Effect of fluoride layer thickness on the leakage current in Au/CaF2/Si (111) heterostructures

Thin films of calcium fluoride with a nominal thickness from 2 up to 20 nm were grown on silicon-(111) surface by means of molecular beam epitaxy. The interest to fluorite insulating films is stipulated by the potential applications in field-effect transistors with a 2D channel. The studied Au/CaF2/Si heterostructures exhibited behavior which is typical for a metal-insulator-semiconductor system. In order to attain a quantitative agreement between the measured current-voltage curves and the theoretical model, spatial fluctuations of the fluoride layer thickness had to be accounted for. The standard deviation of the CaF2 layer thickness was found to be between 0.2 and 1.2 nm tending to increase with the nominal thickness. These values are in agreement with root mean square surface roughness measured by atomic force microscopy. A satisfactory reproducibility of the electric characteristics of the studied films makes CaF2 a promising insulating material for electronic devices.