Searcharxiv⌕ Search

arXiv subjects

Tillmann Kubis

Publications and source records attributed to Tillmann Kubis.

At least 19 recordsLinked to original sources
arXiv2026

Ultrasensitive, universal single-ion nanodetector

In this paper, a carbon nanotube (CNT) based single-ion detector is proposed and its performance is evaluated with atomistic quantum transport models. The sensor can detect any ion type without molecule-specific functionalization and allows for continuous real-time ion monitoring. A single ion temporarily changes the operating principle of the sensor's CNT field-effect transistor into a resonant tunneling diode. The concrete device example of this paper showed a source-drain current increase of 5 orders of magnitude induced by a single ion.

physics.app-ph↗
arXiv2021

Band tail formation in mono and multilayered transition metal dichalcogenides: A detailed assessment and a quick-reference guide

Transition metal dichalcogenides (TMDs) are promising candidates for a wide variety of ultrascaled electronic, quantum computation, and optoelectronic applications. The exponential decay of electronic density of states into the bandgap, i.e. the band tail has a strong impact on the performance of TMD applications. In this work, the band tails of various TMD monolayer and multilayer systems when placed on various dielectric substrates is predicted with density functional theory based nonequilibrium Green's functions. Nonlocal scattering of electrons on polar optical phonons, charged impurities and remote scattering on phonons in the dielectric materials is included in the self-consistent Born approximation. The band tails are found to critically depend on the layer thickness, temperature, doping concentration and particularly on the chosen dielectric substrate. The underlying physical mechanisms are studied in high detail and an analytical interpolation formula is given to provide a quick-reference for Urbach parameters in $MoS_2$, $WS_2$ and $WSe_2$.

cond-mat.mes-hall↗
arXiv2020

First-principles computation of boron-nitride-based ultrathin UV-C light emitting diodes

Short wavelength ultraviolet (UV-C) light deactivates DNA of any germs, including multiresistive bacteria and viruses like COVID-19. Two-dimensional (2D) material-based UV-C light emitting diodes can potentially be integrated into arbitrary surfaces to allow for shadow-free surface disinfection. In this work, we perform a series of first-principles calculations to identify the core components of ultrathin LEDs based on hexagonal boron nitride (hBN). The electrons and holes are predicted to be confined in multiple quantum wells (MQWs) by combining hBN layers with different stacking orders. Various p- and n-doping candidates for hBN are assessed, and the relative p- and n-type metal contacts with low Schottky barrier heights are identified. The findings are summarized in a concrete UV-C LED structure proposal.

physics.app-ph↗
arXiv2020

Mode-space-compatible inelastic scattering in atomistic nonequilibrium Green's function implementations

The nonequilibrium Green's function (NEGF) method is often used to predict transport in atomistically resolved nanodevices and yields an immense numerical load when inelastic scattering on phonons is included. To ease this load, this work extends the atomistic mode space approach of Ref. [1] to include inelastic scattering on optical and acoustic phonons in silicon nanowires. This work also includes the exact calculation of the real part of retarded scattering self-energies in the reduced basis representation using the Kramers-Kronig relations. The inclusion of the Kramers-Kronig relation for the real part of the retarded scattering self-energy increases the impact of scattering. Virtually perfect agreement with results of the original representation is achieved with matrix rank reductions of more than 97%. Time-to-solution improvements of more than 200$\times$ and peak memory reductions of more than 7$\times$ are shown. This allows for the solution of electron transport scattered on phonons in atomically resolved nanowires with cross-sections larger than 5 nm $\times$ 5 nm.

physics.comp-ph↗
arXiv2020

Introducing Open boundary conditions in modeling nonperiodic materials and interfaces: the impact of the periodic assumption

Simulations are essential to accelerate the discovery of new materials and to gain full understanding of known ones. Although hard to realize experimentally, periodic boundary conditions are omnipresent in material simulations. In this work, we intro-duce ROBIN (recursive open boundary and interfaces), the first method allowing open boundary conditions in material and interface modeling. The computational costs are limited to solving quantum properties in a focus area which allows explicitly discretizing millions of atoms in real space and to consider virtually any type of environment (be it periodic, regular, or ran-dom). The impact of the periodicity assumption is assessed in detail with silicon dopants in graphene. Graphene was con-firmed to produce a band gap with periodic substitution of 3% carbon with silicon in agreement with published periodic boundary condition calculations. Instead, 3% randomly distributed silicon in graphene only shifts the energy spectrum. The predicted shift agrees quantitatively with published experimental data. Key insight of this assessment is, assuming periodici-ty elevates a small perturbation of a periodic cell into a strong impact on the material property prediction. Periodic boundary conditions can be applied on truly periodic systems only. More general systems should apply an open boundary method for reliable predictions.

cond-mat.mtrl-sci↗
arXiv2020

Introduction of Multi-particle Büttiker Probes -- Bridging the Gap between Drift Diffusion and Quantum Transport

State-of-the-art industrial semiconductor device modeling is based on highly efficient Drift-Diffusion (DD) models that include some quantum corrections for nanodevices. In contrast, latest academic quantum transport models are based on the non-equilibrium Green's function (NEGF) method that cover all coherent and incoherent quantum effects consistently. Carrier recombination and generation in optoelectronic nanodevices represent an immense numerical challenge when solved within NEGF. In this work, the numerically efficient Büttiker-probe model is expanded to include electron-hole recombination and generation in the NEGF framework. Benchmarks of the new multiple-particle Büttiker probe method against state-of-the-art quantum-corrected DD models show quantitative agreements except in cases of pronounced tunneling and interference effects.

physics.app-ph↗
arXiv2019

Thermal boundary resistance predictions with non-equilibrium Green's function and molecular dynamics simulations

The non-equilibrium Green's function (NEGF) method with Büttiker probe scattering self-energies is assessed by comparing its predictions for the thermal boundary resistance with molecular dynamics (MD) simulations. For simplicity, the interface of Si/heavy-Si is considered, where heavy-Si differs from Si only in the mass value. With Büttiker probe scattering parameters tuned against MD in homogeneous Si, the NEGF-predicted thermal boundary resistance quantitatively agrees with MD for wide mass ratios. Artificial resistances that the unaltered Landauer approach yield at virtual interfaces in homogeneous systems are absent in the present NEGF approach. Spectral information result from NEGF in its natural representation without further transformations. The spectral results show that the scattering between different phonon modes plays a crucial role in thermal transport across interfaces. Büttiker probes provide an efficient and reliable way to include anharmonicity in phonon related NEGF. NEGF including the Büttiker probes can reliably predict phonon transport across interfaces and at finite temperatures.

physics.app-ph↗
arXiv2019

Non-equilibrium Green's function predictions of band tails and band gap narrowing in III-V semiconductors and nanodevices

High-doping induced Urbach tails and band gap narrowing play a significant role in determining the performance of tunneling devices and optoelectronic devices such as tunnel field-effect transistors (TFETs), Esaki diodes and light-emitting diodes. In this work, Urbach tails and band gap narrowing values are calculated explicitly for GaAs, InAs, GaSb and GaN as well as ultra-thin bodies and nanowires of the same. Electrons are solved in the non-equilibrium Green's function method in multi-band atomistic tight binding. Scattering on polar optical phonons and charged impurities is solved in the self-consistent Born approximation. The corresponding nonlocal scattering self-energies as well as their numerically efficient formulations are introduced for ultra-thin bodies and nanowires. Predicted Urbach band tails and conduction band gap narrowing agree well with experimental literature for a range of temperatures and doping concentrations. Polynomial fits of the Urbach tail and band gap narrowing as a function of doping are tabulated for quick reference.

cond-mat.mes-hall↗
arXiv2018

Explicit screening full band quantum transport model for semiconductor nanodevices

State of the art quantum transport models for semiconductor nanodevices attribute negative (positive) unit charges to states of the conduction (valence) band. Hybrid states that enable band-to-band tunneling are subject to interpolation that yield model dependent charge contributions. In any nanodevice structure, these models rely on device and physics specific input for the dielectric constants. This paper exemplifies the large variability of different charge interpretation models when applied to ultrathin body transistor performance predictions. To solve this modeling challenge, an electron-only band structure model is extended to atomistic quantum transport. Performance predictions of MOSFETs and tunneling FETs confirm the generality of the new model and its independence of additional screening models.

physics.app-ph↗
arXiv2017

Control of interlayer delocalization in 2H transition metal dichalcogenides

It is assessed in detail both experimentally and theoretically how the interlayer coupling of transition metal dichalcogenides controls the electronic properties of the respective devices. Gated transition metal dichalcogenide structures show electrons and holes to either localize in individual monolayers, or delocalize beyond multiple layers - depending on the balance between spin-orbit inter- action and interlayer hopping. This balance depends on layer thickness, momentum space symmetry points and applied gate fields. A good quantitative agreement of predictions and measurements of the quantum confined Stark effect in gated MoS2 systems unveils intralayer excitons as major source for the observed photoluminesence.

cond-mat.mes-hall↗
arXiv2017

Thermal Transport Across Metal Silicide-Silicon Interfaces: First-Principles Calculations and Green's Function Transport Simulations

In this work, we use a combination of first-principles calculations under the density functional theory framework and heat transport simulations using the atomistic Green's function (AGF) method to quantitatively predict the contribution of the different scattering mechanisms to the thermal interface conductance of epitaxial CoSi$_2$-Si interfaces. An important development in the present work is the direct computation of interfacial bonding from density functional perturbation theory (DFPT) and hence the avoidance of commonly used `mixing rules' to obtain the cross-interface force constants from bulk material force constants. Another important algorithmic development is the integration of the recursive Green's function (RGF) method with Büttiker probe scattering that enables computationally efficient simulations of inelastic phonon scattering and its contribution to the thermal interface conductance. First-principles calculations of electron-phonon coupling reveal that cross-interface energy transfer between metal electrons and atomic vibrations in the semiconductor is mediated by delocalized acoustic phonon modes that extend on both sides of the interface, and phonon modes that are localized inside the semiconductor region of the interface exhibit negligible coupling with electrons in the metal. We also provide a direct comparison between simulation predictions and experimental measurements of thermal interface conductance of epitaxial CoSi$_2$-Si interfaces using the time-domain thermoreflectance technique. Importantly, the experimental results, performed across a wide temperature range, only agree well with predictions that include all transport processes: elastic and inelastic phonon scattering, electron-phonon coupling in the metal, and electron-phonon coupling across the interface.

cond-mat.mes-hall↗
arXiv2016

Transferable tight binding model for strained group IV and III-V materials and heterostructures

It is critical to capture the effect due to strain and material interface for device level transistor modeling. We introduced a transferable sp3d5s* tight binding model with nearest neighbor interactions for arbitrarily strained group IV and III-V materials. The tight binding model is parameterized with respect to Hybrid functional(HSE06) calculations for varieties of strained systems. The tight binding calculations of ultra small superlattices formed by group IV and group III-V materials show good agreement with the corresponding HSE06 calculations. The application of tight binding model to superlattices demonstrates that transferable tight binding model with nearest neighbor interactions can be obtained for group IV and III-V materials.

cond-mat.mtrl-sci↗
arXiv2016

In-surface confinement of topological insulator nanowire surface states

The bandstructures of [110] and [001] Bi2Te3 nanowires are solved with the atomistic 20 band tight binding functionality of NEMO5. The theoretical results reveal: The popular assumption that all topological insulator wire surfaces are equivalent is inappropriate. The Fermi velocity of chemically distinct wire surfaces differs significantly which creates an effective in-surface confinement potential. As a result, topological insulator surface states prefer specific surfaces. Therefore, experiments have to be designed carefully not to probe surfaces unfavorable to the surface states (low density of states) and thereby be insensitive to the TI-effects.

cond-mat.mtrl-sci↗
arXiv2015

NEMO5: Achieving High-end Internode Communication for Performance Projection Beyond Moore's Law

Electronic performance predictions of modern nanotransistors require nonequilibrium Green's functions including incoherent scattering on phonons as well as inclusion of random alloy disorder and surface roughness effects. The solution of all these effects is numerically extremely expensive and has to be done on the world's largest supercomputers due to the large memory requirement and the high performance demands on the communication network between the compute nodes. In this work, it is shown that NEMO5 covers all required physical effects and their combination. Furthermore, it is also shown that NEMO5's implementation of the algorithm scales very well up to about 178176CPUs with a sustained performance of about 857 TFLOPS. Therefore, NEMO5 is ready to simulate future nanotransistors.

cs.DC↗
arXiv2015

Surface Passivation in Empirical Tight Binding

Empirical Tight Binding (TB) methods are widely used in atomistic device simulations. Existing TB methods to passivate dangling bonds fall into two categories: 1) Method that explicitly includes passivation atoms is limited to passivation with atoms and small molecules only. 2) Method that implicitly incorporates passivation does not distinguish passivation atom types. This work introduces an implicit passivation method that is applicable to any passivation scenario with appropriate parameters. This method is applied to a Si quantum well and a Si ultra-thin body transistor oxidized with SiO2 in several oxidation configurations. Comparison with ab-initio results and experiments verifies the presented method. Oxidation configurations that severely hamper the transistor performance are identified. It is also shown that the commonly used implicit H atom passivation overestimates the transistor performance.

cond-mat.mtrl-sci↗
arXiv2015

Tight binding analysis of Si and GaAs ultra thin bodies with subatomic resolution

Empirical tight binding(ETB) methods are widely used in atomistic device simulations. Traditional ways of generating the ETB parameters rely on direct fitting to bulk experiments or theoretical electronic bands. However, ETB calculations based on existing parameters lead to unphysical results in ultra small structures like the As terminated GaAs ultra thin bodies(UTBs). In this work, it is shown that more reliable parameterizations can be obtained by a process of mapping ab-initio bands and wave functions to tight binding models. This process enables the calibration of not only the ETB energy bands but also the ETB wave functions with corresponding ab-initio calculations. Based on the mapping process, ETB model of Si and GaAs are parameterized with respect to hybrid functional calculations. Highly localized ETB basis functions are obtained. Both the ETB energy bands and wave functions with subatomic resolution of UTBs show good agreement with the corresponding hybrid functional calculations. The ETB methods can then be used to explain realistically extended devices in non-equilibrium that can not be tackled with ab-initio methods.

cond-mat.mtrl-sci↗
arXiv2014

Modeling techniques for quantum cascade lasers

Quantum cascade lasers are unipolar semiconductor lasers covering a wide range of the infrared and terahertz spectrum. Lasing action is achieved by using optical intersubband transitions between quantized states in specifically designed multiple-quantum-well heterostructures. A systematic improvement of quantum cascade lasers with respect to operating temperature, efficiency and spectral range requires detailed modeling of the underlying physical processes in these structures. Moreover, the quantum cascade laser constitutes a versatile model device for the development and improvement of simulation techniques in nano- and optoelectronics. This review provides a comprehensive survey and discussion of the modeling techniques used for the simulation of quantum cascade lasers. The main focus is on the modeling of carrier transport in the nanostructured gain medium, while the simulation of the optical cavity is covered at a more basic level. Specifically, the transfer matrix and finite difference methods for solving the one-dimensional Schrödinger equation and Schrödinger-Poisson system are discussed, providing the quantized states in the multiple-quantum-well active region. The modeling of the optical cavity is covered with a focus on basic waveguide resonator structures. Furthermore, various carrier transport simulation methods are discussed, ranging from basic empirical approaches to advanced self-consistent techniques. The methods include empirical rate equation and related Maxwell-Bloch equation approaches, self-consistent rate equation and ensemble Monte Carlo methods, as well as quantum transport approaches, in particular the density matrix and non-equilibrium Green's function (NEGF) formalism. The derived scattering rates and self-energies are generally valid for n-type devices based on one-dimensional quantum confinement, such as quantum well structures.

cond-mat.mes-hall↗