J. Phys. Chem. B 1999, 103, 3099-3109
3099
Morphological and Compositional Evolution of Pt-Si Intermetallic Thin Films Prepared by the Activated Adsorption of SiH4 on Pt(111) Joseph C. Bondos, Andrew A. Gewirth,* and Ralph G. Nuzzo* School of Chemical Sciences and the Frederick Seitz Materials Research Laboratory, UniVersity of Illinois, Urbana, Illinois 61801 ReceiVed: September 3, 1998; In Final Form: February 5, 1999
We have investigated using scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) the growth and structural evolution of Pt-Si intermetallic phases formed via a chemical vapor deposition (CVD) mediated process. The Pt silicide thin films were prepared though the exposure of a Pt(111) crystal to silane (SiH4) followed by various annealing treatments. The deposition of Si via the decomposition of silane at room temperature preferentially forms clusters at step edges that avoid the centers of Pt terraces. The sizes and coverages of the clusters increases with silane exposure. The clusters are of intermetallic character (composed of both Si and Pt) and coarsen to give cluster heights much larger than a Pt(111) step height. These observations implicitly establish that Si interdiffusion in the near-surface region is weakly activated. Studies performed as a function of the silane 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 (x7 × x7)R19.1° phase, at least one and perhaps two separate overlayers with (x19 × x19)R23.4° symmetry, and a centered, rectangular overlayer. The structure of the adlayers observed by STM generally confirm but expand upon earlier structural studies based primarily on low-energy electron diffraction (LEED), which explored a more restricted sampling of the Pt(111)-Si compositional phase space. In this paper, we also describe several dynamical phenomena that have heretofore not been appreciated as making important contributions to silicide growth processes. They include progressive degradation and incomplete phase formation behaviors, the coexistence of ordered phases, and the metastable growth of multiple hexagonal (i.e., nonbulk) structures. Large-scale growth behavior involving step edge evolution and bunching as well as island structural evolution and coarsening have also been characterized. Notable is that fact that qualitative features of the island structure (e.g., size, angularity, and arrangement) appear to depend only weakly on the method of formation but rather strongly on the elemental composition. Growth and degradation mechanisms have been constructed, and the phenomena observed are contrasted with standard kinetic models based on sequential phase growth.
Introduction Metal-semiconductor thin films, especially silicides, are of fundamental importance in the microelectronics industry where transition metal silicides have been widely used as interconnects, gates in FET’s, Schottky barriers, and ohmic contacts in very large scale integration (VLSI) technologies.1-3 Of particular importance is the self-aligned silicide layer (or salicide) process for fabricating MOS transistors.2 More recently, Pt-Si/Si Schottky diodes (barrier height in the 0.79-0.88 eV range depending on the Pt-Si stoichiometry2) have emerged as effective infrared detectors with sensitivity in the 3-5 µm wavelength range.4 As the dimensions of microelectronic devices have become progressively smaller, it has become increasingly important to understand how to control the phase development of such thin films. For this reason, research has sought to develop a deeper knowledge of the structure, physical properties, and reactivity of metal-semiconductor thin films and to optimize fabrication procedures that yield optimal device performance. It is now understood that there exists a significant need to understand the growth properties of metal silicides at length scales that approach atomic levels, since the thin film reactions frequently exhibit nonequilibrium phase behaviors that
are strongly dependent on the interfacial reaction kinetics.1-8 The nature of such processes has typically been inferred from studies of the formation of thin films from a reaction-limiting component supported on a bulk substrate. Most studies of the reactions occurring at the metalsemiconductor interface involve the deposition of thin transition metal films (
