Atomic displacements on stepped (16,1,1) copper structures: A channeling study

By taking advantage of the channeling phenomenon in solids, stepped structures are particularly well suited for studying surface atomic displacements. In particular 90° double-alignment experiments, in which the blocking of the backscattered particles by axes of the terrace plane is studied, provide exclusive information on the first atomic plane, without any bulk contribution. Surface thermal vibrations and disorder can be studied in this way. Moreover, in these experiments, the specific relaxation of the step-edge atoms can be measured. Channeling experiments in a more standard geometry can also be performed as on flat surfaces in order to extract information on the relaxation of the whole terrace. The (16,1,1) stepped, kinkless copper structure has been studied. The main results obtained for a clean surface are the following: (1) The coherent relaxation of the surface plane is very small (at the limit of our experimental precision); however, the best fit of our results corresponds to a contraction of $3\times {10}^{-2\mathrm{}}$ Å. (2) A coherent relaxation, perpendicular to the terraces, specific to the ledge atoms, has not been detected and is hence certainly smaller than $5\times {10}^{-2\mathrm{}}$ Å. (3) There is evidence for small, quasi-isotropic, static displacements of the surface atoms, all over the surface plane of concern. These displacements, with a mean value around 0.1 Å at 300 K, are somewhat correlated and are seen in channeling in the same way as thermal vibrations, but they have a stronger temperature dependence. This type of defect is perhaps related to the stepped structure. However, since it can only be detected unambiguously in 90° double-alignment backscattering experiments, which require a stepped surface, it may well be present but unobserved in simpler structures. We have also studied the effect of oxygen coverage. The saturation coverage ${\ominus }_s$ has been determined quantitatively by nuclear microanalysis; it corresponds to ${\ominus }_s=0.5\mathrm{}$ of a monolayer. In this case low-energy electron-diffraction patterns show the existence of surface reconstruction and faceting. The channeling results are consistent with a change from the $3\times {10}^{-2\mathrm{}}$ Å contraction observed on the clean surface to a $3\times {10}^{-2\mathrm{}}$ Å dilatation. However, they mainly point to an increase of surface disorder. The very high sensitivity of the channeling technique in the 90°-backscattering double-alignment geometry for studying surface defects and surface thermal vibrations is also discussed.