Desorption of silver atoms from benzene-covered Ag(1 1 1) by energetic Ar bombardment

Experiments have been conducted to gain insight into the processes of desorption of neutral species from surfaces covered with organic molecules due to bombardment with keV particles. The system is comprised of benzene molecules adsorbed onto Ag(1 1 1) and bombarded with 8 keV Ar⁺ ions. Molecular dynamics (MD) simulations of the same system have been performed. Results show that the presence of the benzene alters the yield, the kinetic energy distributions, and the angular distributions of the silver atoms. These changes of the desorption characteristics are the result of collisions between the Ag atoms and the benzene molecules adsorbed to the surface. As more benzene is adsorbed to the surface, the changes to the Ag atom desorption characteristics become more pronounced. The simulations reproduce the modifications to the Ag atom energy and angle distributions.     

Angular distribution of sputtered atoms induced by low-energy heavy ion bombardment

The sputtering yield angular distributions have been calculated based on the ion energy dependence of tohal sputtering yields for Ni and Mo targets bombarded by low-energy Hg^ ion. The calculated curves show excellent agreement with the corresponding Wehner‘s experimental results of sputtering yield angular distribution. The fact clearly demonstrated the intrinsic relation between the ion energy dependence of total sputtering yields and the sputtering yield angular distribution. This intrinsic relation had been ignored in Yamamura‘s papers (1981,1982) due to some obvious mistakes.     

Resonant ion stimulated desorption of O from the TiO2(1 1 0) surface: Effects of oxygenation and ion bombardment

The O⁺ desorption from reduced, oxygenated, and ion-bombarded TiO₂(1 1 0) surfaces has been investigated during He⁺ irradiation. The O⁺ desorption is initiated by creation of an antibonding O 2s core hole state via quasi-resonant charge exchange with the He⁺ 1s state, followed by the intra-atomic Auger decay of the O 2s hole. Upon oxygenation of the reduced TiO₂(1 1 0) surface, the O⁺ yield increases by one order of magnitude. The O₂ molecule is dissociated at the vacancy site of bridging oxygen and the oxygen atoms either fill a vacancy site or chemisorb at a fivefold-coordinated Ti⁴⁺ site as an adatom. The latter is detected with much higher efficiency than the former. The O⁺ yield is increased during He⁺ bombardment of the reduced TiO₂(1 1 0) surface due to formation of lower coordinated oxygen atoms. The oxygen species thus formed by ion bombardment or oxygenation are unstable on the surface and tend to diffuse into bulk vacancy sites or higher coordination surface sites even at room temperature.     

Light emission from oxygen covered Al and Cu surfaces

Ion beam induced light emission is used to investigate the sputtering yield, S_O, of oxygen atoms on the surfaces of a polycrystalline copper and an Al(1 1 1) target. Under Ar⁺ and Ne⁺ ion bombardment of Al(1 1 1) and polycrystalline copper targets, spectral lines of Cu I and Al I emitting from sputtered excited atoms are measured as a function of the oxygen partial pressure, wavelength and beam energy. The light emission for two Al I lines (3082 and 3962 Å) and Cu I lines (3247 and 3274 Å) are proportional to the oxygen partial pressure (1×10⁻⁴ Torr). Above 2×10⁻⁴ Torr, the light intensities start to decrease which is consistent with other measurements. From saturated-oxygen covered target surfaces, light intensities of Al I and Cu I lines are measured as a function of time and oxygen partial pressures. The sputtering yields could be determined from the curves of spectral lines directly. For 10 and 20 keV Ar⁺ ions bombarding the copper surface, the oxygen sputtering yields are 0.34 and 0.22 (atoms/ion), respectively. The same copper target was bombarded by Ne⁺ ions at 5 and 10 keV, the oxygen sputtering yields are 0.87 and 0.59, respectively. For 10, 15, and 20 keV Ar⁺ bombarding an Al(1 1 1) target, the obtained sputtering yields are 0.44, 0.31, and 0.2 (atoms/ion), respectively.     

Neutralization of keV Ne and Ar ions during grazing scattering from an Al(1 1 1) surface

Charge fractions after scattering of Ne⁺ ions, Ne⁰ atoms and Ar⁺ ions with keV energies under a grazing angle of incidence from an atomically clean and flat Al(1 1 1) surface are studied. For incoming Ne⁺ ions we observe defined ion fractions in the scattered beams, whereas for incident Ne⁰ atoms ion fractions are more than one order of magnitude smaller. This experimental result provides clear evidence for a survival of Ne⁺ ions over the whole scattering event. From the dependence of ion fractions on the perpendicular energy component we derive neutralization rates as function of distance from the surface. These rates compare well with recent theoretical calculations for the system He⁺–Al(1 1 1). For incident Ar⁺ ions no survival of ions is found and upper limits for the survival probability and lower limits for the neutralization rate are determined.     

Electron emission in 10–100 keV H and He grazing ion-surface collisions

We have measured energy distributions of electrons ejected during grazing 10–100 keV H⁺ and He⁺ ions scattering from Si surfaces for a broad range of electron observation angles. The distributions have contributions strongly dependent on the directions of incidence and observation. For observation regions around the specular reflection of the ions we have studied the angular dependence of the electron structure resulting from electron transfer to the continuum of the effective ion potential. Far from the ion scattering plane we have observed the Si Auger electron spectrum. We discuss the differences between the Auger peaks obtained by electron and grazing proton bombardment of Si surfaces partially covered with O₂ and Al.     

Surface ionization by ion impact at grazing incidence

We have calculated the energy distribution of electrons produced by ionization of bulk electrons in grazing H⁺ surface collisions. In this first approach we assume a direct ion-surface Coulomb interaction, describe the solid within the jellium approximation, and consider nonparallel ion trajectories near the surface. In agreement with experimental results, the energy distributions present a maximum around 3 eV, strongly dependent on the observation angle and projectile energy.     

The angular distribution of particles sputtered from Cu,Zr, and Au surfaces by ion bombardment at grazing incidence

The angular distribution of particles sputtered from polycrystalline Cu, Zr, and Au targets has been measured for bombardment with Ar⁺- and Xe⁺-ions at perpendicular and grazing ion incidence (80° and 85° with respect to the surface normal). The ion energy was varied between 100 keV and 900 keV. The measured distributions follow approximately a cosine-squared curve rather than the cosine function, they are found to be symmetric with respect to the surface normal and almost independent of the ion species, the ion energy and the angle of incidence. Values of sputtering yield of Cu, Au and Zr are also presented.     

Skipping motion of 30 keV H ions on single crystal surface of KCl

The angular distributions and the energy spectra of scattered H⁺ ions are studied at glancing angle incidence of 30 keV H⁺ ions on the (001) surface of KCl single crystal. Discrete energy loss spectra of the scattered ions are observed. The observed exit angle distributions of the ions which have lost anomalous large amounts of energy at various angles of incidence do not depend on the angle of incidence. From the observed results, it is concluded that skipping motion occurs on the surface of KCl.     

Simulation of ion beam induced crystallization and amorphization in (0 0 1) silicon

The ion beam induced epitaxial crystallization (IBIEC) and the ion beam induced interfacial amorphization (IBIIA) in (0 0 1) silicon caused by 3 MeV Si⁺ and 3 MeV Au⁺ irradiation at 293 K and 623 K are investigated by using a combination of binary collision MC simulations and MD simulations. The energy and angular distribution of the primary recoils is calculated by TRIM and the subcascades caused by the primary recoils are treated by classical MD simulations using a correspondingly large MD cell with 49152 atoms. The resulting topological interface structure is analyzed and compared with that obtained by thermally activated solid phase epitaxy. The rates of crystallization and amorphization are calculated and compared with experimental data. Especially, their dependence on the nuclear deposited energy is discussed.     

Angular effects in the sputtering of Cr from stainless steel by 10 keV H3

Energy and angular distributions of Cr⁺ sputtered from stainless steel by 1.6 × 10⁻¹⁵ J (10 keV) H⁺₃ are reported as a function of angle of incidence. For more normal incidence, the peak in the energy distribution occurs in the vicinity of 3.2 × 10⁻¹⁹ J (2 eV), the average energy is approximately 1.12 × 10⁻¹⁸ J (7 eV), and the angular distribution is close to cosine. Toward glancing incidence, the peak energy increases to ˜6.4 × 10⁻¹⁹ J (4 eV), the average energy increases to ˜1.28 × 10⁻¹⁸ J (8.0 eV), and the angular distribution shows a distinct maximum in the forward direction. These results are discussed in terms of the increasing role of surface recoils in the sputtering mechanism at glancing incidence.     

The density effects in sputtering of amorphous materials

The effects of the target atomic density on sputtering of amorphous targets under 1 keV Ar ion bombardment have been investigated using binary-collision simulation. Attention was given to the sputtering yield, and the angular and energy distributions of sputtered atoms. A large set of targets, from ³Li to ⁹²U was considered and three interatomic potentials were applied. It has been shown that both the sputtering yield and the angular and energy distributions of sputtered atoms are undoubtedly dependent on the target atomic density. Results are compared with the data from the literature.     

Temperature dependence of the chemical sputtering of amorphous hydrogenated carbon films by hydrogen

The temperature dependence of chemical erosion and chemical sputtering of amorphous hydrogenated carbon films due to exposure to hydrogen atoms (H⁰) alone and combined exposure to argon ions and H⁰ was measured in the temperature range from 110 to 950 K. The chemical erosion yield for H⁰ alone is below the detection limit for temperatures below about 340 K. It increases strongly with increasing temperature, goes through a maximum around 650–700 K and decreases again for higher temperatures. Combined exposure to Ar⁺ and H⁰ results in substantial chemical sputtering yields in the temperature range below 340 K. In this range the yield does not depend on temperature, but it increases with energy from about 1 (eroded carbon atoms per impinging Ar⁺ ion) to about 4 if the ion energy is increased from 50 to 800 eV. For temperatures above 340 K the measured erosion rates show the same temperature dependence as for the H⁰-only case, but they are higher than for H⁰-only. The difference between the Ar⁺ and H⁰ and the H⁰-only cases increases monotonically with increasing ion energy.     

Charge transfer in grazing scattering on clean and oxygen covered Cu(1 1 0) surfaces: velocity effects

We have measured negative (F⁻) and positive (Na⁺, Li⁺) charge fractions produced by grazing scattering from the corrugated metallic Cu(1 1 0) surface. A wide range of incident energies and scattered angles were explored in order to analyse the effects of the parallel and perpendicular velocities of the projectiles on the charge transfer process. The obtained dependences of F⁻ formation and of Na⁺ and Li⁺ neutralization with the parallel velocity are in good agreement with previous experimental and theoretical studies, which incorporate parallel velocity effects via the “shifted Fermi sphere” model. The surface structure is then modified by adsorption of oxygen. The dependence of charge fractions with oxygen adsorption and the effects of velocity in the case of the Cu–O surface are analysed.     

Total sputtering yields of solids under MeV-energy Si ion bombardment

Total sputtering yields have been measured for SiO₂ and Cu targets bombarded with Si ions at an incident energy between 500 keV and 5.0 MeV using a quartz crystal microbalance technique. In order to measure total yields accurately, we have developed a beam modulation technique to avoid the effect of thermal drift. In the MeV energy range, an ion penetrates through thin SiO₂ and Cu targets and is implanted into a quartz crystal. Therefore, the thickness of these layers deposited on quartz crystals was carefully controlled to avoid damage of quartz crystal by incident ions. As a result, total sputtering yields of SiO₂ increased with incident Si ion energy, while those of the Cu target decreased. The total yields of the SiO₂ target were represented well by a power low of the electronic stopping power.     

Neutralization rate for slow Ar ions in front of KCl(0 0 1)

Charge state distributions of reflected ions are measured when 5 keV Ar^q+(q = 0−2) ions are incident on a clean KCl(0 0 1) surface at grazing angle, θ_i. Although the charge state distribution does not depend on the incident charge state at larger θ_i, significant dependence of the charge state distribution on incident charge state is observed at smaller θ_i. The ionization of Ar⁰ is completely suppressed at θ_i < 20 mrad, while large neutralization probability is observed for Ar⁺ incidence. These features allow us to derive the position-dependent neutralization rate of Ar⁺ in front of KCl(0 0 1). The obtained neutralization rate decreases exponentially with distance from the surface as it is usually assumed.     

Argon irradiation damage at a Cu/Al2O3 interface for different alumina structures

The evolution of damages at a Cu/Al₂O₃ device interface after Ar⁺ irradiation, depending on alumina structure, and the effect of surface roughness on sputtering have been studied. A polycrystalline Cu/Al₂O₃ bilayer and polycrystalline Cu on amorphous alumina were irradiated with 400 keV Ar⁺ ion beam at doses ranging from 5 × 10¹⁶ to 10¹⁷ Ar⁺/cm² at room temperature. The copper layer thicknesses were between 100 and 200 nm. RBS analysis was used to characterize the interface modification and to deduce the sputtering yield of copper. The SEM technique was used to control the surface topography. A RBS computer simulation program was used to reproduce experimental spectra and to follow the concentration profile evolutions of different elements before and after ion irradiation. A modified TRIM calculation program which takes into account the sputtering yield evolution as well as the concentration variation versus dose gives a satisfactory reproduction of the experimental argon distribution. The surface roughness effect on sputtering and the alumina structure influence at the interface on mixing mechanisms are discussed.     

Effects of Ag and Au ion implantation of lithium niobate

The optical effects of implantation of lithium niobate crystals with 100 keV Ag⁺ and 8 MeV Au³⁺ ions with fluences of 1 × 10¹⁷ ions/cm² have been investigated. Metal nanoparticle formation has been studied as a function of annealing temperature, and the resulting optical extinction curves have been simulated by the Mie theory in the small particle limit. Transmission electron microscopy (TEM) has provided direct evidence for the MNP sizes allowing comparison with the calculated results. A TEM study of an X-cut sample implanted with Ag⁺ ions show that the implanted region is partially amorphised. The differences in the temperature of Au colloid development in X- and Y-cut faces of the lithium niobate crystal are attributed to restoration of crystallinity as a result of annealing.     

Effect of electric charge accumulation on low energy ion implantation in insulators

Charge accumulation at the surface of insulators during low energy ion implantation is related to two processes: ion impinging on the sample and secondary electron emission. Samples composed of a piece of Si (having the size of the ion beam) fixed on the centre of polyethylene (PE) coupons have been implanted with 2.2 keV H₂ ions to a fluence of 2 × 10¹⁶ H/cm². ERD (Elastic Recoil Detection) depth profiles of the implanted ions are shallower with an increase of the PE coupon size. The relative critical Si/PE size to repel all the incident ions is around 1.1 × 1.1 cm²/2.5 × 2.5 cm². The potential of the secondary electron suppressor has been varied from −500 V to +500 V. It changes the secondary electron distribution around the implanted area and, consequently, affects the accumulation of charges at the sample surface. When the potential is 0 V, a uniform ion implantation with little effect of charge accumulation for all sizes of PE coupons is obtained. A two-dimension model has been performed and gives a good explanation for the mechanism of the electric charge neutralisation.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.