Negative cluster emission in sputtering of Si1-xGex alloys: A full spectrum approach

The emission of negative cluster ions in sputtering of silicon, germanium and silicon-germanium alloys was investigated by time of flight secondary ion mass spectrometry using Cs+ ions at 1 keV for sputtering and Ga+ ions at 25 keV for analysis. Various homonuclear ðSin ; Gen Þ and heteronuclear (SiGen ; SinGe) anionic cluster species were studied and their abundance distributions as a function of cluster size n were determined in steady state conditions. Surprisingly Sin with n > 2 are less sensitive to Ge concentration than Si. This implies that the variation of the secondary ion signals as a function of Ge concentration cannot be explained in terms of simple statistical considerations. Also Gen clusters show a peculiar behavior. Very strong matrix effects were observed for Ge with decreasing intensity at high germanium concentrations. An opposite behavior is followed by the other clusters whose intensities are proportional to the Ge content. Moreover at high germanium concentrations, the Ge2 and Ge3 anions exhibit signals higher than Ge. Finally, we observed that the fractions of silicon and germanium atoms in the ionized fraction of the sputtered flux are equivalent to their elemental fractions in the alloy. This behavior suggests the possibility to establish a protocol for the quantification of germanium concentration that can be applied to any Si1xGex alloy avoiding matrix effects.