Ion bombardment of reconstructed metal surfaces: From two-dimensional dislocation dipoles to vacancy pits

By means of scanning tunnel microscopy the surface morphology of reconstructed Au(001) surfaces has been studied after bombardment with 600 eV Ar ions as a function of dose, in the range of 10(13) to 10(16) ions/cm(2), and the experimental results analyzed in the light of molecular dynamics simulations using a glue potential. At low dose (5x10(13) ions/cm(2)) new defects, different from the commonly observed vacancy islands are reported. They appear as depressions 0.06 nm deep, with a characteristic width of 1.44 nm. Bombardment with similar doses of Pt(001) show the same general behavior. Molecular dynamics simulations with a realistic glue potential that reproduces the hexagonal-like Au(001) reconstruction, confirm that these depressions are in fact two-dimensional pi /3 dislocation dipoles originated by the relaxation of vacancy rows on the ridges of the topmost reconstructed layer. These two-dimensional dipoles are seen to dissociate into individual two-dimensional dislocations that display the characteristics of ordinary hulk dislocations, e.g. glide or climb. At higher doses (similar or equal to 3 x 10(14) ions/cm(2)). but well below a nominal removal of 1 monolayer, vacancy islands, one atomic spacing high, are seen to nucleate on these depressions. With increasing ion damage these vacancy islands become the dominant feature. For doses of about 10(15) ions/cm(2), other defects related to the reconstruction, such as perpendicular reconstruction domains and unreconstructed patches of (001) square symmetry, become visible.