Angular spectra of rainbow scattering at glancing keV He bombardment of NiAl(1 0 0) surface with transverse energies in the range 1-10 eV

Previous simulations of glancing incidence ion-surface interaction have demonstrated that classical dynamics using the row-model have successfully reproduced multimodal azimuthal and polar spectra. These studies have also shown considerable sensitivity to the form of the interatomic potential thus making it a strong test of the validity of such potentials and even allow deduction of the ion-surface potentials. In these simulations the individual pairwise interactions between the projectile and the target atoms have been replaced by cylindrical potentials.Comparison to numerous experimental studies have confirmed the existence of rainbow scattering phenomena and successfully tested the validity of the cylindrical potential used in these simulations. The use of cylindrical potentials avoids stochastic effects due to thermal displacements and allows faster computer simulations leading to reliable angular distributions.In the present work we extend the row-model to consider scattering from binary alloys. Using He⁺ scattered at glancing incidence from NiAl surfaces, Al or Ni terminated, a faster method has been developed to easily and accurately quantize not only the maximum deflection azimuthal angle but all the singular points in the angular distribution. It has been shown that the influence of the surface termination on the rainbow angle and the inelastic losses is small.     

Dynamic dependence of the interaction potentials for grazing scattering of fast atoms from metal and insulator surfaces

For scattering of fast atoms from metal and insulator surfaces under axial channeling conditions pronounced peaks in the angular distributions of scattered projectiles are interpreted in terms of rainbow scattering. The angular position of such “rainbow peaks” are closely related to the interaction potential and its corrugation in the topmost surface region. We have scattered N and O atoms, with energies ranging from 10 to 70 keV, from clean and flat Al(0 0 1) and LiF(0 0 1) surfaces along low index axial directions in the surface plane and studied the positions of the rainbow peaks as function of the kinetic energy of the atomic projectiles normal to the surface. For the insulator surface the rainbow angle does not depend on projectile energy for constant normal energy, whereas for the metal surface we find pronounced dynamic effects. We interpret this different behaviour as arising from a projectile energy dependent contribution to the underlying interaction potentials owing to embedding the projectiles into the free electron gas in the selvedge of the surfaces, which is present for the metals but absent for insulators.     

Study of ion emission from a germanium crystal surface under impact of fast Pb ions in channeling conditions

A thin germanium crystal has been irradiated at GANIL by Pb beams of 29 MeV/A (charge state Q_in = 56 and 72) and of 5.6 MeV/A (Q_in = 28). The induced ion emission from the sample entrance surface was studied, impact per impact, as a function of Q_in, velocity v_in and energy loss ΔE in the crystal. The Pb ions transmitted through the crystal were analyzed in charge (Q_out) and energy using the SPEG spectrometer. The emitted ionized species were detected and analyzed in mass by a time-Of-flight multianode detector (LAG). Channeling was used to select peculiar ΔE values in Ge and hence peculiar Pb ion trajectories close to the emitting entrance surface. The experiment was performed in standard vacuum. No Ge emission was found. The dominating emitted species are H⁺ and hydrocarbon ions originating from the contamination layer on top of the crystal. The mean value 〈M〉 of the number of detected species per incoming Pb ion (multiplicity) varies as (Q_in/v_in)^p, with p values in agreement with previous results. We have clearly observed an influence of the energy deposition ΔE in Ge on the emission from the top contamination layer. When selecting increasing values of ΔE, we observed a rather slow increase of 〈M〉. On the contrary, the probabilities of high multiplicity values, which are essentially connected to fragmentation after emission, strongly increase with ΔE.     

Competition of terrace and step-edge sputtering under oblique-incidence ion impact on a stepped Pt(1 1 1) surface

Using molecular-dynamics simulation, we study the sputtering of a Pt(1 1 1) surface under oblique and glancing incidence 5 keV Ar ions. For incidence angles larger than a critical angle ?_c, the projectile is reflected off the surface and the sputter yield is zero. We discuss the azimuth dependence of the critical angle ?_c with the help of the surface corrugation felt by the impinging ion. If a step exists on the surface, sputtering occurs also for glancing incidence ?>?_c. We demonstrate that for realistic step densities, the total sputtering of a stepped surface may be sizable even at glancing incidence.     

Improved calculation of Si sputter yield via first principles derived interatomic potential

Silicon sputter yield under medium energy Ar+ ion bombardment is calculated via molecular dynamics, using a highly accurate interatomic potential for Ar-Si interactions derived from first-principles calculations. Unlike the widely used universal repulsive potentials such as the Moliere or ZBL parameterizations, this new potential, referred to as DFT-ArSi, is developed via localized basis density functional theory. Sputter yields for Si obtained with the DFT-ArSi potential at 500 eV and 1 keV incident energies are found to be within 6% and 2% of experimental results, respectively, while errors using existing potentials are typically on the order of 11%. The DFT-ArSi potential differs from existing empirical potentials in the ∼1 Å interatomic separation range which is shown to be the most important range for modeling low-to-medium energy ion bombardment.     

LEIS: A reliable tool for surface composition analysis?

Different single and polycrystalline surfaces of Cu and Ag have been investigated by time-of-flight low-energy ion scattering using ⁴He⁺ ions. The fraction of ions that survived single scattering from the outermost surface layers, P⁺, was measured in different neutralization regimes. At low energies, a distinct difference in P⁺ was observed for non-equivalent Cu crystal surfaces for projectiles backscattered in a single collision. The polycrystalline surface was found to exhibit similar neutralization behaviour as the (1 1 1) single crystal surface. At higher energies, P⁺ shows a strong dependence on the angular orientation of the single crystal. The impact of these findings on quantitative surface composition analysis by LEIS is discussed.     

Development of secondary ion mass spectroscopy using medium energy C60 ion impact

In this paper, we report time-of-flight (TOF) secondary ion mass spectroscopy using primary C₆₀ ions with an energy range from several tens of keV to several hundreds of keV. Application of the spectroscopy to the analysis of a poly(amino acid) film revealed that characteristic peaks, necessary for identification of the amino acid in proteins, show higher intensities for medium energy C₆₀ (120 keV and 540 keV ) impacts than those for low energy C₆₀ (30 keV ) impacts. This finding demonstrates that medium energy C₆₀ ion impacts are useful for highly sensitive characterization of amino acids.     

Optical emission from Be, Cu and CuBe targets during ion beam sputtering

A spectral structure of the radiation (190-590 nm) emitted during sputtering of polycrystalline Cu, Be and CuBe targets by Kr⁺ ions with 5 keV have been presented. Evolution of surface composition during ion beam sputtering is investigated. Several time scales are distinguished, corresponding to different processes: the elimination of surface contaminants, the removal of the corroded layer. The implications for the use of ion beam optical spectroscopy in surface analysis are discussed. In the case of Be and Cu98 Be2, a molecular structure appears between 492 nm and 502 nm. It is similar for both samples and is ascribed to de-excitation of BeH.     

On the emission of MoCs molecular ions from Cs irradiated molybdenum surface

The present paper deals with the emission of atomic and molecular ions from elemental molybdenum surface under Cs⁺ bombardment to explore the MCs⁺ formation mechanism with changing Cs surface coverage. Integrated count of MoCs⁺ shows a monotonic increase with increasing primary ion energy (1-5 keV). Change in MoCs⁺ intensity is attributed to the variation of surface work function ? and cesium surface concentration c_Cs due to varying impact energies. Variation of c_Cs has been obtained from the expression, c_Cs ∝ 1/(1 + Y) where Y is the elemental sputtering yield estimated from TRIM calculations. Systematic study of the energy distributions of all species emerging from Mo target has been done to measure the changes in surface work function. Changing slopes of the leading parts of Cs⁺ energy distributions suggest a substantial depletion in surface work function ? with decreasing primary ion energies. Δ? shows a linear dependence on c_Cs. The maximum reduction in surface work function Δ?_max = 0.69 eV corresponds to the highest value of c_Cs = 0.5. A phenomenological model, based on the linear dependence of ? on c_Cs, has been employed to explain the MoCs⁺ data.     

Angular distribution of GaAs sputtered under oblique Cs bombardment

The angular distribution of Ga and As sputtered from Gallium Arsenide (1 0 0) by a Cs⁺ ion beam was experimentally measured through a collector technique allowing modifications of the energy and incidence angle of the ion beam. The impact energy was varied in the range of 2-10 keV and the angle of incidence from 30° to 60°.The angular distributions of emitted matter are determined by means of SIMS depth profiles. Our series of experiments show an evolution of the preferential direction of emission as well as the spreading around this direction in function of the characteristics of the ion beam.The second objective is the study of the evolution of the stoichiometry of the deposit in function of the emission angle. A decrease of the As/Ga ratio around the preferential direction of emission and an increase of this ratio for oblique emission are observed for different conditions of primary bombardment. Considering that the angular distribution depends on the depth of origin, our results suggest that the Cs⁺ bombardment changes the stoichiometry of the near-surface layers of the sample with an enrichment of As in the outmost layers while the sub-surface region is impoverished in As due to preferential sputtering.     

The influence of projectile charge state on ionization probabilities of sputtered atoms

The ionization probability of atoms sputtered from a clean polycrystalline metal surface was measured for different charge states of the projectile used to bombard the sample. More specifically, a polycrystalline indium surface was irradiated with Ar⁺ and Ar⁰ beams of energies between 5 and 15 keV, and In⁺ secondary ions and neutral In atoms emitted from the surface were detected under identical experimental conditions regarding the sampled emission angle and energy. The resulting energy integrated ionization probability of sputtered In atoms is consistently found to be smaller for neutral projectiles, the difference decreasing with decreasing impact energy. The observed trends agree with those measured for kinetic electron emission, indicating that secondary ion formation is at least partly governed by kinetic substrate excitation.     

New method of evaluation for interatomic interaction potential in LEIS with large-angle scattering using the two-atom scattering model

The interaction potential between an incident ion and a target atom in impact-collision ion scattering spectroscopy (ICISS), which is a specialization of low energy ion scattering (LEIS) and its variants, i.e. ICISS with detection of neutrals (NICISS), coaxial ICISS (CAICISS) and impact-collision atom scattering spectroscopy with detection of neutrals (NICASS), has been evaluated by the new method using the dependence of the total scattering angle on the impact parameter for the first collision in the numerical calculations based on the two-atom scattering model (TWASM). From the comparison of determined values of scaling factor for the Firsov screening length by three-dimensional computer simulations with calculated ones by TWASM, it became obviously that the interatomic potentials for the various combinations of an incident ion and a target atom in LEIS are suitably given by the Moliere potential with the reduced Firsov screening length employing the scaling factor obtained in TWASM calculations.     

On the origin of the LEIS signal in TOF- and in ESA-LEIS

The ion fraction analysis of ⁴He⁺ ions backscattered from various faces of copper single crystals is performed by using time-of-flight (TOF) and electrostatic analyzer (ESA) low-energy ion scattering (LEIS) techniques. When an experiment that integrates over 2π azimuth (typical ESA-LEIS setup) is used, the yield of ions backscattered from the Cu(1 1 0) surface may be given by projectiles penetrated much deeper than just one or two monolayers. The threshold energy for reionization processes for ⁴He⁺ and Cu found earlier by TOF-LEIS is experimentally confirmed by ESA-LEIS.     

Segregation of atoms in NixPdy alloys during ion irradiation

Sputtering of Ni₅Pd and NiPd₅ alloys by 10 keV Ar ions has been studied using the binary-collision simulation. Special attention was given to the angular distributions of sputtered atoms at the steady-state conditions. The results of simulations were compared with the experimental data published recently. For both targets, the concentrations of Ni and Pd atoms in the top monolayer were extracted from the experimental data. The results of simulations favor segregation of Pd in Ni₅Pd and segregation of Ni in NiPd₅. The total concentration of surface vacancies was found to be about 10-30%.     

Molecular dynamics simulations to explore the effect of chemical bonding in the keV bombardment of Si with C60, Ne60 and Ne60 projectiles

Depth profiling experiments using secondary ion spectrometry (SIMS) have shown effects that are characteristic to the pairing of the projectile with a Si target. Previous molecular dynamics simulations demonstrate that this unusual behavior is due to the fact that strong covalent bonds are formed between the C atoms in the projectile and the Si atoms in the target, which result in the implantation of carbon into the solid. The focus of this paper is to understand how the formation of chemical bonds affects the net sputtered yield. The results of molecular dynamics simulations of the keV bombardment of Si with C₆₀, Ne₆₀ and ¹²Ne₆₀ at normal incidence are compared over a range of incident kinetic energies from 5 to 20 keV. The net yields with Ne₆₀ and ¹²Ne₆₀ are significantly greater than with C₆₀ at all incident kinetic energies, with ¹²Ne₆₀ having the largest values. Application of the mesoscale energy deposition footprint (MEDF) model shows that the initial deposition of energy into the substrate is similar with all three projectiles. Snapshots of the initial pathway of the projectile atoms through the substrate show a similar lateral and vertical distribution that is centered in the region of the energy footprint. Therefore, the reason for the reduced yield with C₆₀ is that the C atoms form bonds with the Si atoms, which causes them to remain in the substrate instead of being sputtered.     

SIMS analysis of xenon and krypton in uranium dioxide: A comparison of two models of gas-phase ionisation

Reasonable detection limits in secondary ion mass spectrometry (SIMS) analysis of inert gases in solids may be achieved by gas-phase ionisation. For Xe in UO₂ Desgranges and Pasquet (D&P) have recently reported that the ion intensity can be enhanced significantly using ion bombardment in combination with an oxygen jet directed at the sputtered area. A similar effect was reported by Portier et al. for Kr in UO₂. The enhancement was attributed to ionisation by an interaction with oxygen atoms and molecules (D&P model). The proposed mechanism is at variance with a recently outlined model of gas-phase ionisation involving charge transfer between the ejected atoms and the incoming primary ions. The purpose of this study was to clarify these contradictory views by a reanalysis of the original data. Access to all relevant original data was kindly provided by L. Desgranges. In contrast to expectation based on the D&P model, step-wise increases in the flow rate of the oxygen jet did not result in an immediate response of . Instead, transient yield changes were observed, with characteristic rise times that were even longer for Xe⁺ than for uranium specific signals. This observation invalidates the idea that ionisation of Xe atoms is due to interaction with oxygen molecules of the jet. Recalling well known transient phenomena in ion-bombardment induced photon emission, it is argued that the enhancement effect may be associated with the increase in the oxidation state of the sample, as a result of which the fraction of Xe (or Kr) atoms leaving the sample in an electronically excited state is presumably increasing, thus enlarging the effective cross section for charge transfer. Alternatively, the enhancement may be due to a lowering of the ejection velocity of rare gas atoms, possibly caused by the increase in near-surface sample oxidation. A second problem with the D&P study is the use of as a reference signal. Literature data as well as new results reveal that and UO⁺ secondary ions dominate the SIMS spectrum of oxidised uranium at energies <60 eV, the U⁺ fraction amounting to only 1% or less. Other issues are (i) the significant variation of the sample erosion rate that occurred upon deliberate changes of the mean primary ion current density, (ii) the associated bombardment induced, progressive oxygen depletion of the sample, (iii) the presence of a background superimposed on the Xe⁺ signals, a significant effect at low current densities and (iv) the space-charge broadening of the primary ion beam at high beam currents. Eliminating all these interfering factors, the (reduced) ionisation probabilities of Xe⁺ were found to be in accordance with gas-phase ionisation by charge exchange, even in the pressure of excessive oxygen flooding.     

Electron exchange between an H ion and a spherical cluster of aluminum atoms

The resonant charge transfer (RCT) between a hydrogen anion and a cluster of aluminum atoms is investigated by means of the wave-packet propagation method that does not exploit the perturbation theory. The RCT on a spherical cluster is found to exhibit quantum size effects due to the finite size of the cluster. The survival amplitude of an ion state has been calculated as a function of the distance to the ion-surface in a normal collision. It is shown that depending on the velocity of the impinging particle, the cluster can behave either as a bulk metal or as a quantum structure with discrete energy states existing over two coordinates.     

Assessment of approximations for efficient topography simulation of ion beam processes: 10 keV Ar on Si

The main assumption of existing efficient topography simulations is that sputtering is a local process that depends only on the angle of incidence and not on the detailed shape of the surface. If redeposition is considered, sputtered atoms are redeposited and cause no further sputtering when they hit another part of the surface. Furthermore the angular distribution of sputtered atoms follows a cosine law. If ion reflection is considered, ions do not lose energy during backscattering. Using binary collision simulations (IMSIL) and comparing them with results obtained by a topography simulator (IonShaper^®) we show that all these assumptions need refinement for the simulation of nanostructures except the neglect of sputtering by sputtered atoms. In addition we show that a nonlocal model is essential for ion beam induced deposition of narrow structures.     

Evolution of ion-induced ripple patterns on SiO2 surfaces

The evolution of nanoscale ripple patterns during sub-keV ion sputtering of thermally grown, fused and single crystalline SiO₂ surfaces has been investigated by means of atomic force microscopy. For all three materials, different dependencies of the ripple wavelength and the surface roughness on the ion fluence have been found. Within the Bradley-Harper model of pattern formation, the observed differences are consistent with different amounts of surface and near-surface mass transport by ion-enhanced viscous flow which might result from different surface energies of the SiO₂ specimens.