arXiv subjects

Mathieu Luisier

Publications and source records attributed to Mathieu Luisier.

At least 55 records · Page 3Linked to original sources

arXiv2022

Dynamics of van der Waals Charge Qubit in 2D Bilayers: Ab initio Quantum Transport and Qubit Measurement

A van der Waals (vdW) charge qubit, electrostatically confined within two-dimensional (2D) vdW materials, is proposed as building block of future quantum computers. Its characteristics are systematically evaluated with respect to its two-level anti-crossing energy difference ($Δ$). Bilayer graphene ($Δ$ $\approx$ 0) and a vdW heterostructure ($Δ$ $\gg$ 0) are used as representative examples. Their tunable electronic properties with an external electric field define the state of the charge qubit. By combining density functional theory and quantum transport calculations, we highlight the optimal qubit operation conditions based on charge stability and energy-level diagrams. Moreover, a single-electron transistor (SET) design based on trilayer vdW heterostructures capacitively coupled to the charge qubit is introduced as measurement setup with low decoherence and improved measurement properties. It is found that a $Δ$ greater than 20 meV results in a rapid mixing of the qubit states, which leads to a lower measurement quantity, i.e. contrast and conductance. With properly optimized designs, qubit architectures relying on 2D vdW structures could be integrated into an all-electronic quantum computing platform.

cond-mat.mes-hall↗

arXiv2022

Asymmetrical contact scaling and measurements in MoS2 FETs

Two-dimensional (2D) materials have great potential for use in future electronics due to their atomically thin nature which withstands short channel effects and thus enables better scalability. Device scaling is the process of reducing all device dimensions to achieve higher device density in a certain chip area. For 2D materials-based transistors, both the channel and contact scalability must be investigated. The channel scalability of 2D materials has been thoroughly investigated, confirming their resilience to short-channel effects. However, systematic studies on contact scalability remain rare and the current understanding of contact scaling in 2D FET is inconsistent and oversimplified. Here we combine physically scaled contacts and asymmetrical contact measurements to investigate the contact scaling behavior in 2D field-effect transistors (FETs). The asymmetrical contact measurements directly compare electron injection with different contact lengths while using the exact same channel, eliminating channel-to-channel variations. Compared to devices with long contact lengths, devices with short contact lengths (scaled contacts) exhibit larger variation, smaller drain currents at high drain-source voltages, and a higher chance of showing early saturation and negative differential resistance. Quantum transport simulations show that the transfer length of Ni-MoS2 contacts can be as short as 5 nm. Our results suggest that charge injection at the source contact is different from injection at the drain side: scaled source contacts can limit the drain current, whereas scaled drain contacts cannot. Furthermore, we clearly identified that the transfer length depends on the quality of the metal-2D interface. The asymmetrical contact measurements proposed here will enable further understanding of contact scaling behavior at various interfaces.

cond-mat.mtrl-sci↗

arXiv2022

An Atomistic Modelling Framework for Valence Change Memory Cells

We present a framework dedicated to modelling the resistive switching operation of Valence Change Memory (VCM) cells. The method combines an atomistic description of the device structure, a Kinetic Monte Carlo (KMC) model for the creation and diffusion of oxygen vacancies in the central oxide under an external field, and an ab-initio quantum transport method to calculate electrical current and conductance. As such, it reproduces a realistically stochastic device operation and its impact on the resulting conductance. We demonstrate this framework by simulating a switching cycle for a TiN/HfO$_2$/TiN VCM cell, and see a clear current hysteresis between high/low resistance states, with a conductance ratio of one order of magnitude. Additionally, we observe that the changes in conductance originate from the creation and recombination of vacancies near the active electrode, effectively modulating a tunnelling gap for the current. This framework can be used to further investigate the mechanisms behind resistive switching at an atomistic scale and optimize VCM material stacks and geometries.

cond-mat.mtrl-sci↗

arXiv2022

Efficient and accurate defect level modelling in monolayer MoS$_2$ via GW+DFT with open boundary conditions

Within the framework of many-body perturbation theory (MBPT) integrated with density functional theory (DFT), a novel defect-subspace projection GW method, the so-called p-GW, is proposed. By avoiding the periodic defect interference through open boundary self-energies, we show that the p-GW can efficiently and accurately describe quasi-particle correlated defect levels in two-dimensional (2D) monolayer MoS$_2$. By comparing two different defect states originating from sulfur vacancy and adatom to existing theoretical and experimental works, we show that our GW correction to the DFT defect levels is precisely modelled. Based on these findings, we expect that our method can provide genuine trap states for various 2D transition-metal dichalcogenide (TMD) monolayers, thus enabling the study of defect-induced effects on the device characteristics of these materials via realistic simulations.

cond-mat.mtrl-sci↗

arXiv2021

Impact of Orientation Misalignments on Black Phosphorus Ultrascaled Field-effect Transistors

Two-dimensional materials with strong bandstructure anisotropy such as black phosphorus BP have been identified as attractive candidates for logic application due to their potential high carrier velocity and large density-of-states. However, perfectly aligning the source-to-drain axis with the desired crystal orientation remains an experimental challenge. In this paper, we use an advanced quantum transport approach from first-principle to shed light on the influence of orientation misalignments on the performance of BP-based field-effect transistors. Both $n$-and $p$-type configurations are investigated for six alignment angles, in the ballistic limit of transport and in the presence of electron-phonon and charged impurity scattering. It is found that up to deviations of $50^{\circ}$ from the optimal angle, the ON-state current only decreases by $30\%$. This behavior is explained by considering a single bandstructure parameter, the effective mass along transport direction.

physics.comp-ph↗

arXiv2021

Performance Prediction of InP/GaAsSb Double Heterojunction Bipolar Transistors for THz applications

The intrinsic performance of "type-II" InP/GaAsSb double heterojunction bipolar transistors (DHBTs) towards and beyond THz is predicted and analyzed based on a multi-scale technology computer aided design (TCAD) modeling platform calibrated against experimental measurements. Two-dimensional hydrodynamic simulations are combined with 1-D full-band, atomistic quantum transport calculations to shed light on future DHBT generations whose dimensions are decreased step-by-step, starting from the current device configuration. Simulations predict that a peak transit frequency $f_{T,peak}$ of around 1.6 THz could be reached in aggressively scaled type-II DHBTs with a total thickness of 256 nm and an emitter width $W_E$ of 37.5 nm. The corresponding breakdown voltage $BV_{CEO}$ is estimated to be 2.2 V. The investigations are put in perspective with two DHBT performance limiting factors, self-heating and breakdown characteristics.

physics.app-ph↗

arXiv2021

Circuit-aware Device Modeling of Energy-efficient Monolayer WS$_2$ Trench-FinFETs

The continuous scaling of semiconductor technology has pushed the footprint of logic devices below 50 nm. Currently, logic standard cells with one single fin are being investigated to increase the integration density, although such options could severely limit the performance of individual devices. In this letter, we present a novel Trench (T-) FinFET device, composed of a monolayer two-dimensional (2D) channel material. The device characteristics of a monolayer WS$_2$-based T-FinFET are studied by combining the first-principles calculations and quantum transport (QT) simulations. These results serve as inputs to a predictive analytical model. The latter allows to benchmark the T-FinFET with strained (s)-Si FinFETs in both quasi-ballistic and diffusive transport regimes. The circuit-level evaluation highlights that WS$_2$ T-FinFETs exhibit a competitive energy-delay performance compared to s-Si FinFET and WS$_2$ double-gate transistors, assuming the same mobility and contact resistivity at small footprints.

cond-mat.mtrl-sci↗

arXiv2020

Efficient partitioning of surface Green's function: toward ab initio contact resistance study

In this work, we propose an efficient computational scheme for first-principle quantum transport simulations to evaluate the open-boundary conditions. Its partitioning differentiates from conventional methods in that the contact self-energy matrices are constructed on smaller building blocks, principal layers (PL), while conventionally it was restricted to have the same lateral dimensions of the adjoining atoms in a channel region. Here, we obtain the properties of bulk electrodes through non-equilibrium Green's function (NEGF) approach with significant improvements in the computational efficiency without sacrificing the accuracy of results. To exemplify the merits of the proposed method we investigate the carrier density dependency of contact resistances in silicon nanowire devices connected to bulk metallic contacts.

cond-mat.mtrl-sci↗

arXiv2020

Ab initio simulation of band-to-band tunneling FETs with single- and few-layer 2-D materials as channels

Full-band atomistic quantum transport simulations based on first principles are employed to assess the potential of band-to-band tunneling FETs (TFETs) with a 2-D channel material as future electronic circuit components. We demonstrate that single-layer (SL) transition metal dichalcogenides are not well suited for TFET applications. There might, however, exist a great variety of 2-D semiconductors that have not even been exfoliated yet; this paper pinpoints some of the most promising candidates among them to realize highly efficient TFETs. SL SnTe, As, TiNBr, and Bi are all found to ideally deliver ON-currents larger than 100μA/μm at 0.5-V supply voltage and 0.1 nA/μm OFF-current value. We show that going from single to multiple layers can boost the TFET performance as long as the gain from a narrowing bandgap exceeds the loss from the deteriorating gate control. Finally, a 2-D van der Waals heterojunction TFET is revealed to perform almost as well as the best SL homojunction, paving the way for research in optimal 2-D material combinations.

cond-mat.mes-hall↗

arXiv2020

Ab initio mobility of mono-layer MoS2 and WS2: comparison to experiments and impact on the device characteristics

We combine the linearized Boltzmann Transport Equation (LBTE) and quantum transport by means of the Non-equilibrium Green's Functions (NEGF) to simulate single-layer MoS2 and WS2 ultra-scaled transistors with carrier mobilities extracted from experiments. Electron-phonon, charged impurity, and surface optical phonon scattering are taken into account with all necessary parameters derived from ab initio calculations or measurements, except for the impurity concentration. The LBTE method is used to scale the scattering self-energies of NEGF, which only include local interactions. This ensures an accurate reproduction of the measured mobilities by NEGF. We then perform device simulations and demonstrate that the considered transistors operate far from their performance limit (from 50% for MoS2 to 60% for WS2). Higher quality materials and substrate engineering will be needed to improve the situation.

cond-mat.mes-hall↗

arXiv2020

Ab Initio Modeling of Thermal Transport through van der Waals Materials

An advanced modeling approach is presented to shed light on the thermal transport properties of van der Waals materials (vdWMs) composed of single-layer transition metal dichalcogenides (TMDs) stacked on top of each other with a total or partial overlap only in the middle region. It relies on the calculation of dynamical matrices from first-principle and on their usage in a phonon quantum transport simulator. We observe that vibrations are transferred microscopically from one layer to the other along the overlap region which acts as a filter selecting out the states that can pass through it. Our work emphasizes the possibility of engineering heat flows at the nanoscale by carefully selecting the TMD monolayers that compose vdWMs.

cond-mat.mtrl-sci↗

arXiv2020

On the suitability of hBN as an insulator for 2D material-based ultrascaled CMOS devices

Complementary metal oxide semiconductor (CMOS) logic circuits at the ultimate scaling limit place the utmost demands on the properties of all materials involved. The requirements for semiconductors are well explored and could possibly be satisfied by a number of layered two-dimensional (2D) materials, like for example transition-metal dichalcogenides or black phosphorus. The requirements for the gate insulator are arguably even more challenging and difficult to meet. In particular the combination of insulator to semiconductor which forms the central element of the metal oxide semiconductor field effect transistor (MOSFET) has to be of superior quality in order to build competitive devices. At the moment, hexagonal boron nitride (hBN) is the most common two-dimensional insulator and widely considered to be the most promising gate insulator in nanoscaled 2D material-based transistors. Here, we critically assess the material parameters of hBN and conclude that while its properties render hBN an ideal candidate for many applications in 2D nanoelectronics, hBN is most likely not suitable as a gate insulator for ultrascaled CMOS devices.

physics.app-ph↗

arXiv2020

Quantum mechanical modeling of anharmonic phonon-phonon scattering in nanostructures

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.

cond-mat.mes-hall↗

arXiv2020

Winterface: An Interface from Wannier90 to Quantum Transport

In this work a framework for quantum transport simulation from first principles is introduced, focusing on the coherent case. The model is based on the non-equilibrium Green's function (NEGF) formalism and maximally localized Wannier functions (MLWFs). Any device simulation, here based on two-dimensional (2-D) materials, starts by identifying a representative unit cell, computing its electronic structure with density functional theory (DFT), and converting the plane-wave results into a set of MLWFs. From this localized representation of the original unit cell, the device Hamiltonian can be constructed with the help of properly designed upscaling techniques. Here, a powerful tool called Winterface is presented to automatize the whole process and interface the initial MLWF representation with a quantum transport solver. Its concepts, algorithms, and general functionality are discussed on the basis of a molybdenum disulfide (2-D) monolayer structure, as well as its combination with tungsten disulfide. The developed approach can be considered as completely general, restricted only by the capability of the user to perform the required DFT calculations and to "wannierize" its plane-wave results.

physics.comp-ph↗

arXiv2020

2-D materials for ultra-scaled field-effect transistors: hundred candidates under the ab initio microscope

Thanks to their unique properties single-layer 2-D materials appear as excellent candidates to extend Moore's scaling law beyond the currently manufactured silicon FinFETs. However, the known 2-D semiconducting components, essentially transition metal dichalcogenides, are still far from delivering the expected performance. Based on a recent theoretical study that predicts the existence of more than 1,800 exfoliable 2-D materials, we investigate here the 100 most promising contenders for logic applications. Their "current vs. voltage" characteristics are simulated from first-principles, combining density-functional theory and advanced quantum transport calculations. Both n- and p-type configurations are considered, with gate lengths ranging from 15 down to 5 nm. From this unprecedented collection of electronic materials, we identify 13 compounds with electron and hole currents potentially much higher than in future Si FinFETs. The resulting database widely expands the design space of 2-D transistors and provides original guidelines to the materials and device engineering community.

cond-mat.mes-hall↗

arXiv2020

First-principles calculations for ferroelectrics at constant polarization using generalized Wannier functions

Localized Wannier functions provide an efficient and intuitive framework to compute electric polarization from first-principles. They can also be used to represent the electronic systems at fixed electric field and to determine dielectric properties of insulating materials. Here we develop a Wannier-function-based formalism to perform first-principles calculations at fixed polarization. Such an approach allows to extract the polarization-energy landscape of a crystal and thus supports the theoretical investigation of polar materials. To facilitate the calculations, we implement a quasi-Newton method that simultaneously relaxes the internal coordinates and adjusts the electric field in crystals at fixed polarization. The method is applied to study the ferroelectric behavior of $\mathrm{BaTiO_3}$ and $\mathrm{PbTiO_3}$ in tetragonal phases. The physical processes driving the ferroelectricity of both compounds are examined thanks to the localized orbital picture offered by Wannier functions. Hence, changes in chemical bonding under ferroelectric distortion can be accurately visualized. The difference in the ferroelectric properties of $\mathrm{BaTiO_3}$ and $\mathrm{PbTiO_3}$ is highlighted. It can be traced back to the peculiarities of their electronic structures.

cond-mat.mtrl-sci↗

arXiv2020

A back-end, CMOS compatible ferroelectric Field Effect Transistor for synaptic weights

Neuromorphic computing architectures enable the dense co-location of memory and processing elements within a single circuit. This co-location removes the communication bottleneck of transferring data between separate memory and computing units as in standard von Neuman architectures for data-critical applications including machine learning. The essential building blocks of neuromorphic systems are non-volatile synaptic elements such as memristors. Key memristor properties include a suitable non-volatile resistance range, continuous linear resistance modulation and symmetric switching. In this work, we demonstrate voltage-controlled, symmetric and analog potentiation and depression of a ferroelectric Hf$_{57}$Zr$_{43}$O$_{2}$ (HZO) field effect transistor (FeFET) with good linearity. Our FeFET operates with a low writing energy (fJ) and fast programming time (40 ns). Retention measurements have been done over 4-bits depth with low noise (1%) in the tungsten oxide (WO$_{x}$) read out channel. By adjusting the channel thickness from 15nm to 8nm, the on/off ratio of the FeFET can be engineered from 1% to 200% with an on-resistance ideally >100 kOhm, depending on the channel geometry. The device concept is using earth-abundant materials, and is compatible with a back end of line (BEOL) integration into complementary metal-oxidesemiconductor (CMOS) processes. It has therefore a great potential for the fabrication of high density, large-scale integrated arrays of artificial analog synapses.

cs.ET↗

arXiv2020

Charge Transport in Semiconductors Assembled from Nanocrystals

The potential of semiconductors assembled from nanocrystals (NC semiconductors) has been demonstrated for a broad array of electronic and optoelectronic devices, including transistors, light emitting diodes, solar cells, photodetectors, thermoelectrics, and phase charge memory cells. Despite the commercial success of nanocrystals as optical absorbers and emitters, applications involving charge transport through NC semiconductors have eluded exploitation due to the inability for predictive control of their electronic properties. Here, we perform large-scale, ab-initio simulations to understand carrier transport, generation, and trapping in NC-based semiconductors from first principles. We use these findings to build the first predictive model for charge transport in NC semiconductors, which we validate experimentally. Our work reveals that we have been thinking about transport in NC semiconductors incorrectly. Our new insights provide a path for systematic engineering of NC semiconductors, which in fact offer previously unexplored opportunities for tunability not achievable in other semiconductor systems.

cond-mat.mtrl-sci↗