Thickness dependence of the work function in double-layer metallic films

The work function of metallic thin films limited by symmetric surfaces is expected to be thickness dependent at a level of 0.1 eV and a thickness range of about 5 nm. Recent experiments, however, demonstrated that Cu films on glass or Ni substrates show a long ranging (10–20 nm) increase of the work function with increasing film thickness 1]. This effect was attributed to a violation of local charge neutrality in films with unlike surfaces. In this paper we show that the barrier height of thin film diodes like metal-insulator-metal (MIM)-, metal-semiconductor (Schottky contacts)-and metal-vacuum-metal (Kelvin capacitors) structures decreases with increasing thickness of one metal electrode. This metal electrode consists of a double layer whose single layer thicknesses are of the order of few tens of nm. The observed effect can be attributed to a decrease of the work function at the counter limiting interface not exposed to the evaporation beam. A possible explanation can be found again in the violation of the local charge neutrality in films with unlike surfaces.