Molecular dynamics study of sputtering of Cu (111) under Ar ion bombardment

Abstract

We have used the molecular dynamics (MD) technique using many-body interaction potentials to analyse in detail the processes leading to sputter emission, in order to gain a microscopic understanding of low energy bombardment phenomena. Calculations were performed for a Cu (111) single crystal surface bombarded with Ar atoms in the energy range from 10–1000 eV. The results presented for low bombarding energies are mainly concerned with the near sputtering threshold behaviour, yields and depth of origin of sputtered atoms. Furthermore, it is found, that in addition to sputtered atoms, a large number of ad-atoms at the surface are generated during the evolution of the collision cascade. At higher energies the question of cluster emission and especially their energy distribution and angular distribution are addressed. It was found that the energy distributions for the dimers and monomer atoms exhibit a similar dependence on emission energy as has been observed recently also experimentally. For atoms good agreement with the theoretical Sigmund-Thompson energy distribution was observed. However, for dimers we found that the energy distributions exhibit an asymptotic behaviour at high energies with E^?3 rather than with E^?5, as predicted in previous modelling of cluster emission. Concerning the angular distributions six emission spots, three strong ones in the <110> and three weak ones in the <100> direction were found for atoms, but for dimers only emission spots in the <110> direction were observed, in agreement with experimental results.     

金属钨中氦行为的分子动力学模拟

采用分子动力学方法模拟了氦在金属钨中的扩散聚集行为. 首先,建立了氦与钨原子间相互作用势,短程部分采用ZBL势形式,长程部分采用从头算法数据,实现了两者之间的平滑连接. 通过计算氦在钨中不同间隙位的形成能发现,单个氦原子更易存在于金属钨中的四面体间隙位,这与最新的研究成果是一致的. 在400-1200 K的温度范围内,考察了氦原子在金属钨中的扩散行为,获得了扩散迁移能,其值介于实验值和从头算法结果之间. 最后,研究了氦的聚集行为,从能量的角度考察了氦团簇形成初期的生长机理. 研究发现,在氦团簇形成初期,氦团簇对氦的结合能随着氦团簇的生长有逐渐增大的趋势,说明氦团簇吸收氦的能力逐渐增强. 关键词： 氦扩散 氦团簇 辐照损伤 分子动力学模拟     

Relative sputtering yields from close-packed directions in nickel under low-energy Ar bombardment

A new approach to low-energy sputtering was developed wherein a monoenergetic Ar⁺ ion beam of the order of 10⁻⁹ A impinged onto a crystalline nickel target. Target preparation consisted of plating about 100 monolayers of high specific activity Ni-63 onto a coldrolled nickel substrate and then heating the target above its recrystallization temperature under ultra-high vacuum. The result was a highly ordered polycrystalline structure which, when sputtered, behaved like the (100) surface of a nickel single crystal. Approximately 25 percent of the surface atoms were Ni-63. Sputtered material was collected on a molybdenum foil which was subsequently analyzed by radiotracer techniques.

Experimental results concerning sputtering from [110] and [100] close-packed directions in nickel under bombardment by Ar⁺ ions of energy 25 eV to 600 eV are reported. The relative sputtering yields from [110] directions are presented as a function of incident ion energy for 75° and 15° ion incidence measured with respect to the [110] directions, and the extrapolated thresholds are compared with theoretical predictions.

In addition to the expected deposits on the foil due to sputtering from individual closepacked directions, secondary deposits occurred which are attributed to specular reflection of sputtered nickel atoms from the molybdenum foil.     

Field-ion microscope observations of helium ion bombardment damage in tungsten

The field-ion microscope has been used in a pilot investigation of the damage caused to tungsten surfaces following bombardment with low-energy helium ions. Such effects could be important because the first wall of a fusion reactor will have to withstand high doses of these particles. The damage observed following bombardment with total doses in the range 10¹³ to 5 × 10¹⁸ ions cm^?2 has been found to be dose dependent. These results have then been related to those derived from studies using more conventional techniques and this has led to the conclusion that the origin of the observed damage lies in the formation of near-surface gas bubbles and surface blisters.     

Molecular dynamics simulations of sputtering of organic overlayers by slow, large clusters

The ion-stimulated desorption of organic molecules by impact of large and slow clusters is examined using molecular dynamics (MDs) computer simulations. The investigated system, represented by a monolayer of benzene deposited on Ag{1 1 1}, is irradiated with projectiles composed of thousands of noble gas atoms having a kinetic energy of 0.1-20 eV/atom. The sputtering yield of molecular species and the kinetic energy distributions are analyzed and compared to the results obtain for PS4 overlayer. The simulations demonstrate quite clearly that the physics of ejection by large and slow clusters is distinct from the ejection events stimulated by the popular SIMS clusters, like C₆₀, Au₃ and SF₅ at tens of keV energies.     

Molecular dynamics simulations of microscopic structure of ultra strong shock waves in dense helium

Hydrodynamic properties and structure of strong shock waves in classical dense helium are simulated using non-equilibrium molecular dynamics methods. The shock speed in the simulation reaches 100 km/s and the Mach number is over 250, which are close to the parameters of shock waves in the implosion process of inertial confinement fusion. The simulations show that the high-Mach-number shock waves in dense media have notable differences from weak shock waves or those in dilute gases. These results will provide useful information on the implosion process, especially the structure of strong shock wave front, which remains an open question in hydrodynamic simulations.     

Ni原子倾斜轰击Pt(111)表面低能溅射现象的分子动力学模拟

采用嵌入原子方法的原子间相互作用势,通过分子动力学模拟详细研究了以不同角度入射的低能Ni原子与Pt (111)基体表面相互作用过程中的低能溅射行为.结果表明:随着入射角度从0°增加到80°,溅射产额Y_s和入射原子钉扎系数S的变化均可以根据入射角θ近似地分为以下三个区域:当θ ≤ 20°时,Y_s和S几乎保持不变,其值与垂直入射时接近,溅射原子的发射角分布和能量分布也与垂直入射时的情 关键词： 分子动力学模拟 入射角 低能溅射     

Molecular dynamics simulation of bipartite bimetallic clusters under low-energy argon ion bombardment

The evolution of bipartite bimetallic atomic clusters within 5 ps under bombardment with monoenergetic argon ions at the initial energy ranging from 1 eV to 1.4 keV has been simulated by the classical molecular dynamics method with a target obtained from Ni?Al and Cu?Au clusters consisting of 78 and 390 atoms, equally divided between the corresponding monometallic parts, the simulated pairs of which have different heats of intermixing. The changes in the potential energy and temperature, the sputtering yields, and the intensity of the ion-stimulated movement of atoms at the interface of the monometallic parts of clusters of both sizes have been determined as functions of the energy of the bombardment.     

氦热力学性质的分子动力学研究

 利用经典分子动力学和第一性原理分子动力学,研究了氦在高压下的熔化曲线、状态方程和非金属-金属转变。得到了氦在温度小于4.5 eV、 密度0.3~5.0 g/cm³范围内的状态方程,并把氦的熔化曲线的压强范围拓展到了50 GPa。氦的能隙宽度曲线表明,温度大大降低了氦的金属化密度。     

氦热力学性质的分子动力学研究

利用经典分子动力学和第一性原理分子动力学,研究了氦在高压下的熔化曲线、状态方程和非金属-金属转变。得到了氦在温度小于4.5 eV、 密度0.3~5.0 g/cm3范围内的状态方程,并把氦的熔化曲线的压强范围拓展到了50 GPa。氦的能隙宽度曲线表明,温度大大降低了氦的金属化密度。     

Synergetic approach to studying material sputtering under ion bombardment

The use of the synergetic approach to studying the sputtering of materials under ion bombardment is considered. The applicability of this approach is based on complete correspondence between the main concepts of ion sputtering and synergetics, i.e., the disequilibrium and nonlinearity of multiparticle processes. The systematic application of synergetic methods within a uniform approach allow us to analyze such characteristics as the formation of mass and energy distributions (mass and energy spectra), and to develop a universal approach to studying the processes of elastic displacements and thermal spike based on the principles of nonlinear dynamics. A conclusion is drawn regarding the usefulness of the synergetic approach to the whole spectrum of problems of radiation physics.     

Molecular dynamics simulations of phase transition of n-nonadecane under high pressure

The phase transition behavior of n-nonadecane under high pressure was investigated with molecular dynamics (MD) simulations method. A simplified model with amorphous structure and periodic boundary conditions in constant-temperature, constant-pressure ensemble was used in this study. The results showed that the whirling and molecules motion of n-nonadecane chains were restrained by the high pressure. The simulated phase transition temperature of n-nonadecane under high pressure is higher than that under atmospheric pressure. The order parameter of n-nonadecane decreases with the increase in temperature. The simulations reveal that MD is an effective method to understand the phase transition of alkane-based phase change materials on molecular and atomic scale.     

Study of gold nanocluster sputtering under 38-keV Au ion bombardment by a classical molecular dynamics method

The computer simulation of the interaction of 38-keV Au₁ ions with isolated spherical Au _N nanoclusters of diameters 2.6 and 18 nm is performed in the framework of the classical molecular dynamics (MD) method. The distribution of the absorbed energy ε per one atom of the irradiated cluster and the sputtering yields are analyzed for different ratios of the nanocluster diameter D to the average projective range R _p of the bombarding ion. It is established that the small values of the absorbed energy (ε ? ε_max = E/N) are most probable for D < R _p, and either small (ε ? ε_max) or the maximum possible (ε ～ ε_max) values are mainly realized for D ≤ R _p. It is shown that the total sputtering yield depends weakly on the impact parameter. It is demonstrated for the first time that the irradiated cluster, as a whole, can be ejected by direct impact with a probability of approximately 6–13%. Such events are realized in the cases where the bombarding ion causes secondary cluster-atom emission in the dominant direction to a substrate, with the result that an unsputtered cluster fraction acquires momentum in the opposite direction. This recoil effect can be one of the mechanisms for desorption of nanoclusters deposited on the surface under ion (or cluster) bombardment.     

The deformation of thin aluminium films under helium ion bombardment

ABSTRACT : Aluminium films 2000 Å thick have been bombarded with various flu-ences of 5 keV He⁺ ions and the resulting bubble distribution observed by Transmission Electron Microscopy. Scanning Electron Microscopy of the films indicates an areal increase in the unsupported regions of the films, which, if it results from three dimensional swelling of the films, is two orders of magnitude larger than would be calculated from the total bubble volume as observed by T. E. M. In addition features closely resembling blisters observed in bulk materials, form on the supported regions of the film. These blisters like features are thought to result from lateral stresses in the films.     

On the existence of an universal energy function for sputtering yields by low-energy light-ion bombardment

Summary The mechanism wherenism whereby heavy solids are sputtered by lowenergy light ions can be explained by a linear-transport theory, by assuming only primary recoils as candidate for sputtering. It is shown that, from a theoretical point of view, scaling properties of the sputtering yield can be introduced if the appropriate dimensionless energy variable is assumed. In fact, the yield expression can be reduced to a formulation in which an energy function common to all the target-ion combinations is multiplied by a constant factor different for each combination. This expression, free from any kind of fitting parameter, gives a good approximation to the experimental data. Further developments are discussed in relation to the spreading of experimental points which do not exibit strict scaling properties.

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Riassunto Il meccanismo di erosione di solidi durante il bombardamento di ioni leggeri di bassa energia può essere spiegato nell'ambito della teoria del transporto assumendo che solo i rinculanti primari contribuiscono all'erosione. L'approccio teorico mostra che il coefficiente di erosione può esprimersi in termini di una funzione universale assumendo un'opportuna variabile energetica adimensionale. Infatti il coefficiente di erosione può essere ricondotto ad un' espressione in cui una funzione della sola energia, comune a tutte le combinazioni ione-bersaglio, è moltiplicata per una fattore costante differente per ogni combinazione. Questa espressione, priva di parametri aggiustabili, fornisco una buona approssimazione per i dati sperimentali. Ulteriori sviluppi sono discussi in relazione al fatto ai dati sperimentali non sono strettamente riconducibili ad una fuzione universale.

     

Molecular dynamics simulations on the dynamics of two-dimensional rounded squares

The collective motion of rounded squares with different corner-roundness ζ is studied by molecular dynamics(MD)simulation in this work. Three types of translational collective motion pattern are observed, including gliding, hopping and a mixture of gliding and hopping. Quantitatively, the dynamics of each observed ordered phase is characterized by both mean square displacement and van Hove functions for both translation and rotation. The effect of corner-roundness on the dynamics is further studied by comparing the dynamics of the rhombic crystal phases formed by different corner-rounded particles at a same surface fraction. The results show that as ζ increases from 0.286 to 0.667, the translational collective motion of particles changes from a gliding-dominant pattern to a hopping-dominant pattern, whereas the rotational motion pattern is hopping-like and does not change in its type, but the rotational hopping becomes much more frequent as ζincreases(i.e., as particles become more rounded). A simple geometrical model is proposed to explain the trend of gliding motion observed in MD simulations.     

Molecular dynamics simulation of defect formation in an aluminum crystal under low-energy ion bombardment

Atomic collision cascades initiated by Ar and Xe ions (with energies of 25, 40, and 50 eV) normally incident on the Al(100) crystal surface at a crystal temperature of 300 K have been simulated by the molecular dynamics technique. The formation of vacancies and radiation-adsorbed and interstitial atoms in a cascade is discussed. It is demonstrated that the numbers of surface and bulk vacancies formed in cascades under bombardment of the Al(100) surface by Xe ions reach two maxima within 0.2–0.3 and 0.7–1.0 ps after the cascade initiation, whereas the number of vacancies generated under bombardment by Ar ions reaches one maximum within 0.2–0.3 ps after the cascade initiation.     

Molecular dynamics simulations of compressed hydrogen

Abstract

Molecular dynamics simulations have been performed for highly compressed fluid hydrogen in the density and temperature regime of recent shock-compression experiments. Both density functional and tight-binding electronic structure techniques have been used to describe interatomic forces. Two tight-binding models of hydrogen have been developed with a single s-type orbital on each atom that reproduce properties of the dimer, of various crystalline structures, and of the fluid. The simulations indicate that the rapid rise in the electrical conductivity observed in the gas-gun experiments depends critically on the dissociated atoms (monomers). We find that the internal structure of warm, dense hydrogen has a pronounced time-dependent nature with the continual dissociation of molecules (dimers) and association of atoms (monomers). Finally, Hugoniots derived from the equations-of-state of these models do not exhibit the large compressions predicted by the recent laser experiments.