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.     

Accommodation coefficients for low-energy ions on metal surfaces

The interaction of low-energy ions (E = 3 to 100 eV) with the surface of a solid cannot be treated in terms of gas dynamics. The scattering of particles at an energy of E _o > 20 eV may be explained in terms of binary collisions with the atoms of the target. The validity of the single collision model with free surface atoms for medium energies from 10 to 100 eV and even down to 1 eV was confirmed on the basis of both experimental and computational data. This paper describes experimental studies of the secondary ion emission from the (100) and (110) faces of a Mo single crystal and both experimental and theoretical studies of alkaline ion accommodation coefficient on polycrystalline Mo within the energy range E = 3 to 50 eV with a varying direction of the primary ion beam from normal to the glancing angle of incidence (ρ = 0 to 75°). On the basis of the retarding potential method using a spherical condenser whose central electrode was the target, we measured the energy distribution of secondary ions. Calculations have been performed for the energy of scattered ions and the high energy portion of accommodation coefficient on the basis of single and double binary collisions using the Born-Mayer potential and taking into consideration the influence of adsorption forces at the surface of the target.     

Pt(111)表面低能溅射现象的分子动力学模拟

利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，详细研究了贵金属原子在Pt (111)表面的低能溅射现象.模拟结果显示：对于垂直入射情况，入射原子的质量对Pt (11 1)表面的溅射阈值影响不大.当入射原子的能量小于溅射阈值时，入射原子基本以沉积为主 ；当入射原子的能量大于溅射阈值时，溅射产额随入射原子能量的增加而线性增大；当入射 原子能量达到200 eV时，各种入射原子的溅射产额都达到或接近1，此时入射原子主要起溅 射作用.溅射原子发射的角分布概率和溅射花样与高能溅射相类似.研究表明：与基于二体碰 撞近似的线性级联溅射理论不同，当入射原子能量大于溅射阈值时，低能入射原子的溅射产 额正比于入射原子的约化能量和入射原子与基体原子的质量比.通过对低能入射原子的钉扎 能力分析，提出了支配低能溅射的入射原子反射物理机理. 关键词： 分子动力学模拟、低能溅射     

Probability of ionization of sputtered particles as a function of their energy: Part I: Negative Si ions

In this study we have investigated how the probability of ionization of sputtered Si atoms to form negative ions depends on the energy of the atoms. We have determined the ionization probability from experimental SIMS energy distributions using a special experimental technique, which included de-convolution of the energy distribution with an instrumental transmission function, found by separate measurements.We found that the ionization probability increases as a power law ∼E^0.677 for particles sputtered with energies of 0-10 eV, then becomes a constant value (within the limits of experimental error) for particles sputtered with energies of 30-100 eV. The energy distributions of Si⁻ ions, measured under argon and cesium ion sputtering, confirmed this radical difference between the yields from low and high-energy ions.To explain these results we have considered ionization mechanisms that are different for the low energy atoms (<10 eV) and for the atoms emitted with higher energy (>30 eV).     

Point-defect properties of,and sputtering events in,the {001} surfaces of Ni3Al. II. Sputtering events at and near surfaces

Atomic recoil events at and near {001} surfaces of Ni₃Al due to elastic collisions between electrons and atoms have been simulated by molecular dynamics to obtain the sputtering threshold energy as a function of atomic species, recoil direction and atomic layer of the primary recoil atom. The minimum sputtering energy occurs for adatoms and is 3.5 and 4.5?eV for Al and Ni adatoms on the Ni–Al surface (denoted ‘M’), respectively, and 4.5?eV for both species on the pure Ni surface (denoted ‘N’). For atoms within the surface plane, the minimum sputtering energy is 6.0?eV for Al and Ni atoms in the M plane and for Ni atoms in the N surface. The sputtering threshold energy increases with increasing angle, θ, between the recoil direction and surface normal, and is almost independent of azimuthal angle, ?, if θ<60°; it varies strongly with ? when θ>60°, with a maximum at ??=?45° due to ?{110}? close-packed atomic chains in the surface. The sputtering threshold energy increases significantly for subsurface recoils, except for those that generate efficient energy transfer to a surface atom by a replacement collision sequence. The implications of the results for the prediction of the mass loss due to sputtering during microanalysis in a FEG STEM are discussed.     

Electron and ion emission of faces of a tungsten crystal

The emission properties of the close-packed (110) and porous (111) faces of a tungsten crystal were studied with band-shaped samples in planar devices at a residual gas pressure of 10^–9 torr. The experimental apparatus has a provision for simultaneous measurement of the emission currents from two samples. The orientation of the samples is determined by chemical etching and by x-ray diffraction. The most probable values of _(hkl) for the (110) and (111) faces were found to be 5. 30 ± 0. 03 and 4. 40 ± 0. 02 eV, respectively, by the method of thermionic emission and the method of positive surface ionization of lithium atoms. The peak observed at temperatures < 1500 °K on the logI⁺ = f(5040/T) curve for the (111) face is explained, following Dobretsov, in terms of an increase in the work function of this surface at low coverages (­ 1) by adsorbed lithium atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 7–11, June, 1970.     

ADATOM, VACANCY AND SPUTTERING YIELDS OF ENERGETIC Pt ATOMS IMPACTING ON Pt(100) BY MOLECULAR DYNAMICS SIMULATION

We have studied the influence of incident atoms with low energy on the Pt(100) surface by molecular dynamics simulation. The interaction potential obtained by the embedded atom method (EAM) was used in the simulation. The incident energy changes from 0.1eV to 200eV, and the target temperature ranges from 100 to 500 K. The target scales are 6×6×4 and 8×8×4 fcc cells for lower and higher incident energies, respectively. The adatom, sputtering, vacancy and backscattering yields are calculated. It was found that there is a sputtering threshold for the incident energy. When the incident energy is higher than the sputtering threshold, the sputtering yield increases with the increase of incident energy, and the sputtering shows a symmetrical pattern. We found that the adatom and vacancy yields increase as the incident energy increases. The vacancy yields are much higher than those obtained by Monte Carlo simulation. The dependence of the adatom and sputtering yields on the incident energy and the relative atomistic mechanisms are discussed.     

Investigation of oxygen adsorption on Cu(110) by low-energy ion bombardment

The interaction of molecular oxygen with a Cu(110) surface is investigated by means of low energy ion scattering (LEIS) and secondary ion emission. The position of chemisorbed oxygen relative to the matrix atoms of the Cu(110) surface could be determined using a shadow cone model, from measurements of Ne⁺ ions scattered by adsorbed oxygen atoms. The adsorbed oxygen atoms are situated 0.6 ± 0.1 Å below the midpoint between two adjacent atoms in a 〈100〉 surface row. The results of the measurements of the ion impact desorption of adsorbed oxygen suggest a dominating contribution of sputtering processes. Ion focussing effects also contributes to the oxygen desorption. The ion induced and the spontaneous oxygen adsorption processes are studied using different experimental methods. Sticking probability values obtained during ion bombardment show a strong increase due to the ion bombardment.     

Acceleration of Mesic Atoms Associated with Auger Transitions in Low Energy Collisions

New mechanisms of mesic atom acceleration associated with Auger transitions in mesic hydrogen at low energy collisions (10^?3<ε<10 eV) with a hydrogen atom are considered. Two possible mechanisms are discussed: Auger-plus and Auger-minus processes, which can accelerate mesic atoms up to kinetic energy ～1 eV and ～100 eV, respectively.     

Structure sensitive factors in molecular cluster formation by ion bombardment of single crystal surfaces

The dynamics of molecular cluster formation from a solid bombarded by a 600 eV Ar⁺ ion have been studied classically by computer simulation. The dimers and trimers are found to establish their identity as clusters within interaction range of the solid, but not by a direct ejection of a bound molecule. The Cu₂/Cu and Cu₃/Cu ratios are found to be strongly dependent on crystal orientation. The (111) face is 2–3 times more likely to produce multimers than the (100) face. We find 9 trimers from (111) but none from (110). The relationship between cluster composition and the original arrangement of those atoms on the surface is presented in detail. We find that each multimer forms from atoms that originate within a roughly circular region of area ～70 Ų or less. This region is not necessarily centered on the ion impact point. A consequence of this observation is that dimers can consist of atoms that were several Ångströms apart on the surface but that most trimers contain at least one nearest neighbor pair of atoms. The calculated energy distribution for the dimers matches well with similar experimental studies.     

UO2 晶体中低密勒指数晶面表面能的分子动力学模拟

采用基于刚性离子势的分子动力学模拟方法初步计算了UO₂晶体中(100), (110)和(111) 3种低密勒指数晶面在300–1500 K范围内的表面能大小. 结果表明, 3种晶面的表面能大小随温度的升高而降低, 与实验结果趋势一致; 原子排列最紧密的(111)晶面具有最低的表面能, 3种晶面的表面能大小从高到低依次为(100), (110)和(111)晶面; 达到平衡状态下的表面层原子相对于体内原子层在表面的法线方向上发生了明显的压缩并且表面层原子的对称性也降低了, 表面原子的弛豫效应一直影响到了第5层. 计算研究结果将有助于深入认识UO₂燃料中裂变气体气泡的聚集长大以及燃料的辐照肿胀开裂行为. 关键词： 分子动力学 2')" href="#">UO₂ 低密勒指数晶面 表面能     

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.     

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.     

NMR-on study of60Co in Fe−Si single crystal

NMR-ON measurements have been performed for⁶⁰CoFe−Si. The samples of Fe−Si (6 at. %) single crystal dish with the (110) surface were used. Rosonance measurements have been carried out with the magnetization direction along the <100> and the <111> axes. Five prominent resonances were found at 166, 162, 151, 147 and 135 Mllz. The resonance at 166 Mllz has been known to be due to⁶⁰Co without neighbourt Si atoms. The resonances at 151 and 135 Mllz are due to⁶⁰Co nuclei with one and two Si atom(s) in the 1st neighbour site, respectively. The resonances at 162 and 147 Mllz would be due to the contribution of the 3rd <111> and the <100> magnetization axes, the differences of the resonance width have been qualitatively explained using the dipolar field.     

The adsorption of CO on Cu surfaces studied by electron energy loss spectroscopy

Electron energy loss spectroscopy (ELS) in the energy range of electronic transitions (primary energy 30 < E₀ < 50 eV, resolution ΔE ≈ 0.3 eV) has been used to study the adsorption of CO on polycrystalline surfaces and on the low index faces (100), (110), (111) of Cu at 80 K. Also LEED patterns were investigated and thermal desorption was analyzed by means of the temperature dependence of three losses near 9, 12 and 14 eV characteristic for adsorbed CO. The 12 and 14 eV losses occur on all Cu surfaces in the whole coverage range; they are interpreted in terms of intramolecular transitions of the CO. The 9 eV loss is sensitive to the crystallographic type of Cu surface and to the coverage with CO. The interpretation in terms of d(Cu) → 2π¹(CO) charge transfer transitions allows conclusions concerning the adsorption site geometry. The ELS results are consistent with information obtained from LEED. On the (100) surface CO adsorption enhances the intensity of a bulk electronic transition near 4 eV at E₀ < 50 eV. This effect is interpreted within the framework of dielectric theory for surface scattering on the basis of the Cu electron energy band scheme.     

Surface coverage measurements by sims for CO adsorption on a number of metals and for CO and H2S CO-adsorption on Ni(110), (100) and (111)

A detailed study of CO adsorption on Ni(100) utilizing static SIMS and a comparison of the data with surface coverage data from the literature shows that there is a linear relationship between CO coverage and the peak intensity ratios (MCO⁺/M⁺ and M₂CO⁺/M⁺₂) of the CO-containing secondary ions, in the region of coverage below which the adlayer becomes compressed. Adsorption isobares were obtained using the intensity ratios and from these, adsorption isosteres were derjved to give heats of adsorption as a function of coverage. These data are in very close agreement with the literature. Confirmatory data were obtained for this relationship for CO adsorption on polycrystalline Ni, Pd, Pt and Cu and Cu(100). The application of this technique of surface coverage measurements to a study of the extent to which H₂S coadsorption reduces the coverage of adsorbed CO on Ni(110), (100) and (111) shows that these faces are poisoned in the order (100) > (111) > (110). Surface coverage measurements on the non-closepacked (110) face are affected by the apparent insensitivity of SIMS to adsorbates located in the “channels”.     

低能原子沉积在Pt(111)表面的分子动力学模拟

采用嵌入原子方法的原子间相互作用势,利用分子动力学方法模拟了六种贵金属原子(Ni,Pd,Pt,Cu,Ag,Au)分别在Pt(111)表面低能沉积的动力学过程.结果表明:随着入射能量从0.1eV升高到200eV,基体表面原子是按层迁移的,沉积过程对基体表面的影响和沉积原子在基体表层的作用均存在两个转变能量(ET1≈5eV,ET2≈70eV).当入射能量低于5eV时,基体表面几乎没有吸附原子和空位形成,沉积原子在基体表层几乎没有注入产生;当入射能量在5—70eV范围内时,沉积原子在基体表层有注入产生,其注入深度小于两个原子层,即为亚注入,此时吸附原子主要由基体表层原子形成,基体表面第三层以下没有空位形成;当入射能量高于70eV时,沉积原子的注入深度大于两个原子层,将会导致表面以下第三层形成空位,并且空位产额随入射能量的升高而急剧增加.基于分子动力学模拟的结果,对低能沉积作用下的薄膜生长以及最优沉积参数的选择进行了讨论.     

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采用分子动力学方法模拟200eV的CH3粒子轰击到不同基底温度的钨样品上,分析了C、H原子在钨表面的沉积、散射及溅射情况,结果表明C、H原子的沉降量均随入射剂量的增加而增加。在基底温度为100K时,相同入射剂量下沉积的C原子最多,而当基底温度为1200K,在入射剂量大于1.5X1016cm-2时,C原子的沉降量小于其它基底温度下的C的沉降量。CH3在轰击样品时发生了分解,各种分解情况随基底温度变化较小,其中不同基底温度下一级分解率在40%上下波动,二级分解在23%左右,而完全分解的CH3在9%左右。C、H原子的散射角主要分布在5°~85°间,散射C原子分布的最大值分布在40%~50°或50°~60°间,散射C原子分布的最小值分布在0°~10°或80°~90°间;而不同基底温度下散射H原子分布的最大值均在40°~50°间,最小值均在0°~10°间。散射C原子的能量在0~140eV之间,散射能量为0~120eV的C原子占散射总量的98%以上,散射C原子平均能量随基底温度的增加而增加,其变化从65.5eV增加到68.5eV;散射H原子的能量也在0~140eV之间,但大约70%的散射H原子能量在40eV以内,散射平均能量随基底温度的增加而减小,其变化从13.92eV减小到13.05eV。     

ZrMn2（110）表面结构及吸氢机理的第一性原理研究

采用基于密度泛函理论(DFT)的平面波赝势(PW-PP)方法,研究了ZrMn2（110）清洁表面结构和氢原子在表面的吸附。弛豫表面结构的计算结果表明表面结构的最表层为曲面,且表面结构的原子间隙变小。由1Zr2Mn原子组成的空位是氢原子吸附在ZrMn2（110）表面的最佳吸附位,吸附能为3.352 eV,氢原子吸附后离表面的距离为1.140 Å。Mulliken电荷布居分析表明吸附的氢原子与表面原子的相互作用主要是接近氢原子的第一层原子与氢原子的相互作用。过渡态计算表明被吸附的氢原子进入表面内部需克服的最大势垒为1.033 eV。