Solid state amorphization in metal/Si systems

The formation of amorphous interlayer (a-interlayer) by solid-state diffusion in diffusion couples has been one of the most challenging problems in condensed matter physics in recent years. The a-interlayer has been found to occur in all refractory metal/Si and a number of rare-earth (RE) metal and platinum group metal and crystalline silicon systems. A systematic survey and review of extensive studies on the subject in the past years showed that (1) a negative heat of mixing provides the driving force for the reaction and fast diffusion of one component in the other preempts the formation of crystalline compounds, (2) the growth follows a linear law at the initial stage with activation energy around 1–1.5 eV for refractory metal/Si systems and 0.5 eV for RE metal/Si systems, (3) the dominant diffusing species is Si, (4) the stability of amorphous interlayer depends on the composition, (5) simultaneous presence of multiphases in the initial stage of metal/Si interaction, and (6) good correlations between physical parameters and kinetic data. From the investigation of amorphous interlayers, mechanisms of roughing of epitaxial RE silicide/(0 0 1)Si interface, formation of stacking faults and pinholes in RE silicides have gained in basic understanding. The insight led to successful growth of pinhole-free epitaxial RE silicide layer on (1 1 1)Si. Furthermore, the enhanced formation of technologically important C54-TiSi2 by high temperature sputtering, a thin interposing Mo layer and tensile stress can all be explained involving some aspects of the amorphous interlayers.