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## Publications

Publications (431)

Due to the potential for technological application, there has been an explosion of interest in heavily polycrystalline ferroelectric nanofilms, such as those of doped hafnium oxide. However, the heavily polycrystalline nature of these materials invalidates conventional modeling approaches as the dynamics have been found to be: 1) nucleation-limited...

Employing novel 2D materials with topologically protected current-carrying edge states is promising to boost the on-current in electronic devices. Using nanoribbons is essential to reduce the contribution of the 2D bulk states to the current. Making the nanoribbon widths narrower allows one to put more current-carrying edge states under the gate of...

We discuss boundary value problems for the characteristic stationary von Neumann equation (stationary sigma equation) and the stationary Wigner equation in a single spatial dimension. The two equations are related by a Fourier transform in the non-spatial coordinate. In general, a solution to the characteristic equation does not produce a correspon...

Continuous miniaturization has brought the feature size of silicon technology down into the nanometer scale, where performance enhancement cannot easily be achieved by further size reduction. The use of new materials with advanced properties has become mandatory to meet the needs for higher performance at reduced power. Topological insulators posse...

The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity,...

The field of thermoelectric materials has undergone a revolutionary transformation over the last couple of decades as a result of the ability to nanostructure and synthesize myriads of materials and their alloys. The ZT figure of merit, which quantifies the performance of a thermoelectric material has more than doubled after decades of inactivity,...

The use of new materials with advanced properties has become mandatory to meet the needs for higher electronics performance at reduced power. Topological insulators (TIs) possess highly conductive topologically protected edge states which are insensitive to scattering and thus suitable for energy-efficient highspeed devices. Here, we evaluate the s...

We have developed a novel Monte Carlo (MC) algorithm to study carrier transport in semiconductors in the presence of electron-electron scattering (EES)Electron-electron scattering. It is well known that the Boltzmann scattering operator for EES is nonlinear in the single-particle distribution function. Numerical solution methods of the resulting no...

The Wigner equation describing stationary quantum transport has a singularity at the point \(k=0\). Deterministic solution methods usually deal with the singularity by just avoiding that point in the mesh (e.g., Frensley’s method). Results from such methods are known to depend strongly on the discretization and meshing parameters.

Experimental observations and quantum mechanical device simulations point to different electronic properties of dislocations in silicon and germanium. Experimental data suggests a supermetallic behavior of dislocations in Si and thus the high strain in the dislocation core is thought to causes the confinement of charge carriers, which leads to the...

A backward Monte Carlo method for the numerical solution of the semiconductor Boltzmann equation is presented. The method is particularly suited to simulate rare events. The general theory of the backward Monte Carlo method is described, and several estimators for the contact current are derived from that theory. The transition probabilities for th...

We report on a first-principles study of the electronic and transport properties of pristine, B-, C-, N-, O-doped blue phosphorene (BlueP) with the adsorption of gas molecules NH3, NO, NO2, CO, and CO2. The adsorption distances, adsorption energies, charge transfer, density of states (DOS), and transmission spectra of these gas molecules on molecul...

There are many exotic scenarios where the Lorenz number of the Wiedemann-Franz law is known to deviate from expected values. However, in conventional semiconductor systems, it is assumed to vary between the values of ~1.49x10^{-8} W {\Omega} K^{-2} for non-degenerate semiconductors and ~2.45x10^{-8} W {\Omega} K^{-2} for degenerate semiconductors o...

Electron energy filtering has been suggested as a promising way to improve the power factor and enhance the ZT figure of merit of thermoelectric materials. In this work, we explore the effect that reduced dimensionality has on the success of the energy-filtering mechanism for power factor enhancement. We use the quantum mechanical non-equilibrium G...

A significant increase of the drain current appears if defined arrangements of dislocations are present in the channel of MOSFETs. Furthermore, analyses of the electronic properties of individual defects refer to a supermetallic behavior of dislocations. The reason is the extremely high strain in the dislocation core exceeding values of ϵ ≅ 0.1. Su...

In this work we present a semi-classical modeling and simulation approach for ultra-narrow channels that has been implemented as part of the Vienna Schrödinger-Poisson (VSP) simulation framework (Baumgartner, J Comput Electron 12:701–721, 2013; http://www.globaltcad.com/en/products/vsp.html (2014)) over the past few years. Our research has been dri...

In this letter, we demonstrate the formation of unique Ga/GaAs/Si nanowire heterostructures, which were successfully implemented in nanoscale light-emitting devices with visible room temperature electroluminescence. Based on our recent approach for the integration of InAs/Si heterostructures into Si nanowires by ion implantation and flash lamp anne...

Due to their tunable properties, silicon nano-crystals (NC) are currently being investigated. Quantum confinement can generally be employed for size-dependent band-gap tuning at dimensions smaller than the Bohr radius (∼5 nm for silicon). At the nano-meter scale, however, increased surface-to-volume ratio makes the surface effects dominant. Specifi...

Cross-plane superlattices composed of nanoscale layers of alternating
potential wells and barriers have attracted great attention for their potential
to provide thermoelectric power factor improvements and higher ZT figure of
merit. Previous theoretical works have shown that the presence of optimized
potential barriers could provide improvements to...

Silicon nano-crystals (NCs) are potential candidates for enhancing and tuning optical properties of silicon for optoelectronic and photo-voltaic applications. Due to the high surface-to-volume ratio, however, optical properties of NC result from the interplay of quantum confinement and surface effects. In this work, we show that both the spatial po...

Energy filtering has been put forth as a promising method for achieving large thermoelectric power factors in thermoelectric materials through Seebeck coefficient improvement. Materials with embedded potential barriers, such as cross-plane superlattices, provide energy filtering, in addition to low thermal conductivity, and could potentially achiev...

Dislocations exhibit a number of exceptional electronic properties resulting in a significant increase in the drain current of MOSFETs if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic conductivity. A model of the electronic structure of dislocations is proposed bas...

The thermoelectric (TE) performance of materials is determined by the figure of merit ZT = σS 2 T/κ, where σ denotes the electrical conductivity, S the Seebeck coefficient and κ the thermal conductivity. Large improvements in ZT have recently been reported in nanoscale materials due to drastic reduction in κ[1]. On the other hand, much less success...

In this work, a comprehensive investigation of the effect of source/drain tunneling in ultra-scaled transistors is presented. A novel approach to efficiently and accurately incorporate the quantum-mechanical effects of source/drain (S/D) tunneling in semi-classical device simulators has been developed. The ballistic quantum transport model has been...

Thermoelectric materials can convert waste heat into usable power and thus have great potential as an energy technology. However, the thermoelectric efficiency of a material is quantified by its figure of merit, which has historically remained stubbornly low. One possible avenue towards increasing the figure of merit is through the use of low-dimen...

Modulation doping is a promising means of increasing the electrical conductivity of thermoelectric (TE) materials and achieving a high figure of merit (ZT). We compared, qualitatively and quantitatively, the TE performance of a field-effect density modulated Si nanowire channel of diameter D = 12 nm with that of its doped counterpart, by use of sel...

We investigate the influence of low-dimensionality and disorder in phonon
transport in ultra-narrow armchair graphene nanoribbons (GNRs) using
non-equilibrium Greens function (NEGF) simulation techniques. We specifically
focus on how different parts of the phonon spectrum are influenced by
geometrical confinement and line edge roughness. With the i...

An experimental-simulation methodology to explore the spatially nonhomogeneous properties of the tunneling current in nanoscaled MOSFET is introduced. The magnetic field B is introduced into the Schrödinger-Poisson system, which allows simulating the effect of the B field on the gate oxide tunneling current and be compared with experimental data. W...

Abstract In this paper we develop several extensions to semi-classical modeling of low-field mobility, which are necessary to treat planar and non-planar channel geometries on equal footing. We advance the state-of-the-art by generalizing the Prange-Nee model for surface roughness scattering to non-planar geometries, providing a fully numerical tre...

A numerical simulation methodology for incorporating thermo-magnetic effects on the MOSFET gate tunneling current is introduced. The methodology is based on the solution of the Schrödinger-Poisson coupled system, which allows simulating the influence of a static magnetic field and temperature on the wave functions and gate tunneling current of MOSF...

The optical properties of 1-D superlattices formed by armchair graphene nanoribbons embedded in hexagonal boron nitride superlattices (BNSLs) are studied. A set of tight-binding (TB) parameters is proposed, which gives results in excellent agreement with first-principle calculations. Based on the tight-binding model, it is demonstrated that in BNSL...

An optimization study of quantum cascade lasers (QCLs) considering the laser instability condition is performed. To model current transport, the Pauli master equation is solved using a Monte Carlo approach. The effects of saturable absorber and pumping strength on the instability threshold are investigated. A particle swarm optimization algorithm i...

We present a Monte Carlo study of heat transport in Si nanomeshes. Phonons are treated semiclassically as particles of specific energy and velocity that undergo phonon-phonon scattering and boundary scattering on the surfaces of the nanomesh pores. We investigate the influence of: (1) geometric parameters such as the pore arrangement/randomness and...

We investigate the effect of electrostatic gating on the thermoelectric power
factor of p-type Si nanowires (NWs) of up to 20nm in diameter in the [100],
[110] and [111] crystallographic transport orientations. We use atomistic
tight-binding simulations for the calculation of the NW electronic structure,
coupled to linearized Boltzmann transport eq...

We demonstrate the concept of diagonal transitions for quantum cascade detectors (QCD). Different to standard, vertical QCDs, here the active transition takes place between two energy levels in adjacent wells. Such a scheme has versatile advantages. Diagonal transitions generally yield a higher extraction efficiency and a higher resistance than ver...

The momentum relaxation time (MRT) is widely used to simplify low-field mobility calculations including anisotropic scattering processes. Although not always fully justified, it has been very practical in simulating transport in bulk and in low-dimensional carrier gases alike. We review the assumptions behind the MRT, quantify the error introduced...

In this paper, a comprehensive investigation of quantum transport in nanoscaled gallium nitride (GaN) high electron mobility transistors (HEMTs) is presented. A simulation model for quantum transport in nanodevices on unstructured grids in arbitrary dimension and for arbitrary crystal directions has been developed. The model has been implemented as...

This works presents a study of the location of 3D band-to-band tunneling barriers in order to create improved tunneling devices. Specifically, the i-Si/n-InAs junction is considered. The large lattice mismatch in this material system causes dislocations in the interface and traps in the bandgap. Alternative device configurations are considered that...

A systematic optimization study of quantum cascade lasers with integrated nonlinearity for third-harmonic generation is performed. To model current transport the Pauli master equation is solved using a Monte Carlo approach. A multi-objective particle swarm optimization algorithm is applied to obtain the Pareto front. Our theoretical analysis indica...

Accurate band structure modeling is an essential ingredient in mobility modeling for any kind of semiconductor device or channel. This is particularly true for holes as the valence band of the most commonly used semiconductor materials is not even close to being parabolic. Instead, valence bands exhibit warped energy surfaces that simply cannot be...

This work focuses on studying band-to-band tunneling in 3D devices, while considering variations in material properties (mass, doping), applied bias, or geometry. A simulation study of cylindrical nanowires, tapered structures and doping concentration variation demonstrates the importance of 3D effects in band-to-band tunneling current computation.

We present a framework for modeling the low-field mobility of ultra-narrow Si channels such as FinFETs based on a full-band description of the electronic structure. Hole mobility is of particular interest since its calculation necessitates a full-band approach. Our approach is entirely based on physical modeling and thus naturally includes effects...

We theoretically investigate thermal conductivity in silicon nanomeshes using Monte Carlo simulations of phonon transport. Silicon membranes of 100 nm thickness with randomly located pores of 50 nm diameter are considered. The effects of material porosity and pore surface roughness are examined. Nanomesh porosity is found to have a strong detriment...

We present a framework for modeling the low-field mobility of ultra-narrow Si channels such as nanowires or FinFETs based on a full-band description of the electronic structure. Hole mobility is of particular interest since its calculation necessitates a full-band approach. Using cylindrical nanowires of different crystal orientation as a model for...

We present atomistic valence force field calculations of thermal transport in
Si nanowires of diameters from 12nm down to 1nm. We show that as the diameter
is reduced, the phonon density-of-states and transmission function acquire a
finite value at low frequency, in contrast to approaching zero as in the bulk
material. It turns out that this effect...

The Vienna Schrödinger-Poisson (VSP) simulation framework for quantum-electronic engineering applications is presented. It is an extensive software tool that includes models for band structure calculation, self-consistent carrier concentrations including strain, mobility, and transport in transistors and heterostructure devices. The basic physical...

Low-dimensional semiconductors are considered promising candidates for thermoelectric applications with enhanced performance because of a drastic reduction in their thermal conductivity, κ
l
, and possibilities of enhanced power factors. This is also the case for traditionally poor thermoelectric materials such as silicon. This work presents atomis...

We conduct a comprehensive simulation study of non-planar n-type channels based on consistent, physical models containing measurable quantities rather than fit-parameters. This contrasts empirical thin-body models used in classical/quantum-corrected TCAD. The method involves the self-consistent solution of the two-dimensional Schrödinger-Poisson sy...

In this work, we describe a novel idea that allows for high thermoelectric power factors in two-phase materials that are heavily doped with an inhomogeneous distribution of dopants. We show that a concurrent increase of the electrical conductivity and Seebeck coefficient and a consequent increase of the power factor can be achieved in such systems....

The sp3d5s*-spin-orbit-coupled atomistic tight-binding (TB) model is used for
the electronic structure calculation of Si nanowires (NWs), self consistently
coupled to a 2D Poisson equation, solved in the cross section of the NW. Upon
convergence, the linearized Boltzmann transport theory is employed for the
mobility calculation, including carrier s...

A comprehensive study of correlated gate leakage and drain current fluctuations in nMOS devices using non-equilibrium Green's function calculations has been carried out. A simulation model combining 3D self-consistent electrostatic potentials accounting for random discrete dopants and charged oxide traps with a 1D and 2D transport description of di...

In this work, we investigate the effect of band structure anisotropy and channel orientation on SRS in non-planar channels such as tri-gate and GAA structures. A new formalism is introduced for calculating SRS rates for non-planar structures. The formalism is an extension of the theory by Prange and Nee for planar structures, which has been widely...

This work focuses on modeling the tunneling mechanism in direct semiconductors. An effective barrier is extracted between the valence and conduction band, by defining the barrier as valence-like near the valence band and conduction bandlike near the conduction band. The transition occurs at a point obtained by momentum matching. Computation of tran...

The bandstructure of p-type gated Si nanowires (NWs) is calculated self-consistently using the sp3d5s* atomistic tight-binding (TB) model and the 2D Poisson equation. The Boltzmann transport formalism is then used for calculation of the low-field mobility. We show that the bandstructures of NWs in the [110] and [111] transport orientations change a...

We analyze the effect of low dimensionality on the electrical conductivity (σ) and Seebeck coefficient (S) in ultra-narrow Si nanowires (NWs) by employing atomistic considerations for the electronic structures and linearized Boltzmann transport theory. We show that changes in the geometrical features of the NWs such as diameter and orientation most...

Large efforts in improving thermoelectric energy conversion are devoted to
energy filtering by nanometer size potential barriers. In this work we perform
an analysis and optimization of such barriers for improved energy filtering. We
merge semiclassical with quantum mechanical simulations to capture tunneling
and reflections due to the barrier, and...

An analytical two-band k•p model for the conduction band of silicon is compared with the numerical nonlocal empirical pseudo-potential method and the sp3d5s* nearest-neighbor tight-binding model. The two-band k•p model gives results consistent with the empirical pseudo-potential method and describes the conduction band structure accurately. The tig...

We study the effect of confinement on the phonon properties of ultra-narrow silicon nanowires of side sizes of 1 nm to 10 nm. We use the modified valence force field (MVFF) method to compute the phononic dispersion and extract the density of states, the transmission function, the sound velocity, the ballistic thermal conductance, and boundary-scatt...

Following our recent study on the electronic properties of rough nanoribbons , in this paper the role of geometrical and roughness parameters on the thermal properties of armchair graphene nanoribbons is studied. Employing a fourth nearest-neighbor force constant model in conjuction with the nonequilibrium Green's function method the effect of line...

We investigate the effect of confinement and orientation on the phonon
transport properties of ultra-thin silicon layers of thicknesses between 1
nm-16 nm. We employ the modified valence force field method to model the
lattice dynamics and the ballistic Landauer transport formalism to calculate
the thermal conductance. We consider the major thin la...