NIOMI zyxwvutsrq B Nuclear Instruments and Methods in Physics Research B 90 (1994) 247-251 North-Holland Beam Interactions with Materials & Atoms 2 keV He and Ne ion scattering on a Cu( 110) surface: experiment vs simulation L. Houssiau * and P. Bertrand Universite’ Catholique de Louvain, PCPM, 1 place Croix du Sud, B-1348 Louvain-la-Neuve, Belgium Scattering experiments performed at 143” with a TOF-ISS system on a Cu(ll0) single crystal indicate the very different sampling depth of He ions as compared with Ne ions. The ISS polar scans measured with He ions show a high sensitivity to the bulk atomic rows of the single crystal, whereas the scans measured with Ne ions indicate a sensitivity to only a couple of atoms in the first atomic layers. In order to evaluate the depth sampled by the backscattered particles, ion trajectories have been calculated with the MARLOWE simulation code. With Ne, the simulation shows that backscattered particles arise only from the 4 first layers (5 &, but with He ions, they come from at least 80 layers (100 A),. A detailed analysis of escaping trajectories indicates that the Ne projectiles are scattered from the first layers after short trajectories. On the other hand, helium trajectories are usually quite complex, due to the possibility to follow privileged directions (channels), so that the projectiles can scatter from deep layers. We see that, even at low energy, channeling may be the major contributor to the variations of the backscattered intensity. 1. Introduction 2. Experiment Ion scattering is currently used to determine the surface structure of monocrystalline samples [1,2]. This work aims to look at the influence of the primary ion species on the structural information available [3]. The unreconstructed surface of Cu(ll0) [4-61 has been chosen to test the sampling depth achieved during structure analysis by low energy ion scattering. For this purpose, the intensity of backscattered particles versus the beam incidence is measured (polar scan) and com- pared for two different primary ions: He and Ne. These scans are directly related to the atomic positions on the surface, due to shadowing, blocking and focus- ing effects [1,4]. However, the sampling depth of the ions is found to be significantly larger for He ions than for Ne ions, so that He scattering can no more be considered as surface sensitive [7]. In order to evaluate this sampling depth, we performed two kinds of MAR- LOWE simulations: in the first one, we evaluate the backscattered particle intensity versus the depth at which the deepest collision occurs, and in the second one, we follow the primary projectile trajectories into the crystal. * Corresponding author, tel. +32 10 473582, fax + 32 10 473452, e-mail houssiau@pcpm.ucl.ac.be. The sample consists of a high purity Cu single crystal, polished to 1 km alumina and then electro- chemically polished. Before analysis, the Cu cIysta1 was cleaned by repeated cycles of 10 nA 2 keV Ar sputter- ing, followed by a 700 K annealing to restore the structure. For the TOF-ISS analyses, the sample is bombarded by a pulsed, low energy ion beam at a frequency of 50 kHz. The ion pulse width is about 100 ns, the average pulsed current is 30 pA and the beam spot is 1 mm in diameter. The particles backscattered at scattering angle 0 = 143” are collected into a mi- crochannel plate located at 335 mm flight distance from the sample. Another detector (channeltron) at a low scattering angle 0 = 22.5” allows direct recoil spec- trometry and then detects low mass elements: this is useful to check the presence of impurities on the surface. The typical acquisition time for a spectrum is 60 s. This corresponds to a total ion dose of 10” ions/cm’, which is certainly non-destructive. The base pressure in the analysis chamber is 2 X 10d9 Torr and rises to 3 X lo-* Torr of noble gas with the beam on. 3. Simulation The MARLOWE code, version 13, has been used to simulate the experiments. This code calculates asymp- 0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0666-5 III. SURFACE PHENOMENA