Effect of Auger neutralization on yields of He ions backscattered and alkali ions sputtered from the surface of alkali halides by keV He ion impact

The yields of ions and neutrals backscattered and alkali ions sputtered from LiF crystals by keV He⁺ ion impact have been measured by means of the coaxial impact collision ion scattering spectroscopy in time of flight analysis mode using the charging-up effect. It is found that as the charging-up potential increases due to continuous irradiation of the pulsed ion beam, the time of flight of the He⁺ ions backscattered shifts toward the shorter time, while that of the neutrals backscattered shifts toward the longer, and that of Li ions sputtered also shifts much more clearly toward the shorter. The charging-up potential has been estimated as a function of irradiation time of the pulsed ion beam from the time of flight data and the ion to neutral ratio in the backscattering yields is estimated to be about 0.15. The mechanisms for ionization on He and sputtering of alkali ions are discussed in terms of charging-up and trion (bihole and electron) produced by Auger neutralization of keV He⁺ ions at the target surface.     

He+离子辐照Inconel 718合金表面形貌改变及机理研究

研究了3种不同剂量He⁺离子辐照后Inconel 718合金的形貌变化规律及其形成机理。结果表明,He⁺离子辐照会在合金表面形成纳米多孔结构,其孔径会随辐照剂量的增加而增大。此外,He⁺离子辐照还会破坏合金表面δ相并导致碳化物的持续溅射损耗,且这一现象会随着辐照剂量的增加而愈发严重。由于辐照过程中氦泡间微观应力σ n作用会引起毗邻材料断裂及氦泡合并长大,且辐照溅射作用又会导致氦泡上层薄膜的损耗甚至破裂,因而这也是He⁺离子辐照Inconel 718合金表面纳米多孔结构的形成机制。     

High ion-beam induced electron yields from polycrystalline diamond

Extraordinarily high ion-beam induced electron yields are seen from polycrystalline diamond surfaces for high energy H⁺, He⁺ and Ar⁺ ions. This is attributed to the negative electron affinity of the diamond surface. However, the yield decays rapidly with dose, limiting the potential applications. The mechanism for decay is suggested to be electron-stimulated desorption of surface hydrogen, which removes the negative electron affinity responsible for high yields.     

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.     

Low-energy ion scattering and sputtering at grazing ion bombardment of clean and oxygen covered Ag(1 1 0) surface

The ion scattering and sputtering processes at low energy grazing N⁺ and Ne⁺ ion bombardment of clean and oxygen covered Ag(1 1 0) surface have been investigated by computer simulation in the binary collision approximation.

The spatial, angular and energy distributions of scattered, sputtered particles and desorbed molecules of oxygen as well as their yields versus the angle of incidence have been calculated. In these distributions the some characteristic peaks were observed and analysed. It was found that an adsorption layer plays a role of the additional surface barrier, i.e. it reflects leaving target atoms back to crystal. The azimuth angular dependencies of Ag sputtering yield and non-dissociative O₂ desorption yield at grazing incidence have been calculated. It was shown that these dependencies correlate the crystal orientation.     

RBS analyses of xenon-irradiated Ti and TiN films

TiN films of 30–300 nm thickness, deposited onto stainless steel via magnetron sputtering, and 10 μm thick Ti foils were irradiated with 80–360 keV Xe⁺ ions at influences of φ = 10¹⁵–10¹⁷ ions/cm². The Xe content was depth-profiled by means of 900 keV He⁺⁺ Rutherford backscattering. Irradiations of films with a thickness exceeding the ion range (at 80 and 250 keV) led to saturation effects due to sputtering and outdiffusion from the near-surface region. The sputtering yields deduced at low Xe fluences were compared to calculations for mono-elemental and compound sputtering. Surface blistering was observed after 250 keV saturation implantation into Ti. For TiN layers with a thickness comparable to the ion range, precipitation of the mixing gas at the interface was observed which finally led to the destruction of the layers. The dependence of the Xe fraction accumulated in the interface is discussed in terms of thermal-spike calculations.     

Electron microscopy and Rutherford backscattering spectrometry characterisation of 6H SiC samples implanted with He

6H SiC single crystals were implanted at room temperature with 1 MeV He⁺ up to a fluence of 2 × 10¹⁷ at./cm². RBS-channeling analysis with a 2 MeV He⁺ beam indicated the formation of extended defects or the generation of point defects at a constant concentration over a depth of about 1 μm. Electron microscopy characterisation revealed the presence of two amorphous buried layers at depths of about 1.75 and 4.8 μm. They are due to the implantation and to the analysing RBS beam, respectively. No extended planar or linear faults were found in the region between the surface and the first amorphous layer. However, at the surface, a 50 nm thick amorphous layer was observed in which crystalline inclusions were embedded. Electron diffraction and HREM data of the inclusions were typical for diamond. These inclusions were even found in the crystalline SiC material below this layer, however at a reduced density.     

Measurements of the erosion of stainless steel, carbon, and SiC by hydrogen bombardment in the energy range of 0.5 – 7.5 keV

Total erosion yields by sputtering and blistering for 1 to 15 keV H₂⁺ bombardment at normal incidence have been measured by weight loss of 304 stainless steel, pyrolytic graphite, carbon fibres, glassy carbon and SiC. The erosion yields are in the range of 3 × 10⁻³ to 2.6 × 10⁻² atoms per incident hydrogen atom. Observation in the scanning electron microscope shows that blisters occur in stainless steel and SiC at doses of 5 × 10¹⁸ particles/cm², but disappear at doses of 5 × 10 particles/cm² . The surface roughening observed depends largely on grain orientation. On carbon no blistering could be found. After bombardment the carbon surfaces are generally more smooth than before.     

Static and dynamic deformations stimulated by ion implantation

The laser interferometric technique has been applied for the investigation of the quasi-static and dynamic components of the elastic deformation of crystalline silicon wafers during implantation of Ar⁺ and He⁺ ions. The absolute values of the integral stress and the amplitude of deformation vibrations are measured as a functions of implanted ions dose, energy and current density and compared with the dose dependencies of the surface acoustic waves (SAW) velocity in the implanted layer. The correlation between integral stress, amplitude of vibrations and SAW velocity variations is shown.     

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.     

Autoionising state excitation in collisions of inert gas projectiles with an Al surface and effects of initial stages of oxidation

Results of an angle resolved electron spectroscopy study of low keV He⁺, Ne and Ne⁺ collisions with an Al surface are presented. Production of projectile autoionising states is demonstrated to exist not only for the previously known Ne⁺ case but also for Ne and He⁺. The characteristics of autoionising peak line shapes suggest differences in angular distributions of the different Ne^** states. Initial stages of the oxidation of Al are studied. Saturation coverages of oxygen are reached for exposures greater than 100 L. An initial decrease in the work function is observed amounting to 160 meV for a 30 L exposure. Exposure to oxygen results in a strong decrease of autoionising state production. State-selective attenuation is observed and reflects changes in electron transfer rates.     

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.     

Investigation of the radiation hardness on semiconductor devices using the ion micro-beam

Variation of the ion beam induced charge (IBIC) pulse heights due to ion irradiation was investigated on a Si pn diode and a 6H-SiC Schottky diode using a 2 Mev He⁺ micro-beam. Each diode was irradiated with a focused 2 MeV He⁺ micro-beam to a fluence in the range of 1×10⁹–1×10¹³ ions/cm². Charge pulse heights were analyzed as a function of the irradiation fluence. After a 2 MeV ion irradiation to the Si pn junction diode, the IBIC pulse height decreased by 15% at 9.2×10¹² ions/cm². For the SiC Schottky diode, with a fluence of 6.5×10¹² ions/cm², the IBIC pulse height decreased by 49%. Our results show that the IBIC method is applicable to evaluate irradiation damage of Si and SiC devices and has revealed differences in the radiation hardness of devices dependent on both structural and material.     

Investigation of films of YBa2Cu3O7 using Rutherford backscattering spectrometry

Rutherford backscattering spectrometry (RBS) has been applied to films of the high-temperature superconductor YBa₂Cu₃O₇. The films were prepared by rf and dc sputtering and silk-screen printing onto substrates of alumina, sapphire and magnesia. Some of the samples were exposed to various heat treatments in order to promote the superconducting phase. A 2 MeV beam of He⁺ particles from the 3 MV Dynamitron accelerator at Birmingham University's School of Physics and Space Research was employed for the RBS measurements. The composition and the depth profile of the elements in the films were determined by simulating the observed RBS spectra with a simulation software package. The simulations show that the heat treatment causes marked interdiffusion of the film and substrate. The experimental results and the computer simulations shown in this paper illustrate the advantages of using RBS to characterise films of the new superconducting materials.     

Kinetic electron emission yield induced by H and He ions versus stopping power for Al, Cu, Ag and Au

For H⁺ and He²⁺ ions impinging on Al, Cu, Ag and Au targets we measured simultaneously the yield, γ, of emitted electrons and the electronic energy loss, S_e, in the energy range 0.5 to 4.8 MeV. The targets were prepared under high-vacuum conditions before they were transferred to an ultra-high-vacuum chamber without breaking the vacuum. The targets were sputter cleaned and their composition was examined by Auger electron spectrometry. The values of γ were obtained by current integration and S_e was determined from the energy width of Rutherford backscattering spectra. For H⁺ ions impinging on Cu, Ag or Au and He²⁺ on Al and Cu, the expected proportionality between γ and S_e was found within the experimental errors of 2%. For H⁺ ions on Al and He²⁺ ions on Ag and Au targets, significant deviations were observed.     

Ion beam induced luminescence of materials

Ion beam interactions with many insulating materials produces strong luminescence and this may be collected and analysed to gain information on the defect structures present. The use of light ions such as H⁺ and He⁺ with MeV energies allows penetration to a depth of several microns in most materials. The variation of ion species and energy allows precise control of ion-induced damage and ionisation within the material. This control of depth, ionisation, and damage can be superior to that of the traditional cathodo-luminescence technique and comparison with spectra from the two methods often yields differences in sensitivity to different emission bands and features.     

Observation of micro-cavities in nickel during He and D irradiation and post-irradiation annealing

Changes in sizes and morphology of small cavities in nickel irradiated by 25 keV helium ions and 20 keV deuterons were investigated during irradiation and on annealing after irradiation by means of transmission electron microscopy. In the early stage of He⁺ irradiations at 600 and 700° C, roundish cubes appeared, gradually changed to octahedra. and, then, by the truncation of apexes, finally reached cubo-octahedra. Nucleation and growth behavior of cubic cavities in D⁺ irradiated nickel was different from the case of He⁺ irradiation. On annealing of the He⁺ irradiated specimen, only octahedral cavities showed marked growth, finally changing to roundish cubes at 750° C. Cavities of roundish cubes and cubo-octahedra did not grow nor change their shapes remarkably by the annealing. The cubic cavities formed by D⁺ irradiation at 360° C showed gradual shrinkage on annealing at 600° C and disappeared at 625° C. The changes of cavities during irradiation and on annealing were interpreted by the effect of the internal gas pressure.     

Spin effects in the screening and Auger neutralization of He ions in a spin-polarized electron gas

The screening of a He⁺ ion embedded in a free electron gas is studied for different spin-polarizations of the medium. Density functional theory and the local spin density approximation are used to calculate the induced electronic density for each spin orientation, i.e. parallel or antiparallel to the spin of the electron bound to the ion. Since both the He⁺ ion and the electron gas are spin-polarized, we analyze in detail the spin state of the screening cloud for the two different possibilities: the spin of the bound electron can be parallel to either the mayority spin or the minority spin in the medium. Finally, the spin-dependent Kohn–Sham orbitals are used to calculate the Auger neutralization rate of the He⁺ ion. The polarization of the Auger excited electron is influenced by the spin-polarization of the medium. The results are discussed in terms of the spin-dependent screening and the indistinguishability of electrons with the same spin state.     

Irradiation-induced nanostructures in cadmium niobate pyrochlores

This paper reports the formation processes of crystalline Cd nanostructures on ion-cut surfaces of cadmium niobate pyrochlores (Cd₂Nb₂O₇). Irradiation with 3 MeV He⁺ ions has been performed at low temperatures (295 K) to induce material decomposition and aggregation of host atoms. The irradiation also leads to surface exfoliation due to rupture of gas (He and O₂) filled blisters. Nanoparticles and nanowires are observed on the ion-cut surfaces at low and higher doses, respectively. These structures are examined and characterized using a suite of experimental tools. Both the particles and wires are found to be single crystals that primarily consist of metallic Cd.