Low-Energy Ion Scattering by Various Crystallographic Planes of Tungsten Single Crystals

The dependence of the low-energy ion scattering signal intensity on the crystallographic orientation of the single-crystal surface was studied to show that it by no means always is proportional to the atomic density in the uppermost atomic layer. Based on a comparison of signals from various planes of a single crystal of high-purity tungsten, it is shown that for a surface with a more open structure ions scattered from the atoms of deeper atomic layers give a significant contribution to the measured signal. Thus, for the (111) plane, the contribution from the deeper layers even exceeds the contribution from the uppermost layer. It is shown that, to quantitatively analyze the surface composition by the low-energy ion scattering (LEIS) method, standard samples with a well-known surface density are required. The best standard sample are crystallographically well-oriented single crystals with close-packed surface planes, since in this case the signal upon low-energy ion scattering is proportional to the atomic density of the upper atomic layer. If single-crystal samples with an open structure of the surface are used as standard samples, then contributions from deeper atomic layers should be taken into account to ensure precise quantitative analysis. Cleaning of the surface of standard samples using ion sputtering can lead to amorphization and the appearance of roughness at the surface. Ion etching of close-packed planes of refractory metals can lead to a decrease in the signal by about 30%. If a surface prepared by this method is used as a standard sample, then the error in the quantitative analysis may be of the same order of magnitude. The high mobility of the surface atoms for the materials with a relatively low melting temperature leads to smaller roughness of the surface and smaller amorphization, which yields a decrease in the signal of scattered ions by only 5-10%.