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International audience; The understanding of hydrodynamic heat transport in finite-sized graphitic materials remains elusive due to the lack of an efficient methodology. In this paper, we develop a computational framework enabling an accurate description of heat transport in anisotropic graphite ribbons by a kinetic theory approach with full quantum mechanical first-principles input. A unified analysis of the size scaling of the thermal conductivity in the longitudinal and transverse directions of the system is made within the computational framework complemented with a macroscopic hydrodynamic approach. As a result, we demonstrate a strong end effect on the phonon Knudsen minimum, as a hallmark of the transition from ballistic to hydrodynamic heat transports, along a rectangular graphite ribbon with finite length and width. The phonon Knudsen minimum is found to take place only when the ribbon length is ∼5-10 times the upper limit of the width range in the hydrodynamic regime. This paper contributes to a unique methodology with high efficiency and a deeper understanding of the size effect on phonon hydrodynamics, which would open opportunities for its theoretical and experimental investigation in graphitic micro-and nanostructures.

Understanding heat transport at solid-solid interface is crucial for both the fundamentals of thermal science and the engineering applications such as thermal management in semiconductor devices. The role of anharmonic phonon-phonon scattering at interface has been investigated by the classical molecular dynamics (MD) simulation and also by quantum modeling via the non-equilibrium Green’s function (NEGF) formalism recently. However, the investigations on the role of lattice anharmonicity on heat transport at interface remain inconclusive.

Nanophononic metamaterials have broad applications in fields such as heat management, thermoelectric energy conversion, and nanoelectronics. Phonon resonance in pillared low-dimensional structures has been suggested to be a feasible approach to reduce thermal conductivity (TC). In this work, we study the effects of imperfections in pillared nanostructures based on graphene nanoribbons (GNR), using classical molecular dynamics simulations and harmonic lattice dynamics. The TC of perfect pillared GNR is only about 13% of that of pristine GNR due to the strong phonon resonant hybridization in pillared GNR. However, introducing imperfections such as vacancy defects and mass mismatch between the pillars and the base material, and alloy disorder in the pillars, can weaken the resonant hybridization and abnormally increase the TC. We show that both vacancy defects and mass mismatch can reduce the penetration of the resonant modes from the pillars into the base material, while the alloy disorder in the pillars can scatter the phonons inside them, which turns regular resonance into a random one with weaker hybridization. Our work provides useful insight into the phonon resonance mechanisms in experimentally relevant low dimensional nanostructures containing various imperfections.

International audience; The understanding and modeling of inelastic scattering of thermal phonons at a solid/solid interface remain an open question. We present a fully quantum theoretical scheme to quantify the effect of anharmonic phonon-phonon scattering at an interface via nonequilibrium Green's function (NEGF) formalism. Based on the real-space scattering rate matrix, a decomposition of the interfacial spectral energy exchange is made into contributions from local and nonlocal anharmonic interactions, of which the former is shown to be predominant for high-frequency phonons whereas both are important for low-frequency phonons. The anharmonic decay of interfacial phonon modes is revealed to play a crucial role in bridging the bulk modes across the interface. The overall quantitative contribution of anharmonicity to thermal boundary conductance is found to be moderate. The present work promotes a deeper understanding of heat transport at the interface and an intuitive interpretation of anharmonic phonon NEGF formalism.

Hot-carrier solar cells could overcome the Shockley-Queisser limit by having electrons and holes at a higher temperature than the lattice. To generate these hot carriers under concentrated sunlight, the thermalization rate should be as low as possible. Our objective in this presentation is to quantify the influence of different thermalization mechanisms. We determine the carrier temperature in ultrathin GaAs absorbers using continuous-wave photoluminescence and identify distinct surface and volume thermalization contributions. We explain the origin of these contributions using theoretical models involving non-equilibrium LO phonon populations and thermionic emission. We implement these mechanisms in detailed balance calculations for further understanding.

Engineering low-frequency phonon transport in nanostructures with the phonon resonant mechanism has become an important research direction. On the basis of non-equilibrium molecular dynamics simulations, the thermal transport in pristine and resonant Si-membranes bounded with {100}, {110} and {111} facets is investigated. It is found that the creation of surfaces can introduce anisotropic thermal transport due to the lattice symmetry breaking. Besides, ballistic phonon transport is found in pristine membranes with lengths up to 500 nm at low-frequencies with a critical frequency mainly dependent on the crystallographic orientation. Moreover, although surface resonances can dramatically reduce the thermal conductivity of all membranes, the resonant effect strongly relies on membrane orientation. Among the three studied membrane orientations, the resonant effect is maximized in the {111}-membrane, where the thermal conductivity is tuned from the largest one to the smallest one among the three membrane types by resonant pillars. The large thermal conductivity reduction in the {111}-membranes by resonances originated from the reduced spectral heat flux between 3 and 12 THz. Furthermore, the resonant coupling strength can be tuned by the interface vacancy between resonant pillars and the base material, which can enhance phonon transport at an intermediate frequency range. Our work provides further insights on thermal transport engineering by phonon resonances and could be useful for thermal conductivity engineering with surface orientations and resonances.

This theoretical study sheds light on questions raised by inter-subband transition in quantum dot intermediate band solar cells. Based on a dedicated analytical model that correctly treats, from a quantum point-of-view, the trade-off between the absorption, the recombination and the electronic transport, we clearly show that it is essential to control the transit rate between the excited state of the quantum dot and the embedding semiconductor with a tunnel barrier. Such a barrier, matching the recombination and the tunnel rates, allows to strongly improve the current. On the other hand, by better controlling the retrapping, such a barrier can also improve the voltage. Finally this work, by giving a framework to design efficient inter-subband transitions, opens new opportunities for quantum dot intermediate-band solar cells.

The coherent quantum effect becomes increasingly important in the heat dissipation bottleneck of semiconductor nanoelectronics with the characteristic size shrinking down to few nano-meters scale nowadays. However, the quantum mechanical model remains elusive for anharmonic phonon-phonon scattering in extremely small nanostructures with broken translational symmetry. It is a long-term challenging task to correctly simulate quantum heat transport including anharmonic scattering at a scale relevant to practical applications. In this article, we present a clarified theoretical formulation of anharmonic phonon non-equilibrium Green function (NEGF) formalism for both 1D and 3D nanostructures, through a diagrammatic perturbation expansion and an introduction of Fourier representation to both harmonic and anharmonic terms. A parallelized computational framework with first-principle force constants input is developed for large-scale quantum heat transport simulation. Some crucial approximations in numerical implementation are investigated to ensure the balance between numerical accuracy and efficiency. A quantitative validation is demonstrated for the anharmonic phonon NEGF formalism and computational framework by modeling cross-plane heat transport through silicon thin film. The phonon-phonon scattering is shown to be appreciable and to introduce about 20% reduction of thermal conductivity at room temperature even for a film thickness around 10 nm. The present methodology provides a robust platform for the device quantum thermal modeling, as well as the study on the transition from coherent to incoherent heat transport in nano-phononic crystals. This work thus paves the way to understand and to manipulate heat conduction via the wave nature of phonons., Comment: 47 pages, 16 figures

Ballistic transport in monolayer Germanane MOSFETs is investigated for high-performance (HP) applications. Characteristics of both n- and p-type transistors having channel lengths of 7, 5, and 3 nm are studied and compared against the International Technology Roadmap for Semiconductor (ITRS) target of 2028. Our simulation approach is based on a self-consistent quantum ballistic transport model within the framework of the nonequilibrium Green’s function formalism and relies on a single-band and a two-band ${k}\cdot {p}$ Hamiltonian for n- and p-type channels, respectively. We found that, even for a gate length scaled down to 3 nm, the ON current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) in n- and p-MOSFETs for a fixed OFF current ${I}_{ \mathrm{\scriptscriptstyle OFF}} =100\,\,{\text nA/\mu m}$ is as high as ~890 and 700 $\mu \text {A}/\mu \text {m}$ , respectively. For longer channel lengths, the p-MOSFET can outperform the n-MOSFET in terms of ${I}_{ \mathrm{\scriptscriptstyle ON}}$ requirements, as the direct source-to-drain tunneling gets suppressed. Other performance metrics, including gate capacitance, intrinsic switching delay, and switching energy, have also been calculated and found to be comparable to the ITRS 2028 HP technology requirements.

International audience; Anderson localization of thermal phonons has been shown only in few nanostructures with strong random disorder by the exponential decay of transmission to zero and a thermal conductivity maximum when increasing the system length. In this work, we present a path to demonstrate the phonon localization with distinctive features in graded superlattices with short-range order and long-range disorder. A thermal conductivity minimum with system length appears due to the exponential decay of transmission to a non-zero constant, which is a feature of partial phonon localization caused by the moderate disorder. We provide clear evidence of localization through the combined analysis of the participation ratio, transmission, and real-space phonon number density distribution based on our quantum transport simulation. The present work would promote heat conduction engineering by localization via the wave nature of phonons.

Electron-phonon coupling limited transport in phosphorene metal oxide semiconductor field effect transistors (MOSFETs) is studied along the armchair (AC) and zigzag (ZZ) directions. In a multiscale approach, the unit cell of phosphorene is first relaxed, and the band structure is calculated using hybrid density functional theory (DFT). The transport equations are then solved quantum mechanically under the nonequilibrium Green’s function formalism using DFT-calibrated two-band k ⋅ p hamiltonian. The treatment of electron-phonon scattering is done under the self-consistent Born approximation in conjunction with deformation potential theory. It is found that optical phonon modes are largely responsible for degradation of ON-current apart from p-channel AC MOSFET where acoustic phonon modes play a stronger role. It is further observed that electron-phonon scattering is more pronounced in the ZZ direction, whereas the diffusive ON-current of p-MOSFET in a given direction is higher than n-MOSFET. Further study on the complex band structure of phosphorene reveals band wrapping within the bandgap region in the AC direction and multiple crossings in the ZZ direction. This signifies strong phonon-assisted tunneling in the ZZ direction in comparison with the AC direction. For completeness, drain current in the AC tunnel field effect transistor is calculated, and electron-phonon scattering is observed only in the near vicinity of the OFF-current.

The nonequilibrium Green's function formalism is interesting for studying quantum electronic transport in nanostructures, including the treatment of polar optical (PO) phonon scattering, but the latter is usually described within a local approximation. This study uses an analytically derived rescaling technique to describe the interaction between carriers and PO phonons, including the effect of nonlocality. Simulation of double-gate transistors based on $I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I\ensuremath{-}V$ semiconductors shows the approach to be well-defined, very simple to implement, and effective. It seems that $I\phantom{\rule{0}{0ex}}I\phantom{\rule{0}{0ex}}I\ensuremath{-}V$ transistors will not significantly outperform Ge and Si devices, due in part to strong PO phonon scattering.

This paper proposes a theoretical analysis of electronic transport in ultrathin (220 nm) single-junction GaAs solar cell. Using an in-house electronic quantum transport model, we shed light on two detrimental phenomena, namely the “back-diffusion” and the “contact-to-contact diffusion.” While the back-diffusion degrades both the short-circuit current and the fill factor, the contact-to-contact diffusion reduces the open-circuit voltage. The so-called window and back-surface-field barriers used to reflect minority carriers away from contacts reduce these two detrimental phenomena. In a second part, we then show a synthesis of performance optimization of window/GaAs/back-surface-field heterojunctions varying thicknesses, materials, and material composition profiles. Our results conclude that the $\mathrm{\text{Al}_{0.4}\text{Ga}_{0.6}\text{As}(10\; nm)/\text{GaAs}/\text{In}_{0.49}\text{Ga}_{0.51}P(10\; nm)}$ structure provides the best output power.

We investigate the effect of the geometrical model adopted for the gate electrode and for the insulator enveloping the access regions on the full-quantum simulation of ultrascaled nanowire FETs (NW-FETs). We compare the results obtained in the “minimal” geometry commonly used in simulations with those obtained in a more realistic one, able to fully account for the gate fringing effects. We evaluate the impact of the model geometry on the static performance of NW-FETs and discuss the interplay with the surface roughness and the random distribution of dopants. We find that the ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio evaluated in the minimal geometry can be remarkably underestimated in short devices, notably in the case of small length-to-width ratio. The roughness-induced current degradation and the sensitivity to the surface roughness variability can also suffer from nonnegligible underestimations when evaluated in this geometry. Finally, we point out that an inaccurate description of the device electrostatics is expected to result in an overestimation of the sensitivity to the doping-induced variability.

International audience; We study by means of full quantum simulations the operating principle and performance of a semiconductor heterostructure refrigerator combining resonant tunneling filtering and thermionic emission. Our model takes into account the coupling between the electric and thermal currents by self-consistently solving the transport equations within the non-equilibrium Green's function framework and the heat equation. We show that the device can achieve relatively high cooling power values, while in the considered implementation, the maximum lattice temperature drop is severely limited by the thermal conductivity of the constituting materials. In such an out-of-equilibrium structure, we then emphasize the significant deviation of the phonon temperature from its electronic counterpart which can vary over several hundred Kelvin. The interplay between those two temperatures and the impact on the electrochemical potential is also discussed. Finally, viable options toward an optimization of the device are proposed.

Based on the nonequilibrium Green's function formalism, we show a numerically efficient method to treat inelastic scattering in multidimensional atomistic codes. Using a simple rescaling approach, we detail the calculations of the lowest-order approximation (LOA) [Y. Lee et al., Phys. Rev. B 93, 205411 (2016)] series to the usual, computationally intensive, self-consistent Born approximation (SCBA). This, combined with the analytic continuation technique of Pad\'e approximants, is applied to an atomistic code based on a tight-binding $s{p}^{3}{d}^{5}{s}^{*}$ model for electrons and holes, and a modified valence-force-field method for phonons. Currents in Si and Ge gate-all-around nanowire transistors are then computed considering the main crystallographic transport directions ($\ensuremath{\langle}100\ensuremath{\rangle}$, $\ensuremath{\langle}110\ensuremath{\rangle}$, $\ensuremath{\langle}111\ensuremath{\rangle}$) for both $n$-type and $p$-type devices. Our results show that in most configurations, third-order LOA currents are enough to achieve a high agreement with SCBA results, while reducing the calculation time by about one order. In addition, we propose a criterion to determine the validity of such expansion techniques.

International audience; We theoretically investigate GaN/InGaN/GaN tunnel junctions grown along the wurtzite c-axis. We developed a dedicated quantum electronic transport model based on an 8-band k.p Hamiltonian coupled to the non-equilibrium Green's function formalism. We first show that the transmission is dominated by quantum states localized at the heterojunction. We also confirm that, for a thin InGaN layer, current strongly increases with doping. On the other hand, for thick InGaN layers (> 8 nm), our results show an unexpected low impact of doping on current. In this latter case, the spontaneous and the piezoelectric polarizations reduce the tunnel-barrier width to the InGaN layer thickness. We conclude that quantum electronic transport in such tunnel junctions is mainly controlled by interfaces with both polarizations and localized states. Published by AIP Publishing.

International audience; Due to the constant size reduction, single-donor-based nanowire transistors receive an increasing interest from the semi-conductor industry. In this work we theoretically investigate the coupled influence of electron-phonon scattering, temperature and size (cross-section and channel length) on the properties of such systems. The aim is to determine under what conditions the localized character of the donor has a remarkable impact on the current characteristics. We use a quantum non-equilibrium Green's function approach in which the acoustic electron-phonon scattering is treated through local self-energies. We first show how this widely used approach, valid at high temperatures, can be extended to lower temperatures. Our simulations predict a hysteresis in the current when reducing the temperature down to 150 K. We also find that acoustic phonons degrade the current characteristics while their optical counterparts might have a beneficial impact with an increase of the ON-current. Finally we discuss the influence of nanowire length and cross-section and emphasize the complexity of precisely controlling the dopant level at room temperature. (C) 2016 Elsevier Ltd. All rights reserved.

Based on electronic quantum transport modeling, we study the transition between the intermediate-band and the conduction-band in nano-structured intermediate-band solar cell. We show that a tunnel barrier between the quantum well (QW) and the host material could improve the current. The confinement generated by such a barrier favors the inter-subband optical coupling in the QW and then changes the excitation-collection trade-off. More surprisingly, we also show that tunneling impacts the radiative recombination and then the voltage. Using a detailed balance model we explain and we propose a broadening factor for this Voc modification. Finally we show that a thin tunnel barrier is beneficial for both current and voltage.

This study presents ionized impurity impacts on silicon nanowire MOS transistors. We calculate the current characteristics with a self-consistent three-dimensional (3D) Green’s function approach and show the effects of both acceptor and donor impurities on the physical electron properties. In particular, we emphasize that the presence of a donor induces different transport phenomena according to the applied gate bias. Our results show that the influence of a single impurity strongly depends on its position and induces high transistor performance variability with current modifications from 50% to two orders of magnitude.

This study presents two relevant effects influencing the electronic transport of nanowire transistors. We first focus on the ionized impurity impacts and calculate the current characteristics with a self-consistent three-dimensional (3D) Green’s function approach. The results show the effects of both acceptor and donor impurities on the physical electron properties. In particular, we emphasize that the presence of a donor induces different transport phenomena according to the applied gate bias. In a second part, we report a numerical study of the self-energy correction due to correlation effects from dynamic screening of the moving electron in silicon nanowire transistors. This many-body effect, which is not included in the usual Hartree approximation, is then incorporated self-consistently into a non-equilibrium Green’s function (NEGF) code. The results pinpoint the importance of dielectric confinement whose magnitude can not be neglected compared to its quantum counterpart in ultimate nanowire transistors.

This work will review the development of 2D/3D device simulators based on the Non-Equilibrium Green Function (NEGF) formalism, which has been developed in order to study variability issues. We have concentrated on the variability deriving from the particular arrangement of dopants, unintentional defects and roughness of the Si/SiO2 interface. The device simulators developed have the capability to address very strong local potential inhomogeneities that stem from discrete dopants. We have implemented volume discretisation techniques that allow us a more precise representation of interfaces and dopant localization. The simulators have been parallelized and recursive algorithms have been implemented in order to reduce the computational time, which is a killer effect in the Poisson-NEGF loop in a 3D quantum simulator. The simulator is designed for an effective mass tensor approximation. Finally, we have carried out studies of the performance of nanotransistors due to the effect of the discreteness of dopant and interface roughness

3C-SiC is a promising material for high power and high-speed electronic devices as well as in sensors operating at high temperatures or hostile environments. For these reasons, we solved self-consistently the Poisson equation within the quantum Non Equilibrium Green Function Formalism (NEGF) in order to model and compare 3C-SiC and Si nanowire (NW) Field Effect Transistors (FETs) operating in ballistic regime (at room temperature 300 K). As a general conclusion of our calculations, Si and SiC NW FETs have almost the same electrical behavior: they depict the same subthreshold slope and have similar on currents [ION/IOFF (SiC)~81 % ION/IOFF (Si) in case of 4 nm NW cross section side].

A method for simultaneous nondestructive evaluation of WC-Co coating thickness, elastic moduli, and density is presented. The technique, known as laser-ultrasonics, is used to generate and detect surface acoustic waves in a noncontact and nondestructive manner. The surface acoustic wave velocity dependence on frequency is compared to a model and an optimization procedure is used to evaluate the coating properties. The results obtained demonstrate the ability of the technique to simultaneously determine such properties with a single and possibly in situ measurement.

We theoretically investigate the impact of uniaxial strain in extremely thin Si and Ge p-type double gate transistors. Quantum transport modeling is treated using a 6-band k.p Hamiltonian and the non-equilibrium Green's function formalism including hole-phonon scattering. Based on this framework we analyze the influence of strain on current characteristics considering different transport directions and gate length's. The results first confirm the dominance of Ge in long devices (15 nm gate length) for which best electrical performances are obtained for channels along with a uniaxial compressive strain. Situation is reversed for shorter devices (7 nm gate length) where the small effective masses of Ge deteriorate the off-regime of transistors regardless the considered strain. Due to weaker hole-phonon scattering, Si devices with a tensile strain are interestingly found to be more competitive than their -compressive counterparts.

In this work we investigate the dependence of electron-phonon scattering in single dopant-based nanowire transistor with respect to temperature and dimensions. We use a 3D real-space non-equilibrium Green’s function (NEGF) approach where electron-phonon scattering is treated within the selfconsistent Born approximation (SCBA) through self-energies. We also use an analytic model to extend the validity of the acoustic phonon self-energy at low temperatures. Based on this model our simulations show the presence of a current hysteresis when reducing the temperature down to 150 K. The influence of channel length and nanowire cross-section on the dopant level contribution to the current is also discussed.

This theoretical work analyzes the photo-generation and the escape of carrier in InGaN/GaN core-shell nanowires. Our electronic transport model considers quantum behaviors such as confinement, tunneling, electron-phonon scattering and electron-photon interactions. The large lattice mismatch between InN and GaN requires the use of multiple quantum well design, in which either In content or well thickness is limited. Since thick GaN barriers are required in these stressed devices, we show that tunneling has a negligible impact on carrier escape, which is mostly achieved by the phonon absorption. This thermionic escape strongly depends on the quantum confinement. Our conclusions demonstrate that a thick quantum well with a low In content, in which the confinement is moderate, is more efficient.

This chapter is devoted to the theory and modeling of the main physical phenomena occurring in semiconductor nanowires. As nanowire represents a central structure for nanoelectronic applications, most of the discussed physical phenomena will be presented in the scope of nanowire transistors. The chapter will first introduce the quantum nonequilibrium Green's function formalism, which represents one of the most suited techniques to treat transport in nanostructures. Based on this formalism, we will focus on three principal effects: (1) the interface-induced correlation effects, (2) the influence of single dopant atoms for both electron and hole transport, and (3) the phonon-scattering and self-heating effects.

4th European Workshop on Ultimate Integration of Silicon (ULIS 2003), Udine, ITALY, MAR, 2003; International audience; In this work, we present an atomic-scale modeling of a single conduction channel double-gate MOSFET based on the self-consistent solving of the two-dimensional Poisson equation with the Schrodinger equation. This later, expressed in tight-binding, is solved using the Green's function formalism in the axial part of the device (i.e. the source-channel-drain region) modeled as a single atomic linear chain and sandwiched between two gate dielectrics treated as perfect insulating media. For device with channel length typically below 10 nm, our results show that source-to-drain tunneling effect and electron reflection in the channel severely impact the device characteristics. Nevertheless, essential field-effect transistor behaviors are preserved, which demonstrates that the operation of double-gate devices can be still considered even at this ultimate limit of a single conduction channel depicted at the atomic-scale. (C) 2003 Elsevier Ltd. All rights reserved.

Metal oxide semiconductor field-effect transistors with channel lengths in the nanometer range have recently been the subject of many new modeling and simulation approaches. Unfortunately, semi-classical methods currently used in microelectronics are not suitable to describe accurately all physical effects when device integration approaches atomic scale. The aim of this study is to compare theoretical results obtained from different atomistic approaches with those classically deduced with the effective mass approximation. We calculated the transmission coefficient in the simple case of linear atomic chains or of several coupled chains by successively considering: (i) the effective mass approximation, (ii) a direct calculation of semi-infinite chains transmission using a simple one band tight binding approximation, (iii) the diffusion theory in the Green’s functions formalism expressed in the frame of tight binding. This last method offers several advantages since it is able to capture the essential physics of nanoscale devices (electron–electron and electron–phonon interactions). Finally, we summarize the discussion of the extension of this last approach to realistic devices.

A three-dimensional self-consistent non-equilibrium Green's function approach is used to investigate the influence of phonon scattering in single dopant nanowire transistors. Phonon interactions are described within the self-consistent Born approximation in which both acoustic and optical phonons are included. Transport properties are then analyzed in the ballistic and scattering regimes. Ballistic results first confirm the current hysteresis due to two different screening mechanisms of the dopant reported by Mil'nikov et-al [1]. The transition between them is smoothed by the interactions with acoustic phonons which suppress the current hysteresis. Interestingly our findings also show a beneficial impact of the optical phonon interactions. They generate a phonon-assisted resonant tunneling from which can result a higher current than in the ballistic regime. Finally a temperature dependance analysis shows that the hysteresis should be restored at lower temperatures.

Thanks to recent technological developments, single-atom electronics is now conceivable. So far very few works, whether experimental [1] or theoretical [2], focused on new applications with single dopant semiconductor devices. In that context, simulations performed by Mil'nikov et-al predicted a remarkable intrinsic bistability in single dopant nanowire Metal-Oxide-Semiconductor (MOS) transistor [3]. This bistability results from two distinct screening mechanisms of the Coulomb potential impurity and could lead to relevant applications. However the study only considers the ballistic regime and optical-phonon interactions in the scope of the resonant tunneling model [4].

Phonon scattering can drastically influence transport properties of nanodevices. From a simulation point of view, the non-equilibrium Green's function formalism provides a natural way to include inelastic scattering in quantum transport codes, by means of self-energies. Phonon scattering is usually treated with the so-called self-consistent Born approximation which involves the evaluation of the SCBA self-energy together with the electrostatic potential; a computationally expensive self-consistent procedure. In this work we present an alternative one-shot current conserving method to treat phonon scattering and apply it to the modeling of silicon n-type nano-wire and p-type double-gate MOSFETs.

We propose an alternative approach to self-consistency and conservation laws in the theory of nonequilibrium Green's functions (NEGF's), which provides an infinite family of conserving but, generally, non-self-consistent approximations. Within any $\ensuremath{\Phi}$-derivable approximation the associated Born series for the NEGF is shown to be conserving. Expectation values calculated from the Born series are then used to build a Pad\'e table of approximations, while conservation laws are naturally preserved. We implement this technique for the $\ensuremath{\Phi}$-derivable self-consistent Born approximation (SCBA), for which we obtain a recursion relation that yields the Born series for the NEGF up to any desired order. The expectation values calculated from the Born series are then postprocessed to build a Pad\'e table of conserving approximations. The calculation of the SCBA photocurrent in a biased molecular junction model provides an example where, in addition to conservation laws, a substantial convergence acceleration relative to standard techniques is achieved. The present reformulation of the SCBA might aid convergence to the fully self-consistent results in a wide variety of problems.

International audience; We describe a lowest-order approximation (LOA) to the nonequilibrium Green's function in the presence of interactions, and generally address how one can build-derivable one-shot approximations that satisfy the continuity equation. These approximations produce conserved electronic currents in one shot, requiring only one self-energy evaluation and, when applicable, they are as accurate as but much faster than the corresponding self-consistent approximation. This challenges the currently adopted view that heavy self-consistent calculations are necessary to get a satisfactory prediction of transport in nanoscale structures. We illustrate this with the case of electron-phonon scattering expressed within the self-consistent Born approximation (SCBA). In the LOA, the SCBA is further approximated by accounting only for one-phonon processes. LOA and SCBA are compared in one-dimensional wire where electrons interact with one optical phonon mode at room temperature. The LOA is found to provide a considerable reduction in computational time. Its limitations and extensions to include two-phonon processes are discussed.

In this work we present a quantum transport model considering the electron-phonon scattering within the Non-Equilibrium Green Function formalism. The model which explicitly considers transverse mode coupling is applied to study the electronic transport in double-gate transistors. Original underlying behaviors due to transverse-mode coupling related to phonon are then reported.

We have developed a full three-dimensional real-space quantum transport simulator for p-type nanowire field effect transistor. The model is based on a six-band k·p Hamiltonian expressed within the non-equilibrium Green's function formalism. Based on this simulator we have studied the influence of a single donor and acceptor dopant on the transport properties. Numerical calculations show that an acceptor impurity induces both resonant and anti-resonant interferences which degrade the transistor performances.

We address the use of the tight-binding and effective mass approximations (TB and EMA) for the calculation of transport properties in silicon nanowires (SiNWs). Three different transport scenarios are considered within a non-equilibrium Green's function approach: ballistic transport, tunneling under a potential barrier, and transport through a potential well. In the first two cases TB and EMA provide roughly similar conductances. For the potential-well case, however, the EMA approach fails to reproduce fano resonances. These discrepancies are strongly reduced at room temperature.

We reconsider spin-orbit contribution to strain effects in semiconductors within six-valence-band k · p model. We demonstrate that standard procedure for including that contribution is generally wrong. Indeed, we show that taking into account the spin-orbital strain term of the effective hamiltonian does not modify the matrix form of the strain model in terms of deformation components. Consequently, part of the spin-orbital strain interaction is formally included in the a, b and d deformation potential parameters of the conventional Bir-Pikus matrix. This result brings into question the parameter values: do they or not take into account that spin-orbital strain contribution? Moreover, the remaining part, which involves the split-off valence band, is included in a form similar to that of Pidgeon-Brown (set up for P i P j products) using three additional parameters a Δ , b Δ and d Δ (in place of γ1Δ, γ2Δ and γ3Δ). Following this original approach, we propose new parametrized models for Si, Ge, GaAs and InAs materials. Impacts of spin-orbit contribution are presented for Si, Ge, GaAs and InAs bulk and [001] quantum wells.

We have developed a ballistic self-consistent code which couples the six-band k.p Hamiltonian to the Green function formalism. We investigate the influence of channel material (Si, Ge, SiGe and GaAs), crystallographic orientation ([100] and [110]) and strain (biaxial and uniaxial) on the ultimate double-gate pMOSFET performances. The results show that the best configuration is obtained with strained [100]-oriented Si devices.

We report a theoretical investigation of long-range correlation effects induced by the presence of interfaces in realistic semiconductor based nanoscale structures. This is performed within the so-called GW approximation of Hedin and Lundqvist in which we isolate the contribution of the interfaces with dielectrics or metallic electrodes to the exchange-correlation self-energy. We incorporate these correlation effects self-consistently into the solution of the Schr\"odinger equation and calculate its influence on transport properties in realistic nanoscale transistors. Numerical results show that the self-energy correction due to dielectric mismatch can be comparable to the direct quantum confinement effect. With the decrease in size, this correlation effect has a significant impact on the current-voltage characteristics and contributes to the increase in variability in ultimate nanoscale transistors.

We address the transport properties of narrow gate-all-around silicon nanowires in the presence of surface-roughness (SR) scattering at the Si/SiO2 interface, considering nanowire transistors with a cross section of 3 times 3 nm2 and gate length of 15 nm. We present transfer characteristics and effective-mobility calculations based on a full 3-D real-space self-consistent Poisson-Schrodinger solver within the nonequilibrium Green's function formalism. The effect of SR is included via a geometrical method consisting in a random realization of potential fluctuations described via an exponential autocorrelation law. The influence on transfer characteristics and on low-field mobility is evaluated by comparison with the clean case and for different values of the root mean square of potential fluctuations. The method allows us to exactly account for mode-mixing and subband fluctuations and to evaluate the effect of SR up to all orders of the interaction. We find that SR scattering is mainly responsible for positive threshold-voltage shift in the low-field regime, whereas SR-limited mobility slowly depends on the linear charge density, showing the inefficiency of mode-mixing scattering mechanism for very narrow wires.

This study presents ionized impurity impacts on silicon nanowire MOS transistors. We first calculate the current characteristics with a self-consistent three-dimensional (3D) Green's function approach and show the effects of both acceptor and donor impurities on the physical electron properties. In particular, we emphasize that the presence of a donor induces different transport phenomena according to the applied gate bias. Considering an attractive Coulomb potential, we then evaluate the effective mass validity by comparing the localized states of cubic dots with those obtained through a sp 3 third-neighbor tight-binding model. Our results show that in first approximation, the effective mass is still adapted to treat ionized impurities.

We report a numerical study of the self-energy correction due to correlation effects from dynamic screening of the moving electron in silicon nanowire transistors. This many-body effect, which is not included in the usual Hartree approximation, is then incorporated self-consistently into a non-equilibrium Green's function (NEGF) code. The results pinpoint the importance of dielectric confinement whose magnitude can not be neglected compared to its quantum counterpart in ultimate nanowire transistors.

We report a numerical study of both donor- and acceptor doping impurity effects in the quantum transport of silicon nanowire metal-oxide-semiconductor (MOS) transistors. The code is based on a full three-dimensional (3D) real-space non-equilibrium green function (NEGF) formalism self-consistently coupled with the 3D Poisson equation. The general results show that the influence of an impurity strongly depends on its type. Indeed, an acceptor or a donor will create a repulsive or attractive potential, giving rise to tunneling effect or resonances, respectively. Our calculations analyze the impact on electron density, transmission coefficient and drain current (I"D) which undergoes variations up to 50%. This pinpoints the importance of intrinsic fluctuations due to doping in ultimate nano-transistors whose magnitude cannot be neglected in the next generations of integrated circuits.

We present a theoretical method to simulate magnetotransport in silicon nanowire (Si-NW) MOSFET including the effect of Surface Roughness (SR). We use a full three dimensional (3D) real-space self-consistent Poisson-Schrodinger solver based on Non Equilibrium Green’s function Formalism (NEGF) which can treat the influence of an external magnetic field on the device. By comparing magnetoconductance curves with the classical Drude formula we extract magnetoresistance (MR) mobility for nanowires with and without roughness. From the preliminary results it seems that the MR mobility is not dramatically reduced for the SR parameters considered in this work.

The aim of this paper is to study the effect on a Double Gate (DG) MOSFET of unintentional dopants located in the channel region using a ballistic 2D self-consistent Non-equilibrium Green’s function (NEGF) approach. The effect of mass anisotropy and the particular configuration of the unintentional dopant atoms on the I D-V G characteristic have been studied.

The authors report an atomistic study of the ballistic current through silicon nanowire metal-oxide-semiconductor transistors. A self-consistent quantum ballistic transport model is used to calculate the current in gate-all-around nanowire transistors, taking into account the full-band structure of the quantum wire with a sp3 tight-binding approach. The authors demonstrate the occurence of an optimal wire cross section for which the on-state/off-state current ratio is maximum, a result which cannot be obtained in a standard bulk effective mass description.

The subband structure of square Ge 〈100〉-oriented nanowires using a sp 3 tight-binding model is studied. Starting from the bulk Ge structure, we describe the bands obtained in nanowires before showing the dependence of the band-gap energy and the effective masses as a function of the wire thickness. For this orientation, the Ge nanowire properties are found to be very sensitive to transverse confinement. In particular the band-gap goes very close to the one of the silicon nanowire, reducing then the impact of the band-to-band tunneling and making it more suitable for nanoelectrics applications.

This work investigates the conduction band structure of silicon nanowires, its dependence with the wire width and its influences on the electrical performances of Si nanowire-based MOSFET’s working in the ballistic regime. The energy dispersions relations for Si nanowires have been calculated using a sp 3 tight-binding model and the ballistic response of n-channel devices with a 3D Poisson–Schrodinger solver considering a mode-space approach and open boundary conditions (NEGF formalism). Results are compared with data obtained considering the parabolic bulk effective mass approximation, highlighting in this last case the overestimation of the I on current, up to 60% for the smallest (1.36 nm × 1.36 nm Si wire) devices.