Ionization probability of secondary ions sputtered from Si(1Â 1Â 1) and Ge(1Â 1Â 1) surfaces

We have measured the energy distributions of the secondary ions sputtered from the Si(1 1 1) and Ge(1 1 1) surfaces and investigated the ionization probabilities of sputtered Si+ and Ge+ ions for clarifying their ionization mechanisms. The observed ionization probabilities depend on the velocity of Si+ and Ge+ ions. This velocity dependence can be successfully analyzed by a theoretical expression, which was proposed originally for the metal surfaces. This implies that the ionization mechanism of Si+ and Ge+ ions is the same as ions sputtered from the metal surface, i.e., the resonant electron transfer in the high velocity regime and the thermal excitation process in the low velocity regime. The difference in the ionization probability between Si+ and Ge+ ions is well explained by the difference in the band gap energy.