Kinetic Monte Carlo Simulations Research Articles

Percolation theory-based KMC simulation for scaled Fe-FET based multi-bit computing-in-memory with temperature compensation strategy

In this letter, we investigate the impact of percolation transport mechanisms on ferroelectric field effect transistor (FeFET) multi-value storage with kinetic Monte-Carlo (KMC) simulation considering aspect ratio and temperature dependencies. It is found that the portion of the ferroelectric polarization, which dominates the threshold voltage shift of the FeFET, increases when aspect ratio of device decreases. Moreover, randomness of percolation path formation and variations of equivalent conductance can be suppressed, indicating mitigation of device-to-device variations and enhancement of separation of individual states. Besides, to further investigate an amorphous channel promising in multi-bit applications, disorder effects in channel contribute to intrinsic percolation transport, coupling with multi-domain dynamics in Fe-layer, are studied by the high temperature characterization. On this basis, the KMC scheme is further modified to predict multi-value distribution from 300 K to 400 K. To tackle such critical reliability issues induced inaccuracy for in-memory computing (CIM), an efficient write-verify scheme is proposed to mitigate state overlapping and provide in-depth insights for co-design of device reliability and multi-bit CIM performances.

Hybrid simulation method for agglomerate evolution in driven granular gases

We present a new hybrid simulation method for protoplanetary dust evolution that is efficient and takes into account the complex fragmentation and agglomeration dynamics. We applied it to simulate the evolution of agglomerate size distributions for turbulent, charged systems. The hybrid method combines kinetic Monte Carlo and discrete element simulations in such a way that the expensive latter is only deployed when two agglomerates collide. This method can easily be extended to include additional driving mechanisms, interactions, and the effects of inhomogeneities. Our simulations reveal an emerging steady state in the size distribution. Due to the efficiency of our method, we are able to extend previous results with improved statistics for the size distribution of large agglomerates; in addition to the previously reported power law, we find a regime with an exponential decay.

The effects of surface kink nucleation on the dislocation mobility for bcc metals: a kinetic Monte Carlo study

Abstract Dislocation plasticity in bcc metals at low temperatures and stresses is well known to be controlled by screw dislocation mobility. Screw dislocations move by the nucleation of kink-pairs on the dislocation line and is a relatively well studied phenomenon. However, how screw dislocation mobility is influenced by interfaces and surfaces has not been well-studied. To provide insight into the role interfaces play in screw dislocation mobility, this study proposes an empirical model to treat a grain boundary as a dislocation source, which is then implemented into kinetic Monte Carlo simulations for body-centered-cubic (bcc) metals. This effort focuses on the roles of kink nucleation and migration processes at the interface, comparing the energetics of these events on interfaces versus those on the interior of the dislocation line. The findings reveal that interfaces can either enhance dislocation motion or not affect it at all, depending on temperature, stress, and dislocation length. This work provides insights into the mesoscale behavior of bcc metals and bridges the gap between experimental observations and computational models at small scales.

Application of Time Series Analysis on Kinetic Monte Carlo Simulations of Hyperthermal Oxidation of Graphite

Application of Time Series Analysis on Kinetic Monte Carlo Simulations of Hyperthermal Oxidation of Graphite

Gas-phase and water-mediated mechanisms for the OCS + OH reaction.

We report a computational study of the gas-phase and water-mediated mechanisms for the oxidation of carbonyl sulfide (OCS) by the hydroxyl radical. To achieve reliable results, we employ a dual-level strategy within interpolated single-point energies (VTST-ISPE) at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level of theory. In the gas-phase mechanism, we have determined the rate constants by kinetic Monte Carlo simulation in the interval of temperatures of 250-550 K. The calculated rate constant, at room temperature, is 4.86 × 10-16 cm3 molecule-1 s-1, in agreement with experimental measurement: 6.00 ± 4.00 × 10-16 cm3 molecule-1 s-1 [M. T. Leu and R. H. Smith, J. Phys. Chem., 1981, 85, 2570-2575]. The water-mediated mechanism, a more complex process than the gas-phase, revealed six reaction pathways. The application of the pre-equilibrium model allowed us to determine termolecular thermal rate constants. Considering the concentrations of water as a function of the relative humidity at 0 km altitude, we estimated effective rate constants. The magnitude of the rate coefficients for this mechanism suggested a negligible effect of the water in the OCS + OH reaction.

Layer-by-layer growth, apparent instability, and mound coarsening in thin film deposition controlled by thermally activated surface diffusion

Thin film deposition from vapor with adsorbate diffusion and Ehrlich-Schwoebel (ES) barriers at step edges is studied by kinetic Monte Carlo (KMC) simulations and scaling approaches. Mound coarsening with slope selection is shown at the largest thicknesses, where roughness (W) and correlation length (ξ) scale in time with exponents β≈n≈0.25, consistently with theoretical works but contrasting with previous simulations that systematically showed temperature-dependent exponents. At a given film thickness, we show that the mounded morphology is independent of the ES barrier, so that interlayer transport is suppressed, while the temperature increase of W and ξ is consistent with intralayer transport limited by adatom detachment from flat terrace borders. Initial layer-by-layer (LBL) growth occurs above a characteristic temperature that increases with the activation energies of terrace diffusion and of ES barrier, but weakly depends on the external flux. Subsequently, as the surface roughness is near the atom/molecule size, there is rapid roughening where, counterintuitively, larger effective growth exponents (β>0.5, suggesting unstable growth) are correlated with longer LBL regimes. Similar LBL-rough transitions were reported in organic film deposition and the model application to diindenoperylene films suggests a large ES barrier ∼0.4eV. In the mound coarsening regime, W/ξ and θ1/2/Wξ converge to constant values dependent on surface diffusion coefficients (θ is the film thickness). This convergence suggests mound coarsening with slope selection in SnTe films whose effective β is anomalously large, showing how film morphology can be interpreted beyond exponent measurements.

Molecular-scale kinetic Monte Carlo simulation of pattern formation in photoresist materials for EUV nanolithography

A kinetic Monte Carlo (KMC) simulation tool for modeling the pattern formation process in photoresist materials for extreme ultraviolet (photon energy 92 eV) nanolithography is presented. The availability of such a tool should support the progress toward novel materials and experimental procedures that lead to an improved pattern resolution. The molecular-scale simulations describe the process in a stochastic and mechanistic manner and include the excitation of high-energy electrons upon light absorption, the creation of a charged-particle cloud, electron-induced chemical degradation of the photoresist molecules, the resulting bond formation between neighboring degraded molecules, and a chemical development step after which a pattern of the remaining non-dissolved molecules is obtained. The method is applied to the application-relevant class of Sn-oxocore photoresist materials and uses their known electronic structure and optical electron energy loss function. The validity of the approach is tested by comparing measured and simulated total electron yield spectra and photoelectron spectra. A demonstration of the method is given by calculating the dose and pitch dependent average shape and stochastic variability (line edge roughness) of line patterns that are obtained for rectangular and sine-wave illumination, assuming various scenarios that determine how molecular-scale degradation will lead to bond creation. We show how from these simulations the ultimate pattern resolution can be deduced. The findings are analyzed systematically using results of KMC simulations that reveal the size of the cloud of degraded molecules around a point of absorption (blur length) and that further reveal the sensitivity to uniform illumination (contrast curves), and using percolation theory. We find that KMC modeling captures the consequences of the strong gradients in the density of degraded molecules and of the stochasticity of the patterning process that simplified models do not include, leading to a significantly improved view of the final pattern quality.

Tight Binding Molecular Dynamics Study of Growth of Nanostructure Materials

Abstract The growth dynamics of mixed In and Se films on the MoSe2 substrate is modeled using quantum mechanical tight-binding-based molecular dynamics simulations. The substrate temperature, the ratio of In and Se, and the deposition flux are chosen to mimic the experimental conditions as closely as possible. The conversion of individual adatoms and clusters into alternate layers of Se-In-
Se is captured during our simulations. One of the main outcomes of our model is that we are able to identify and obtain energy barriers for the concerted motion of some species on the substrate surface. Such kind of estimates of energy barriers are important for larger scale kinetic Monte Carlo simulations. Furthermore, we have identified different diffusion regimes of In and Se.

Geregelter Ladungstransfer in metallorganischen Donor‐Akzeptor‐Gerüstverbindungen für hochempfindliche Photodetektoren

AbstractBei der photoinduzierten Ladungstrennung sind organische Dünnschichten mit Donor‐ und Akzeptor‐Chromophoren für Anwendungen wie künstliche Photosynthese und Photodetektion von entscheidender Bedeutung. Eine der größten Herausforderungen ist die Optimierung der Ladungstrennungseffizienz und die Bestimmung des optimalen Akzeptor/Donor‐Verhältnisses. Dies ist aufgrund der Variabilität der molekularen Packung in diesen typischerweise amorphen organischen Aggregaten nur schwer zu erreichen. Metallorganische Gerüstverbindungen (MOFs) bieten einen rationalen Ansatz, indem sie die systematische Herstellung von Donor/Akzeptor‐Mischungen mit einstellbaren Verhältnissen innerhalb eines kristallinen Gitters ermöglichen. Wir demonstrieren diesen Ansatz, indem wir Naphthalindiimid (NDI)‐Donor‐ und Akzeptor‐Chromophore als Linker in eine hoch orientierte, monolithische MOF‐Dünnschicht einbauen. Durch die Anpassung der NDI‐Akzeptor‐Linkerkonzentration während der Schicht‐für‐Schicht‐Synthese der oberflächenmontierten MOF‐Dünnschichten (SURMOFs) konnten wir die Effizienz der Ladungstrennung erheblich steigern. Überraschenderweise lag die optimale Akzeptorkonzentration bei nur 3 %, womit eine vierzigfache Steigerung in der Effizienz der Photodetektion im Vergleich zu rein NDI‐Donor‐basierten SURMOFs erreicht wurde. Dieses unerwartete Verhalten wurde durch eine theoretische Analyse erklärt, die durch die gut definierte kristalline Struktur der SURMOFs ermöglicht wurde. Mit Hilfe der Dichtefunktionaltheorie und kinetischer Monte‐Carlo‐Simulationen konnten wir zwei gegensätzliche durch Akzeptoren bewirkte Effekte feststellen: Dem positive Effekt der Unterdrückung einer unerwünschten Rekombination von Ladungsträgern wirkt bei hohen Konzentrationen eine Verringerung der Ladungsträgermobilität entgegen.

Regulated Charge Transfer in Donor-Acceptor Metal-Organic Frameworks for Highly-Sensitive Photodetectors.

In photo-induced charge separation, organic thin films with donor and acceptor chromophores are vital for uses such as artificial photosynthesis and photodetection. The main challenges include optimizing charge separation efficiency and identifying the ideal acceptor/donor ratio. Achieving this is difficult due to the variability in molecular configurations within these typically amorphous organic aggregates. Metal-organic frameworks (MOFs) provide a structured solution by enabling systematic design of donor/acceptor blends with adjustable ratios within a crystalline lattice. We demonstrate this approach by incorporating donor and acceptor naphthalenediimide (NDI) chromophores as linkers in a highly oriented, monolithic MOF thin film. By adjusting the NDI acceptor linker concentration during the layer-by-layer assembly of surface-anchored MOF thin films (SURMOFs), we significantly enhanced charge separation efficiency. Surprisingly, the optimum acceptor concentration was only 3 %, achieving a forty-fold increase in photodetection efficiency compared to baseline NDI donor-based SURMOFs. This unexpected behaviour was clarified through theoretical analysis enabled by the well-defined crystalline structure of the SURMOFs. Using density functional theory and kinetic Monte Carlo simulations, we identified two opposing effects from acceptors: the positive effect of suppressing undesirable charge carrier recombination is offset at high concentrations by a reduction in charge-carrier mobility.

Mechano-chemical competition in driven complex concentrated alloys

Mechanically driven complex concentrated alloys can be formed by three or more types of elemental powders mixing into a solid solution through severe plastic deformation at temperatures much lower than the melting point of each element. While competition between the thermal and mechanical driving forces during forced mixing of binary systems with positive enthalpy of mixing is relatively well understood, the physics of mechanically driven mixing of multiple elements with negative mixing enthalpies remained unclear. In this work, we combined mechanical alloying (MA), X-ray diffraction (XRD), and kinetic Monte Carlo (kMC) simulations to systematically study the interplay between the chemical pairing potential and the mechanical strength of elemental pairs during the formation process of mechanically driven complex concentrated alloys. We demonstrate that the chemical and mechanical forces play a competing role in the mixing of ternary complex concentrated alloys with negative mixing enthalpies. The chemical driving force favors a chemically ordered atomic structure, while the mechanical force encourages a random atomic arrangement. We reveal the energetic basis of this competition as the gain and loss in mixing enthalpy and configurational entropy. Following this fundamental understanding, three types of mixing mechanisms and their corresponding steady-state phases are defined. We show that the molar content of the element with the lowest average mixing enthalpy governs the mixing mechanisms and thus determines the energetic stabilization of the steady-state phases. A theoretical phase prediction map is provided for alloy design. We synthesized a nanocrystalline equiatomic NiCoCr coating under the guidelines of the map, which presents exceptional mechanical properties achieved by the mechano-chemical mixing.

Cathodic Corrosion of Copper: A DFT and Kmc Simulation Study

The electrochemical CO2 reduction reaction (eCO2RR) is one of the promising pathways which primarily leverages electrocatalysts such as copper (Cu) to facilitate the conversion of CO2 into higher value carbon products; C1 (e.g., CO, methane, formic acid, and methanol etc.) and C2 (e.g., ethylene, ethane, and ethanol etc.) using renewable electricity. Cu is the only monometallic electrocatalyst able to electrochemically transform CO2 into higher hydrocarbons with appreciable activity and selectivity. However, Cu and oxide-derived Cu surfaces (OD-Cu) fall short in stability or durability during eCO2RR. One significant challenge is the cathodic corrosion of Cu. Cathodic corrosion leads to degradation of Cu surfaces, reducing their catalytic activity, selectivity, and long-term operability. In this study, using a combination of density functional theory (DFT) and kinetic Monte Carlo (KMC) simulations, we delve into the intricate mechanisms driving this phenomenon. Our hypothesis posits the pivotal role of alkaline hydrogen evolution reaction (HER) in facilitating cathodic corrosion. Through comprehensive analysis, we establish correlations between various microenvironments, including subsurface oxygen (Osub) diffusion, pH-dependent OH adsorption on the Cu surface, and Cu dissolution as a (Cu-OH) complex into the electrolyte solution. We rigorously calculated the pH-dependent OH adsorption on the surface, as well as the stability and diffusion behavior of Osub. Furthermore, our investigation explores the significance of under-coordinated sites in Cu corrosion. By integrating DFT-derived thermodynamic data into our KMC model, we successfully predict the formation of experimentally observed corrosion pits on Cu surfaces. This combined approach not only advances our fundamental understanding of Cu cathodic corrosion but also offers insights crucial for developing effective corrosion mitigation strategies. The data obtained from theoretical calculations were also corroborated with available experimental observations. Finally, the details of this study will be presented during the conference. Figure 1

Structural Origins of High Performance in Partially Disordered Li- and Mn-Rich Cathodes

The discovery of Li-excess cation-disordered rocksalt (DRX) cathodes lifts the restriction on a specific Li-TM ordering to enable the exploration of cathode materials in a chemical space of earth abundant materials. Recently, phase transformation in disordered Li- and Mn-rich rock-salts (Li1.05Mn0.85Ti0.1O2) have been shown to form high energy density, capacity retention and rate capability cathode materials (named delta) with partial Spinel-like ordering at short coherence lengths.1 However, atomic scale structural features of the material which determine its high performance such as extent of disordering and Li-transport pathways are not well-understood yet. We first perform symmetry analysis to reveal aspects of the transformation which are central to the nanoscale domain formation in the delta phase and crucial for characterizing its structure. We develop a simple structural order parameter to distinguish the Spinel-like ordering from the disordered rocksalt (DRX) structure, characterizing the extent of disordering, sizes and spatial location of the domains as the transformation proceeds. We perform multi time-scale Kinetic Monte Carlo Simulations for understanding the DRX-to-delta transformation. Our results reveal both thermodynamic and kinetic factors causing the formation of spinel-like nano domains and spatial distribution of Ti and Li-excess in the delta phase of Li1.05Mn0.85Ti0.1O2, providing key insights into its high rate capability. Our simulations reveal principles for designing the structure at nanoscale and electrochemical performance through changes in composition. Cai, Z.; Ouyang, B.; Hau, H.-M.; Chen, T.; Giovine, R.; Koirala, K. P.; Li, L.; Ji, H.; Ha, Y.; Sun, Y.; Huang, J.; Chen, Y.; Wu, V.; Yang, W.; Wang, C.; Clément, R. J.; Lun, Z.; Ceder, G. In Situ Formed Partially Disordered Phases as Earth-Abundant Mn-Rich Cathode Materials. Nat. Energy 2023, 1–10. https://doi.org/10.1038/s41560-023-01375-9.

Role of Short-Range Order on Diffusion Coefficients in the Li-Mg Alloy.

Li-Mg alloys are important because of their beneficial role in fostering uniform plating and stripping of lithium in all-solid-state batteries. The alloy forms a solid solution on the BCC crystal structure when the lithium content is greater than x ≈ 0.3. The activation barriers of lithium and magnesium exchanges with a vacancy, crucial for substitutional diffusion, are predicted to be exceptionally low and almost identical, with negligible dependence on the alloy composition. The equilibrium vacancy concentration at room temperature is predicted to be very low, and it also remains almost constant with no dependence on Mg content in the alloy (for ). Nevertheless, both experiments and kinetic Monte Carlo simulations indicate that the tracer diffusion coefficients decrease by almost an order of magnitude with the addition of Mg to the alloy. In this contribution, the crucial role that chemical short-range order plays in affecting the diffusion coefficients is studied. Chemical short-range order is found to increase the effective activation barrier for lithium and magnesium diffusion by making successive atomic hops with vacancies correlated.

Variable‐Range Hopping Conduction in Amorphous, Non‐Stoichiometric Gallium Oxide

AbstractAmorphous, non‐stoichiometric gallium oxide (a‐GaOx, x < 1.5) is a promising material for many electronic devices, such as resistive switching memories, neuromorphic circuits and photodetectors. So far, all respective measurements are interpreted with the explicit or implicit assumption of n‐type band transport above the conduction band mobility edge. In this study, the experimental and theoretical results consistently show for the first time that for an O/Ga ratio x of 0.8 to 1.0 the dominating electron transport mechanism is, however, variable‐range hopping (VRH) between localized states, even at room temperature and above. The measured conductivity exhibits the characteristic exponential temperature dependence on T−1/4, in remarkable agreement with Mott's iconic law for VRH. Localized states near the Fermi level are confirmed by photoelectron spectroscopy and density of states (DOS) calculations. The experimental conductivity data is reproduced quantitatively by kinetic Monte Carlo (KMC) simulations of the VRH mechanism, based on the ab‐initio DOS. High electric field strengths F cause elevated electron temperatures and an exponential increase of the conductivity with F1/2. Novel results concerning surface oxidation, magnetoresistance, Hall effect, thermopower and electron diffusion are also reported. The findings lead to a new understanding of a‐GaOx devices, also with regard to metal|a‐GaOx Schottky barriers.

Deciphering the reaction network for the acetoxylation of propylene to allyl acetate on PdCu catalysts combined DFT with kMC analysis

The acetoxylation of propylene to allyl acetate on PdCu catalysts is a critical step in the Lyondell method for BDO production. By combining Density Functional Theory and kinetic Monte Carlo simulations, the reaction network and the formation pathways of allyl acetate and CO2 were systematically studied. Propylene and acetic acid were both activated by oxygen-assisted dehydrogenation to obtain CH3COO* and CH2CHCH2*, which would participate in the C-O coupling reaction to form allyl acetate, with the apparent activation energy of 0.64 eV. Besides, the dominant paths for the formation of CO2 from the complete oxidation of propylene and acetic acid were identified with the apparent activation energy of 1.80 eV. As the temperature rises, the TOF of allyl acetate and CO2 formation increase gradually, while the molar fraction of allyl acetate in the products decreases, indicating that temperature exerts a greater influence on the formation of CO2 than allyl acetate.

Pressure-Dependent Shape and Edge Configurations of MoS2 by Kinetic Monte Carlo Simulation.

Understanding the influence of precursor pressures is crucial for optimizing the properties of MoS2 grown through the chemical vapor deposition (CVD) process. In this study, we use kinetic Monte Carlo (KMC) simulations to investigate how varying the pressures of molybdenum (PMo) and sulfur (PS) impacts the structural properties of MoS2, such as grain shape and edge configurations. The simulations differentiate three distinct regimes─growth, steady-state, and etching─each defined by specific PMo, PS, and the most probable atomic sites for filling or etching. We further explore how these regimes influence the atomic configuration of MoS2, particularly the formation of different edge structures like sulfur zigzag (ZZS), molybdenum zigzag (ZZMo), and their respective derivatives. A pressure diagram based on the equations of state and most probable atomic sites was constructed for each regime and validated by comparing predicted ZZ-derived edges to experimental observations. Additionally, the study examines the impact of etching on various line defects, providing insights into the evolution of the MoS2 edges during the CVD process. These findings underscore the importance of controlling both growth and cessation phases in the CVD process to customize edge configurations, with significant implications for chemical functionalization, catalysis, and the electronic properties of transition metal dichalcogenides.

Real-space investigation of polarons in hematite Fe2O3.

In polarizable materials, electronic charge carriers interact with the surrounding ions, leading to quasiparticle behavior. The resulting polarons play a central role in many materials properties including electrical transport, interaction with light, surface reactivity, and magnetoresistance, and polarons are typically investigated indirectly through these macroscopic characteristics. Here, noncontact atomic force microscopy (nc-AFM) is used to directly image polarons in Fe2O3 at the single quasiparticle limit. A combination of Kelvin probe force microscopy (KPFM) and kinetic Monte Carlo (KMC) simulations shows that the mobility of electron polarons can be markedly increased by Ti doping. Density functional theory (DFT) calculations indicate that a transition from polaronic to metastable free-carrier states can play a key role in migration of electron polarons. In contrast, hole polarons are significantly less mobile, and their hopping is hampered further by trapping centers.

Adsorption and evolution of N2 molecules over ZnO monolayer: a combined DFT and kinetic Monte-Carlo insight

A large surface area, wide band gap, and unique bonding property between Zn and O atoms make the hexagonal ZnO monolayer attractive as a gas sensor. In the present work, the adsorption and evolution of nitrogen (N2) molecules over a ZnO monolayer have been studied using two different theoretical methods: van der Waals density functional theory (vdW-DFT) and kinetic Monte-Carlo (kMC) simulation. The adsorption and diffusion (hopping over the surface) energy of a N2 gas molecule has been calculated considering the different sites over the ZnO substrate using the revPBE-vdW functional. Bader charge, electron localization function analysis, density of states and band structure plotting have been used to understand the adsorption mechanism. Lateral repulsive interaction between two N2 molecules limits the maximum packing number of gas molecules within one hexagonal ring. The output of the vdW-DFT calculation has been fed to the kMC code to predict the rate of adsorption, desorption, and diffusion, along with the overall surface coverage at different temperatures and pressures. Finally, the change in the N2 adsorption energy has been predicted with the increase of the ZnO layer number.

Effects of copper on hydroxyapatite thin films by pulsed laser deposition on silicon

AbstractThis study deals with the effect of Cu on hydroxyapatite (HAp) thin films on silicon substrates prepared by pulsed laser deposition (PLD) technique for Cu concentrations ranging from 0 to 0.8 wt.%. Fourier transform infrared spectroscopy and Raman spectroscopy provide additional evidence of HAp in the films, which exhibit characteristic HAp bands such as the 960 cm−1 phosphate ion band. The X‐ray diffraction patterns confirm the presence of HAp, without copper‐related peaks, suggesting that it is not incorporated into the HAp lattice, which is present as an amorphous phase. X‐ray photoelectron spectroscopy confirms the presence of Cu on the HAp thin films, possibly related to the adsorption of Cu2+ on HAp surfaces through electrostatic or interactions with (PO4)3− groups. Atomic force microscopy shows that the roughness of the films varies depending on the presence or absence of copper ions. Finally, density functional theory and kinetic Monte Carlo simulations explain the effects of atomic diffusion on roughness and HAp clustering. These changes correlate with the contact angle values, indicating the formation of hydrophobic films due to copper inclusions.