Electronic Structure of Silicide-Silicon Interfaces

Over the past few years the electronic structure of transition metal-silicon and silicide-silicon interfaces (and of bulk silicide compounds) has been revealed for the first time, using surface spectroscopies (photoemission and Auger) and theoretical calculations. These investigations, which have included palladium, platinum, nickel, vanadium, chromium, molybdenum and tungsten, have elucidated the important role played by the chemical bond between the transition metal d and the Si 3p electrons. They have also shown the high chemical reactivity of the atomically clean transition metal-silicon interface, which leads to interfacial silicide formation at relatively low temperatures (i.e. markedly below those needed for bulk silicide formation on chemically cleaned silicon surfaces). As a result, silicide-like chemical bonding dominates the interface electronic structure of such contacts under a wide variety of conditions. Detailed comparisons of interface spectra (observed at low metal coverages) with those of the bulk silicide reaction products give strong evidence for additional electronic states of relatively high density (about 0.1 states per interface atom) which lie in or near the silicon band gap region. These states are believed to be true interface states associated with localized bonding configurations unique to the atoms at the interface, and they could explain the silicon-rich silicide composition of a thin (about 3–5 Å) interfacial region which has been observed by Auger composition analysis and suggested by chemical shifts in core and valence electron densities of states. Finally, metal atom diffusion into the silicon substrate has been observed in ion channeling studies and suggested from surface spectroscopy results; these impurity atoms should produce defect states localized near the interface and lying within the silicon band gap, although these electronic states have not yet been directly observed. As a whole these results present a fairly detailed picture of the electronic structure and chemistry of the silicide-silicon interface.However, correlations with the interface electrical properties are needed to ascertain which electronic features and chemical mechanisms in fact determine the Schottky barrier height of the contact.