Role of buried ultra thin interlayer silicide on the growth of Ni film on Si(100) substrate

The presence of a buried, ultra-thin amorphous interlayer in the interface of room temperature deposited Ni film with a crystalline Si(100) substrate has been observed using cross sectional transmission electron microscopy (XTEM). The electron density of the interlayer silicide is found to be 2.02 e/?³ by specular X-ray reflectivity (XRR) measurements. X-ray diffraction (XRD) is used to investigate the growth of deposited Ni film on the buried ultra-thin silicide layer. The Ni film is found to be highly textured in an Ni(111) plane. The enthalpy of formation of the Ni/Si system is calculated using Miedema’s model to explain the role of amorphous interlayer silicide on the growth of textured Ni film. The local temperature of the interlayer silicide is calculated using enthalpy of formation and the average heat capacity of Ni and Si. The local temperature is around 1042 K if the interlayer compound is Ni₃Si and the local temperature is 1389 K if the interlayer compound is Ni₂Si. The surface mobility of the further deposited Ni atoms is enhanced due to the local temperature rise of the amorphous interlayer and produced highly textured Ni film. Received: 2 March 2000 / Accepted: 28 March 2000 / Published online: 11 May 2000     

Solid-state crystal-to-amorphous transition in metal‐metal multilayers and its thermodynamic and atomistic modelling

In this review article, first a brief summary is presented concerning the formation of amorphous alloys (or metallic glasses) in binary metal systems by solid-state reaction of metallic multilayers. Secondly, under the framework of Miedema's model, thermodynamic modelling of crystal-to-amorphous transition is developed with special consideration of the excess interfacial free energy in metallic multilayers. Thirdly, the results of molecular dynamics simulations in some representative systems are presented, revealing the detailed kinetics of the crystal-to-amorphous transition on the atomic scale, such as the temperature/time dependence of interfacial reactions, the asymmetric growth of amorphous interlayers, and the nucleation and/or presence of growth barriers resulting from the interfacial texture. Fourthly, the critical solid solubilities of some representative systems are directly determined from the inter-atomic potentials through molecular dynamics simulations and then correlated with the metallic-glass-forming ability of the systems as well as their asymmetric growth during solid-state amorphization observed in experiments and/or simulations.     

First-principles calculation and molecular dynamics simulation of fracture behavior of VN layers under uniaxial tension

We develop the second nearest-neighbor modified embedded atom method (2NN MEAM) potential for vanadium nitride (VN) in terms of the individual vanadium and nitrogen. The potential parameters are determined by fitting the cohesive energy, lattice parameter, and elastic constants of the VN with the NaCl-type structure, which are obtained by first-principles calculations. We find that the developed potentials can be used to describe the fundamental physical properties of the V–N system with other lattice structures. The calculated tensile stress–strain curves of the VN layers by first principles agree with those obtained by molecular dynamic simulations, validating the use of the developed potential. The bond breaking and its growth and coalescence are found to play an important role in the formation of fracture. We also find that temperature influences markedly the breaking of bonds, which can be attributed to the deviation of atoms from their equilibrium positions due to the thermal activated vibration, or to the superposition of the thermal energy to the deformation energy. Moreover, no dislocations and slips are found throughout the deformation process.     

Zr基大块非晶中添加元素对非晶形成能力及耐蚀性的影响

运用实空间递归方法研究了添加元素Nb,Ta,Y,La对Zr基非晶合金的非晶形成能力和耐腐蚀性能的影响.用计算机编程构造了Zr基非晶中初始晶化相Zr₂Ni的原子结构模型,用Zr₂Ni中的二十面体原子团簇模拟非晶中的二十面体团簇.计算了替代二十面体中心或顶角位置原子前后Ni,Zr及合金元素的局域态密度、团簇中心Ni与近邻Zr原子及Ni与替代元素Nb,Ta,Y,La间的键级积分,还计算了合金元素替代前后团簇的费米能级.局域态密度计算结果表明：合金元素Cu占据二十面体团 关键词： 电子结构 Zr基大块非晶 非晶形成能力 耐蚀性     

The effects of strain and atomic mobility on the amorphization of Al by Mn implantation at RT

The crystalline to amorphous phase transformation was studied by performing Mn implantations into Al thin films and single crystals at RT. Structural changes and the lattice site occupation of the Mn atoms were directly observed as a function of Mn concentration by X-ray diffraction, Rutherford backscattering (RBS) and channeling experiments. A detailed analysis of the strain distribution upon implantation is given taking into account the lattice site occupation of the Mn atoms and the atomic mobility during implantation at RT. At Mn concentrations 1–2 at% the mainly substitutionally incorporated Mn atoms distort the Al lattice considerably. These distortions provide the driving force for local lattice rearrangements that lead to the formation of amorphous clusters. Experimental results are further consistent with the assumption that the atomic density of these amorphous clusters is equal to the fcc value thus minimizing the strains after transformation. From the results it is concluded that the strain energy is the single most important factor in the amorphization process, determining both the instability of the supersaturated solid solution and the transformation to the amorphous state.     

Analytical calculation of atom ejection from the Ni (111), Ni (001), and Au (001) surfaces in frames of a three-dimensional model

Atom ejection from lattice sites at the Ni (111) and Ni (001) surfaces in the azimuthal direction toward the center of a lens consisting of two nearest neighboring atoms in the surface plane is calculated using a developed analytical three-dimensional model. The types of scattering of ejected atoms are classified in frames of the constructed model. It is found that the first and second ejection cones are observed in the sputtering pattern in the case of atom ejection from the Ni (111) surface and that the contribution of strongly blocked atoms to sputtering is considerable. The focusing of sputtered atoms at some angle from the surface normal is observed. A maximum of the polar angular distribution of sputtered atoms is shifted nonmonotonically as the energy increases. It is shown that the energy spent by the ejected atom on the recoiling of the lens atoms can be larger than that spent by this atom to overcome the potential barrier. It is found that small changes in the potential hardness and the binding energy at the magnetic phase transition can lead to a qualitative change in the ejection pattern. The expressions for the final ejection angle and energy in the case of Ni in the f-state are found in the form of an expansion in terms of two small parameters. As one passes from the case of atom ejection from the Ni (001) face to the case of atom ejection from the Au (001) face, the interaction cross section increases significantly because of an increase in the atomic number and the effects of blocking and focusing turn out to be considerable.     

Simulations of C60 bombardment of Si, SiC, diamond and graphite

Molecular dynamics simulations of the 20-keV C₆₀ bombardment at normal incidence of Si, SiC, diamond and graphite targets were performed. The unique feature of these targets is that strong covalent bonds can be formed between carbon atoms from the C₆₀ projectile and atoms in the solid material. The mesoscale energy deposition footprint (MEDF) model is used to gain physical insight into how the sputtering yields depend on the substrate characteristics. A large proportion of the carbon atoms from the C₆₀ projectile are implanted into the lattice structure of the target. The sputtering yield from SiC is ∼twice that from either diamond or Si and this can be explained by both the region of the energized cylindrical tract created by the impact and the number density. On graphite, the yield of sputtered atoms is negligible because the open lattice allows the cluster to deposit its energy deep within the solid. The simulations suggest that build up of carbon with a graphite-like structure would reduce any sputtering from a solid with C₆₀⁺ bombardment.     

XAFS研究Ni-P和Ni-Ce-P超细非晶合金的退火晶化

采用原子配位分布函数为Gaussian函数Ｐ_G和指数函数P_E直积的非 对称模型进行拟合计算,XAFS定量地研究化学还原法制备的Ni-P和Ni-Ce-P超细非晶合金大 无序度体系中Ni原子的局域环境结构随退火温度升高而产生的变化.结果表明Ni-P和Ni-Ce-P 原样的Ni-Ni配位的平均键长Ｒ_j、配位数N、热无序度σ_T、结构无 序度σ_S分别为0271nm,100,00060nm,0028n 关键词： XAFS Ni-P Ni-Ce-P 超细非晶合金 局域结构     

Molecular dynamics simulations of atomic assembly in the process of GaN film growth

Molecular dynamics simulations using a Coulomb–Buckingham potential have been used to investigate the process of wurtzite GaN films growth, including the appearance of films in early stage, regulation of growth, structure of the surface and the dynamic features. The simulations show that the N atoms and Ga atoms are absorbed on the lattice of substrate and take on a distinct sandwich structure. Time evolution of the mean square displacements and diffusion coefficient of the deposited atoms are observed, the results show that the clusters will become stable with the increase of time steps and the atoms reach the initial stable state after 25 ps; N atoms reach the equilibrium positions more quickly than Ga atoms. It is proved by radial distribution function and the ratio of vacancy of every deposited layer that the crystalline characters of the films will become better as the time steps increase and weaker from bottom to top.     

Theoretical study of the diffusion of lithium in crystalline and amorphous silicon

The effect of the lattice deformation on potential barriers for the motion of a lithium atom in crystalline silicon has been studied through ab initio density functional calculations. A new universal method of calculating the diffusion coefficient of an admixture in amorphous solid media through the activation mechanism has been proposed on the basis of these data. The method is based on the calculation of the statistical distribution of potential barriers for the motion of an admixture atom between minima depending on the position of neighboring atoms. First, the amorphous structure, which is generated by annealing from the crystalline structure with vacancies, has been simulated. Then, the statistical distribution of the potential barriers in the amorphous structure for various local environments of the admixture atoms has been calculated by means of linear regression with the parameters determined for barriers in crystalline silicon subjected to different deformations. The diffusion coefficient of the admixture has been calculated from this distribution by using the Arrhenius formula. This method has been tested by the example of crystalline and amorphous silicon with admixture of lithium atoms. The method demonstrates that the diffusion of lithium in amorphous silicon is much faster than that in crystalline silicon; this relation is confirmed experimentally.     

Dynamical Analysis of Sputtering at Threshold Energy Range： Modelling of Ar＋Ni（100） Collision System

The sputtering process of Ar＋Ni（100） collision systems is investigated by means of constant energy molecular dynamics simulations. The Ni（100） slab is mimicked by an embedded-atom potential, and the interaction between the projectile and the surface is modelled by using the reparametrized ZBL potential. Ni atom emission from the lattice is analysed over the range of 20-50 eV collision energy. Sputtering yield, angular and energy distributions of the scattered Ar and of the sputtered Ni atoms are calculated, and compared to the available theoretical and experimental data.     

computer simulations of ion scattering spectra for 2.4 keV Ne+ incident on Ni(001)

Monte-Carlo computer simulations of Buck's Time-of-Flight Ion Scattering Spectroscopy data collected for 2.4 keV Ne⁺ incident on a Ni (001) surface were used to examine the importance of various input parameters in the computational model. The binary collision approximation was found to yield satisfactory results for this particular projectile, energy, and target. The calculated energy spectra were quite sensitive to the screening length of the Molière potential used for calculating the binary collision interactions and to the magnitude of the mean square vibrational amplitudes chosen for the Ni atoms in the lattice. The agreement between the calculated and experimental spectra was good, but the calculations probably could have been improved a little further by fine tuning the parameters in the simulation. The simulations showed that the ISS spectra for 2.4 keV Ne⁺ ion beam incidence directions along the [110] azimuth of Ni(001) were dominated by multiple scattering events and that the second layer of the surface contributed most to the backscattering because of the focusing effect of the first layer atoms.     

玻色-爱因斯坦凝聚系统中原子的空间混沌分布

研究了非对称周期势阱中玻色-爱因斯坦凝聚原子的空间混沌分布结构. 在凝聚体相位为常数的情况下, 凝聚体内部不存在原子流,凝聚原子的空间分布结构可以用一个无阻尼双驱动Duffing方程描述. 理论分析给出了原子间呈排斥作用系统的Mel'nikov混沌判据.数值模拟结果显示,化学势的增大能够对原子混沌分布产生明显的抑制作用,甚至使混沌完全消失. 对于原子间呈吸引作用的系统,在一定参数条件下,调节光格势强度比可以使凝聚原子由周期状态进入到空间混沌分布状态,随着化学势的增大这种空间混沌分布被完全抑制. 关键词： 玻色-爱因斯坦凝聚 Mel'nikov函数 混沌     

Multiscale simulation from atomistic to continuum – coupling molecular dynamics (MD) with the material point method (MPM)

A new multiscale simulation approach is introduced that couples atomistic-scale simulations using molecular dynamics (MD) with continuum-scale simulations using the recently developed material point method (MPM). In MPM, material continuum is represented by a finite collection of material points carrying all relevant physical characteristics, such as mass, acceleration, velocity, strain and stress. The use of material points at the continuum level provides a natural connection with the atoms in the lattice at the atomistic scale. A hierarchical mesh refinement technique in MPM is presented to scale down the continuum level to the atomistic level, so that material points at the fine level in MPM are allowed to directly couple with the atoms in MD. A one-to-one correspondence of MD atoms and MPM points is used in the transition region and non-local elastic theory is used to assure compatibility between MD and MPM regions, so that seamless coupling between MD and MPM can be accomplished. A silicon single crystal under uniaxial tension is used in demonstrating the viability of the technique. A Tersoff-type, three-body potential was used in the MD simulations. The coupled MD/MPM simulations show that silicon under nanometric tension experiences, with increasing elongation in elasticity, dislocation generation and plasticity by slip, void formation and propagation, formation of amorphous structure, necking, and final rupture. Results are presented in terms of stress–strain relationships at several strain rates, as well as the rate dependence of uniaxial material properties. This new multiscale computational method has potential for use in cases where a detailed atomistic-level analysis is necessary in localized spatially separated regions whereas continuum mechanics is adequate in the rest of the material.     

The effect of deposition temperature on the intermixing and microstructure of Fe/Ni thin film

The physical vapour deposition of Ni atoms on α-Fe(001) surface under different deposition temperatures were simulated by molecular dynamics to study the intermixing and microstructure of the interfacial region. The results indicate that Ni atoms hardly penetrate into Fe substrate while Fe atoms easily diffuse into Ni deposition layers. The thickness of the intermixing region is temperature-dependent, with high temperatures yielding larger thicknesses. The deposited layers are mainly composed of amorphous phase due to the abnormal deposition behaviour of Ni and Fe. In the deposited Ni-rich phase, the relatively stable metallic compound B₂ structured FeNi is found under high deposition temperature conditions.     

Microscopic collective dynamics of atoms in the amorphous metallic alloy Ni33Zr67

The structural properties and microscopic collective dynamics of atoms in the amorphous metallic alloy Ni₃₃Zr₆₇ are studied using molecular dynamics simulations with a pair-additive model potential. The calculated equilibrium structural and dynamic characteristics are compared with experimental data on neutron diffraction and inelastic X-ray scattering. Theoretical analysis of the structural relaxation of microscopic density fluctuations for amorphous metallic alloys is performed within the Lee’s recurrent relation approach. The results of theoretical calculations for the intensity of scattering I(k, ω) for the amorphous metallic alloy Ni₃₃Zr₆₇ are in good agreement with the results of computer simulation and experimental inelastic X-ray scattering data. The low-frequency excitations observed in the longitudinal current spectra are related to the vibrational motions of individual atom clusters, which include Ni and Zr atoms.     

Molecular simulations of dewetting

We have studied the breakup and subsequent fluid flow in very thin films of partially wetting liquid on solid substrates, using molecular dynamics simulations. The liquid is made of short chain molecules interacting with Lennard-Jones interactions, and the solid is modeled as a clean crystal lattice whose atoms have thermal oscillations. Films below a critical thickness are found to exhibit a spontaneous spinodal-like instability leading to dry patches, as predicted theoretically and observed in some experiments. Liquid withdrawing from a dry patch collects in a moving rim whose fluid dynamics is only partially in agreement with earlier predictions.     

间隙原子对ZnNb2O6光电特性影响的第一性原理研究

基于密度泛函理论第一性原理，研究Zn、Nb、O间隙原子对ZnNb₂O₆体系光电特性的影响。分析显示：间隙原子对体系晶格畸变的影响与间隙原子几何尺寸有关。缺陷结构中，由于间隙原子电负性存在差异，也是产生晶格畸变的因素。光电特性分析显示：含有Zn、Nb间隙原子的体系表现为n型简并半导体。且Nb_i表现出较强的介电效应，主要与Nb的离子势与电离能有关。O_i表现为p型简并半导体，对光电效应贡献较小。结果表明Nb_i体系有良好的光电特性，在实际应用中具有较大潜力。     

Low-energy thermal excitations in the V-O system

The thermal excitations of the atoms of the V-O solid solution have been investigated by the slow-neutron scattering method for low-energy transfers. The features of the low-frequency excitations of a light impurity in the metal lattice for energy transfers ? ≈ 1.3 and 2.4 meV have been revealed. It is assumed that the observed low-energy excitations appear in V-O due to strong local distortions around an impurity atom, which are responsible for the formation of the effective potential of a complex shape with the almost flat bottom. Analysis of the interaction of oxygen with the nearest matrix atoms shows that the low-energy excitations of oxygen atoms can be attributed to the splitting of the ground state in the complex potential. The level scheme of the split state can be obtained in the representation of decelerated circular quantum rotation about the axis passing through the two nearest metal atoms or the motion in a two-dimensional potential well. The results indicate that rotational low-energy excitations can exist in (metal-interstitial impurity) solid solutions.