Computer simulation of the penetration and backscattering of low energy krypton ions into single crystal tungsten

Abstract

The trajectories of Kr ions in the energy range 50 eV to 5 keV incident on the (100) and (1 10) surfaces of W have been followed using digital computational techniques, in which the interaction was treated as a multiparticle process rather than a sequence of binary collisions. At the higher energies (>2 keV), the latter process was found to be a good description also of the multiparticle event but at lower energies, many atoms were involved simultaneously in the ion scattering process. Probabilities of ion penetration into the surfaces were derived and compared with experimental data of Kornelsen and Sinha and penetration probability-energy functions shown to be similar. For the (100) surface penetration was prohibited below an ion energy of-400 eV and angular and energy distributions of backscattered particles in this energy range were determined. Angular analysis showed a pattern illustrative of surface atomic symmetry but the energy distribution shows a sharply defined probability of backscattering in narrow energy range which lies between the maximum recoil energy in a binary collision and recoil from an infinitely hard surface. This indicates that, as energy decreases, the multiple collision process can be approximately characterised as a 'soft' surface of effective mass greater than the target atom mass. Because of the sharply defined energy of recoil and a quite well defined narrow cone of backscattering, these results suggest a method of producing quite mono-energetic neutral atomic beams by recoil of ions fromsurfaces, since neutralization is usually highly efficient at low ion energies. The energy of ions recoiling in a collision on an axis of symmetry between surface atoms is compared with the theoretical non-binary scattering model of Andersen and Sigmund.

Low yield sputtering of monocrystalline metals

“…la). The measured distributions are very similar to the angular distributions of scattered Ne on Au(100) and Ar on Cu (100) [5], derived by us and by Smith et al [6] from trajectory calculations.…”

“…The factor Ao accounts for the enhancement of the sputtered intensity in preferential directions and the parameter q for refraction effects; e.g., for perpendicular ejection from a (I00) surface we take Ao = 1 (no preferential sputtering) and q = 1 giving the usual expression (1 -Eb/Eo) following from the uniform potential barrier model for surface ejection. In oblique directions values of Ao and q are found either by surface ejection calculations [5,8] (2) of sputtered atoms, which follows from the above assumptions with Eo =E s +E b (E s = the energy of the sputtered atoms), to experimental energy spectra e.g., measured by Stuart and Wehner [6]. We found generally a close correspondence with the measured curves for Cu (100) surfaces if q = 1 substituted for the perpendicular and q = 2 for the oblique direction, while EM(E ) steadily increases with the projectile energy E.…”

“…We were not able to measure accurately the averaged energy at energies near the threshold energy for sputtering owing to the energy contribution of backscattered projectiles. From the average energy given by Stuart and Wehner [6] and measurements by Weijsenfeld at higher energy [4] we derived that E M first increases linearly with E, but at higher energy, if preferential ejection directions are concerned, E M increases markedly slower.…”