Quantum ATK - Atomistic Simulation Applications

1a. Modeling and Optimization of Metal-semiconductor Contacts

As transistor scaling reduces the area available for metal-semiconductor contacts, the impact of contact resistance on transistor performance increases. Therefore, the detailed modeling of metal-semiconductor contacts has become an essential component of transistor optimization.

Atomic-scale modeling of metal-semiconductor contacts in QuantumATK provides key insights on how to optimize contact resistance, including:  

• how the atomic structure of the interface impacts the metal-semiconductor contact resistance,
• the intrinsic processes that limit the resistance of the contact,
• doping-dependence of contact resistance.

The nanoscale of today’s and future metal-semiconductor contacts render its experimental characterization challenging. The atomic-level modeling in QuantumATK provides the key benefit of a first-principles description to complement experimental characterization to guide transistor optimization.  

1b. Bandstructure and Carrier Transport Modeling in Transistor Channels

With transistor scaling, new channel structures and materials need to be investigated as a means to improve carrier mobility and achieve the target performance. The change in composition and 2D quantum-mechanical confinement of semiconductor structure in, for example, a nanosheet, significantly alters the bandstructure, resulting in changes to the effective masses and mobility of the carriers relative to bulk materials. This, consequently, has a primary impact on the carrier transport in the channel and requires TCAD models to be equipped with new relevant parameters. Although it is possible to measure some of these parameters experimentally, it is much simpler and faster to obtain them from atomic-scale simulations with QuantumATK.

Key Benefits of QuantumATK

• Calculate bandstructures and extract effective masses from the bands to be inserted in TCAD advanced transport simulations.
• Use our efficient implementation to calculate phonon-limited electron mobility (less complex and time-consuming compared to other implementations).
• Generate random alloys for any type of geometry using our built-in genetic Special Quasi-Random Structures (SQS) algorithm, which is faster than open-ended Monte Carlo simulations in other codes.

1c. Spintronic Memory Simulation

Spin-transfer torque RAM (STT-RAM) is a novel non-volatile memory with unique properties that make it attractive for embedded and solid-state drive applications. QuantumATK enables the investigation and optimization of the material stacks that comprise the magnetic tunnel junctions (MTJs) in the STT-RAM bit cells. Since the first-principles calculations in QuantumATK do not require empirical inputs, users can efficiently investigate the very large set of material stacks using key attributes such as:

• Tunnel magnetoresistance ratio (TMR)
• Spin-polarized tunneling current
• Bias-dependent spin-transfer torque based on non-collinear spin NEGF transport

Non-ideal material stacks, such as oxidized interfaces and impure materials, can also be handled, thereby allowing the modeling of realistic structures. QuantumATK is a valuable simulation platform to support the R&D of next-generation spin-based non-volatile memories.

1d. Modeling of Novel Photovoltaic Materials and Devices

Abundance of new materials and lack of efficient devices for solar cells (silicon, thin film, etc.) highlight the need for ab initio atomic-scale simulations, which include important effects such as confinement of electrons and phonons, surface effects, and strain. QuantumATK can take these effects into account and simulate band alignment in interfaces (front, back end, etc.) in solar cells comprised of different materials, impact of defects, simulate photocurrent and open circuit voltage (OCV) measurements. Temperature effects has a significant impact on OCV and photocurrent, and electron-phonon scattering can be included with QuantumATK to take these effects into account.

Key Benefits of QuantumATK

• Obtain photocurrent density as a function of applied voltage and photon energy at different temperatures.
• Investigate how open circuit voltage depends on light intensity and temperature.
• Calculate power density as a function of applied voltage at different temperatures.
• Investigate band alignment in various interfaces (front end, back end, etc.).
• Simulation diffusion of defects.

2a. Calculation of Electronic Properties

Nanoscale dimensions and increasing complexity of materials render its experimental characterization challenging and thus there is a need for atomic-level modeling to complement experimental characterization of electronic properties. Completeness of methods, ease-of-use and advanced post-processing capabilities make QuantumATK superior to other tools for calculation of electronic properties.

QuantumATK is the only code including pseudopotential-based Density Functional Theory (DFT) with LCAO and plane-wave basis sets in one framework. Being fully integrated into the QuantumATK NanoLab environment, QuantumATK DFT-PlaneWave code is probably the most flexible and user friendly plane-wave code available.  This makes it possible to shift seamlessly from LCAO to Plane-Wave basis sets, and, thus, easily adjust and test tradeoffs between speed and accuracy. It is also very easy to plot projections of band structure and density of states (DOS) onto atoms, spin, orbitals, or angular momenta, in any desired combination, combine plots, e.g. band structure and DOS, turn a 2D plot into a Python script, which can be modified or batch-processed.

Key Benefits of QuantumATK

• Investigate how band structure, DOS and their projections, molecular spectrum, Fermi surfaces, exchange coupling constants, spin life time, magnetic anisotropy energy, and many more electronic properties change with material composition and structure.
• Examine how defect formation energies and thermodynamic transition levels depend on the type of defect (vacancy, substitutional, interstitial), charge state and supercell repetition.
• Predict reaction mechanisms (transition states, reaction pathways, and reaction barriers) using the nudged elastic band (NEB) method with and without an electric field. 

2b. Calculation of Optical Properties

Optic and electro-optical analysis tools are of paramount importance when characterizing emerging bulk, 2D materials and nanostructures, extracting information about vibrational, and chemical properties, inhomogeneities, strain, crystallinity, electron-phonon coupling and anharmonicities in a local environment, and detecting different structural phases. QuantumATK enables simulation and advanced analysis of an exceptionally large range of optical and electro-optical parameters. The fully automated workflows in NanoLab GUI, from initial relaxation to calculation of advanced spectroscopic properties, highly reduce the chance of errors and turnaround time (TAT). Furthermore, QuantumATK stands out for offering advanced features for polar materials, i.e. including ionic contribution to optical properties, and including temperature dependencies through electron-phonon coupling.

Key Benefits of QuantumATK

• Simulate Raman spectrum: either polarization dependent for one or multiple angles between incoming and scattered light, or polarization averaged spectrum
• Simulate infrared spectrum
• Obtain refractive indices, extinction coefficients, reflectivity, susceptibility, optical conductivity
• Simulate optical spectrum including a possibility to calculate the intraband contribution for metals
• Predict second order susceptibility
• Calculate electro-optical tensor
• Conveniently resolve different phonon contributions to optical properties

2c. Calculation of the Phonon-limited Electron Mobility of Bulk Materials and Nanoscale Systems

Electron mobility is an important performance indicator of materials; however, it is very challenging to model at the atomic scale. In order to calculate mobility accurately, multiple effects must all be considered: electron-phonon interactions (i.e., electron scattering due to interactions with the vibrations of the lattice), surface effects, strain, and the dimensions and shapes of the material on the nanoscale. Including such effects, in particular for amorphous materials, systems with low lattice symmetry, nanostructures and devices is very complex. In QuantumATK we strive to make this type of calculations easily accessible, accurate, fast, and able to handle large systems. Choose between three different methods to calculate phonon-limited mobility: Boltzmann Transport Equation, Molecular Dynamics-Landauer and Special Thermal Displacement-Landauer methods, depending on the size and the type of your system.

Key Benefits of QuantumATK

• Determine how electron mobility changes with material composition, structure, and lattice symmetries.
• Predict how electron mobility depends on carrier density and temperature,
• Obtain good understanding of electron transport in materials,
• Investigate the impact of defect scattering and grain boundary scattering on mobility.

2d. Calculation of Mechanical Properties

There is a need for new materials with improved mechanical properties to be used in various fields, such as aeronautics, automotive, construction, etc.  QuantumATK atomic-scale modeling tools are optimized for large scale simulations of mechanical properties and are able to predict and screen new materials or processes, thus significantly reducing the number and cost of experiments. Two aspects make QuantumATK tools superior to other available tools. First, the QuantumATK ForceField database contains more than 300 empirical classical potentials, in addition to the possibility of adding your own and literature potentials and combining them, or easily switching to density functional theory or semi-empirical methods. Secondly, Python scripting interface makes it easy to create and analyze both standard and highly customized simulations of mechanical properties.

Key Benefits of QuantumATK

• Examine how elastic constants and more general moduli, such as bulk, shear, and Young’s modulus change with materials composition and structure, and how defects affect these properties.
• Set up advanced structures, such as polycrystalline materials.
• Gain atomistic insight into interplay between the properties of the crystal and grain boundaries.
• Perform molecular dynamics (MD) simulations to gain insight into physical processes (for example, creep simulation) in order to access atomic structures for which little experimental information is available. 

2e. Simulation of Polymers

There is a need for polymers with improved thermomechanical and other material properties for automotive and manufacturing industries. QuantumATK atomic-scale modeling tools are optimized not only for studying novel polymer systems, but also for investigating how known polymer properties change upon blending them with other polymers, molecules and nanoparticles.  NanoLab's advanced GUI is extremely flexible and user friendly streamlining the building of such polymer systems while having full control of tacticity, monomer composition, and end groups. Advanced polymer building and equilibration tools provide reliable polymer models, which can then be simulated using fully automated workflows, powered by a highly scalable MPI parallelized molecular dynamics engine. In addition to this, three other aspects make QuantumATK special and superior to other available tools. First, QuantumATK enables simulations of long time-scales through time-stamped force-bias Monte Carlo simulations, e.g. to get over barriers in establishing molecular distributions in polymer systems. Secondly, it enables simulation of heat transport. Lastly, it provides advanced analysis tools in the GUI, such as a glass transition temperature analyzer, temperature profile for heat transport and others to advance polymer engineering.

Key Benefits of QuantumATK

• Advanced tools for building and equilibrating polymer melts
• Obtain thermomechanical properties, such as glass transition temperature, elastic and dynamic moduli, e.g., Young’s modulus, at different temperatures
• Study polymer performance under high deformation / shear rates, high cooling / heating rates
• Simulate heat transport
• Calculate optical properties

Supported polymer systems

• Linear homo- and co-polymers
• Polymer blends, such as polymer/molecule, e.g., small photo acid generator (PAG) molecules in PMMA polymer matrix for photoresist applications, and polymer/polymer, e.g., poly-butadiene/styrene copolymer melt for tire industry
• Polymers on surfaces, e.g., polymer-polymer interface, polymer-metal interfaces, polymer on SiO2, etc. surface
• Polymer/nanoparticle composites, e.g., polymer melts with SiO2 nanoparticles for tire industry
• Polymers with specific chemical composition

2f. Simulation of Thermal Transport in Crystals, Nanostructures, and Interfaces

With continuing increase in the thermal loads on materials and devices, new materials with improved thermal properties need to be investigated. Nanoscale dimensions and increasing complexity of materials present a couple of challenges in computationally predicting thermal conductivity of materials, such as modeling the interplay of grain boundaries and modeling interfaces between different materials. QuantumATK offers two methods for efficient and insightful simulation of thermal transport in crystals, nanostructures, through grain boundaries and interfaces: calculation of the phonon transmission spectrum based on Non-Equilibrium Green’s Function technique (NEGF) and Non-Equilibrium Molecular Dynamics (NEMD) technique in which thermal flux is imposed in the system.

Key Benefits of QuantumATK

• Obtain thermal conductance/conductivity of solid materials, nanostructures, interfaces and grain boundaries and its dependence on temperature.
• Investigate how to maximize-minimize thermal conductance by constructing interfaces between different materials.
• Determine grain size impact on thermal conductance.

2g. Simulation of Thin Film Growth with Various Deposition Techniques

The structures of deposited films are often clouded with much uncertainty and contradictory results as no experimental technique can give us a precise determination of the exact molecular topology. With QuantumATK molecular dynamics (MD) simulation tools you can complement experiments (diffraction, STEM, ADF, etc.) by gaining an atomistic insight into various deposition processes (vapor deposition, sputtering, ion implantation, etc.). Two aspects make QuantumATK MD tool highly suitable for simulating thin film growth. First, it allows newly deposited atoms or molecules to be added to the current configuration at each cycle. Secondly, it allows users to use Python scripting to exactly specify how the deposition is supposed to take place.

Key Benefits of QuantumATK

• Determine underlying microscopic mechanisms of deposition.
• Gain atomistic insight into microstructure of deposited films and how it depends on process parameters, such as pressure, temperature, etc.
• Obtain structural differences in films deposited using different deposition or preparation techniques (e.g. melt-quench).

2h. Simulation of Thin Film/Surface Heterostructures

Investigation of thin film/surface heterostructures is of particular importance in various fields, such as electronics and (photo)electrochemistry. For example, thin insulating films on metal substrates were showed to be very promising in inducing changes in work function of the metal support. Complexity of thin film/surface heterostructures requires accurate, insightful, and computationally efficient modeling. Two aspects make QuantumATK tools superior to other tools available for studying thin film/surface heterostructures. First, QuantumATK provides user-friendly tools to build low-strain interfaces and optimize their structure. Secondly, the surface Green’s function (SGF) method in QuantumATK simulates the properties of a truly semi-infinite system, which is divided into a finite surface region and a semi-infinite bulk region. Thus, the SGF method accounts well for the bulk states in the surface region and ensures that calculated metal work functions are almost independent of the surface region thickness.  In contrast, the traditional slab approach, de facto standard for first principles atomistic simulations of surfaces, models a surface structure with just a few atomic layers and, thus, suffers from the finite-size effects.

Key Benefits of QuantumATK

• Obtain band alignment between the surface and a thin-film at different doping levels of the surface region.
• Calculate band alignment parameters: induced charge in the film, interface potential (Schottky barrier) at the heterostructure interface, offsets between the film and bulk valence (conduction) band minima.
• Investigate how thin film thickness, thin film structure, and lattice parameters impact a metal work function.
• Examine how the nature of thin film – metal interface affects a metal work function. 

2i. Simulation of Electronic Surface States in External Electric Fields

Modeling electronic surface states in noble metals, topological insulators, etc. with or without external electric fields is often a very challenging task with the commonly-used slab method due to interaction of two equivalent surface states. QuantumATK offers an alternative unique tool, surface Green’s functions (SGF) method, in which the system is described as truly semi-infinite solid, with a surface region coupled to an electron reservoir. This ensures that in SGF there is only one surface and there is no change of the chemical potential of the surface in the presence of electric fields. Furthermore, the procedure of applying electric field in QuantumATK, i.e., shifting the potential near the surface in the vacuum, resembles the scanning tunneling spectroscopy (STS) experiments, and provides good comparison with STS and PES (Photo emission spectroscopy) results.  

Key Benefits of QuantumATK

• Calculate band structure and density of states (DOS) of surfaces for different external electric fields and identification of surface states.
• Obtain field-induced difference in the microscopic in-plane averaged Hartree potential in a surface region for different external electric fields.
• Determine energy shifts of surface states for different external electric fields.