonian manages to accurately reproduce the DFT results for an isolated boron-doped graphene nanoribbon. Using a 3D Poisson/Schr\"odinger solver within the Non-Equilibrium Green's Functions (NEGF) formalism, self-consistent calculations of the gate-screened scattering potentials induced by the boron impurities have been performed, allowing the theoretical exploration of the tunability of transistor characteristics. The boron-doped graphene transistors are found to approach unipolar behavior as the boron concentration is increased, and by tuning the density of chemical dopants the electron-hole transport asymmetry can be finely adjusted. Correspondingly, the onset of a mobility gap in the device is observed. Although the computed asymmetries are not sufficient to warrant proper device operation, our results represent an initial step in the direction of improved transfer characteristics and, in particular, the developed simulation strategy is a powerful new tool for modeling doped graphene nanostructures., Comment: 7 pages, 5 figures, published in ACS Nano

In recent years, the understanding and accurate simulation of carbon nanotube-based transistors has become very challenging. Conventional simulation tools of microelectronics are necessary to predict the performance and use of nanotube transistors and circuits, but the models need to be refined to properly describe the full complexity of such novel type of devices at the nanoscale. Indeed, many issues such as contact resistance, low dimensional electrostatics and screening effects, demand for more accurate quantum approaches. This article reviews recent progresses on multiscale simulations which aim at bridging first principles calculations with compact modelling, including the comparison between semiclassical Monte Carlo and quantum transport approaches. © 2013 Elsevier Ltd. All rights reserved., We acknowledge support from the French National Research Agency, through the ANR/PNANO project ACCENT. A part of this work was supported by the European Community, through Network of Excellence NANOSIL (ICT-216171).

International audience; Mobility in high-k/metal-gate Ultra-Thin Body and Box Fully Depleted SOI devices has been extensively investigated by means of multi-scale simulations and experimental data. Split-CV mobility measurements have been performed for various Interfacial Layer Equivalent Oxide Thickness allowing an investigation of the physical mechanisms responsible for the mobility degradation at highk/ Interfacial layer interface. The impact of the back bias on transport properties is investigated and mobility enhancement in the reverse regime (back gate inversion) is studied. A multi-scale simulation strategy is ranging from quantum Non-equilibrium Green's Functions to semi-classical Kubo Greenwood approach. These advanced solvers made possible a throughout calibration of empirical TCAD mobility models.

We compute the contact resistances $R_{\rm c}$ in trigate and FinFET devices with widths and heights in the 4 to 24 nm range using a Non-Equilibrium Green's Functions approach. Electron-phonon, surface roughness and Coulomb scattering are taken into account. We show that $R_{\rm c}$ represents a significant part of the total resistance of devices with sub-30 nm gate lengths. The analysis of the quasi-Fermi level profile reveals that the spacers between the heavily doped source/drain and the gate are major contributors to the contact resistance. The conductance is indeed limited by the poor electrostatic control over the carrier density under the spacers. We then disentangle the ballistic and diffusive components of $R_{\rm c}$, and analyze the impact of different design parameters (cross section and doping profile in the contacts) on the electrical performances of the devices. The contact resistance and variability rapidly increase when the cross sectional area of the channel goes below $\simeq 50$ nm$^2$. We also highlight the role of the charges trapped at the interface between silicon and the spacer material., 16 pages, 15 figures

The microelectronics activity regroups the study, design, and manufacturing of very small electronic components. These devices are essentially based on interconnected transistors, sort of “switches” which allow controlling the electric current, and are made of semiconductor materials. Depending on the voltage applied to its “gate” electrode, a transistor is in ON state (high current) or OFF state (smallest possible current and low power consumption). Since the invention of the first transistor in 1948, technological progress allowed miniaturizing drastically electronic circuits, and the industry grew fast up to now. For example, the first microprocessor of INTEL (the “4004”) contained 2300 transistors while the Pentium 4 in the early 2000’s got 55 millions of transistors and the dual core more than 150 millions. To have a clear idea on the fast growing of this industry, in the 60’s and 70’s, the number of transistors in integrated circuits was doubled every year. Since the 80’s, the standard rule is a factor 2 every 18 months. This evolution is more known as the “Moore’s law”. Of course, such an industry implies several companies. Microelectronics is become very competitive in performances as well as for economical aspects. The price of 1 million of transistor was 75000 € in 1973, while it was of 6 cents in 2000 then 0.5 cent in 2005. The common objective in microelectronics is so to go ahead with the improvement of transistor in all aspects (electronic performances and economical).

6 páginas.-- et al., We present first-principles transport calculations of graphene nanoribbons with chemically reconstructed edge profiles. Depending on the geometry of the defect and the degree of hydrogenation, spectacularly different transport mechanisms are obtained. In the case of monohydrogenated pentagon (heptagon) defects, an effective acceptor (donor) character results in strong electron−hole conductance asymmetry. In contrast, weak backscattering is obtained for defects that preserve the benzenoid structure of graphene. Based on a tight-binding model derived from ab initio calculations, evidence for large conductance scaling fluctuations are found in disordered ribbons with lengths up to the micrometer scale.

4 páginas, 5 figuras.-- et al., As is common knowledge, armchair graphene nanoribbons (aGNRs) share many electronic features with carbon nanotubes (CNTs). Nevertheless, crucial differences emerge when disorder comes into play. It is thus instructive, both from a theoretical and a technological perspective, to analyze the impact of possible types of disorder on the transport properties of these graphene-based materials. Here we report such a comparative study between CNTs and GNRs, which points out the similarities and differences emerging as a consequence of doping by substitutional boron and nitrogen impurities. The role of edge defects (absent in CNTs) is also contrasted with chemical doping disorder. All disorder models have been derived from accurate ab initio calculations of the electronic structures., B.B. is indebted to the Juan de la Cierva program of the Spanish MICINN for financial support. A.C. acknowledges support from CARNOT graphene from LETI.

We report a first-principles based study of mesoscopic quantum transport in chemically doped graphene nanoribbons with a width up to 10 nm. The occurrence of quasibound states related to boron impurities results in mobility gaps as large as 1 eV, driven by strong electron-hole asymmetrical backscattering phenomena. This phenomenon opens new ways to overcome current limitations of graphene-based devices through the fabrication of chemically-doped graphene nanoribbons with sizes within the reach of conventional lithography., Comment: Nano Letters (in press)

We report on the effect of electron-phonon coupling on quantum transport in carbon nanotubes. The vibrational atomic displacements as well as the electron-phonon coupling strength are introduced through a time-dependent perturbation of the $\ensuremath{\pi}$-electron Hamiltonian. The effect of dephasing on the Kubo conductance is studied for metallic and semiconducting nanotubes, and from a phenomenological law, coherence length (time) scales are found to fluctuate within the range 10 to 150 nm (0.01 to 4 ps) depending on the energy of charge carriers and phonon amplitude.

International audience; We report on the first experimental study of the magnetoresistance of double-walled carbon nanotubes under magnetic field as large as 50 Tesla. By varying the field orientation with respect to the tube axis, or by gate-mediated shifting the Fermi level position, evidences for unconventional magnetoresistance are presented and interpreted by means of theoretical calculations.

We report on theoretical results about contact-dependent effects and tunneling currents through DNA molecules. A tetranucleotide PolyGACT chain, connected in between metallic contacts, is studied as a generic case, and compared to other periodic sequences such as PolyAT or PolyGC. Remarkable resonance conditions are analytically derived, indicating that a strong coupling does not always result in a larger conductance. This result is properly illustrated by considering intrinsic features of bias-dependent tunneling currents in the coherent regime.

For carbon nanotubes, we determine the role of disorder and helicity in the transport length scales and intrinsic conductance. Our results highlight different physical phenomena originating from defect scattering and multishell conduction. Those effects are sensitive to the position of the Fermi level, and allow for a consistent interpretation of recent transport experiments in doped nanotubes., This work was supported by the Micro and Nanotechnology Program from French Ministry of Research under the grant RTB: Post CMOS moléculaire 200 mm. A.R. thanks support from the EC research training network COMELCAN (Grant No. HPRN-CT-2000-00128), Spanish MCyT (Grant No. MAT2001-0946), M-DNA (Grant No. IST-2001-38051), and Universidad del País Vasco (Grant No. 9/UPV 00206.215-13639/2001).

We investigate the properties of electronic states in two and three-dimensional quasiperiodic structures: the generalized Rauzy tilings. Exact diagonalizations, limited to clusters with a few thousands sites, suggest that eigenstates are critical and more extended at the band edges than at the band center. These trends are clearly confirmed when we compute the spreading of energy-filtered wavepackets, using a new algorithm which allows to treat systems of about one million sites. The present approach to quantum dynamics, which gives also access to the low frequency conductivity, opens new perspectives in the analyzis of two and three-dimensional models., Comment: 4 pages, 8 EPS figures

Geometry incommensurability between weakly coupled shells in multi-walled carbon nanotubes is shown to be the origin of unconventional electronic conduction mechanism, power-law scaling of the conductance, and remarkable magnetotransport and low temperature dependent conductivity when the dephasing mechanism is dominated by weak electron-electron coupling.

We present a theoretical study of electron transport in silicon nanowire (SNW). A self-consistent 2D-Poisson-Schrodinger solver provides the band structure. Then, both electron velocity and low-field electron mobility along the SNW axis are computed with an ensemble Monte-Carlo method. Scattering mechanisms due to phonons (acoustic phonons, zero-order and first-order intervalley phonons) and surface roughness are taken into account. We investigate the effect of cross section size and transverse electric field on electron mobility in SNW.

Magnetotransport measurements in large diameter multiwall carbon nanotubes (20-40 nm) demonstrate the competition of a magnetic-field dependent bandstructure and Altshuler-Aronov-Spivak oscillations. By means of an efficient capacitive coupling to a backgate electrode, the magnetoconductance oscillations are explored as a function of Fermi level shift. Changing the magnetic field orientation with respect to the tube axis and by ensemble averaging, allows to identify the contributions of different Aharonov-Bohm phases. The results are in qualitative agreement with numerical calculations of the band structure and the conductance., 4 figures, 5 pages

We report on a theoretical study of quantum transport in carbon nanotubes in the presence of two different sources of scattering: a static short-range random potential that simulates lattice defects, superimposed onto a long-range time-dependent perturbation that mimics the phonon-induced real-space atomic displacements. In the weak-localization regime, fluctuations of the coherent length scales are shown to be driven by band-structure features, whereas the phonon-induced delocalization effect occurs in the stronger-localization regime.

We compute the electron and hole mobilities in Trigate and gate-all-around silicon nanowires (SiNWs) within the nonequilibrium Green’s Function framework. We then derive a simple model for the dependence of the mobility on the SiNW width and height. This model interpolates between the square SiNW and thin film limits. In order to provide a complete description of the mobility in SiNW devices, we calculate the phonon, surface roughness, and remote Coulomb-limited mobilities of square nanowires and of thin films with side or thickness ${t}=5, 7$ , and 10 nm. The mobility of arbitrary rectangular SiNWs with width ${W}={t}$ or height ${H}={t}$ can then be reconstructed from these partial mobilities using Matthiessen’s rule. We show that these models successfully reproduce the trends measured on ${n}$ - and ${p}$ -type devices with different widths and orientations.

Resistive non-volatile memories based on oxides (OxRAM) are recently acquiring a wide interest for their performances, which make them promising candidates as storage memories to replace flash technology, and as embedded memories for neural network applications. Nevertheless, emerging OxRAM devices still present some drawbacks, like non-uniformity of switching parameters and switching failures. Overcoming these drawbacks requires a deeper comprehension of the OxRAM working principles, so far not completely understood. This can be achieved by means of textit{ab initio} simulations. Hence this work presents a careful characterization of HfO₂, which is within the most promising materials to build OxRAM devices, by means of accurate quasi-particle (QP) calculations. A study of the electronic transport properties in OxRAM devices is also of primary importance. However, this requires a robust and reliable theoretical framework to compute the conductance of bulk metal/insulator junctions. The standard approach, based on density functional theory, Green function formalism, and Landauer formula, has some limitations and reliability issues. This work proposes a more reliable approach based on QP calculations, which provide a more accurate electronic structure to compute the conductance, and largely tests this new method on different junctions mimicking OxRAM devices., Les mémoires résistives non volatiles basées sur les oxydes (OxRAM) acquièrent récemment un grand intérêt pour leurs performances, ce qui en fait des candidats prometteurs comme mémoire de stockage pour remplacer la technologie flash, et comme mémoires intégrées pour les applications réseau de neurones. Néanmoins, les dispositifs OxRAM émergents présentent encore certains inconvénients, comme la non-uniformité des paramètres de commutation et les défaillances de commutation. Surmonter ces inconvénients exige une compréhension plus profonde des principes de fonctionnement de l’OxRAM, jusqu’à présent pas complètement compris. Ceci peut être réalisé au moyen de simulations textit{ab initio}. Ce travail présente donc une étude approfondie de HfO₂, qui fait partie des matériaux les plus prometteurs pour la construction de dispositifs OxRAM, au moyen de calculs précis des états de quasi-particules (QP). Une étude des propriétés du transport électronique dans les dispositifs OxRAM est également de première importance. Toutefois, cela nécessite un cadre théorique solide et fiable afin de calculer la conductance des jonctions métal/isolant. L’approche standard, basée sur la théorie fonctionnelle de la densité, le formalisme de la fonction de Green et la formule de Landauer, a quelques limites et soucis de fiabilité. Ce travail propose une approche plus fiable basée sur les calculs QP, qui fournissent une structure électronique plus précise pour calculer la conductance, et teste en grande partie cette nouvelle méthode sur différentes jonctions imitant les dispositifs OxRAM.

One of the most important device for semiconductor industry nowadays is the Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) which is hugely applied in the development of a vast number of electronic applications. The downscaling of MOSFET geometry has been a very successful process to improve the performances of Complementary Metal-Oxide Semiconductor (CMOS) devices. The scaling of transistors dimensions according to scaling rules enabled the performance improvements up to the 90 nm technology node, but the continuous shrinking of MOSFET dimensions faces both physical and economical limitations. In order to overcome these limitations and achieve the performance requirement, several “boosters” have been explored by the semiconductor industries, notably the use of alternative device structures such as “Fully Depleted Silicon On Insulator” (FDSOI), whose architecture has been chosen to be explored in this work.For advanced CMOS technology, robust and predictive electronic transport modeling is a major concern. This PhD work intended to improve the device modeling for ultimate FDSOI devices, with a particular focus on carrier transport. In this scenario, Technological Computer-Aided Design (TCAD) based on Density-Gradient and Drift-Diffusion models arise as a fast and powerful tool to support the technological development within the industry, however we have shown that their accuracy for predicting advanced nodes is often doubtful. In order to overcome this issue, we presented a two-dimensional simulation tool (UTOXPP) based on physical models which makes use of state of the art C++ architecture and accounts for a complete and friendly GUI. By means of Finite-Difference method, we describe a complete modeling strategy for the most important parts of the solver, namely 1.5D Poisson-Schrödinger, Quantum Drift-Diffusion and the mobility models from Kubo-Greenwood formulation and Nonequilibrium Green’s function (NEGF). Simulation results showed the efficiency of UTOXPP for solving electrostatics and the quantum effects for both carrier distribution and transport for the given devices. The objective of this PhD work has been achieved as UTOXPP delivers reliable results for advanced nodes in a timely manner, being an excellent choice for the industrial daily use., Parmi les plus important dispositifs pour l'industrie des semi-conducteurs, le transistor “Metal Oxide Semiconductor Field-Effect Transistor” (MOSFET) est largement utilisé pour le développement d'un grand nombre d'applications électroniques. La miniaturisation de ces dispositifs MOSFET a été un processus très efficace pour améliorer la performance de la technologie “Complementary Metal-Oxide Semiconductor” (CMOS). La mise à l'échelle des transistors selon “scaling rules” a permis l'amélioration des performances jusqu'à nœud technologique 90 nm, mais la diminution continue des dimensions MOSFET fait face à des limitations physiques et économiques. Afin de surmonter ces limitations et de parvenir à l'exigence de performance, plusieurs “boosters” ont été explorées par l’industrie des semi-conducteurs, notamment l'utilisation de dispositifs efficaces tel que "Fully Depleted Silicon On Insulator" (FDSOI), dont l'architecture a été choisie pour être explorée dans ce travail.Pour la technologie CMOS ultime, la modélisation fiable du transport électronique est une préoccupation majeure. Ce travail de thèse vise à améliorer la modélisation de dispositifs ultimes FDSOI, concentré sur le transport des porteurs. Dans ce scénario, “Technological Computer-Aided Design” (TCAD) basé sur des modèles Densité-Gradient et de Dérive-Diffusion se présente comme un outil rapide et puissant pour soutenir le développement technologique dans le secteur technologique. Cependant, nous avons montré que leur précision pour prédire les nœuds avancés est souvent douteuse. Afin de surmonter ce problème, nous avons présenté un outil de simulation à deux dimensions (UTOXPP) basé sur des modèles physiques et qui est implementé sur une efficace architecture C++ avec une bonne interface graphique. Au moyen de la méthode Finite-Difference, nous décrivons une stratégie de modélisation complète pour les parties les plus importantes de ce outil, à savoir 1.5D Poisson-Schrödinger, Quantum Drift-Diffusion et les modèles de mobilité de la formulation Kubo-Greenwood et de la fonction de Green hors equilibrium (NEGF). Les résultats de simulation ont montré l'efficacité de UTOXPP pour résoudre les effets quantiques à la fois pour la distribution de charge et également pour le transport des dispositifs choisis. L'objectif de ce travail de thèse a été réalisée puisque UTOXPP se montré capable de fournir des résultats fiables et rapides pour les nœuds avancés, raison d'être un excellent choix pour l'usage quotidien dans la industrie.