THE GROWTH OF EPITAXIAL YB SILICIDE - A STUDY WITH STM

The formation of epitaxial Yb silicide phases on Si(111) substrates has been investigated by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The STM images demonstrate that the morphology of the silicide thin film phases depends critically on the conditions of preparation. For evaporation of Yb at room temperature followed by annealing up to 600-degrees-C a closed silicide film with flat terraces and many domain boundaries is observed, in which epitaxial order can only be inferred from the hexagonally shaped terrace boundaries. Heating of this phase to 650-700-degrees-C introduces the break-up of the silicide film and the formation of large flat silicide islands with outdiffused Si agglomerates on their top surfaces and a (2 x 1) Yb-Si superstructure in between them. Evaporation of Yb onto heated Si(111) substrates yields crystallographically well defined silicide islands of completely different shape: ''three-dimensional'' hexagonal or trigonal structures surrounded by a (2 x 1) Yb-Si layer. The pyramidal island shapes suggest that the islands have grown in a layer-by-layer fashion, and the epitaxial order in this phase is directly reflected in STM images displaying atomic resolution. The overall silicide growth mechanism, however, appears to follow the Stranski-Krastanov type irrespective of the detailed growth conditions.