Growth and electron quantization of metastable silver films on Si(001)

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The growth morphology and the electronic structures of thin metastable Ag films grown on the $\mathrm{Si}(001)2\ifmmode\times\else\texttimes\fi{}1$ surface at low temperatures are investigated by scanning tunneling microscopy and angle-resolved photoemission spectroscopy using synchrotron radiation. The morphology of Ag films exhibits a strong thickness and temperature dependence indicating an intriguing growth mechanism. The as-deposited film at \ensuremath{\sim}100 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 more three-dimensional (3D) character above \ensuremath{\sim}5 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 the optimally annealed epitaxial films, discrete $\mathrm{Ag}5s$ 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 (QWS's) based on phase-shift quantization. No such well-defined QWS is observed for the films with a coverage less than \ensuremath{\sim}5 ML. The phase shift, the energy dispersion, and the thickness-versus-energy relation of the QWS's of the epitaxial Ag(111) films are consistently derived. The QWS's in photoemission spectra show two distinctive types of the photon-energy dependence in their binding energies; the oscillatory shifts for $h\ensuremath{\nu}=5--15\mathrm{eV}$ and no such shift at $h\ensuremath{\nu}=20--25\mathrm{eV}.$ This can be explained in terms of the different final states in the photoemission process.