Kinetics Of Vacancies Research Articles

Configurable Crack Wall Conduction in a Complex Oxide

Mobile defects in solid-state materials play a significant role in memristive switching and energy-efficient neuromorphic computation. Techniques for confining and manipulating point defects may have great promise for low-dimensional memories. Here, we report the spontaneous gathering of oxygen vacancies at strain-relaxed crack walls in SrTiO3 thin films grown on DyScO3 substrates as a result of flexoelectricity. We found that electronic conductance at the crack walls was enhanced compared to the crack-free region, by a factor of 104. A switchable asymmetric diode-like feature was also observed, and the mechanism is discussed, based on the electrical migration of oxygen vacancy donors in the background of Sr-deficient acceptors forming n+-n or n-n+ junctions. By tracing the temporal relaxations of surface potential and lattice expansion of a formed region, we determine the diffusivity of mobile defects in crack walls to be 1.4 × 10-16 cm2/s, which is consistent with oxygen vacancy kinetics.

Interlayer Hopping Kinetics of Vacancies in CrI3 Layers Leading to Monolayer/Bilayer Heterostructures

Abstract2D magnets, especially their multilayer structures exhibit promising applications in magnetic sensing and data storage due to magnetoresistance effect. Defects have significant influence on the performance of devices and are widely investigated for typical 2D magnets. However, the defects in multilayer structures are rarely studied. Here, based on density functional theory, an interesting synergy of the interlayer migration of I and Cr vacancies in bilayer CrI3 is found. Cr vacancies attract I vacancies in adjacent layers and greatly reduce the intermigration barrier of I. On the other hand, I vacancies also facilitate the interlayer migration of Cr vacancies. A monolayer/bilayer heterostructure is expected to form after annealing at about 500 K due to the fast hopping of vacancies between layers and the strong driving force for vacancies clustering. The interlayer hopping and merge of vacancies help eliminate defect states in bilayer CrI3. These results provide an atomic‐scale understanding of vacancy migration between CrI3 layers, which is important for defect engineering and applications of bilayer and multilayer CrI3.

Absorption kinetics of vacancies by cavities in aluminum: Numerical characterization of sink strengths and first-passage statistics through Krylov subspace projection and eigenvalue deflation

Modeling the microstructural evolution of metal and alloys, specifically under irradiation, is essential to predict the aging properties of materials. Many models are based on a transition rate matrix describing the jump frequencies of defects and involve a master equation governing the time evolution of a state probability vector. Here, we present non-stochastic numerical techniques to characterize the motion of individual defects migrating over long distances prior to recombining or being absorbed by another defect, resorting to the theory of absorbing Markov chains. These important events are fully determined by their first-passage time distribution to distant locations, no-passage distribution, and walker fluxes to the sinks. We show that these functions can be efficiently computed using a method combining Krylov subspace projection and eigenvalue deflation. For a model system describing the absorption of a vacancy by a cavity in aluminum, the use of a small Krylov subspace deflated by the unique eigenmode corresponding to the quasi-stationary distribution is sufficient to capture the kinetics of the defect absorption faithfully. This method can be used in kinetic Monte Carlo simulations to perform stochastic non-local moves or in cluster dynamics simulations to compute sink strengths.

Strain Dependence of Energetics and Kinetics of Vacancy in Tungsten.

We investigate the influence of hydrostatic/biaxial strain on the formation, migration, and clustering of vacancy in tungsten (W) using a first-principles method, and show that the vacancy behaviors are strongly dependent on the strain. Both a monovacancy formation energy and a divacancy binding energy decrease with the increasing of compressive hydrostatic/biaxial strain, but increase with the increasing of tensile strain. Specifically, the binding energy of divacancy changes from negative to positive when the hydrostatic (biaxial) tensile strain is larger than 1.5% (2%). These results indicate that the compressive strain will facilitate the formation of monovacancy in W, while the tensile strain will enhance the attraction between vacancies. This can be attributed to the redistribution of electronic states of W atoms surrounding vacancy. Furthermore, although the migration energy of the monovacancy also exhibits a monotonic linear dependence on the hydrostatic strain, it shows a parabola with an opening down under the biaxial strain. Namely, the vacancy mobility will always be promoted by biaxial strain in W, almost independent of the sign of strain. Such unexpected anisotropic strain-enhanced vacancy mobility originates from the Poisson effect. On the basis of the first-principles results, the nucleation of vacancy clusters in strained W is further determined with the object kinetic Monte Carlo simulations. It is found that the formation time of tri-vacancy decrease significantly with the increasing of tensile strain, while the vacancy clusters are not observed in compressively strained W, indicating that the tensile strain can enhance the formation of voids. Our results provide a good reference for understanding the vacancy behaviors in W.

Kinetics of radiation defect formation in iron under high displacement rate irradiation

In this study, the kinetics of vacancy and interstitial formation has been simulated in iron at a temperature of 600 K under irradiation at high radiation defect production rate 6.5·10-3,3.25·10-3,1.63·10-3dpa/s and displacement energies of 10 keV, 15 keV, 20 keV. The reaction-diffusion equations and molecular dynamics simulation of displacement cascades have been combined to calculate the kinetics of radiation defect formation. We have introduced the criterion to estimate homogeneity of distribution of radiation defects in the system. The kinetics of radiation defect formation has been calculated up to doses of ~5·10−5 dpa, whereas stationary values of their concentration are achieved at ~10−5 dpa. It has been shown, that high defect production rate of irradiation causes inhomogeneous distribution of the radiation defects with a finite life-time. Also, we consider the effect of high defect production rate and displacement energy on kinetics of radiation defect formation and the self-diffusion mechanism in iron at the considered temperature.

Oxygen Vacancy Kinetics Mechanism of the Negative Forming-Free Process and Multilevel Resistance Based on Hafnium Oxide RRAM

Switching between high resistance states and low resistance states in a resistive random access memory device mainly depends on the formation and fracture of conductive filaments. However, the randomness of the conductive filament growth and the potential breakdown of the large voltage in the forming process will lead to unstable resistive switching and memory performance. We studied the possible natural forming process of conductive filaments for intrinsic defects under the influence of top electrode material based on the structure of W/HfO2-x/Pt. Such a simple device shows long retention time and great endurance cycles. The dendritic oxygen vacancy (VO) conductive filament model was constructed, and the dynamic VO migration under directional external bias was described according to the characteristic electrical performance. In addition, we also explored the relationship between the multilevel resistance and the evolution of a dendritic VO conductive filament, signifying the potential application of multilevel storage in the future. Furthermore, a Ag/HfO2-x/Ag selector was fabricated to assemble the memory device in wire connection which exhibits the potential of eliminating leaky current in the memory array. The connection also indicates that the fabrication process of the 1S1R structure can be simplified by using the same functional layer.

Structure, Stability, and Kinetics of Vacancy Defects in Monolayer PtSe2: A First-Principles Study.

The recent epitaxial growth of monolayer PtSe2 has raised hope for its novel applications in valleytronic, spintronic, and energy-harvesting devices. Compared with 2H-phase transition-metal dichalcogenides, the 1T-phase PtSe2 is much less studied and this is especially true for its defects behaviors and their influence on electronic properties. In this article, we systemically explore the structure, stability, and kinetics of both Pt and Se vacancies in monolayer PtSe2 using first-principles calculations. By examining the relative energies of these vacancies, we identify the most stable Se/Pt single and double vacancies. In particular, we reveal a new type of Se double vacancy structure with the lowest energy. Energetically, both Se and Pt single vacancies prefer to combine to form double vacancies. All Se and Pt vacancies have remarkable influence on the electronic properties. Moreover, Pt single and double vacancies can introduce strong spin polarization in PtSe2, which may be promising for spintronic applications. These findings not only enrich the fundamental understanding of 1T-phase PtSe2 but also provide useful guidance to design PtSe2 for its optoelectronic and spintronic applications.

Kinetics of vacancy annealing upon time-linear heating applied to dilatometry

A kinetic model for the diffusion-controlled annealing of excess vacancies under the experimentally relevant, non-isothermal condition of time-linear heating is presented and applied to dilatometry. The evolution of the vacancy concentration with time is quantitatively analyzed, considering as ideal sinks either dislocations or grain boundaries of spherical- or cylindrical-shaped crystallites. The validity of the model is tested using dilatometry data that were obtained for ultrafine-grained Ni prepared by high-pressure torsion. The entire two-stage annealing curve of the dilatometric length change can be analyzed by combining the present kinetic model of vacancy annealing at grain boundaries with established non-isothermal kinetics of recrystallization.

Entropy and kinetics of point defects in two-dimensional dipolar crystals

We study in experiment and with computer simulation the free energy and the kinetics of vacancy and interstitial defects in two-dimensional dipolar crystals. The defects appear in different local topologies, which we characterize by their point group symmetry; Cn is the n-fold cyclic group and Dn is the dihedral group, including reflections. The frequency of different local topologies is not determined by their almost degenerate energies but is dominated by entropy for symmetric configurations. The kinetics of the defects is fully reproduced by a master equation in a multistate Markov model. In this model, the system is described by the state of the defect and the time evolution is given by transitions occurring with particular rates. These transition rate constants are extracted from experiments and simulations using an optimization procedure. The good agreement between experiment, simulation, and master equation thus provides evidence for the accuracy of the model.

Attachment–detachment limited kinetics on ultra-flat Si(111) surface under etching with molecular oxygen at elevated temperatures

Abstract We have used in situ ultrahigh vacuum reflection electron microscopy to investigate the process of two-dimensional negative islands (etching pits) nucleation on extremely wide (up to 120 μm in diameter) terraces at Si(111) surface during thermal etching with molecular oxygen at temperatures above 1000 °C. By analyzing nucleation kinetics at different temperatures and comparing the results with theoretical model, we determined the dominant surface mass transport mechanism. The squared critical terrace size D crit 2 for nucleation of new island is found to follow power law form with scaling exponent χ changing from 0.95 to 1.15 as the temperature increased T crit = 1180 °C. At temperatures above T crit we estimated the energy of the vacancy–step interaction E AD as 1.5 ± 0.15 eV. We conclude that observed changes of the D crit in high temperature region (above T crit ) can occur through detachment–attachment kinetics of vacancy–step exchange process rather than via surface diffusion.

Energetics and kinetics of vacancies in monolayer graphene boron nitride heterostructures

Graphene and boron nitride (GPBN) heterostructures provide a viable way to realize tunable bandgap, promising new opportunities in graphene-based nanoelectronic and optoelectronic devices. In the present study, we investigated the interplay between vacancies and graphene/h-BN interfaces in monolayer GPBN heterostructures. The energetics and kinetics of monovacancies and divacancies in monolayer GPBN heterostructures were examined using first-principle calculations. The interfaces were shown to be preferential locations for vacancy segregation. Meanwhile the kinetics of vacancies was found to be noticeably modified at interfaces, evidenced by the minimum energy paths and associated migration barriers calculations. The role of interfacial bonding configurations, energy states and polarization on the formation and diffusion of vacancies were discussed. Additionally we demonstrated that it is important to recognize the dissimilarities in the diffusion prefactor for different vacancies for accurate determination of the vacancy diffusion coefficient. Our results provide essential data for the modeling of vacancies in GPBN heterostructures, and important insights towards the precise engineering of defects, interfaces and quantum domains in the design of GPBN-based devices.

Kinetics study of vacancy–oxygen‐related defects in monocrystalline solar silicon

In this work, diffusion and dissociation mechanisms related to the formation and evolution of vacancy–oxygen complexes have been studied. Czochralski‐grown silicon samples have been irradiated at room temperature using fast electrons resulting in the formation of several defects including vacancy–oxygen complexes (VO). The samples were isothermally annealed at different temperatures in the range of 370–470 C. Fourier‐transform infrared spectroscopy has been employed to measure the local vibrational modes associated with the individual defects. The evolution and generation kinetics of vacancy–oxygen complexes have been simulated within the framework of the theory for diffusion‐limited reactions and compared with the experimental data.

A phase field model for dislocation climb

We propose a phase field method to model consistently dislocation climb by vacancy absorption or emission. It automatically incorporates the exact balance between the vacancy flux and the phase field associated with the dislocation evolution, enforced by the conserved character of the total population of vacancies. One of its major advantage is the natural introduction of a dynamic coefficient controlling the kinetics of vacancy emission/absorption by the dislocation. We also derived a closed-form expression of the climb rate valid from the diffusion-limited to the attachment-limited regimes.

Nanoscale mapping of oxygen vacancy kinetics in nanocrystalline Samarium doped ceria thin films

The position-dependent oxygen vacancy dynamics induced by a biased scanning probe microscopy tip in Samarium doped ceria thin films grown on MgO (100) substrates is investigated. The granularity of the samples gives rise to spatially dependent local electrochemical activity, as explored by electrochemical strain microscopy. The kinetics of the oxygen vacancy relaxation process is investigated separately for grain boundaries and grains. Higher oxygen vacancy concentration variation and slower diffusion are observed in the grain boundary regions as compared to the grains.

Plane-strain discrete dislocation plasticity with climb-assisted glide motion of dislocations

A small-strain two-dimensional discrete dislocation plasticity (DDP) framework is developed wherein dislocation motion is caused by climb-assisted glide. The climb motion of the dislocations is assumed to be governed by a drag-type relation similar to the glide-only motion of dislocations: such a relation is valid when vacancy kinetics is either diffusion limited or sink limited. The DDP framework is employed to predict the effect of dislocation climb on the uniaxial tensile and pure bending response of single crystals. Under uniaxial tensile loading conditions, the ability of dislocations to bypass obstacles by climb results in a reduced dislocation density over a wide range of specimen sizes in the climb-assisted glide case compared to when dislocation motion is only by glide. A consequence is that, at least in a single slip situation, size effects due to dislocation starvation are reduced. By contrast, under pure bending loading conditions, the dislocation density is unaffected by dislocation climb as geometrically necessary dislocations (GNDs) dominate. However, climb enables the dislocations to arrange themselves into lower energy configurations which significantly reduces the predicted bending size effect as well as the amount of reverse plasticity observed during unloading. The results indicate that the intrinsic plasticity material length scale associated with GNDs is strongly affected by thermally activated processes and will be a function of temperature.

Diffusion-induced growth and morphological stability of a two-dimensional cavity in an adatomic monolayer on the crystal surface

Diffusion-induced growth of a two-dimensional cavity (a pore) in a monolayer of adsorbed atoms (adatoms) on the crystal surface is considered in the case of the vacancy mechanism for diffusion in the monolayer. In the case in which the cavity radius is smaller than the typical diffusion length of vacancies, the stability of the growing cavity against arbitrary small perturbations of its shape is analyzed with the boundary kinetics of vacancies at the cavity boundary taken into account. Threshold cavity radii are found. Exceeding these radii leads to an increase in both the amplitude of the shape distortion and the amplitude of the relative deformation of the cavity shape.

Homogeneous and Heterogeneous Nucleation of Oxygen in Si-CZ

We present numerical simulations of nucleation kinetics of vacancies and interstitials during RTA and we study the impact of annealing temperature on bulk micro defect concentration. Since the concentration of vacancies and oxygen and also its diffusion kinetics are significantly different inside Czochralski silicon, we assume the nucleation of vacancies and oxygen independent on each other. We show that different populations of voids formed during RTA can influence formation of oxygen precipitate nuclei. According to classical nucleation theory the homogeneous nucleation dominates around temperatures 500 °C while the calculation of oxygen diffusion into the voids shows that the oxygen clusters over the critical size can be formed above temperatures 700 °C. The nuclei concentration of BMD is thus the superposition of homogeneous nucleation below 700 °C and heterogeneous one prevailing above 700 °C.

Vacancies and Self-Interstitials Dynamics in Silicon Wafers

Successful theoretical models of vacancies, self-interstitials and oxygen dynamics during an ingot growth lead us to apply these models results also to silicon wafers. The modern silicon technology uses successfully the RTA (rapid thermal annealing) in order to form MDZ (magic denude zone) in individual CZ Si wafers. The effect of RTA is based on the utilization of vacancies for control of oxygen precipitation. The question about the theory of kinetics of vacancies and interstitials, which describes its behavior within RTA, is still opened up to now. This work deals mainly with the nucleation of vacancies during RTA concerning various cooling rates and initial states.

Oxygen vacancy kinetics in ferroelectric PbZr0.4Ti0.6O3

Oxygen vacancy kinetics in ferroelectric PbZr0.4Ti0.6O3 were studied by oxygen18 (O18) tracer self-diffusion in epitaxial thin films as well as bulk polycrystalline samples. O18 exchange annealing was carried out at an oxygen partial pressure of 250mbar and temperatures between 250 and 400°C. Isotope depth profiling was performed by secondary ion mass spectrometry as well as neutral secondary mass spectrometry. The observed concentration depth profiles of the polycrystalline samples show two distinct diffusion paths, namely, bulk diffusion and grain boundary (GB) diffusion. It appears to be of type B-kinetics in the investigated temperature range, with DGB∕Dbulk⪢100. Donor doped samples with different levels of Nb5+ (1–4mol.%) were also investigated. The effect on the diffusion depth profiles, however, were negligible and can solely be attributed due to the change in the samples microstructure as induced by the dopants. A diffusion coefficient for the bulk diffusion of the O18 isotope, Dbulk=10±5×10−8cm2∕sexp(−0.87±0.1eV∕kT) was found. The faster GB diffusion process shows an activation enthalpy of only EA=0.66±0.2eV.

Kinetic activation-relaxation technique: An off-lattice self-learning kinetic Monte Carlo algorithm

Many materials science phenomena are dominated by activated diffusion processes and occur on time scales that are well beyond the reach of standard molecular-dynamics simulations. Kinetic Monte Carlo (KMC) schemes make it possible to overcome this limitation and achieve experimental time scales. However, most KMC approaches proceed by discretizing the problem in space in order to identify, from the outset, a fixed set of barriers that are used throughout the simulations, limiting the range of problems that can be addressed. Here, we propose a flexible approach---the kinetic activation-relaxation technique ($k$-ART)---which lifts these constraints. Our method is based on an off-lattice, self-learning, on-the-fly identification and evaluation of activation barriers using ART and a topological description of events. Using this method, we demonstrate that elastic deformations are determinant to the diffusion kinetics of vacancies in Si and are responsible for their trapping.