Date of Award
Doctor of Philosophy
Patricia A. Thiel
We investigate the epitaxial growth of Pt and Pd on Pd(100) via spot profile analysis using conventional low-energy electron diffraction (LEED). Despite the limited transfer width of our instrument (ca. 160 A) we resolve a central-spike and diffuse component in the spot profiles, reflecting the layer-occupations and pair-correlations, respectively;Kinetic limitations inhibit layer-by-layer growth at low temperatures. Our data suggest diffusion switches on at ca. 150 K for Pt and ca. 170 K for Pd indicating activation barriers to surface diffusion of ca. 10 and ca. 13 kcal/mol, respectively. We observe intensity oscillations of the central-spike, analogous to those measured via RHEED during MBE, even at substrate temperatures below which surface diffusion is appreciably operative. To clarify the role of diffusion in determining the resulting film morphology, we develop a growth model that incorporates the adsorption-site requirement. Our model predicts intensity oscillations, even in the absence of diffusion. The effect of diffusion is quite complex, since lateral diffusion leads to clustering, making growth less layer-by-layer like, yet interlayer diffusion generally enhances the layer-by-layer quality of the growth;Above ca. 250 K, overlayer reconstruction and/or dissolution interferes with the development of pseudomorphic layers of Pt on Pd(100). We study the homoepitaxy of Pd in a wider temperature range. In-phase LEED data suggest that in the limit of very small islands (one to a few atoms) the interlayer spacing is dependent on the number of atoms in an island. We present a new procedure to experimentally determine out-of-phase scattering conditions. At these energies, ring-structure is evident in the profiles during Pd growth between ca. 200 and 400 K. The appearance of ring-structure is correlated with the onset of diffusion. We report ring intensity oscillations as a function of coverage, which demonstrate the filling of individual layers. Growth at higher temperatures (ca. 500 K) results in "step propagation", wherein deposited atoms generally migrate to existing step edges between deposition events, and nucleation of new layers proceeds only on very large terraces.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Diane K. Flynn-Sanders
Flynn-Sanders, Diane K., "Low-energy electron diffraction investigation of epitaxial growth: Pt and Pd on Pd(100) " (1990). Retrospective Theses and Dissertations. 9412.