P. Dollfus

Université Paris-Sud 11, Orsay, Île-de-France, France

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Publications (234)283.71 Total impact

  • 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. With a chosen transport direction (Ox-axis), we define two classes of rotated graphene lattice distinguished by the different properties of their lattice symmetry and, hence, Brillouin zone, i.e., the two Dirac cones are located either at the same $k_y$-point ($K_y' = K_y = 0$) or at different $k_y$-points ($K_y' = -K_y = 2\pi/3L_y$, where $L_y$ is the periodic length along the Oy axis). As a consequence, a misalignment of Dirac cones of two layers occurs and a significant energy-gap ($\sim$ a few hundreds of meV) of transmission is achieved in devices made of two layers of different lattice classes. We also shown that strain engineering can be used to strongly enlarge the gap in this type of device.
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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.
    Physica E Low-dimensional Systems and Nanostructures 05/2015; 73. DOI:10.1016/j.physe.2015.05.020 · 1.86 Impact Factor
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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.
    Semiconductor Science and Technology 05/2015; 30(5). DOI:10.1088/0268-1242/30/5/055002 · 2.21 Impact Factor
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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.
    Journal of Physics Condensed Matter 03/2015; 27(13):133204. DOI:10.1088/0953-8984/27/13/133204 · 2.35 Impact Factor
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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.
    Applied Physics Letters 02/2015; 106(7). DOI:10.1063/1.4913303 · 3.52 Impact Factor
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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.
    Nanotechnology 12/2014; 26(11). DOI:10.1088/0957-4484/26/11/115201 · 3.67 Impact Factor
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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.
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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.
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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.
    Applied Physics Letters 08/2014; 105(13). DOI:10.1063/1.4896915 · 3.52 Impact Factor
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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.
    Journal of Applied Physics 08/2014; 116(7):074514. DOI:10.1063/1.4893646 · 2.19 Impact Factor
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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.
    Applied Physics Letters 08/2014; 105(7):073114. DOI:10.1063/1.4893697 · 3.52 Impact Factor
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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.
    2014 International Workshop on Computational Electronics (IWCE); 06/2014
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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.
    2014 International Workshop on Computational Electronics (IWCE); 06/2014
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    ABSTRACT: By means of atomistic Tight Binding simulations, we study heterostructures made of an armchair BN nanoribbon sided by two armchair graphene ribbons where a high band gap can be opened. We show that this band gap can be significantly suppressed by applying a relatively weak transverse electric field. This effect can be used to strongly enhance the on/off current ratio higher in graphene transistors.
    2014 International Workshop on Computational Electronics (IWCE); 06/2014
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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.
    2014 International Workshop on Computational Electronics (IWCE); 06/2014
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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.
    2014 International Workshop on Computational Electronics (IWCE); 06/2014
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    ABSTRACT: We report a numerical study on Aharonov-Bohm (AB) effect and parity selective tunneling in pn junctions based on zigzag graphene nanoribbon rings. We find that when applying a magnetic field to the ring, the AB interference can reverse the parity symmetry of incoming waves and hence can strongly modulate the parity selective transmission through the system. Therefore, the transmission between two states of different parity exhibits the AB oscillations with a \pi-phase shift, compared to the case of states of same parity. On this basis, it is shown that interesting effects such as giant (both positive and negative) magnetoresistance and strong negative differential conductance can be achieved in this structure. Our study thus presents a new property of the AB interference, which could be helpful to further understand the transport properties of graphene mesoscopic-systems.
    Journal of Physics Condensed Matter 05/2014; 26:205301. DOI:10.1088/0953-8984/26/20/205301 · 2.35 Impact Factor
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    ABSTRACT: Today, the availability of high mobility graphene up to room temperature makes ballistic transport in nanodevices achievable. In particular, p-n-p transistors in the ballistic regime give access to Klein tunneling physics and allow the realization of devices exploiting the optics-like behavior of Dirac Fermions (DFs) as in the Veselago lens or the Fabry–Pérot cavity. Here we propose a Klein tunneling transistor based on the geometrical optics of DFs. We consider the case of a prismatic active region delimited by a triangular gate, where total internal reflection may occur, which leads to the tunable suppression of transistor transmission. We calculate the transmission and the current by means of scattering theory and the finite bias properties using non-equilibrium Greenʼs function (NEGF) simulation.
    04/2014; 1(1):011006. DOI:10.1088/2053-1583/1/1/011006
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    ABSTRACT: By means of numerical simulation, in this work we study the effects of uniaxial strain on the transport properties of strained graphene heterojunctions and explore the possibility of achieving good performance of graphene transistors using these hetero-channels. It is shown that a finite conduction gap can open in the strain junctions due to strain-induced deformation of the graphene bandstructure. These hetero-channels are then demonstrated to significantly improve the operation of graphene field-effect transistors (FETs). In particular, the ON/OFF current ratio can reach a value of over 10(5). In graphene normal FETs, the transconductance, although reduced compared to the case of unstrained devices, is still high, while good saturation of current can be obtained. This results in a high voltage gain and a high transition frequency of a few hundreds of GHz for a gate length of 80 nm. In graphene tunneling FETs, subthreshold swings lower than 30 mV /dec, strong nonlinear effects such as gate-controllable negative differential conductance, and current rectification are observed.
    Nanotechnology 03/2014; 25(16):165201. DOI:10.1088/0957-4484/25/16/165201 · 3.67 Impact Factor
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    ABSTRACT: It has been shown in a recent study that unstrained/strained graphene junctions could be promising candidates to improve the poor performance resulting from the gapless nature of the material in graphene devices. Although the energy bandgap of strained graphene still 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 even with a strain of only a few percent. However, since essentially depending on the magnitude of Dirac point shift, such a conduction gap strongly depends on the direction of applied strain and the transport direction. In this work a systematic study of conduction gap properties with respect to these quantities is hence presented and the obtained results are carefully analyzed. Our study thus provides useful information for further investigations to exploit graphene strain junctions in electronic applications.
    Semiconductor Science and Technology 03/2014; 29(11). DOI:10.1088/0268-1242/29/11/115024 · 2.21 Impact Factor

Publication Stats

2k Citations
283.71 Total Impact Points

Institutions

  • 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
  • 1996–2011
    • Institute of Fundamental Electronics
      Orsay, Île-de-France, France
  • 1995
    • France Télécom
      Lutetia Parisorum, Île-de-France, France