SCANNING-TUNNELING-MICROSCOPY STUDY OF THE INITIAL-STAGES OF GROWTH OF CU SI(111) - THE FORMATION AND THERMAL EVOLUTION OF THE QUASI-5X5 INCOMMENSURATE STRUCTURE/

The epitaxial growth and the structure of Cu on Si(111)7X7 was investi gated by scanning tunneling microscopy (STM) and Auger electron spectr oscopy. The so-called quasi-''5x5'' incommensurate structure of Cu/Si( lll) was observed by STM with atomic resolution only probing the surfa ce empty states with 2.5 V bias. STM analysis showed the ''5 x 5'' str ucture as made out of rounded shaped clusters with periodicity between 5.4a(Si) and 5.7a(Si) where a(Si) is the lattice parameter of the Si( lll)l X 1 surface. The quasi-''5 X 5'' incommensurate structure was ob served to grow layer by layer, and silicon atoms must be available for reacting with copper to form the two-dimensional (2D) incommensurate layer. The formation of the incommensurate quasi-''5 x 5'' structure a t room temperature was only observed on stepped crystals and is consid ered to be due to the consumption of silicon from the step edges. On 0 degrees miscut Si(lll) crystals, the quasi-''5 X 5'' structure is for med upon thermal annealing, which is necessary to break the Si-Si bond s on the terrace. Films of about 2 ML of Cu/Si(lll) heated up to less than 800 K present a very interesting and uncommon arrangement. A firs t, a complete layer of the incommensurate structure is formed and over it a second layer of 2D crystals with irregular interfaces starts nuc leating. Over these 2D crystals, more 2D crystals can grow. All the la yers are composed of the same clusters and the periodicity of the arra ngement can differ from layer to layer. The organized 2D crystals of t he quasi-''5 x 5'' layer grow upon annealing, and point defects and di slocations are formed to relieve stress coming from the incommensurabi lity between the layer and the Si(lll) substrate. These defect lead to faults in the tiling of the clusters on the quasi-''5x5'' layer. The formation of three-dimensional (3D) crystals made out of Cu3Si was als o observed on 1-2 ML Cu/Si(lll) films annealed to temperatures above 9 50 K; their electronic structure is completely different from that of the incommensurate ''5 X 5'' structure. (C) 1997 American Vacuum Socie ty.