arXiv subjects

Philippe Dollfus

Publications and source records attributed to Philippe Dollfus.

At least 19 recordsLinked to original sources

arXiv2025

Quenching statistics in Si and Ge SPADs using particle Monte Carlo simulation

Si- and Ge-based single-photon-avalanche-diodes (SPAD) are investigated by using self-consistent 3D Monte Carlo simulation, in a mixed-mode approach including the presence of a passive quenching circuit. This approach of transport allows us to capture all stochastic features of carrier transport and SPAD operation, not only for the avalanche triggering but also for the quenching process. Beyond the comparison of Si and Ge devices, we show in particular the strong inverse correlation between avalanche and quenching probabilities when tuning the bias voltage, which highlights the importance to find a tradeoff between these two probabilities for the optimization of SPAD operation.

cond-mat.other↗

arXiv2022

Minimum thermal conductance of twisted-layer graphite nanofibers

We study the thermal transport properties of twisted-layer graphite nanofibers. We show that in the presence of a twisted layer, the phonon thermal conductance of a graphite nanofiber varies remarkably with the twisted angle and can reach minimum values either at two critical angles $θ_1$ and $θ_2$ that conform to the rule $θ_1$ + $θ_1$ = $180^0$ or exactly at the angle $θ$ = $90^0$. A reduction of roughly 50% of the phonon thermal conductance can be achieved in some structures. We unveil that the twisting effect mainly influences the optical modes, leaving almost unaltered the acoustic ones. The effect is also visible in the higher and more numerous van Hove singularities of the phonon density of states. We also point out that the behavior of the thermal conductance with the twisted angle is associated with and dominated by the alteration in the overlap area between the twisted and non-twisted layers. The finite-size effect is demonstrated to play an essential role in defining the critical angles at the local minimums, where these angles are dependent on the size of the investigated nanofibers, in particular on the proportion between the widths of zigzag and armchair edges.

cond-mat.mes-hall↗

arXiv2020

A cold-source paradigm for steep-slope transistors based on van der Waals heterojunctions

The availability of transistors capable of operating at low supply voltage is essential to improve the key performance metric of computing circuits, i.e., the number of operations per unit energy. In this paper, we propose a new device concept for energy-efficient, steep-slope transistors based on heterojunctions of 2D materials. We show that by injecting electrons from an isolated and weakly dispersive band into a strongly dispersive one, subthermionic subthreshold swings can be obtained, as a result of a cold-source effect and of a reduced thermalization of carriers. This mechanism is implemented by integrating in a MOSFET architecture two different monolayer materials coupled through a van der Waals heterojunction, combining the subthermionic behavior of tunnel field-effect transistors (FETs) with the robustness of a MOSFET architecture against performance-degrading factors, such as traps, band tails and roughness. A further advantage with respect to tunnel FETs is that only an n-type or p-type doping is required to fabricate the device. In order to demonstrate the device concept and to discuss the underlying physics and the design options, we study through abinitio and full-quantum transport simulations a possible implementation that exploits two recently reported 2D materials.

cond-mat.mes-hall↗

arXiv2020

Electron transport properties of graphene nanoribbons with Gaussian deformation

Gaussian deformation in graphene structures exhibits an interesting effect in which flower-shaped confinement states are observed in the deformed region [Carrillo-Bastos et al., Phys. Rev. B 90 041411 (2014)]. To exploit such a deformation for various applications, tunable electronic features including a bandgap opening for semi-metallic structures are expected. Besides, the effects of disorders and external excitations also need to be considered. In this work, we present a systematic study on quantum transport of graphene ribbons with Gaussian deformation. Different levels of deformation are explored to find a universal behavior of the electron transmission. Using a tight-binding model in combination with Non-Equilibrium Green Functions formalism, we show that Gaussian deformation influences strongly the electronic properties of ribbons in which the electron transmission decreases remarkably in high energy regions even if small deformations are considered. Interestingly, it unveils that the first plateau of the transmission of semi-metallic armchair ribbons is just weakly affected in the case of small deformations. However, significant large Gaussian bumps can induce a strong drop of this plateau and a transport gap is formed. The transmission at the zero energy is found to decrease exponentially with increasing the size of the Gaussian bump. Moreover, the gap of semi-conducting ribbons is enlarged with large deformations. The opening or the widening of the transport gap in large deformed armchair structures is interpreted by a formation of a three-zone behavior along the transport direction of the hopping profile. On the other hand, a transport gap is not observed in zigzag ribbons regardless of the size of Gaussian bumps. This behavior is due to the strong localization of edge states at the energy point E = 0...

cond-mat.mes-hall↗

arXiv2018

High performance Tunnel Field Effect Transistors based on in-plane transition metal dichalcogenide heterojunctions

In-plane heterojunction tunnel field effect transistors based on monolayer transition metal dichalcogenides are studied by means of self-consistent non-equilibrium Green's functions simulations and an atomistic tight-binding Hamiltonian. We start by comparing several heterojunctions before focusing on the most promising ones, i.e WTe2-MoS2 and MoTe2-MoS2. The scalability of those devices as a function of channel length is studied, and the influence of backgate voltages on device performance is analysed. Our results indicate that, by fine-tuning the design parameters, those devices can yield extremely low sub-threshold swings (below 5mV/decade) and Ion/Ioff ratios higher than 1e8 at a supply voltage of 0.3V, making them ideal for ultra-low power consumption.

cond-mat.mes-hall↗

arXiv2017

High thermoelectric performance of graphite nanofibers

Graphite nanofibers (GNFs) have been demonstrated to be a promising material for hydrogen storage and heat management in electronic devices. Here, by means of first-principles and transport simulations, we show that GNFs can also be an excellent material for thermoelectric applications thanks to the interlayer weak van der Waals interaction that induces low thermal conductance and a step-like shape in the electronic transmission with mini-gaps, which are necessary ingredients to achieve high thermoelectric performance. This study unveils that the platelet form of GNFs in which graphite layers are perpendicular to the fiber axis can exhibit outstanding thermoelectric properties with a figure of merit ZT reaching 3.55 in a 0.5 nm diameter fiber and 1.1 for a 1.1 nm diameter one. Interestingly, by introducing 14C isotope doping, ZT can even be enhanced up to more than 5, and more than 8 if we include the effect of finite phonon mean-free path, which demonstrates the amazing thermoelectric potential of GNFs.

cond-mat.mes-hall↗

arXiv2017

Third nearest neighbor parameterized tight biding model for graphene nano-ribbons

The ab initio band structure of 2D graphene sheet is well reproduced by the third nearest neighbor tight binding model proposed by Reich et al [Phys. Rev. B 66, 035412]. For ribbon structures, the existing sets of tight binding parameters can successfully explain semi-conducting behavior of all armchair ribbon structures. However, they are still failing in describing accurately the slope of the bands while this feature is directly associated to the group velocity and the effective mass of electrons. In this work, both density functional theory and tight binding calculations were performed and a new set of tight binding parameters up to the third nearest neighbors including overlap terms is introduced. The results obtained with this model offer excellent agreement with the predictions of the density functional theory in most cases of ribbon structures, even in the high-energy region. Moreover, this set can induce electron-hole asymmetry as manifested in density functional theory. Relevant outcomes are also demonstrated for armchair ribbons of various widths as well as for zigzag structures, thus opening a route for multi-scale simulations.

cond-mat.mes-hall↗

arXiv2016

Valley filtering and electronic optics using polycrystalline graphene

In this Letter, both the manipulation of valley-polarized currents and the optical-like behaviors of Dirac fermions are theoretically explored in polycrystalline graphene. When strain is applied, the misorientation between two graphene domains separated by a grain boundary can result in a mismatch of their electronic structures. Such a discrepancy manifests itself in a strong breaking of the inversion symmetry, leading to perfect valley polarization in a wide range of transmission directions. In addition, these graphene domains act as different media for electron waves, offering the possibility to modulate and obtain negative refraction indexes.

cond-mat.mes-hall↗

arXiv2016

Optimizing isotope and vacancy engineering in graphene ribbons to enhance the thermoelectric performance without degrading the electronic properties

The enhancement of thermoelectric figure of merit ZT requires to either increase the power factor or reduce the phonon conductance, or even both. In graphene, the high phonon thermal conductivity is the main factor limiting the thermoelectric conversion. The common strategy to enhance ZT is therefore to introduce phonon scatterers to suppress the phonon conductance while retaining high electrical conductance and Seebeck coefficient. Although thermoelectric performance is eventually enhanced, all studies based on this strategy show a significant reduction of the electrical conductance, most often leading to a lower electronic performance. In this study we show that appropriate sources of disorder, including isotopes and vacancies at lowest electron density positions, can be used as phonon scatterers to reduce the phonon conductance in graphene ribbons without degrading the electrical conductance, particularly in the low-energy region which is the most important range for device operation. By means of atomistic calculations using semi-empirical Tight-Binding and Force Constant models in combination with Non-Equilibrium Green function formalism, we show that the natural electronic properties of graphene ribbons can be fully preserved while their thermoelectric efficiency is strongly enhanced. For ribbons of width M = 5 dimer lines, room-temperature ZT is enhanced from less than 0.26 for defect-free ribbons to more than 2.5. This study is likely to set the milestones of a new generation of nano-devices with dual electronic, thermoelectric functionalities.

cond-mat.mes-hall↗

arXiv2016

Transport properties through graphene grain boundaries: strain effects versus lattice symmetry

As most materials available in macroscopic quantities, graphene appears in a polycrystalline form and thus contains grain boundaries. In the present work, the effect of uniaxial strain on the electronic transport properties through graphene grain boundaries is investigated using atomistic simulations. A systematic picture of the transport properties with respect to the strain and the lattice symmetry of graphene domains on both sides of the boundary is provided. In particular, it is shown that the strain engineering can be used to open a finite transport gap in all graphene systems where two domains exhibit different orientations. This gap value is found to depend on the strain magnitude, on the strain direction and on the lattice symmetry of graphene domains. By choosing appropriately the strain direction, a large transport gap of a few hundred meV can be achieved when applying a small strain of only a few percents. For a specific class of graphene grain boundary systems, the strain engineering can also be used to reduce the scattering on defects and hence to significantly enhance the conductance. With a large strain-induced gap, these graphene heterostructures are proposed to be possible candidates for highly sensitive strain sensors, flexible transistors and p-n junctions with a strong non-linear I-V characteristics.

cond-mat.mes-hall↗

arXiv2015

Enhancement of thermoelectric performance in Graphene/BN heterostructures

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.

cond-mat.mes-hall↗

arXiv2015

Enhanced Seebeck effect in graphene devices by strain and doping engineering

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.

cond-mat.mtrl-sci↗

arXiv2015

Transport gap in vertical devices made of incommensurately misoriented graphene layers

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 ($K_y' = -K_y = 2π/3L_y$, 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.

cond-mat.mes-hall↗

arXiv2015

Dispersive hybrid states and bandgap in zigzag Graphene/BN heterostructures

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.

cond-mat.mes-hall↗

arXiv2014

Strain-induced modulation of Dirac cones and van Hove singularities in twisted graphene bilayer

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.

cond-mat.mes-hall↗

arXiv2014

Strain-induced conduction gap in vertical devices made of twisted graphene layers

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.

cond-mat.mes-hall↗

arXiv2014

Klein-tunneling transistor with ballistic graphene

Today the availability of high mobility graphene up to room temperature makes ballistic transport in nanodevices achievable. In particular, p-n-p transistor in the ballistic regime gives access to the Klein tunneling physics and allows the realization of devices exploiting the optics-like behavior of Dirac Fermions (DF) as in the Vesalego lens or the Fabry Pérot cavity. Here we propose a Klein tunneling transistor based on geometrical optics of DF. 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 the transistor transmission. We calculate the transmission and the current by means of scattering theory and the finite bias properties using Non Equilibrium Green's Function(NEGF) simulation.

cond-mat.mes-hall↗

arXiv2014

Conduction gap in graphene strain junctions: direction dependence

It has been shown in a recent study [Nguyen et al., Nanotechnol. \textbf{25}, 165201 (2014)] that unstrained/strained graphene junctions are promising candidates to improve the performance of graphene transistors that is usually hindered by the gapless nature of graphene. Although the energy bandgap of strained graphene still remains zero, the shift of Dirac points in the \textbf{\emph{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 junctions with a strain of only a few percent. However, since it depends essentially on the magnitude of Dirac point shift, this 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 presented and the results are carefully analyzed. Our study provides useful information for further investigations to exploit graphene strained junctions in electronic applications.

cond-mat.mes-hall↗