Study of atomic relaxations on clean and oxygen covered (100), (410) and (510) copper surfaces by channeling

Abstract

Atomic relaxations on Cu(100) and stepped Cu(410) and (510) surfaces were measured using a few hundred keV He + ions in channeling and double alignment geometries. On stepped surfaces, we have also attempted to determine the specific relaxation of edge atoms (both the components parallel and perpendicular to the (100) terraces). For this purpose, we have analyzed blocking effects on particles backscattered near θ lab = 90°, around directions parallel to various major crystallographic axes of the terraces, the incident beam being aligned along the [100] axis perpendicular to the terraces. The studies were performed on clean and on oxygen covered surfaces. On (100) flat surfaces, we find a small contraction of the surface plane on clean Cu (about 2% of the bulk interplanar spacing), and a marked expansion (about 10%) on oxygen covered Cu (at saturation coverage, whose value was measured by nuclear reaction analysis). On stepped surfaces, it appears difficult to discriminate between the specific relaxation of the [001] edge row perpendicular to the terraces and the relaxation of the other [001] rows inside the terraces. Concerning the component of the [001] edge row relaxation in the terrace plane, our results provide an unambiguous estimate: we find a small contraction (about 0.04 Å) on bare Cu and a larger expansion (about 0.18 Å) under oxygen coverage. In all cases, we observed that the presence of oxygen induces a definite surface disordering which can be described as random small atomic displacements of Cu surface atoms around their mean position (on the order of 0.1 Å). The results obtained on the (100) surface covered with oxygen can be interpreted in terms of a missing row model which has been proposed by some authors. However, our data can also be fitted with no missing row. Similarly, on the oxygen covered (410) stepped surface the data interpretation does not require the assumption of a missing row, but the results are not inconsistent with a description of the terrace where the fourth or possibly the third [001] row, parallel to step edge, is missing and where the edge [001] row suffers a considerable outward displacement in the terrace plane ( ≈ 0.3 A ̊ ). The limits of high energy ion channeling for the measurement of surface atomic displacements, in particular when many sites are involved (for instance in the case of alternate relaxation) are discussed and illustrated.