Interaction of H with epitaxial Er silicide layers on Si(111): Adsorption versus absorption

We present evidence from angle-resolved photoemission that a well-ordered H monolayer is chemisorbed on both two-dimensional (1 x 1) and bulk-like (root 3 x root 3)R30 degrees Er silicide epitaxially grown on Si(111) when exposed to atomic H at room temperature. The data indicate a complete saturation of the Si surface dangling bonds in a geometry quite similar to the ideal Si(111)-(1 x 1)H surface. Most interestingly, in the case of bulk-like Er silicide it is found that H is not only adsorbed on but also absorbed in the layer, i.e. formation of a hydride takes place. H absorbed in the bulk-like (greater than or equal to 2 ML) films causes dramatic changes in their electronic structure and is the more tightly bound form in ultrathin (similar to 2 ML) films. The spectroscopic data suggest a model where H in bulk silicide passivates the internal Si dangling bonds related to the ordered array of Si vacancies formed in the graphite-like Si planes of defected (root 3 x root 3)R30 degrees Er silicide.