INTERACTION OF H WITH EPITAXIAL ER SILICIDE LAYERS ON SI(111) - ADSORPTION VERSUS ABSORPTION

We present evidence from angle-resolved photoemission that a well-orde red H monolayer is chemisorbed on both two-dimensional (1 x 1) and bul k-like (root 3 x root 3)R30 degrees Er silicide epitaxially grown on S i(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 interestingl y, 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 hydri de 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. T he spectroscopic data suggest a model where H in bulk silicide passiva tes 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.