Correlations in secondary-ion yields from Cs-implanted semiconductors

Abstract

Elemental and compound semiconductors (Si, Ge, InP, InSb, ZnSe, CdS, CdSe and CdZnTe) were implanted with Cs+ ions of 5.5 and 14.5 keV energy and fluences ranging from 2 × 1015 Cs+/cm2 up to saturation values (>1 × 1017 cm−2). These implants were depth profiled by secondary-ion mass spectrometry (SIMS) employing 3 keV Ar+ primary ions and monitoring positive secondary ions. Specifically, Cs-carrying molecular species like Cs2+ and MCs+ (here M stands for an element of the sample) were recorded during Cs removal in order to investigate a possible correlation between the yields of those ions and that of Cs+. It is observed that for all specimens with the exception of Si the flux of MCs+ ions scales linearly with the Cs+ intensity over a wide range of cesium concentration (several orders of magnitude in some cases); for silicon deviations from this scaling are found at the highest Cs content and are tentatively ascribed to changes of the sputtering yield in the heavily doped Si specimens. The Cs2+ Cs + ratio, on the other hand, is found to decrease more than linearly (possibly exponentially) upon reduction of the Cs concentration, indicative of a decreasing formation probability of Cs2 dimers at low concentrations due to an increasing (average) distance between individual Cs atoms. By contrast, the linearity of MCs+ and Cs+ yields corroborates a mechanism of formation of MCs+ molecular species through an association of neutral M atoms and Cs+ ions in double-collision sputtering events.