Morphological and compositional evolution of Pt-Si intermetallic thin films prepared by the activated adsorption of SiH4 on Pt(111)

We have investigated using scanning tunneling microscopy (STM) and Auger el ectron spectroscopy (AES) the growth and structural evolution of Pt-Si inte rmetallic phases formed via a chemical vapor deposition (CVD) mediated proc ess. The Pt silicide thin films were prepared though the exposure of a Pt(l ll) crystal to silane (SiH4) followed by various annealing treatments. The deposition of Si via the decomposition of silane at room temperature prefer entially forms clusters at step edges that avoid the centers of Pt terraces . The sizes and coverages of the clusters increases with silane exposure. T he clusters are of intermetallic character (composed of both Si and Pt) and coarsen to give cluster heights much larger than a Pt(lll) step height. Th ese observations implicitly establish that Si interdiffusion in the near su rface region is weakly activated. Studies performed as a function of the si lane exposure and annealing temperature reveal a complicated phase behavior that incorporates seven separate atomically ordered phases in addition to large-scale surface features such as three-dimensional islands. The ordered overlayers we have characterized include a complex, multilayer (root 7 x r oot 7)R19.1 degrees phase, at least one and perhaps two separate overlayers with (root 19 x root 19)R23.4 degrees symmetry, and a centered, rectangula r overlayer. The structure of the adlayers observed by STM generally confir m but expand upon earlier structural studies based primarily on low-energy electron diffraction (LEED), which explored a more restricted sampling of t he Pt(111)-Si compositional phase space. In this paper, we also describe se veral dynamical phenomena that have heretofore not been appreciated as maki ng important contributions to silicide growth processes. They include progr essive degradation and incomplete phase formation behaviors, the coexistenc e of ordered phases, and the metastable growth of multiple hexagonal (i.e., nonbulk) structures. Large-scale growth behavior involving step edge evolu tion and bunching as well as island structural evolution and coarsening hav e also been characterized. Notable is that fact that qualitative features o f the island structure (e.g., size, angularity, and arrangement) appear to depend only weakly on the method of formation but rather strongly on the el emental composition. Growth and degradation mechanisms have been constructe d, and the phenomena observed are contrasted with standard kinetic models b ased on sequential phase growth.