Sputtering of the gallium-indium eutectic alloy in the liquid phase

A central assumption of a recent theory explaining isotopic and chemical fractionation during sputtering is that the sputtered atoms come predominantly from the top monolayer. We first describe ion scattering and Auger spectroscopy results that show, in agreement with rough theoretical expectations, that the surface monolayer of a gallium-indium alloy containing 16.5% indium in bulk is greater than 94% indium, while the next layer can be only slightly enriched. We then report measured sputtering yields and angular distributions of sputtered atoms from both the solid and liquid phases of gallium, indium, and gallium-indium eutectic alloy. These were obtained by Rutherford backscattering analysis of graphite collector foils. The sputtering of the liquid eutectic alloy by 15 keV Ar+ results in a ratio of indium to gallium sputtering yields which is 28 times greater than would be expected from the target stoichiometry. Furthermore, the angular distribution of gallium is much more sharply peaked about the normal to the target surface than the indium distribution. When the incident Ar+ energy is increased to 25 keV, the gallium distribution broadens to the same shape as the indium distribution. With the exception of the sharp gallium distribution taken from the liquid eutectic at 15 keV, all angular distribution from liquid targets fit a cos2θ function. The sputtering yields fro the autectic allow us to infer that 85% of the sputtered atoms originate in the surface monolayer at 15 keV incident energy, while 70% do so at 25 keV.