COLLISIONAL DESORPTION OF CS+ AND CSX+ IONS FROM GAAS BY CS+ IMPACT

During Cs+ bombardment of a GaAs matrix there is a high flux of sputte red Cs+ ions which rises exponentially with fluence; equilibrium is re ached within a projectile Cs+ dose comparable with the surface density of a Cs monolayer on GaAs (4 x 10(14) atoms cm-2). Signals of CsGa+ a nd CsAs+ are respectively two and three orders of magnitude smaller bu t exhibit similar behavior. The observations are consistent with the i ncident Cs+ ions not occupying sites related to their range distributi on but rather precipitating immediately to the surface. On the basis t hat the subsequent ejection process is collisional desorption the cros s sections for Cs desorption from a GaAs surface is evaluated as 1.61 x 10(-15) cm2 for 12 keV impact at 45-degrees from the surface normal. Cross sections for desorption of the molecular species are similar. T he variations of the cross sections of the three species have been stu died for impact energies from 4 to 12 keV and impact angles from O to 60-degrees. Implications for understanding Cs+ profiling in SIMS will be discussed.