FORMATION OF 2-DIMENSIONAL SULFIDE PHASES ON AL(111) - AN STM STUDY

The structure and growth of incommensurate phases, formed by dissociat ion of H2S on Al(111) at 350 or 570 K, were studied by scanning tunnel ing microscopy. Various hexagonal structures were found, with lattice constants decreasing from 3.55 to 3.47 Angstrom and rotational angles increasing from 0 degrees to 8 degrees with respect to the [1 ($) over bar 10] direction of the substrate. This points to rotational epitaxy , indicative of a flat interaction potential in relation to strong int eractions within the adlayer. The exact structure of this phase varies slightly across the surface, local variations are introduced by edge dislocations or other defects. The adlayer nucleates at steps, after w hich triangular islands with very stable edges grow on the terraces. T he initial nuclei are distinctly different from the islands, appearing much higher in the STM images. Larger coverages initiate restructurin g and facetting of the Al substrate, indicating mass transport during formation of the adlayer. Our observations provide strong evidence for a composite, sulfide-like structure with strong internal interactions which involves a reconstruction of the substrate. The misfit of this hexagonal multilayer structure and the substrate leads to the moire pa ttern observed by STM. The relatively flat interaction potential of th is composite adlayer with the substrate allows various rotational orie ntations, depending on the exact, local coverage and adlayer lattice p arameters.