Study of the quantum-well states in ultra-thin silver films on Si surfaces

The growth morphology and the electronic structures of thin metastable and epitaxial Ag films grown on Si(001)2 X I and Si(111)7X7 surfaces at low temperature were investigated by scanning tunneling microscopy and angle-resolved photoelectron spectroscopy using synchrotron radiation. The morphology of Ag films on Si(001)2X1 exhibits a strong thickness and temperature dependence, indicating an intriguing growth mechanism. The as-deposited film at similar to100 K is composed of nanoclusters with flat tops in a uniform quasi-layer-by-layer film at 2-3 ML and of homogeneous clusters having a more three-dimensional (3D) character above similar to5 ML. By subsequent annealing at 300-450 K, flat epitaxial Ag(111) films are formed at a nominal coverage larger than 5 ML, while a percolating network of 2D islands is formed at a lower coverage. For optimally annealed epitaxial films, discrete Ag 5s states are observed at binding energies of 0.3-3 eV, together with the Ag(111) surface state. The discrete electronic states are consistently interpreted by a standard description of the quantum-well states (QWSs) based on phase-shift quantization rules. The phase shift, the energy dispersion and the thickness-versus-energy relation of the QWSs of epitaxial Ag(Ill) films are consistently derived. The in-plane band structures of the QWSs of epitaxial Ag films grown on Si(111)7 X 7 and Si(001)2 X 1 surfaces were investigated in detail. In contrast to the expected free-electron-like behavior, the QWSs show intriguing in-plane dispersions, such as (i) a significant enhancement of the in-plane dispersion with decreasing binding energy and (ii) a splitting of a QWS into two electronic states with different dispersions at off-normal emission. Such unexpected electronic properties of a QWS are obviously related to the substrate band structure. Further, the QWS splitting is explained by the energy-dependent phase shift of the film-substrate interface occurring at the substrate band edge. (C) 2002 Elsevier Science B.V. All rights reserved.