Near‐surface disorder studied with slow positrons

We report on measurements of the distribution of open‐volume defects in regions close to solid surfaces. Data are obtained by analyzing the fraction of implanted positrons which diffuse back to the entrance surface as a function of the incident positron energy. The method is applied to detect damage profiles of clean metal surfaces in ultrahigh vacuum, sputtered with keV Ar⁺ and N⁺₂ ions. The extracted vacancy‐type defect distributions have a surface peak typically 10–30 Å wide, with a tail extending down to ∼100 Å. The dependence of defects on various sputtering parameters and annealing treatment is discussed, and we also observe strong interaction between defects and sputtering particles. The results are compared with molecular dynamics and binary collision simulations. Furthermore, we have grown epitaxial Ag/Cu(111), Ag/Ag(111), Cu/Ag(111), and Cu/Cu(111) structures in situ. Defects at the interface and in the overlayer are studied in different growth conditions. Significant open‐volume defect concentrations were recorded only in the Cu/Ag(111) system at Cu thicknesses 10–1000 Å. Evaporation on sputtered surfaces resulted in a partial loss of defects, and we also observed an enhanced level of interdiffusion.