SPUTTERING OF GROUP-IIIA ELEMENTS - PROPERTIES OF THE METAL CLUSTER FORMATION MECHANISM

Neutral and positively charged clusters have been produced by 4 keV ar gon ion bombardment of liquid gallium and the liquid gallium-indium eu tectic alloy. The yields, kinetic energy distributions, and the abunda nce distributions have been studied by time-of-flight mass spectrometr y coupled with single photon post-ionization of the neutrals. The resu lts are compared to experimental data on cluster sputtering from polyc rystalline aluminum and indium previously obtained in our laboratory. The abundance distributions of the neutral and ionic mixed gallium-ind ium clusters with equal number of atoms are found to be statistical. S ince the first atomic layer of the gallium-indium system is labeled du e to surface segregation, the depth of origin of these clusters become s experimental assessable. An increasing depth of origin was found for increasing duster size. While 94% of the sputtered atoms originate fr om the first atomic layer, only 68% of the atoms comprising dusters co ntaining eight atoms come from this outermost layer. A comparison of t he relative yields and kinetic energy distributions for clusters sputt ered from the alloy and from the two pure constituents indicates that these parameters are not controlled by the first atomic layer (indium rich) but by the bulk (gallium rich). The kinetic energies of neutral clusters sputtered from pure aluminum, gallium and indium reveal a cor relation between this property and the binding energies of the materia ls. The mean kinetic energy of the clusters is largest for aluminum fo llowed by gallium and indium. The relative cluster yields decrease wit h increasing cluster size, the slope of which is least far indium, fol lowed by gallium and aluminum.