SEARCH FOR QUANTUM SIZE EFFECTS IN ULTRATHIN EPITAXIAL METALLIC-FILMS

In order to investigate quantum size effects in ultrathin metal films, tunneling spectroscopy measurements have been performed in the epitaxial CoSi2/Si system, with a metal thickness ranging from 1000 angstrom down to 35 angstrom, i.e. a few de Broglie wavelengths of electrons in CoSi2. The resulting spectra show extremely rich sets of features, the origin of which are investigated. The peaks observed at low energy (-100 meV, +100 meV) are thickness independent and attributed to phonon emission by hot electrons. The peaks observed at higher energy (up to 600 meV) are thickness dependent but their physical origin is not yet fully ascertained. The absence of unambiguous electron quantization effects in these epitaxial films is discussed and tentatively attributed to small thickness fluctuations (of the order of a few monolayers), which tend to blur the quantization of the electronic energies.