Investigation of surface defects on Ni(110) with a low-energy positron beam

The trapping of positrons at surface defects that develop during the epitaxial growth of a Ni film on a Ni(110) substrate is established. Trapping is detected by the resultant decrease in positronium that is formed by electron capture at the surface after the incident 900-eV positrons thermalize in the target. Ion-bombardment-induced defects are also investigated. It is found that ledges are the dominant trapping site on both surfaces. The average ledge spacing on the ion-bombarded surface, after annealing at a specified temperature, is determined by analyzing the broadening of low-energy electron diffraction (LEED) spot profiles. This spacing is then used to calibrate the positronium formation results on each of the annealed surfaces. These results, when applied to the Ni-on-Ni(110) film (where LEED spot broadening is too small to be resolved), yields a measurement of the island nucleation density