Impact enhanced surface diffusion during impurity induced sputter cone formation

Abstract

The impact of an energetic ion onto a surface can result in enhanced surface diffusion of nearby impurity adatoms. With moderate current densities, in the range of mA cm 2 , the impact enhanced diffusion can exceed the normal thermal surface diffusion at 200–400°C. This effect is observed in impurity induced sputter cone formation. These sputter cones have been shown to be related to the surface diffusion of adatoms. In the pure thermal surface diffusion case, the spacing of the sputter cones is dependent both on the bulk temperature and on the (sputtering) ion current density. The dependence on the ion current density ( R) for thermal diffusion is R −1 2 . In impact enhanced diffusion, this dependence is changed to R +1. This can also be described in terms of an effective surface temperature: that temperature at which thermal surface diffusion alone would account for the observed thermal and impact enhanced diffusion. In one observation, a bulk temperature of 300°C and a current density of 1 mA cm 2 combine to an effective temperature of 525°C.