Atomic-scale surface studies of alkaline-earth metals on silicon(001)

Surface phases consisting of sub-monolayer amounts of the alkaline-earth metal (AEM) atoms Sr and Ba on Si(001) have been studied using X-ray standing waves (XSW), low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and X-ray photoemission spectroscopy (XPS). XSW triangulation indicates that the same position relative to the bulk unit cell is occupied by Sr atoms in 1/2 ML (2x1) and 1/6 ML (2x3) phases. The same position appears to be occupied in a (3x1) phase as well. The XSW results, in conjunction with nearest-neighbor bond length considerations, are consistent with Sr atoms occupying valley-bridge sites in both the (2x3) and (2x1) phases. XSW results and surface symmetry suggest that the in-plane confinement of the thermal vibration amplitude of Sr is greater in the (2x3) phase than it is in the (2x1) phase. LEED, AES and X-ray fluorescence (XRF) measurements indicate that Sr/Si(001) surfaces with Sr coverage between 1/6 ML and 1/3 ML consist of a mixture of (2x3) and c(2x6) phases.; XSW measurements of a 1/2 ML(2x1) Ba/Si(001) phase indicate that the in-plane position of Ba atoms on this surface is the same as that of Sr in the (2x1) and (2x3) Sr/Si(001) phases. Ordered Ba/Si(001) and Sr/Si(001) surfaces have been observed to degrade due to the presence of residual gases in the ultra-high vacuum (UHV) environment. Ba/Si(001) appears to be the more sensitive of the two systems. This degradation influences both the coherent positions and the coherent fractions measured by XSW.; XSW evidence indicates that oxygen contamination on Sr/Si(001) can have two different effects on the atomic-scale structure of the surface. The particular effect that dominates depends on whether the contamination is introduced before or after the formation of a long-range ordered phase. XPS measurements of Sr/Si(001) surfaces intentionally exposed to O2 have been used to monitor accumulation of oxygen in two different bonding environments. The rate constant for the accumulation of oxygen bonded to Sr is several orders of magnitude higher than that of oxygen bonded to Si. The formation of Si-O bonding occurs more rapidly on the (2x3) Sr/Si(001) surface than it does on (2x1) Sr/Si(001).