Electron Tunneling and the Emission of Secondary Ions from Silicon Surfaces

It has been observed in many systems that lowering of the work function Ф of the sample surface enhances the formation of negative secondary ions [1, 2] while suppressing the formation of positive ions [3]. A model based on the tunneling of electrons between the surface and the sputtered atom has been proposed [3,4,5] to explain these results. According to the model, the work-function dependence of the ionization probability should be independent of how the work-function change ΔФ is being induced, provided that the external atom used to induce ΔФ does not affect the chemical state of the target atoms. We have tested this prediction by studying the emission of secondary ions from Si(111) surfaces using Cs and Li overlayers to induce changes in Ф. These systems are chosen because the adsorption of Cs and Li does not change the chemical state of the surface Si atoms in any appreciable way.