A better understanding of metal-silicon and metal-SiO₂ interfaces would be applicable in many areas of study, including heterogeneous catalysis, the semiconductor industry, and catalytic field effect gas sensors. Due to the buried nature of these interfaces in applications, they have not been well studied. To study these interfaces, a model system consisting of sub-monolayer coverages of silicon containing molecules adsorbed on single crystal metal surfaces has been investigated, using the metal as the substrate rather than the silicon/silica. The primary advantage to such a model system is being amenable to spectroscopic techniques and modeling studies.

In this contribution, we report on a multi-technique surface science investigation of silane adsorption and reaction on clean and O-covered Pd(111). Silane adsorption on Pd(100) and other metals has been studied previously. These studies found that silane desorbs dissociatively, producing H adatoms and SiH_X adsorbates at cold temperatures, followed by complete dissocation of the surface SiH_X and the formation of a metal silicide phase at higher temperatures. To our knowledge, reaction of silane on O-covered metal single crystal surfaces has not been studied.

Silane adsorption and decomposition on Pd(111) was characterized using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and auger electron spectroscopy (AES). Silane exposures of less than 0.5L led to desorption of only H₂ during TPD; after larger exposurses, silane was also observed. HREEL spectra of the surface show that SiHX species on the surface are stable up to ca. 200 K. AES scans of the surface confirm the presence of silicon on the surface after hydrogen desorption, but do not show peak splitting of the Si AES LVV peak indicative of a Pd-silicide. Deposited Si atoms diffuse into the Pd bulk upon heating.

Silane and oxygen coadsorption was studied using the same experimental techniques. Coadsorption leads to low temperature oxidation of surface Si atoms, forming a layer of SiO_X. This is indicated in AES experiments by a shift in the Si AES LVV peak from an energy of 92 eV to an energy of 84 eV and appears similar to an intermediate Si oxidation state seen in previous studies. More evidence of Si-O bonding was seen in HREELS experiments showing losses consistent with Si-O-Si modes. Additional AES experiments show that excess Si diffuses into the Pd bulk during annealing, but the SiO_X layer remains on the surface and must be removed by sputtering. The results of TPD experiments demonstrate that SiO_X formation is highly favorable, and that water formation is only observed when surface oxygen is in excess for the SiO_X formation reaction. The stoichiometric ratio, X, was estimated to be less than 2 through analysis of desorption yields. CO adsorption has also been used to probe of the effect of the SiO_X layer on Pd surface chemistry. Density functional theory studies have also been performed to investigate the structures formed during surface silicon oxidation.

Extended Abstract Status: Not Uploaded