Broadening effects in Auger neutralization of 130–430 eV Ar ions at Al surfaces

We present a model for electron emission from Al surfaces by Auger neutralization of 130–430 eV Ar⁺ ions, that includes the singular response of the metal conduction band to the abrupt change of the surface potential caused by electron capture by the incident ion. This effect, previously identified in X-ray studies, produces a broadening that plays a significant role in reproducing the higher energy part of the experimental electron kinetic energy distributions.     

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.     

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.     

Plasmon excitation and electron promotion in the interaction of slow Na ions with Al surfaces

We studied the energy distributions of electrons emitted in the interaction of slow Na⁺ ions with polycrystalline Al surfaces. To study sub-threshold plasmon excitation we performed measurements as a function of emission angle which showed the excitation of bulk plasmons, confirming the kinetic nature of the excitation process.

Electron spectra show narrow transition lines due to Auger decay in vacuum of sodium projectiles excited in the 2p-shell by electron promotion in collisions with Al target atoms. Several previously unidentified transition lines could be attributed to the autoionization of doubly excited projectiles.     

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.     

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.     

Studies of the interaction of slow very highly charged ions with solid surfaces using the LLNL EBIT facility

A summary of the present status of the LLNL EBIT based research program on highly charged ion-solid surface interaction is presented. New results for coincidence measurements of K−K^β X-ray emission for Fe²⁶⁺ incident on Al, using two Ge detectors are presented. It is shown that excellent resolution of the satellite and hypersatellite lines are achieved, in excess of what is possible with single Ge detector measurements. Recent results from ongoing investigations on electron emission, sputtered ion and energy loss phenomena are presented. Results from charge-state analysis of grazing incidence specular scattering of highly charged ions off atomically flat surfaces of gold, carbon and mica indicate that the incident ion essentially reaches ground state of its electronic distribution within the ion-surface interaction time of 100 fs. Atomic force microscopy is used to investigate the formation of a new type of ion induced surface defect, peculiar to highly charged ion-surface interactions. A linear correlation, between the defect volume and the incident ion charge, supports a mechanism of defect formation due to potential energy induced electron emission and subsequent collective displacement due to local charge imbalance.     

Z variations in yields of multicharged ions scattered and recoiled from a silicon surface

The doubly charged and triply charged ion yields from keV ion-silicon surface scattering are found to have a strong dependence on the atomic number Z₁ of the incident ion. For Z₁ < Z₂ the yield of scattered multicharged ions increases with Z₁, so that these ions dominate the recoiling Si²⁺ and Si³⁺ ions by Z₁ = 9. In contrast, when Z₁ > Z₂, there are large yields of Si²⁺ and Si³⁺ ions, and the multicharged scattered ion yields are too small to detect. The interaction radius at which shell vacancies are produced is also found to change, suggesting that electron promotion occurs at a different level crossing on either side of Z₁ = Z₂.     

Radiation damage induced by 5 keV Si ion implantation in strained-Si/Si0.8Ge0.2

The damage distributions induced by ultra low energy ion implantation (5 keV Si⁺) in both strained-Si/Si_0.8Ge_0.2 and normal Si are measured using high-resolution RBS/channeling with a depth resolution better than 1 nm. Ion implantation was performed at room temperature over the fluence range from 2 × 10¹³ to 1 × 10¹⁵ ions/cm². Our HRBS results show that the radiation damage induced in the strained Si is slightly larger than that in the normal Si at fluences from 1 × 10¹⁴ to 4 × 10¹⁴ ions/cm² while the amorphous width is almost the same in both strained and normal Si.     

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.     

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.     

Effect of dielectronic recombination on charge-state distribution in laser-produced plasma based on steady-state collisional-radiative models

Armed with four different steady-state collisional-radiative (CR) models, we investigated the effect of dielectronic recombination (DR) on the charge-state distribution in laser-produced silicon plasma. To assess this effect, we performed a series of temporally resolved spectra of highly charged Si ions in the extreme ultraviolet region. Ab initio calculations of the DR rate coefficients were done for Si⁶⁺–Si⁴⁺ ions. We also analyzed the evolution of the collisional ionization, radiative recombination, three-body recombination, photo-ionization, and DR rate coefficients as a function of electron temperature. The electron temperature and electron density for different delay times were obtained by comparing the normalized experimental and simulated spectra. The ion fraction and average charge state from the four different CR models were also obtained. The results indicate that the DR process has a greater influence in the stage of plasma evolution that cannot be neglected in plasma diagnoses.     

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.     

Structural evolution in Fe ion implanted Si upon thermal annealing

We have performed high-dose Fe ion implantation into Si and characterized ion-beam-induced microstructures as well as annealing-induced ones using transmission electron microscopy (TEM) and grazing-incidence X-ray diffraction (GIXRD). Single crystals of Si(1 0 0) substrate were irradiated at 623 K with 120 keV Fe⁺ ions to a fluence of 4 × 10¹⁷ cm⁻². The irradiated samples were then annealed in a vacuum furnace at temperatures ranging from 773 K to 1073 K. Cross-sectional TEM observations and GIXRD measurements revealed that a layered structure is formed in the as-implanted specimen with ε-FeSi, β-FeSi₂ and damaged Si, as component layers. A continuous β-FeSi₂ layer was formed on the topmost layer of the Si substrate after thermal annealing.     

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.     

Surface modifications and erosion yields of silicon and titanium doped graphites due to low energy D bombardment

Influences of low energy D⁺ ion bombardment and target temperature on surface topography, surface concentration and erosion yield of carbon based binary compounds were investigated. The samples contained 10 at.% Si and 10 at.% Ti, respectively. The surface concentration was determined in situ by Auger electron spectroscopy and the topography ex situ by scanning electron microscopy. During low energy D⁺ bombardment a pronounced conelike surface developed with silicon respective titanium rich ‘caps’ protecting the underlaying carbon rich shafts from erosion. The average dopant surface concentration was up to 7 × the bulk concentration. The erosion mechanism was determined by surface concentration and chemical state of the surface: At high temperatures carbidic bindings dominated, while at room temperature a mixture of graphite and carbide covered the surface.     

Mechanisms of subthreshold atomic emission from solid surfaces

Computer simulation of the dynamics of ions and atoms on the surfaces of solids has been carried out. The Coulomb, Pauli, exchange and Van der Waals potentials have been taken into account. The semi-empirical quantum-chemical method has been used also. In the case of alkali halide surfaces it is shown that if recharge of an anion (X⁻ →X⁺) occurs in two surface layers, it may initiate the ejection of positive metal ions (M⁺) and, assisted by the capture of an electron by a departing M⁺ of metal atoms M⁰. Besides the Coulomb repulsion the Pauli shock is shown to play an essential role in the driving of the ejection process. This mechanism of desorption has large efficiency when the excitation of a core electron occurs in case of alkali halide crystals and has a strong dependence on the crystal ionicity. We obtained the energy distribution of ejected particles for different mechanisms of electron-ion emission.     

Atomic electron energy spectra of slow He ions impinging on metallic surfaces

Changes in the KLL Auger electron spectra of slow He²⁺ ions impinging on surfaces have been proposed as diagnostics of local spin polarization at ferromagnetic surfaces. For a variety of targets Gd(0 0 0 1), Ni(1 1 0), Fe(1 1 0) and Al(1 1 0) and different ion energies (20-1000 eV) and angles of incidence (5-75°) the KLL spectra have been studied. The line ratios depend on the scattering conditions and on the target used, which may give access to the exact behavior of the electronic states in the vicinity of the surface. Additional information on the ion neutralization processes may be available from the angular distributions of the emitted electrons.     

Effects of ion implantation on poly(dimethylsilylene-co-methylphenylsilylene)

The effects of ion implantation on the electrical and structural properties of poly(dimethylsilylene-co-methylphenylsilylene), (DMMPS) thin films have been investigated. Ionic species of krypton, arsenic, fluorine, chlorine, and sulfur were implanted at energies ranging from 35 to 200 keV and with doses of up to 1 × 10¹⁶ ion cm². The conductivity of the polymer increased upon implantation reaching a maximum value of 9.6 × 10⁻⁶ (Ω cm)⁻¹ for the case of arsenic ion at a dose of 1 × 10¹⁶ ion cm² and energy of 100 keV. The results showed that ion implantation induced conduction in DMMPS was primarily due to structural modifications of the material brought about by the, energetic ions. Infrared analysis and Auger electron spectroscopy showed evidence for the formation of a silicon carbide-like structure upon implantation.     

Influence of SHI irradiation on the formation of buried SiC

Silicon carbide (SiC) precipitates buried in Si(1 0 0) substrates were synthesized by ion implantation of 50 keV and 150 keV C⁺ ions at different fluences. Two sets of samples were subsequently annealed at 850 °C and 1000 °C for 30 min. Fourier transform infrared (FTIR) spectroscopy studies and X-ray diffraction (XRD) analysis confirmed formation of β-SiC precipitates in the samples. Ion irradiation with 100 MeV Ag⁷⁺ ions at room temperature does not induce significant change in the precipitates. It could be interpreted from the FTIR observations that ion irradiation may induce nucleation in Si + C solution created by ion implantation of C in Si. Modifications induced by swift heavy ion irradiation are found to be dependent on implantation energy of C⁺ ions.