Interatomic potential effects on dynamical heterogeneities in liquid SiO2

Dynamical heterogeneities (DH) in low density liquid SiO₂ have been investigated by molecular dynamics (MD) method. Simulations were done in the basic cube under periodic boundary conditions containing 3000 particles with the pair interatomic potentials, which have a weak electrostatic interaction and a Morse type short range interaction (PMSI). We have evaluated the non-Gaussian parameter for the self part of the van Hove correlation function and we found a clear evidence of the existence of DH in low density liquid SiO₂. Moreover, the atomic displacement distribution (ADD) in a model has been obtained and it deviates from a Gaussian form. The results have been compared with those obtained in another liquid SiO₂ system with the Born-Mayer interatomic potentials (BMP) in order to observe the interatomic potential effects on the DH in the system and indeed, the effects are strong. Calculations showed that particles of extremely low or fast mobility have a tendency to form a cluster and mean cluster size of most mobile and immobile particles in PMSI models increases with decreasing temperature. In contrast, no systematic changes have been obtained for the most mobile and immobile particles in BMP models. Calculations show that there is no relation between local particle environment and particle mobility in the system.     

Investigation of multicomponent chemical short-range order in NiZr2

The local atomic configuration of multicomponent chemical short-range order (MCSRO) in NiZr2 has been investigated by means of molecular dynamics simulation (MD) in a wide temperature range. The potential functions for the system based on the embedded atom method are constructed and the parameters are obtained by fitting the structure and properties of NiZr2 crystal. The static structures such as pair distribution functions and the distribution of coordination number have been calculated. The local atomic configurations of the MCSI~Os in the melt were demonstrated as distorted coordination polyhedron of the compound structure and/or the structure similar to cubooctahedron analogues. It is indicated by the results of MD simulation that above the melting point the atomic packing of long-range order disappears, but the chemical interaction of coordinated atoms still exists, which leads to the formation of various MCSROs with atomic configurations similar to the stable or metastable unit cell of NiZr2 compound. When the system is just melted, many icosahedral polyhedron configurations appear, which decrease as the over-heating temperature increases.     

Percolation mechanism of the diffusion of impurity atoms in dense surface layers

The molecular dynamics method is used to study the migration of an impurity atom on an unfilled square lattice. The calculations are performed on a lattice containing 2¹² × 2¹⁴ sites at various values of the ratio p of the frequencies of jumps impurity and lattice atoms and various relative concentrations of vacancies ? _V . In the limit of vanishingly small concentrations of vacancies, ? _V ? 1, the present simulation results are in agreement with our previous analytical results. With increasing ? _V , the diffusion coefficient of impurity atoms predicted by the simulations exceeds the result of the analytical theory, a behavior that can be explained by the growing influence of vacancy clusters, voids on the surface, in which the impurity atom can travel long distances. This is most clearly seen in the case of highly mobile impurity atoms (p ? 1), where within the characteristic time of displacement of impurity atoms, lattice atoms remain practically immobile, and the problem appears to be closely related to the percolation problem. In this case, up to ? _V ≈ 0.3, the diffusion coefficient is independent of p; then, such a dependence appears, and the diffusion coefficient increases sharply with ? _V .     

Simulation of the structural state of amorphous phases in nanoscale SiO2 synthesized via different methods

The structural state in nanoscaled SiO₂ is probed experimentally via X-ray diffraction and the simulation method. The aerosil nanoparticles and nanoparticles synthesized via the electron beam evaporation are compared. The nanoparticles for all samples are shown to be in the amorphous state. The amorphous state of a SiO₂ unit lattice is simulated via the molecular dynamics. The full-profile refinement of parameters for a simulated SiO₂ phase (the Rietveld method) has allowed the complete structural information to be established at varying the specific surface. The unit cell parameters, the spatial atomic distribution and the degree of cell node occupation are determined, as well. The specific surface area is shown to decrease in aerosil nanoparticles and to increase in tarkosil nanoparticles with the increasing binding energy of atoms in a cell.

     

Formation of copper islands on a native SiO2 surface at elevated temperatures

A combination of in situ X-ray photoelectron spectroscopy analysis and ex situ scanning electron- and atomic force microscopy has been used to study the formation of copper islands upon Cu deposition at elevated temperatures as a basis for the guided growth of copper islands. Two different temperature regions have been found: (I) up to 250 °C only close packed islands are formed due to low diffusion length of copper atoms on the surface. The SiO₂ film acts as a barrier protecting the silicon substrate from diffusion of Cu atoms from oxide surface. (II) The deposition at temperatures above 300 °C leads to the formation of separate islands which are (primarily at higher temperatures) crystalline. At these temperatures, copper atoms diffuse through the SiO₂ layer. However, they are not entirely dissolved in the bulk but a fraction of them forms a Cu rich layer in the vicinity of SiO₂/Si interface. The high copper concentration in this layer lowers the concentration gradient between the surface and the substrate and, consequently, inhibits the diffusion of Cu atoms into the substrate. Hence, the Cu islands remain on the surface even at temperatures as high as 450 °C.     

Analysis of the heterogeneous dynamics of imidazolium-based [Tf2N−] ionic liquids using molecular simulation

The complex dynamic behaviour of the imidazolium-based ionic liquids [C_nmim⁺][Tf₂N^?], n = 4, 8, 12 is examined at various temperatures and at atmospheric pressure using molecular dynamics simulation. An existing all-atom force field is further optimised in order to attain reasonable agreement with experimental data for transport properties, such as self-diffusivities and viscosities. Dynamical heterogeneity phenomena are quantified through the calculation of the non-Gaussian parameter and the deviation of the self-part of the van Hove correlation function from the expected normal distribution. From this analysis, ions that move faster or slower than expected are detected in the system. These subsets of ‘fast’ and ‘slow’ ions form individual clusters consisting of either mobile or immobile ions. Detailed analysis of the ions’ diffusion reveals preferential motion along the direction of the alkyl tail for the cation and along the vector that connects the two sulphur atoms for the anion. For the longest alkyl tails, the heterogeneity in the dynamics becomes more pronounced and is preserved for several nanoseconds, especially at low temperatures.     

Imaging and spectroscopy of defects in semiconductors using aberration-corrected STEM

The distribution of single dopant or impurity atoms can dramatically alter the properties of semiconductor materials. The sensitivity to detect and localize such single atoms has been greatly improved by the development of aberration correctors for scanning transmission electron microscopes. Today, electron probes with diameters well below 1 Å are available thanks to the improved electron optics. Simultaneous acquisition of image signals and electron energy-loss spectroscopy data provides means of characterization of defect structures in semiconductors with unprecedented detail. In addition to an improvement of the lateral spatial resolution, depth sensitivity is greatly enhanced because of the availability of larger probe forming angles. We report the characterization of an alternate gate dielectric interface structure. Isolated Hf atoms are directly imaged within a SiO₂ thin film formed between an HfO₂ layer and the silicon substrate. Electron energy-loss spectroscopy shows significant changes of the silicon valence state across the interface structure.     

Mobile effect of hydrogen on intergranular decohesion of iron: first-principles calculations

Atomistic mechanisms of hydrogen-induced cracking along a bcc Fe Σ3(111) symmetrical tilt grain boundary (GB) have been studied by first-principles calculations. The mobile and immobile effects of hydrogen on the GB decohesion are analyzed by calculating the dependence of hydrogen segregation energy on the coverage relevant to the repulsive interaction among segregated hydrogen atoms at the GB and on its fracture surfaces, together with generalizing McLean's formula. It was found that the segregation of combined mobile and immobile hydrogen atoms from the bulk and/or GB on the fracture surfaces causes much stronger reduction (70–80%) in the GB cohesive energy. It can occur even at a very low bulk hydrogen content of about 10^?9 atomic fraction during slow cracking. This is in contrast to only 10–20% decohesion induced by immobile hydrogen at much higher hydrogen content during fast cracking. The mobile effect of hydrogen, giving rise to a profound reduction in the GB cohesive energy, is a key factor controlling the mechanism of hydrogen-induced GB cracking.     

Simulation of diffusion instability of a mercury atomic distribution in the cadmium-mercury-tellurium alloy

The formation of inhomogeneities in Cd_xHg_1-x Te alloys upon post-growth cooling or upon low-temperature annealing is simulated numerically. The mechanism of the formation of inhomogeneities is based on the diffusion instability in a system involving mercury atoms located at lattice sites, interstitial mercury atoms, and cation vacancies. It is revealed that, upon prolonged annealing of the Cd_xHg_1-x Te alloys with a cadmium content x = 0.2 at a temperature of ∼200°C, the concentrations of mercury atoms at lattice sites, interstitial mercury atoms, and vacancies are characterized by an inhomogeneous nearly periodical distribution arising from a small fluctuation when the initial equilibrium concentration of interstitial mercury atoms exceeds a threshold value (∼3 × 10¹⁷ cm⁻³). The spatial and time scales of the concentration distribution are determined primarily by the equilibrium concentration of vacancies and do not depend on the type of fluctuation involved. The spatial period of the concentration distribution increases linearly from 0.01 to 3.00 μm as the equilibrium concentration of vacancies changes from 10¹⁹ to 10¹⁴ cm⁻³. At lower concentrations of vacancies, the periodic structure is formed for a considerably longer time.     

Approach to local thermodynamic equilibrium and the evolution to a glassy core following neutron/ion radiation impact

Using molecular dynamics simulations and statistical-mechanical metrics, we make quantitative predictions on the local thermodynamic and dynamic states following an ion or neutron impact in three materials – copper, silicon and solid argon. Through a two-energy distribution, we first capture the non-equilibrium temperature evolution and the approach to the local thermal equilibrium in three generic stages. By examining the time-resolved van Hove self-correlator, we then demonstrate that the impact core of all the three materials shows the dynamic characteristics of a jammed or glassy state. We delineate a dynamic atom-hopping mechanism that attests to a rapid defect recovery stage in copper; silicon, on the contrary, accommodates only small displacements which resist recovery. The dissimilitude between copper with a close-packed structure and silicon with an open network structure is further drawn out through an isoconfigurational analysis of displacements, which shows a compact dendritic-like condensation front for the mobile atoms in copper through atom hopping. In contrast, silicon portrays larger-scale spatial oscillations of dynamically separated regions, which appear to be a precursor to dynamic lattice instability and eventual amorphisation.     

Influence of annealing on the concentration profiles of boron implantations in silicon

The influence of annealing on the concentration profiles of boron implanted into silicon with does of 10¹⁴ ions/cm² up to 10¹⁶ ions/cm² and an energy of 70 keV was studied. The concentration profiles were measured with Secondary Ion Mass Spectrometry (SIMS). The broadening of the concentration profiles during annealing can be described as a superposition of effects resulting from a relatively immobile and a mobile boron fraction. The properties of the immobile boron fraction were studied by measuring the influence of a boron implantation on the distribution of a homogeneous boron background dope. From these experiments it was concluded that the immobile boron fraction consists of boron precipitates. The properties of the mobile fraction were studied from concentration profiles that were obtained after annealing during different periods at the same temperature. It was found that during the initial stage of the annealing process a fast broadening of the profile occurs; this was assumed to be due to an interstitial type boron diffusion. After prolonged annealing the much slower substitutional type diffusion prevails, due to trapping of the interstitial boron atoms by vacancies. The reliability of the SIMS method, as applied to profile measurements, was checked for the high boron doses used in this investigation. Excessive boron precipitates, obtained after annealing of a high dose, such as 10¹⁶ ions/cm² at about 1000°C, appear to give some increase of the ion yield.     

Interpretation of TOF–SIMS depth profiles from ultrashallow high-k dielectric stacks assisted by hybrid collisional computer simulation

The TRIDYN collisional computer simulation has been modified to account for emission of ionic species and molecules during sputter depth profiling, by introducing a power law dependence of the ion yield as a function of the oxygen surface concentration and by modelling the sputtering of monoxide molecules. The results are compared to experimental data obtained with dual beam TOF–SIMS depth profiling of ZrO₂/SiO₂/Si high-k dielectric stacks with thicknesses of the SiO₂ interlayer of 0.5, 1, and 1.5 nm. Reasonable agreement between the experiment and the computer simulation is obtained for most of the experimental features, demonstrating the effects of ion-induced atomic relocation, i.e., atomic mixing and recoil implantation, and preferential sputtering. The depth scale of the obtained profiles is significantly distorted by recoil implantation and the depth-dependent ionization factor. A pronounced double-peak structure in the experimental profiles related to Zr is not explained by the computer simulation, and is attributed to ion-induced bond breaking and diffusion, followed by a decoration of the interfaces by either mobile Zr or O. PACS 68.49; 79.20; 81.65; 82.80     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.     

Structure and dynamics of liquid MgO under high pressure

Microstructure, dynamics, and diffusion mechanism in liquid MgO have been studied by molecular dynamics simulation. Models consisting of 2000 atoms were constructed under a wide range of pressure and at a temperature of 3800 K. The local structure is analyzed through the coordination number distribution and topology statistics of coordination units (basic structural units) MgO _x (x=2, 3, 4, 5, 6, 7). As regards the structural dynamics, the nearest-neighbor atomic exchange among coordination units, spatially heterogeneous dynamics, clustering, and structural stability (lifetime of basic structural units) are investigated in detail. Investigation of structural dynamics allows us to gain insight into various important atomic (molecular) properties and to clarify the diffusion mechanism in liquid MgO under high pressure.     

Persistent local chemical bonds in intermetallic phase formation

We found a direct evidence for the existence of the local chemical Bi–In bonds in the BiIn₂ melt. These bonds are strong and prevail, dominating the structure evolution of the intermetallic clusters. From the local structure of the melt-quenched BiIn₂ ribbon, the chemical Bi–In bonds strengthen compared with those in the equilibrium solidified alloy. The chemical bonds in BiIn₂ melt retain to solid during a rapid quenching process. The results suggest that the intermetallic clusters in the melt evolve into the as-quenched intermetallic phase, and the intermetallic phase originates from the chemical bonds between unlike atoms in the melt. The chemical bonds preserve the chemical ordered clusters and dominate the clusters evolution.     

Fe- and Al-doped apatite-type lanthanum silicates: Structure and property characterization

For the first time the local structure of Al- and Fe-doped apatite-type lanthanum silicates (ATLS) of different stoichiometries was systematically studied by using IR spectroscopy. In addition to previous ²⁹Si MAS NMR study, data obtained confirm the possibility of [SiO₄] group interaction in ATLS, being favored by the presence of cation vacancies in the apatite. Transport properties of the systems were studied by impedance spectroscopy and oxygen isotope exchange technique. The latter indirectly shows that not only channel/interstitial oxygen atoms can be involved in the oxide-ion migration but also those of tetrahedrons.     

Formation of metallic nanophases in silica by ion beam mixing. Part II: cluster formation

2 matrix by ion-beam mixing of SiO₂/Ag multilayers is studied via Rutherford backscattering spectrometry, optical absorption, and transmission electron microscopy experiments. In a first step, irradiation with MeV heavy ions transforms the continuous Ag layers into a string of micrometer-sized Ag inclusions. This mechanism can be attributed to lateral segregation of metallic atoms induced by irradiation. In a second step, the Ag inclusions are broken up by incoming ions and Ag nanoclusters are formed by agglomeration of mobile Ag atoms. The latter mechanism is likely due to a combination of ballistic mixing and radiation-induced segregation or radiation-enhanced diffusion processes. The size of the metallic nanoclusters formed depends also on the irradiation temperature. Received: 27 October 1997/Accepted: 3 February 1998     

Spatial correlations of mobility and immobility in a glass-forming Lennard-Jones liquid

Using extensive molecular dynamics simulations of an equilibrium, glass-forming Lennard-Jones mixture, we characterize in detail the local atomic motions. We show that spatial correlations exist among particles undergoing extremely large ("mobile") or extremely small ("immobile") displacements over a suitably chosen time interval. The immobile particles form the cores of relatively compact clusters, while the mobile particles move cooperatively and form quasi-one-dimensional, stringlike clusters. The strength and length scale of the correlations between mobile particles are found to grow strongly with decreasing temperature, and the mean cluster size appears to diverge near the mode-coupling critical temperature. We show that these correlations in the particle displacements are related to equilibrium fluctuations in the local potential energy and local composition.     

Research on Electron Emission from Dielectric Materials by a Monte Carlo Method

The charge accumulation in an insulating material under an electron beam bombardment exerts a significant influence to scanning electron microscopic imaging. This work investigates the charging formation process by a self-consistent Monte Carlo simulation of charge production and transportation based on a charge dynamics model. The charging effect in a semi-infinite SiO₂ bulk and SiO₂ trapezoidal lines on a SiO₂ or Si substrate has been studied. We used two methods to calculate the spatial distributions of electric potential and electric field for two different systems respectively: the image charge method was used to deal with a semi-infinite bulk, and, random walk method to solve the Poisson equation for a complex geometric structure. The dynamic charging behavior depending on irradiation time has been investigated for SiO₂. The simulated CD-SEM images of SiO₂ trapezoidal lines with charging effect included were compared well with experimental results, showing the contrast change of SEM image along with scanning frames due to charging.     

SiO2-羟基表面上金属原子的第一性原理研究

采用第一性原理方法研究了SiO₂-羟基表面上几种金属原子的吸附性质，发现In和Ga在SiO₂-羟基表面上的结合很弱，而Fe，Co， Ni在该表面上与Si，O形成强的化学键.等势能面和扩散势垒计算表明In (Ga)的扩散激活能只有0.1—0.3 eV，表明这两种原子容易在表面上扩散.这些结果可以定性地解释纳米合成中的一些实验现象. 关键词： 第一性原理 表面扩散 结合能 金属原子