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Diffusion of substrate material to the surface of some epitaxial thin-film systems has been reported to occur very rapidly, even at temperatures below 40% of the melting points of film and substrate, where bulk interdiffusion should be negligible. In situ scanning tunneling microscopy reveals that the Cu diffusion through ultrathin Co films grown under ultrahigh vacuum onto Cu(100) substrates occurs via a surface diffusion process, not via bulk interdiffusion. During annealing, pores nucleate in weak points of the films, e.g. , near step bands of the substrate. As Cu diffuses from the substrate through the pores to the top of the film, pits of up to several tens of nm linear dimensions are formed.

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The structure of the low-coverage Al phases on the Si{100)2X1surface was determined by scanning tunneling microscopy {STM) with varying bias voltage and bias polarity. Surface structures of 2X2, 2X 3, and 2X 5 phases formed at below 350'C consist of Al-dimer lines perpendicular to the underlying Si-dimer rows. The STM images of the Al-dimer lines taken at positive and negative bias between 1 and 3 V agree with those of the theoretical simulation by assuming the parallel Al-dimer structure. Moreover we found that filled states of Al-Si backbonds and empty states of Al-Al dimer bonds of the parallel Al-dimer lines are observed prominently at -3 and + 1 eV at positions on the underlying Si-dimer rows and between Si-dimer rows, respectively. Atomic configurations at the ends of the Al-dimer lines combined with the underlying Si missing dimer defects are discussed on the basis of the observed STM images.

Electronic states of graphite overlayers formed on the TaC(111}surface have been investigated with the use of scanning tunneling microscopy and photoelectron spectroscopy. The graphite film grows on the substrate layer by layer. The thickness of the overlayer has been adjusted precisely to be either one or two monolayers. The physical properties of the monolayer graphite film are modified by chemical bonding at the interface. This interfacial bonding becomes weak upon the formation of the second layer of graphite, which makes the properties of the double-layer graphite film similar to those of bulk graphite.