P. Dollfus

Université Paris-Saclay, Lutetia Parisorum, Île-de-France, France

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Publications (241)299.82 Total impact

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    ABSTRACT: In this work, we investigate the transport properties of devices made of graphene strained heterochannels. Due to the effects of local strain on the band structure, the Klein tunneling is strongly suppressed and transport gaps can appear in the unstrained/strained graphene junctions. The gap regions can be modulated in k-space and in energy by strain and doping engineering, respectively. We show that these effects can be exploited to achieve a strong negative differential conductance (NDC) in single gate-induced barrier structures and in p–n junctions. When the local strain is suitably applied, the peak-to-valley ratio (PVR) of the current-voltage characteristics can be as high as a few hundred. The dependence of NDC effect on structure parameters is investigated systematically. In particular, a strong NDC is obtained in single barrier structures with large strained region, while the PVR is not strongly sensitive to the transition length in p–n junctions.
    No preview · Article · Dec 2015 · Journal of Applied Physics
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    ABSTRACT: To study the thermal effect in nano-transistors, a simulator solving self-consistently the Boltzmann transport equations for both electrons and phonons has been developed. It has been used to investigate the self-heating effects in a 20 nm-long double-gate MOSFET (Fig. 1). A Monte Carlo solver for electrons is coupled with a direct solver for the steady-state phonon transport. The latter is based on the relaxation time approximation. This method is particularly efficient to provide a deep insight of the out-of-equilibrium thermal dissipation occurring at the nanometer scale when the device length is smaller than the mean free path of both charge and thermal carriers. It allows us to evaluate accurately the phonon emission and absorption spectra in both real and energy spaces.
    Full-text · Article · Dec 2015 · Journal of Computational Electronics
  • Van-Truong Tran · Jérôme Saint-Martin · Philippe Dollfus
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    ABSTRACT: The thermoelectric properties of in-plane heterostructures made of Graphene and hexagonal boron nitride (BN) have been investigated by means of atomistic simulation. The heterostructures consist in armchair graphene nanoribbons to the sides of which BN flakes are periodically attached. This arrangement generates a strong mismatch of phonon modes between the different sections of the ribbons, which leads to a very small phonon conductance, while the electron transmission is weakly affected. In combination with the large Seebeck coefficient resulting from the BN-induced bandgap opening or broadening, it is shown that large thermoelectric figure of merit ZT > 0.8 can be reached in perfect structures at relatively low Fermi energy, depending on the graphene nanoribbon width. The high value ZT = 1.48 may even be achieved by introducing appropriately vacancies in the channel, as a consequence of further degradation of the phonon conductance.
    No preview · Article · Nov 2015 · Nanotechnology
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    Van-Truong Tran · Jérôme Saint Martin · Philippe Dollfus
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    ABSTRACT: The thermoelectric properties of in plane heterostructures made of Graphene and hexagonal Boron Nitride (BN) have been investigated by means of atomistic simulation. The heterostructures consist in armchair graphene nanoribbons to the sides of which BN flakes are periodically attached. This arrangement generates a strong mismatch of phonon modes between the different sections of the ribbons, which leads to a very small phonon conductance, while the electron transmission is weakly affected. In combination with the large Seebeck coefficient resulting from the BN-induced bandgap opening or broadening, it is shown that large thermoelectric figure of merit ZT > 0.8 can be reached in perfect structures at relatively low Fermi energy, depending on the graphene nanoribbon width. The high value ZT = 1.48 may even be achieved by introducing appropriately vacancies in the channel, as a consequence of further degradation of the phonon conductance.
    Full-text · Article · Aug 2015
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    Viet Hung Nguyen · Philippe Dollfus
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    ABSTRACT: By means of atomistic tight-binding calculations, we investigate the transport properties of vertical devices made of two incommensurately misoriented graphene layers. For a given transport direction (Ox-axis), we define two classes of rotated graphene lattice distinguished by difference in lattice symmetry and, hence, in Brillouin zone. In particular, these two classes correspond to two different cases where the position of their Dirac cones in the k y -axis is determined differently, i.e. or (L y is the periodic length along the Oy axis). As a consequence, in devices made of two layers of different lattice classes, the misalignment of Dirac cones between the left and right graphene sections opens a finite energy-gap of conductance that can reach a few hundreds of meV. We also show that strain engineering can be used to further enlarge the transport gap and to diminish the sensitivity of the gap on the twist angle and on the commensurateness of the layer stack.
    Preview · Article · Jun 2015 · Journal of Physics D Applied Physics
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    ABSTRACT: In this work, we investigate the possibility of enhancing the thermoelectric power (Seebeck coefficient) in graphene devices by strain and doping engineering. While a local strain can result in the misalignment of Dirac cones of different graphene sections in the k-space, doping engineering leads to their displacement in energy. By combining these two effects, we demonstrate that a conduction gap as large as a few hundreds meV can be achieved and hence the enhanced Seebeck coefficient can reach a value higher than 1.4 mV/K in graphene doped heterojunctions with a locally strained area. Such hetero-channels appear to be very promising for enlarging the applications of graphene devices as in strain and thermal sensors.
    Full-text · Article · May 2015 · Physica E Low-dimensional Systems and Nanostructures
  • V Talbo · J Mateos · S Retailleau · P Dollfus · T González
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    ABSTRACT: By means of three-dimensional self-consistent simulation, we investigate the time- and frequency-dependent shot noise in multi-level quantum dot (QD)-based double-tunnel junction by analyzing auto-correlation functions and distribution of waiting times between consecutive tunnel events through a given barrier. We derive analytic expressions for correlation and waiting time distributions (WTDs) in the case of a maximum of two electrons in the QD. We separate the contributions of the different evolution paths of the number of electrons in the dot between two consecutive current pulses, called ‘basic paths’. The close relation revealed between probabilities, WTDs and correlation functions associated to basic paths allows a good understanding of the specific dynamics in spectral densities. The analytic results show a perfect agreement with those obtained from numerical Monte-Carlo simulation.
    No preview · Article · May 2015 · Semiconductor Science and Technology
  • V. Talbo · J. Mateos · S. Retailleau · P. Dollfus · T. Gonzalez
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    ABSTRACT: In this work, we investigate and explain the time-dependent behavior of shot noise in Silicon quantum dot-based double-tunnel junctions by means of a three-dimensional self-consistent simulation and a Monte-Carlo algorithm following the time evolution of the system. We demonstrate the strong link between autocorrelation functions and electron waiting time distributions, i.e, the time between two consecutive tunnel events through a given junction. Moreover, we separate and analyze the contribution of each different path - evolution of the number of electrons in the quantum dot between two consecutive tunnel events through the same junction - to understand clearly the behavior of auto-correlations and waiting time distributions in the case of a 3-state system.
    No preview · Article · Apr 2015
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    Philippe Dollfus · Viet Hung Nguyen · Jérôme Saint-Martin
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    ABSTRACT: The thermoelectric properties of graphene and graphene nanostructures have recently attracted significant attention from the physics and engineering communities. In fundamental physics, the analysis of Seebeck and Nernst effects is very useful in elucidating some details of the electronic band structure of graphene that cannot be probed by conductance measurements alone, due in particular to the ambipolar nature of this gapless material. For applications in thermoelectric energy conversion, graphene has two major disadvantages. It is gapless, which leads to a small Seebeck coefficient due to the opposite contributions of electrons and holes, and it is an excellent thermal conductor. The thermoelectric figure of merit ZT of a two-dimensional (2D) graphene sheet is thus very limited. However, many works have demonstrated recently that appropriate nanostructuring and bandgap engineering of graphene can concomitantly strongly reduce the lattice thermal conductance and enhance the Seebeck coefficient without dramatically degrading the electronic conductance. Hence, in various graphene nanostructures, ZT has been predicted to be high enough to make them attractive for energy conversion. In this article, we review the main results obtained experimentally and theoretically on the thermoelectric properties of graphene and its nanostructures, emphasizing the physical effects that govern these properties. Beyond pure graphene structures, we discuss also the thermoelectric properties of some hybrid graphene structures, as graphane, layered carbon allotropes such as graphynes and graphdiynes, and graphene/hexagonal boron nitride heterostructures which offer new opportunities. Finally, we briefly review the recent activities on other atomically thin 2D semiconductors with finite bandgap, i.e. dichalcogenides and phosphorene, which have attracted great attention for various kinds of applications, including thermoelectrics.
    Full-text · Article · Mar 2015 · Journal of Physics Condensed Matter
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    Li Su · Weisheng Zhao · Youguang Zhang · Damien Querlioz · JO Klein · Philippe Dollfus
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    ABSTRACT: In this work, we present a graphene-based all-spin logic gate (G-ASLG) that integrates the functionalities of perpendicular anisotropy magnetic tunnel junctions (p-MTJs) with spin transport in graphene-channel. It provides an ideal integration of logic and memory. The input and output states are defined as the relative magnetization between free layer and fixed layer of p-MTJs. They can be probed by the tunnel magnetoresistance and controlled by spin transfer torque effect. Using lateral non-local spin valve, the spin information is transmitted by the spin-current interaction through graphene channels. By using a physics-based spin current compact model, the operation of G-ASLG is demonstrated and its performance is analyzed. It allows us to evaluate the influence of parameters, such as spin injection efficiency, spin diffusion length, contact area, the device length, and their interdependence, and to optimize the energy and dynamic performance. Compared to other beyond-CMOS solutions, longer spin information transport length (∼μm), higher data throughput, faster computing speed (∼ns), and lower power consumption (∼μA) can be expected from the G-ASLG.
    Full-text · Article · Feb 2015 · Applied Physics Letters
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    ABSTRACT: We investigate the effects of uniaxial strain on the transport properties of vertical devices made of two twisted graphene layers, which partially overlap each other. We find that because of the different orientations of the two graphene lattices, their Dirac points can be displaced and separated in the $k-$space by the effects of strain. Hence, a finite conduction gap as large as a few hundred meV can be obtained in the device with a small strain of only a few percent. The dependence of this conduction gap on the strain strength, strain direction, transport direction and twist angle are clarified and presented. On this basis, the strong modulation of conductance and significant improvement of Seebeck coefficient are shown. The suggested devices therefore may be very promising for improving applications of graphene, e.g., as transistors or strain and thermal sensors.
    Full-text · Article · Dec 2014 · Nanotechnology
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    Viet Hung Nguyen · Philippe Dollfus
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    ABSTRACT: By means of atomistic tight-binding calculations, we investigate the effects of uniaxial strain on the electronic bandstructure of twisted graphene bilayer. We find that the bandstructure is dramatically deformed and the degeneracy of the bands is broken by strain. As a conseqence, the number of Dirac cones can double and the van Hove singularity points are separated in energy. The dependence of these effects on the strength of strain, its applied direction and the twist angle is carefully clarified. As an important result, we demonstrate that the position of van Hove singularities can be modulated by strain, suggesting the possibility of observing this phenomenon at low energy in a large range of twist angle (i.e., larger than $10^\circ$). Unfortunately, these interesting/important phenomena have not been clarified in the previous works based on the continuum approximation. While they are in good agreement with available experiments, our results provide a detailed understanding of the strain effects on the electronic properties and may motivate other investigations of electronic transport in this type of graphene lattice.
    Preview · Article · Dec 2014 · 2D Materials
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    Van Truong Tran · Jérôme Saint-Martin · Philippe Dollfus
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    ABSTRACT: We study the properties of edge states in in-plane heterostructures made of adjacent zigzag graphene and BN ribbons. While in pure zigzag graphene nanoribbons, gapless edge states are nearly flat and cannot contribute significantly to the conduction, at BN/Graphene interfaces the properties of these states are significantly modified. They are still strongly localized at the zigzag edges of graphene but they exhibit a high group velocity up to 4.3x10^5 m/s at the B/C interface and even 7.4x10^5 m/s at the N-C interface. For a given wave vector the velocities of N/C and B/C hybrid interface states have opposite signs. Additionally, in the case of asymmetric structure BN/Graphene/BN, a bandgap of about 207 meV is open for sub-ribbon widths of 5 nm. These specific properties suggest new ways to engineer and control the transport properties of graphene nanostructures.
    Full-text · Article · Dec 2014 · Semiconductor Science and Technology
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    ABSTRACT: In this work, we investigate thermoelectric properties of junctions consisting of two partially overlapped graphene sheets coupled to each other in the cross-plane direction. It is shown that because of the weak van-der Waals interactions between graphene layers, the phonon conductance in these junctions is strongly reduced, compared to that of single graphene layer structures, while their electrical performance is weakly affected. By exploiting this effect, we demonstrate that the thermoelectric figure of merit can reach values higher than 1 at room temperature in junctions made of gapped graphene materials, for instance, graphene nanoribbons and graphene nanomeshes. The dependence of thermoelectric properties on the junction length is also discussed. This theoretical study hence suggests an efficient way to enhance thermoelectric efficiency of graphene devices.
    Full-text · Article · Aug 2014 · Applied Physics Letters
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    ABSTRACT: Electro-thermal effects become one of the most critical issues for continuing the downscaling of electron devices. To study this problem, a new efficient self-consistent electron-phonon transport model has been developed. Our model of phonon Boltzmann transport equation (pBTE) includes the decay of optical phonons into acoustic modes and a generation term given by electron-Monte Carlo simulation. The solution of pBTE uses an analytic phonon dispersion and the relaxation time approximation for acoustic and optical phonons. This coupled simulation is applied to investigate the self-heating effects in a 20 nm-long double gate MOSFET. The temperature profile per mode and the comparison between Fourier temperature and the effective temperature are discussed. Some significant differences occur mainly in the hot spot region. It is shown that under the influence of self-heating effects, the potential profile is modified and both the drain current and the electron ballisticity are reduced because of enhanced electron-phonon scattering rates. (C) 2014 AIP Publishing LLC.
    Full-text · Article · Aug 2014 · Journal of Applied Physics
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    Van-Truong Tran · Jérôme Saint-Martin · Philippe Dollfus
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    ABSTRACT: We propose an approach to design efficient electronic switches based on Graphene/BN heterostructures. By means of atomistic Tight Binding simulations, we investigate heterostructures made of an armchair BN nanoribbon sided by two armchair graphene ribbons where a significant bandgap can be opened. In particular, we show that a bandgap of about 0.55 eV can be strongly suppressed by applying a relatively weak transverse electric field of 10 mV/A. Additionally, by using non-equilibrium Green's function simulation, we show that this effect can be used to control the current in electron devices even at small bias and small length. An on/off current ratio higher than 10(4) is achieved at room temperature. (C) 2014 AIP Publishing LLC.
    Full-text · Article · Aug 2014 · Applied Physics Letters
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    ABSTRACT: We propose a Klein tunneling transistor based on the geometrical optics of DFs. We consider the case of a prismatic active region generated by a triangular gate. In this region, total internal reflection may occur, which leads to the controllable suppression of transistor transmission. We study the transmission and the current in this device by means of non-equilibrium Green's function (NEGF) simulation.
    No preview · Conference Paper · Jun 2014
  • J. Larroque · J. Saint-Martin · P. Dollfus
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    ABSTRACT: We show that with a Full-Band dispersion, the specific heat is closer to the experimental value than with an isotropic quadratic dispersion. So we use a Full-Band dispersion in the transport algorithm. A Monte Carlo algorithm has been developed to simulate phonon transport in silicon nanowire. It has been successfully used to simulate the thermal conductivity.
    No preview · Conference Paper · Jun 2014
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    ABSTRACT: Though the energy bandgap of strained graphene remains zero, the shift of Dirac points in the k-space due to strain-induced deformation of graphene lattice can lead to the appearance of a finite conduction gap of several hundreds meV in strained/unstrained junctions with a strain of only a few percent. This conduction gap strongly depends on the direction of applied strain and the transport direction. Here, we study its properties with respect to these quantities. This work provides useful information for further exploiting graphene strained junctions in electronic applications.
    No preview · Conference Paper · Jun 2014
  • V. Talbo · J. Mateos · S. Retailleau · P. Dollfus · T. Gonzalez
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    ABSTRACT: The simulation of a double-tunnel junction with the SENS simulator gives access to the frequency-dependent and static behavior of shot noise. The concept of basic paths in a multi-state process provides a clear interpretation of the noise regimes, and allows locating cut-offs in autocorrelation functions and spectral densities.
    No preview · Conference Paper · Jun 2014

Publication Stats

2k Citations
299.82 Total Impact Points

Institutions

  • 2015
    • Université Paris-Saclay
      Lutetia Parisorum, Île-de-France, France
  • 1970-2015
    • Université Paris-Sud 11
      • Institut d'Electronique Fondamentale
      Orsay, Île-de-France, France
  • 1970-2014
    • French National Centre for Scientific Research
      Lutetia Parisorum, Île-de-France, France
  • 1995-2011
    • Institute of Fundamental Electronics
      Orsay, Île-de-France, France