Today’s supercomputers power scientific calculations in very different areas, ranging from nanotechnology to climate studies and astronomy. Research groups in nascent areas of science and engineering develop their own scientific software, since it cannot be obtained otherwise. These groups are typically not funded to spend time on performance optimization and software design, but to address their original research questions. In more established fields, several accepted research applications emerge as a standard and get adopted by research groups that are users, with little software development experience. The variability of applications yields a hard to optimize and sub-optimal supercomputer resource usage. Large computing times or large memory requirements often limit the scope of the research exploration and a more optimal use can reduce overall compute time or increase the scientific scope of questions being asked. This work introduces a resource manager that requires minimal intrusion and only superficial understanding of the scientific code, but still schedules the execution of the calculations to optimally utilize memory, central processing unit (CPU) or both. This is exemplified with Quantum Espresso and recursive open boundary and interfaces (ROBIN) calculations on regional and national computer infrastructures. The resource manager presented here reduces compute times by 13–30% in those two scientifically relevant computational codes.

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 MoS2, WS2 and WSe2.

van der Waals p–n heterojunctions using both 2D–2D and mixed-dimensional systems have shown anti-ambipolar behavior. Gate tunability in anti-ambipolar characteristics is obtained in special heterojunction geometries, such as self-aligned black phosphorus/MoS2 p–n heterojunctions. Although the device physics of anti-ambipolar characteristics has been investigated using finite-element or semi-classical device models, an atomic-level description has not yet been developed. This work models the interface physics with quantum transport including incoherent scattering and carrier recombination. Densities of electrons and holes are calculated in DFT-based maximally localized Wannier functions with 2% strain. Qualitative agreement with our experiments is found for both the anti-ambipolar (or Gaussian) behavior and the tunability of Gaussian function in a dual-gated geometry. Carrier recombination is found to determine the overall current density. The two gates control the recombination by regulating the density of electrons in MoS2 and holes in black phosphorus reaching the heterojunction area.

The nonequilibrium Green's function (NEGF) method has become an integral part of computational nanodevice R&D. The reliability of the NEGF method is intimately coupled to the quality of its self-energies as they describe incoherent scattering of electrons on impurities, various types of phonons, and any type of irregularity in the device environment. In this work, we emphasize the impact of approximations on self-energies and ways to avoid them, including approximations on the nonlocality of long-range scattering, the Kramers-Kronig relation of state broadenings and shifts, and the impact of idealized periodicity assumptions in the device vicinity.

Organic reactions in microdroplets can be orders of magnitude faster than their bulk counterparts. We hypothesize that solvation energy differences between bulk and interface play a key role in the intrinsic rate constant increase and test the hypothesis with explicit solvent calculations. We demonstrate for both the protonated phenylhydrazine reagent and the hydrazone transition state (TSB) that molecular orientations which place the charge sites at the surface confer high energy. A pathway in which this high-energy form transforms into a fully solvated TSB has a lower activation energy than bulk by some 59 kJ/mol, a result that is consistent with experimental rate acceleration studies.

Ultra-scaled FET technology requires simulations at the atomic scale. We present the Victory Atomistic tool inherited from Nemo5. Thanks to a combination of non-equilibrium Green's functions and state-of-the-art band structure calculations, versatile, predictive, and fast simulations become accessible within the self-consistent Born approximation, optimized by a generalized low-rank projection.

The nonequilibrium Green’s function 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 Mil’nikov et al. (Phys Rev B 85(3):035317, 2012) 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.

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 introduce 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 that allows explicitly discretizing millions of atoms in real space and to consider virtually any type of environment (be it periodic, regular, or random). The impact of the periodicity assumption is assessed in detail with silicon dopants in graphene. Graphene was confirmed 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 periodicity 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.

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.

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.

A detailed theoretical study of the optical absorption in doped self-assembled quantum dots is presented. A rigorous atomistic strain model as well as a sophisticated 20-band tight-binding model are used to ensure accurate prediction of the single particle states in these devices. We also show that for doped quantum dots, many-particle configuration interaction is also critical to accurately capture the optical transitions of the system. The sophisticated models presented in this work reproduce the experimental results for both undoped and doped quantum dot systems. The effects of alloy mole fraction of the strain controlling layer and quantum dot dimensions are discussed. Increasing the mole fraction of the strain controlling layer leads to a lower energy gap and a larger absorption wavelength. Surprisingly, the absorption wavelength is highly sensitive to the changes in the diameter, but almost insensitive to the changes in dot height. This behavior is explained by a detailed sensitivity analysis of different factors affecting the optical transition energy.

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.

The non-equilibrium Green function (NEGF) method is capable of nanodevice performance predictions including coherent and incoherent effects. To treat incoherent scattering, carrier generation and recombination is computationally very expensive. In this work, the numerically efficient Buttiker-probe model is expanded to cover recombination and generation effects in addition to various incoherent scattering processes. The capability of the new method to predict nanodevices is exemplified with quantum well III-N light-emitting diodes and anti-ambipolar 2D material hetero junctions.

The performance of nitride‐based light emitting diodes is determined by carrier transport through multi‐quantum‐well structures. These structures divide the device into spatial regions of high carrier density, such as n‐GaN/p‐GaN contacts and InGaN quantum wells, separated by barriers with low carrier density. Wells and barriers are coupled to each other via tunneling and thermionic emission. Understanding of the quantum mechanics‐dominated carrier flow is critical to the design and optimization of light‐emitting diodes (LEDs). In this work a multi‐scale quantum transport model, which treats high densities regions as local charge reservoirs, where each reservoir serves as carrier injector/receptor to the next/previous reservoir is presented. Each region is coupled to its neighbors through coherent quantum transport. The non‐equilibrium Green's function (NEGF) formalism is used to compute the dynamics (states) and the kinetics (filling of states) of the entire device. Electrons are represented in multi‐band tight‐binding Hamiltonians. The I–V characteristics produced from this model agree quantitatively with experimental data. Carrier temperatures are found to be about 60 K above room temperature and the quantum well closest to the p‐side emits the most light, in agreement with experiments. Auger recombination is identified to be a much more significant contributor to the LED efficiency droop than carrier leakage.

Transition metal dichalcogenides (TMDCs) based 2D materials appear to very promising materials for tunnel field-effect transistors (TFETs). [1], [2]. The intrinsic subthreshold slope (SS) of these devices is determined by exponentially decaying band tail states/Urbach tails below conduction and valence band edges which places a lower bound on SS. Though there have been few recent studies on calculating and extracting band tails using simple models and qualitative analysis [3], [4], a rigorous study based on atomistic approach is still lacking.

State of the art quantum transport models for semi-conductor 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 work 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.

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.

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.

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.