Energieverteilungen bei der Festkörperzerstäubung durch Ionenbeschuß

The energy distributions of the neutral particles sputtered from polycrystalline targets of Al, Ti, Ni, Cu and Ag by normally incident Ar⁺-ions in the 1 keV region have been determined for ejection energies below 20 eV in a direction close to the normal to the target surface. The experimental method employed is strongly connected to the characteristic properties of a low pressure electrodeless hf plasma used as an effective ion source as well as an ionizing medium for the ejected target atoms. The resulting curves always show a maximum at most probable ejection energies between 1 and 5 eV, being approximately half of the surface binding energy. These curves are converted to the corresponding energy distributions for the recoil atoms within the target by an energy dependent factor. It is found that the energy distribution within the bombarded solid decreases monotonically with the inverse square of the energy of the recoils.     

Dynamic Potential of Interaction between Nitrogen Atoms and Aluminum Crystal Surface

Specific features of the angular distributions of accelerated neutral nitrogen atoms at the grazing angles of incidence on the Al(001) crystal surface have been investigated by the computer simulation method. The N–Al pair interaction potential is approximated by the three-parameter Morse potential with the energydependent coefficients. The angular distributions of scattered atoms have been simulated taking into account the interaction between atoms and several atomic layers in the lattice and the atomic displacement during thermal oscillations. The parameters of the pair potential of accelerated neutral nitrogen atoms in the energy range from 10 to 70 keV have been determined according to the best agreement between the calculated dependence of the rainbow scattering angle on the energy of particles incident on the crystal surface and the available experimental data.

     

Modeling of neutral-atom scattering at the surface of a crystal for the case of grazing incidence

The features of the angular distributions of accelerated neutral atoms at grazing angles of incidence on the Al(001) surface are studied using the mathematical modeling method. The interaction of accelerated atoms with crystal-lattice atoms and the electronic properties and atomic structure of the Al(001) surface are calculated using the electron-density-functional method. The angular distributions of scattered atoms are modeled by taking into account their interaction with several atomic layers in the crystal lattice and atomic displacements during thermal oscillations. The influence of crystal surface-layer relaxation on rainbow scattering, i.e., the difference between the distances of planes on the surface and in the volume, is established. The possibilities of using the effect of rainbow scattering to study the structural features of a crystal surface are discussed.     

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.     

Mechanisms and energetic of atomic processes in surface diffusion

Surface diffusion is a subject of basic importance for understanding mass transport phenomena in surface and nano science. In the particle aspect of surface diffusion of single atoms and simple molecules, information of interest is the detail atomic mechanisms and the activation energy of various atomic processes, and also the binding energy of atoms at different surface sites. In the absence of an external force, atoms will perform random walk without a preferred direction. When an atom is subjected to an external force, or when a chemical potential gradient exists, it will move preferentially in the direction of the force, or in the direction of decreasing chemical potential, thus the random walk becomes directional. Using atomic resolution microscopy, it is now possible to observe random walk diffusion of atoms, molecules and atomic clusters directly as well as to study the dynamic behavior of atoms as perturbed by the electronic interactions of the surface in great detail. Here, methods of studying quantitatively the particle aspect of surface diffusion and how it affects the dynamic behavior of the surface are very briefly reviewed.     

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.     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

Blocking effect for fast ionized recoils

The angular and energy distributions of fast ionized recoils produced under copper crystal irradiation by argon ions have been investigated. The distributions have been found to be strongly determined by regular arrangement of the target atoms. The obtained results are discussed in terms of the blocking model.     

Classical trajectory calculations of the energy distribution of ejected atoms from ion bombarded single crystals

The energy distribution of particles ejected from single crystal surfaces has been calculated using classical dynamics. The model utilizes a microcrystallite of 4 layers with ~60 atoms/layer which is bombarded by 600 eV Ar⁺ at normal incidence. Calculations have been performed for the clean (100) face of copper as well as for copper with oxygen placed in various coverages and site geometries. The energy distributions for Cu, O, Cu₂, CuO and Cu₃ are reported for this system. The distribution for clean Cu exhibits structure which is shown to arise from preferred ejection mechanisms in the crystal. For oxygen adsorbates, the effect of the oxygen binding energy on the peak in the energy distribution of the ejected oxygen atoms is examined by arbitrarily varying the well-depth of the Cu-O pair potential. In general, higher values of the binding energy produce a maximum in the curve at higher energies and also produce a broader energy distribution. The O₂ and Cu₂ dimer distributions are found to peak at approximately the same energy as the O and Cu curves when compared on a kinetic energy/particle basis, although their widths are considerably smaller. Finally, we predict that the CuO energy distribution should be wider than either the Cu₂ or O₂ distributions since it results from the convolution of the Cu and O distributions which are quite different.     

Computer simulation of low-energy displacement events in pure HCP metals

Abstract

Molecular dynamics simulations of low-energy atomic recoils have been carried out for α-Ti (HCP) with a view to investigating the displacement threshold energy, E_d, in detail. These have been undertaken at 0 K and 100 K, using a many-body interatomic potential modified to reflect the dominant two-body interaction at short range. This is the first systematic study of atomic displacement events in the HCP structure using such a potential. The mechanisms of these threshold events have been investigated and the strong orientation dependence of the threshold energy has been interpreted in terms of the HCP crystal structure. Also, a correlation has been found between the magnitude of the threshold displacement energy and the maximum number of atoms temporarily displaced into interstitial positions during a cascade.     

Range of recoil atoms in isotropic stopping materials

A calculation of the stopping power, range, and range straggling of low energetic atoms (E?1 MeV) is reported. The computation applied the biatomic repulsive potential of Firsov and thus had to be carried out numerically. The results, however, are presented in an analytical form. — Experimental checks were performed with recoil atoms. 96 keV Ra-224 atoms from the α-decay of Th-228 were used to measure the range distributions in several light gases. — The fast neutron induced reactions, such as (n, p)-or (n, α)-reactions, led to recoils of higher energies (up to more than 1 MeV). Their mean ranges in polycrystalline metals or amorphous oxide layers could be obtained by measuring the fraction of radioactive recoils which had left the target surface. — The experimental data are in good agreement with the theory, except at the highest energies where the observed ranges are slightly larger than predicted.     

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.     

The effect of surface channeling of neutral atoms

Special features of reflection of neutral atoms from crystal surfaces are considered in the case of grazing incidence. It is shown that polarization effects and the long-range part of the potential of interaction of ions with atomic chains and planes can play an important role in describing processes of reflection of atoms from the surface. The analysis in this paper shows that the polarization of accelerated neutral atoms in the case of grazing incidence on the surface can lead to the formation of bound surface states. The threshold condition for the angle of arrival of atoms at the plane at which the capture by such states is possible is obtained.     

Effect of temperature on the sputtering of surface metallic clusters

Sputtering of a cluster of 75 copper atoms from a copper-substrate surface by 200-eV argon ions is simulated using a molecular-dynamics method for target equilibrium temperatures of 0, 300, and 500 K. The sputtering coefficients of the substrate and the cluster and the angular and energy distributions of the sputtered atoms are studied. The mechanisms behind the influence of the thermal atomic vibrations on the sputtering yield of surface metallic clusters are discussed.     

Temperature and concentration effects in electron-stimulated desorption of sodium atoms from sodium layers adsorbed on tungsten coated with a gold film

The yield and energy distributions of sodium atoms upon electron-stimulated desorption from sodium layers adsorbed on tungsten coated with a gold film are investigated for the first time as functions of the thickness of the gold film, the concentration of deposited sodium, and the surface temperature. It is found that the energy distributions exhibit two peaks, namely, a narrow peak with a maximum at about 0.15 eV, whose intensity continuously increases with increasing temperature, and a broad peak with a maximum at about 0.35 eV, whose intensity either decreases or remains constant with increasing temperature. It is shown that both peaks arise as a result of the same excitation, which gives rise to different channels of electron-stimulated desorption of sodium atoms. Possible mechanisms of electron-stimulated desorption and the kinetics of destruction of the surface coating are discussed.     

Sputtering studies with the Monte Carlo Program TRIM.SP

The Monte Carlo Program TRIM.SP (sputtering version of TRIM) was used to determine sputtering yields and energy and angular distributions of sputtered particles in physical (collisional) sputtering processes. The output is set up to distinguish between the contributions of primary and secondary knock-on atoms as caused by in- and outgoing incident ions, in order to get a better understanding of the sputtering mechanisms and to check on previous theoretical models. The influence of the interatomic potential and the inelastic energy loss model as well as the surface binding energy on the sputtering yield is investigated. Further results are sputtering yields versus incident energy and angle as well as total angular distributions of sputtered particles and energy distributions in specific solid angles for non-normal incidence. The calculated data are compared with experimental results as far as possible. From this comparison it turns out that the TRIM.SP is able to reproduce experimental results even in very special details of angular and energy distributions.     

The physical process of melting and freezing

It is suggested that at the melting temperature the wavelength of the average thermal phonon vibration is equal to or is a harmonic of the distance separating the atomic layers in the crystal. This resonance between the phonon and lattice vibrations equals out the energy of the vibrating atoms in the surface layer. If this “uniform” energy is higher than the energy corresponding to the metastable transition state then all the surface atoms lose its position stability. In order to make the jump to the next potential well energy is required to overcome on the viscous resistance of the liquid. If this energy, latent heat of fusion, is supplied then the atomic/molecular sheet or platelets from the surface are detached and melting occurs. The proposed model is consistent with all of the characteristic features of melting and freezing. Equations calculating the average phonon wavelength and the corresponding lattice distance at the melting temperature are derived from fundamental thermodynamic relationships. The required thermodynamic parameters are determined from experiments of the nine selected highly symmetrical solids. The calculated wavelengths of the phonon vibration are equal to or is a harmonic of the $d$-spacing of the atomic/molecular layers in agreement with theory.     

Angular distributions of Au and Cu atoms sputtered from Au-Cu alloys by keV Ar+ ion bombardment

Angular distributions of Au and Cu atoms sputtered from Au-Cu alloys under 3 keV AR⁺ ion bombardment were measured to understand the preferential sputtering. The surface composition of sputter-deposited Au-Cu films on substrates mounted at different ejection angles was analyzed by Auger electron spectroscopy and electron probe microanalysis. Although the result indicated that the proportion of sputtered Cu atoms to the Au atoms in the Au-Cu alloy depends on the ejection angle, marked enhancement of the lighter component in the direction normal to the surface has not been observed in spite of the larger mass ratio of the constituent atoms of the Au-Cu alloy.     

Arq+(q=0-12)离子掠射到单晶铜表面的能损谱

采用蒙特卡罗模拟方法,对低速高电荷态Arq+离子掠射到单晶铜表面时的能损谱与表面结构的依赖关系进行研究。在能损计算中,包含了四种可能的能损机制。对于Ar原子沿着晶列方向掠射时,发现能损谱为一两峰结构,其中在能损比较大的区间新出现一个明显的小峰结构。通过研究Arq+以不同条件掠射到表面的能损,对观察到的沟道效应进行论述。能损谱的计算结果与实验结果吻合得比较好。     

Neutron diffraction investigation of specific features of the structure of ultrafine (nano) materials

The specific features of the atomic crystal structure of solids with small (<100 nm) sizes of particles in ultrafine (nano) powders have been investigated. It has been shown that, in contrast to the case of coarsegrained materials, distortions of the atomic order occur as an inhomogeneous deformation in surface layers. The distributions of impurity atoms (in the case of a solid solution) and the second phase (in the case of multiphase systems) from the center to the periphery are also inhomogeneous. The atomic distribution function along the radius of a nanoparticle, which characterizes its atomic structure as intermediate between the crystalline and amorphous structures, has been constructed.