SURFACE-STRUCTURES OF SOME STAGES PRECEDING FORMATION OF A PD2SI PHASE

The evolution of the surface structures for room temperature Pd deposition on a (7 x 7)-Si(111) surface and after mild annealing of deposited layers has been studied by means of scanning tunnelling microscopy (STM), low electron energy diffraction (LEED) and Anger electron spectroscopy (AES). Comparative studies of the surface structure and composition at room temperature have shown that at Pd coverages ranging from 1.2 to 3.0 ML amorphous silicide islands with a constant thickness of 3.4 Angstrom are growing. Mild annealing of a 3.0 ML Pd deposited at room temperature produces an inhomogeneous surface layer which contains an ordered (root 3 x root 3)R +/- 30 degrees phase and features of the Si-rich (3 root 3 x 3 root 3)R +/- 30 degrees structure. On the basis of the LEED and STM structural data as well as STM and Auger Pd coverage evaluations, the (root 3 x root 3)R +/- 30 degrees and (3 root 3 x 3 root 3)R +/- 30 degrees structures were described as "chemisorption stages" mediating the formation of a true Pd2Si phase.