arXiv subjects

P. Dollfus

Publications and source records attributed to P. Dollfus.

17 recordsLinked to original sources

arXiv2021

Study of phonon transport across several Si/Ge interfaces using full-band phonon Monte Carlo simulation

A Full Band Monte Carlo simulator has been developed to consider phonon transmission across interfaces that are perpendicular to the heat flux. This solver of the Boltzmann transport equation which does not require any assumption on the shape the phonon distribution can naturally consider all phonon transport regimes from the diffusive to the fully ballistic regime. Hence, this simulator is used to study single and double Si/Ge heterostructures from the micrometer scale down to the nanometer scale i.e. in all phonon transport regime from ballistic to fully diffusive. A methodology to estimate the thermal conductivities and the thermal interfaces is presented.

physics.app-ph↗

arXiv2020

Revisiting thermal conductivity and interface conductance at the nanoscale

A semi-analytical model for studying thermal transport at the nanoscale, able to accurately describe both the effect of out of equilibrium transport and the thermal transfer at interfaces, is presented. Our approach is based on the definition of pseudo local temperatures distinguishing the phonon populations according to the direction of their velocity. This formalism leads to a complete set of equations capturing the heat transfer in nanostructures even in the case of hetero-structures. This model only requires introducing a new intrinsic thermal parameter called ballistic thermal conductance and a geometric one called the effective thermal conductivity. Finally, this model is able to reproduce accurately advanced numerical results of Monte Carlo simulation for phonons in all phonon transport regime: diffusive (as the Fourier heat transport regime is included), ballistic, and intermediate ones even if thermal interface are involved. This formalism should provide new insights in the interpretation of experimental measurements.

cond-mat.mes-hall↗

arXiv2020

A steep-slope MoS2-nanoribbon MOSFET based on an intrinsic cold-contact effect

We propose a steep-slope MoS2-nanoribbon field-effect transistor that exploits a narrow-energy conduction band to intrinsically filter out the thermionic tail of the electron energy distribution. We study the device operation principle and the performance dependence on the design parameters through atomistic self-consistent quantum simulations. Our results indicate that the device can provide high ION/IOFF ratios, compatible with electronic applications, albeit biased at ultralow voltages of around 0.1 V.

cond-mat.mes-hall↗

arXiv2018

Heat transfer in rough nanofilms and nanowires using Full Band Ab Initio Monte Carlo simulation

The Boltzmann transport equation is one of the most relevant framework to study the heat transport at the nanoscale, beyond the diffusive regime and up to the micrometer-scale. In the general case of three-dimensional devices, the particle Monte Carlo approach of phonon transport is particularly powerful and convenient, and requires reasonable computational resources. In this work, we propose an original and versatile particle Monte Carlo approach parametrized by using ab-initio data. Both the phonon dispersion and the phonon-phonon scattering rates have been computed by DFT calculation in the entire 3D Brillouin zone. To treat the phonon transport at rough interfaces, a combination of specular and diffuse reflections has been implemented in phase space. Thermal transport has been investigated in nanowires and thin films made of cubic and hexagonal Silicon, including edge roughness, in terms of effective thermal conductivity, phonon band contributions and heat flux orientation. It is shown that the effective thermal conductivity in quasi-ballistic regime obtained from our Monte Carlo simulation cannot be accurately fitted by simple semi-analytical Matthiessen-like models and that spectral approaches are mandatory to get good results. Our Full Band approach shows that some phonon branches exhibiting a negative group velocity in some parts of the Brillouin zone may contribute negatively to the total thermal flux. Besides, the thermal flux clearly appears to be oriented along directions of high density of states. The resulting anisotropy of the heat flux is discussed together with the influence of rough interfaces.

cond-mat.mes-hall↗

arXiv2014

Enhanced thermoelectric figure of merit in vertical graphene junctions

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.

cond-mat.mes-hall↗

arXiv2011

Large peak-to-valley ratio of negative-differential-conductance in graphene p-n junctions

We investigate the transport characteristics of monolayer graphene p-n junctions by means of the non-equilibrium Green's function technique. It is shown that thanks to the high interband tunneling of chiral fermions and to a finite bandgap opening when the inversion symmetry of graphene plane is broken, a strong negative-differential-conductance behavior with peak-to-valley ratio as large as a few tens can be achieved even at room temperature. The dependence of this behavior on the device parameters such as the Fermi energy, the barrier height, and the transition length is then discussed.

cond-mat.mes-hall↗

arXiv2010

Electron transport properties in high-purity Ge down to cryogenic temperatures

Electron transport in Ge at various temperatures down to 20 mK has been investigated using particle Monte Carlo simulation taking into account ionized impurity and inelastic phonon scattering. The simulations account for the essential features of electron transport at cryogenic temperature: Ohmic regime, anisotropy of the drift velocity relative to the direction of the electric field, as well as a negative differential mobility phenomenon along the <111> field orientation. Experimental data for the electron velocities are reproduced with a satisfactory accuracy. Examples of electron position in the real space during the simulations are given and evidence separated clouds of electrons propagating along different directions depending on the valley they belong.

cond-mat.mes-hall↗

arXiv2009

Controllable spin-dependent transport in armchair graphene nanoribbon structures

Using the non-equilibrium Green's functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbon (GNR) structures controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effect of width-dependent band gap and the nature of contacts. The oscillation of spin polarization in the GNRs with a large band gap is strong in comparison with 2D-graphene structures. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference of electronic structure between normal conductor and graphene generates confined states in the device which have a strong influence on the transport quantities. It suggests that the device should be carefully designed to obtain high controllability of spin current.

cond-mat.mes-hall↗

arXiv2008

A Pearson Effective Potential for Monte-Carlo simulation of quantum confinement effects in various MOSFET architectures

A Pearson Effective Potential model for including quantization effects in the simulation of nanoscale nMOSFETs has been developed. This model, based on a realistic description of the function representing the non zero-size of the electron wave packet, has been used in a Monte-Carlo simulator for bulk, single gate SOI and double-gate SOI devices. In the case of SOI capacitors, the electron density has been computed for a large range of effective field (between 0.1 MV/cm and 1 MV/cm) and for various silicon film thicknesses (between 5 nm and 20 nm). A good agreement with the Schroedinger-Poisson results is obtained both on the total inversion charge and on the electron density profiles. The ability of an Effective Potential approach to accurately reproduce electrostatic quantum confinement effects is clearly demonstrated.

cond-mat.mtrl-sci↗

arXiv2007

Monte Carlo study of coaxially gated CNTFETs: capacitive effects and dynamic performance

Carbon Nanotube (CNT) appears as a promising candidate to shrink field-effect transistors (FET) to the nanometer scale. Extensive experimental works have been performed recently to develop the appropriate technology and to explore DC characteristics of carbon nanotube field effect transistor (CNTFET). In this work, we present results of Monte Carlo simulation of a coaxially gated CNTFET including electron-phonon scattering. Our purpose is to present the intrinsic transport properties of such material through the evaluation of electron mean-free-path. To highlight the potential of high performance level of CNTFET, we then perform a study of DC characteristics and of the impact of capacitive effects. Finally, we compare the performance of CNTFET with that of Si nanowire MOSFET.

cond-mat.other↗

arXiv2007

Monte Carlo study of apparent mobility reduction in nano-MOSFETs

The concept of mobility is discussed in the case of unstrained and strained nanoscale DG MOSFET thanks to particle Monte Carlo device simulation. Without the introduction of specific scattering phenomenon for short channel devices, the apparent mobility extracted from simulated electrical characteristics decreases with the shrinking of the channel length, as experimentally observed elsewhere. We show that this reduction at room temperature is caused by non stationary effects. Moreover, both simulation results and experimental data may be well reproduced by a Mathiessen-like model, using a "ballistic mobility" extracted from MC simulations together with the usual long channel mobility.

cond-mat.other↗

arXiv2007

Scattering approach to current and noise in interacting mesoscopic systems

We propose an extension of the Landauer-Buttiker scattering theory to include effects of interaction in the active region of a mesoscopic conductor structure. The current expression obtained coincides with those derived by different methods. A new general expression for the noise is also established. These expressions are then discussed in the case of strongly sequential tunneling through a double-barrier resonant tunneling structure.

cond-mat.mes-hall↗

arXiv2005

Electron effective mobility in strained Si/Si1-xGex MOS devices using Monte Carlo simulation

Based on Monte Carlo simulation, we report the study of the inversion layer mobility in n-channel strained Si/ Si1-xGex MOS structures. The influence of the strain in the Si layer and of the doping level is studied. Universal mobility curves mueff as a function of the effective vertical field Eeff are obtained for various state of strain, as well as a fall-off of the mobility in weak inversion regime, which reproduces correctly the experimental trends. We also observe a mobility enhancement up to 120 % for strained Si/ Si0.70Ge0.30, in accordance with best experimental data. The effect of the strained Si channel thickness is also investigated: when decreasing the thickness, a mobility degradation is observed under low effective field only. The role of the different scattering mechanisms involved in the strained Si/ Si1-xGex MOS structures is explained. In addition, comparison with experimental results is discussed in terms of SiO2/ Si interface roughness, as well as surface roughness of the SiGe substrate on which strained Si is grown.

cond-mat.other↗

arXiv2005

Comparison of multiple-gate MOSFET architectures using Monte Carlo simulation

Multiple-gate SOI MOSFETs with gate length equal to 25 nm are compared using device Monte Carlo simulation. In such architectures, the short channel effects may be controlled with much less stringent body and oxide thickness requirements than in single-gate MOSFET. Our results highlight that planar double-gate MOSFET is a good candidate to obtain both high current drive per unit-width and weak subthreshold leakage with large integration density and aggressive delay time, compared to non planar devices such as triple-gate or quadruple-gate structures.

cond-mat.other↗

arXiv2004

On the ballistic transport in nanometer-scaled double-gate MOSFET

The scattering effects are studied in nanometer-scaled double-gate MOSFET, using Monte Carlo simulation. The non-equilibrium transport in the channel is analyzed with the help of the spectroscopy of the number of scatterings experienced by electrons. We show that the number of ballistic electrons at the drain end, even in terms of flux, is not the only relevant characteristic of ballistic transport. Then the drive current in the 15 nm-long channel transistor generations should be very close to the value obtained in the ballistic limit even if all electrons are not ballistic. Additionally, most back-scattering events which deteriorates the ON current, take place in the first half of the channel and in particular in the first low field region. However, the contribution of the second half of the channel can not be considered as negligible in any studied case i.e. for a channel length below 25 nm. Furthermore, the contribution of the second half of the channel tends to be more important as the channel length is reduced. So, in ultra short channel transistors, it becomes very difficult to extract a region of the channel which itself determine the drive current Ion.

cond-mat.mes-hall↗

arXiv2004

Effect of discrete impurities on electron transport in ultra-short MOSFET using 3D Monte Carlo simulation

This paper discusses the influence of the channel impurity distribution on the transport and the drive current in short-gate MOSFET. In this purpose, a careful description of electron-ion interaction suitable for the case of discrete impurities has been implemented in a 3D particle Monte Carlo simulator. This transport model is applied to the investigation of 50 nm MOSFET operation. The results show that a small change in the number of doping impurities or in the position of a single discrete impurity in the inversion layer may significantly influence the drain current. This effect is not only related to threshold voltage fluctuations but also to variations in transport properties in the inversion layer, especially at high drain voltage. The results are analyzed in terms of local fluctuations of electron velocity and current density. In a set of fifteen simulated devices the drive current Ion, determined at VGS = VDS = 0.6 V, is found to vary in a range of 23% according to the position of channel impurities.

cond-mat.mes-hall↗

arXiv2003

Electron transport in Si/SiGe modulation-doped heterostructures using Monte Carlo simulation

The electron transport in the two-dimensional gas formed in tensile-strained Si1-xGex/Si/Si1-xGex heterostructures is investigated using Monte Carlo simulation. At first the electron mobility is studied in ungated modulation doped structures. The calculation matches very well the experimental results over a wide range of electron density. The mobility typically varies between 1100 cm2/Vs in highly-doped structures and 2800 cm2/Vs at low electron density. The mobility is shown to be significantly influenced by the thickness of the spacer layer separating the strained Si channel from the pulse-doped supply layers. Then the electron transport is investigated in a gated modulation-doped structure in which the contribution of parasitic paths is negligible. The mobility is shown to be higher than in comparable ungated structures and dependent on the gate voltage, as a result of the electron density dependence of remote impurity screening.

cond-mat.mes-hall↗