C. Benazeth

An apparatus for ion-surface collisional process studies and surface analysis

Abstract An experimental setup allowing both surface and interface analysis and ion-surface collisional process studies is described. It consists of an ion beam line which provides 1–80 keV mass analysed rare gas and alkali-metal ions, and an UHV chamber. This one is equipped with two time of flight systems which allow energy and mass analysis of scattered and recoiled particles at scattering angles 180° and in the range 0–165°. In addition a fixed electrostatic hemispherical analyser can record either the positive ions or the emitted electrons, at an angle of 90° from the ion beam. The versatility of the apparatus is illustrated with data on clean Si (111) surface and on impurity- or Cs-covered Si surface: chemical and crystallographic structure analysis, depth profile, ion fractions and ion-induced electron spectroscopy.

INTERFACE FORMATION IN SILICON BY CESIUM BOMBARDMENT

Abstract Experiment and computer simulation with the TRIDYN code are performed in order to study the contribution of collision processes to the interface formation induced by 4–80 keV Cs ions incident on an amorphous silicon surface. The simulations show evidence for the fluence dependence of Cs range profiles and of the surface elevation (swelling) of the irradiated target. Range profiles become constant above some fluence whose value depends on the incidence conditions. In the experiment, saturation is traced by measuring the fluence dependence of the electron emission coefficient and is found to be of the same order as that found for simulated yields and ranges. Depth profiles are determined by TOF-spectroscopy in combination with layer-by-layer sputtering. The results at 40 keV with a fluence above the saturation value are in qualitative agreement with simulation. In contrast, strong deviations are found at 4 keV which are attributed to surface Cs segregation.

Inelastic energy loss of light particles scattered by solid surfaces at low energy: influence of the `gap'

Abstract The energy spectra of particles scattered by solid surfaces are used to determine the inelastic energy loss at low energy. Assuming the binary collision approximation, a modified TRIM code provides length distributions which are converted to time-of-flight (TOF) spectra by using the friction coefficient as an adjustable parameter. Owing to the nonlinear effects occurring in this energy range, the theoretical value of the electronic stopping power is performed from electron-particle scattering cross-section using a screened potential and so, the phase shifts, obtained self-consistently in the framework of density functional theory (DFT). In the case of He/a:Si interaction at 4 keV, the standard model leads to a largely overestimated value. This fact has been attributed to the presence of the electron energy gap E G and to the structure of the valence band. We verify this assumption in a non-static model involving all electrons of the valence band with a threshold condition v e ′2 > v F 2 +2 E G , where v F is the Fermi velocity and v e the electron velocity after scattering (non-static extended collisional model). The theoretical results agree very well with the experimental ones for He colliding three targets: a:Si, a:Ge and polycrystalline Ni at 4 keV. The calculations performed for the velocity range below 1 a.u. confirm the important role of the gap and the band structure in the lowering of stopping power at low velocity.

Image potential effect on the specular reflection coefficient of alkali ions scattered from a nickel surface at low energy

Abstract The resonant charge exchange in the incoming path of alkali ions scattered at low energy from a polycrystalline nickel surface is studied by using the image effect occurring at glancing incidence (2–10° from the surface plane) and for specular reflection. The part of the experimental artefacts (geometrical factor, surface roughness…) is extracted from the reflection coefficient of almost completely neutralised projectiles (He+ or Ne+) compared with the coefficient obtained from numerical simulations (TRIM and MARLOWE codes). The present model explains very well the lowering of the reflection coefficient measured at grazing incidence (below 4°). Furthermore, the optimised values of the charge fraction in the incoming path and the image potential are in agreement with the theoretical calculations in the case of Na+/Ni at 4 keV.

Transport theory and Monte Carlo simulation of the scattering of low energy Li+ ions from a polycrystalline nickel surface

Abstract In this paper, we present experimental results about the scattering of 4 keV Li+ ions by a polycrystalline nickel surface. Incidence angle was λ=4° and different values of the scattering angle α are considered. Two simulation methods are used in order to calculate the angular distributions of the total path length in solids, the reflection coefficient and the energetic scattering spectra of reflected particles. The first method is based on a Monte Carlo code (TRIM). The second method is based on the solution of the Boltzmann equation in the transport theory frame and is valuable for low incidence and scattering angles. In both cases, the binary-collision approximation is assumed and multiple scattering of incident particles is included. Comparison between simulated curves is done without any normalization and shows good agreement. It is important to note here, that comparison of simulated energetic spectra to experimental ones allows the determination of inelastic stopping power (dE/dx)ine which is difficult to evaluate otherwise in this energy range.

Charge exchange in scattering of atomic and molecular deuterium ions on a NaCl surface

Abstract Fast deuterium atomic and molecular ions (a few keV) are scattered from a NaCl (1xa00xa00) surface at an incidence angle of 15°. Time-of-flight (TOF) of both neutrals and ions are measured as a function of the scattering angle. Data show that neutralisation occurs with very high efficiency. Positive and negative charge fractions are measured for the different types of projectile. Different charge exchange processes are discussed to account for the results.

Experimental change fractions and angular distributions of particles scattered in Ne+/Si interactions

Abstract Low energy ion scattering spectroscopy by a time of flight technique has been used to study both the angular distributions and the charge fractions of particles scattered in collisions of 4 keV Ne+ ions with amorphous silicon. The angular distributions versus the scattering angle at near grazing incidence display for both single (QSS) and total scattering events a shift towards high scattering angles of the ion distribution maximum relative to the neutral curves. The experimental distributions were compared to computer simulation (MARLOWE) profiles for two incidence angles. Ne+ charge fractions versus the scattering angle θ were determined; for near grazing incidence, charge fractions as high as 70% are measured for the QSS event. At a constant scattering angle (θ = 60°), a decrease of the Ne+ charge fraction as a function of the incidence angle is observed; this result illustrates the neutralization probability, inside the target, taking place after the violent collision.