The (3×2) phase of Ba adsorption on Si(001)-2×1

The initial stages and surface structures of the (3×2) phase of Ba adsorption on an Si(100)-2×1 surface held at 900°C have been studied by low-energy electron diffraction, Auger electron spectroscopy, and ultra-high vacuum scanning tunneling microscopy (STM). At low coverages (<16 ML), the Ba atoms form atomic chains across the Si dimer rows by occupying valley bridge sites, as well as on fourfold sites by replacing Si dimers and exhibiting a local (3×2) phase, with the 3×-phase along the Si dimer row direction. Two different configurations for the (3×2) phase, namely, mono- and dimer-Ba models, are proposed based on the STM images. Below a coverage of 16 ML, the (3×2) phase is formed by single Ba atoms at fourfold sites by replacing original Si dimers. For a Ba coverage of 13 ML, the (3×2) phase is formed by buckled Ba dimers, as revealed by high-resolution STM images.