The morphology of the (100) face after sputtering with 600eV Ar+ ions was investigated, using scanning tunnelling microscopy, as a function of beam flux and fluence. The treatment generated vacancy and adatom islands which were bounded by monatomic steps. These islands had equilibrium shapes (square with rounded corners). At low fluxes, vacancy as well as adatom islands were present, and the removal mode was 3-dimensional. At high fluxes, the adatom islands disappeared and the removal mode became 2-dimensional (layer-by-layer). The observation of distinct vacancy or adatom islands depleted zones in the vicinity of pre-existing monatomic steps demonstrated that the adsorption kinetics of vacancies or adatoms at a step depended upon the side from which they reached the edge. The energy barriers which controlled these processes also affected inter-layer mass transport. It was concluded that the observations could be explained in a consistent manner if the heights of the barriers were assumed to decrease for steps which bounded small islands.

J.C.Girard, Y.Samson, S.Gauthier, S.Rousset, J.Klein: Surface Science, 1994, 302[1-2], 73-80