Anomalous metal-to-insulator transition in FeSi films deposited on SiO2∕Si substrates

Abstract

In-plane conductivity measurements of FeSi films deposited on boron-doped silicon substrates exhibited an anomalous metal-to-insulator transition near 250K. In the temperature range of 250–215K the resistance of the films increased by more than three orders of magnitude. For temperatures >250K, metallic conductivity consistent with the conductivity of the doped silicon substrate was observed. This indicates an ohmic contact between the film and the silicon substrate across the native SiO2 layer. Below the transition temperature (<250K), the temperature dependence of the resistance implies hopping conduction between localized states that is observed in disordered FeSi films. This metal-to-insulator transition observed in these films suggests switching of the current percolation path from substrate to the film due to a rapid increase in the interfacial resistance. The experimental results agree well with a three-layer model that incorporates an exponentially increasing interfacial resistance with decreasing temperature. The presence of a thin native oxide layer between the deposited film and the silicon substrate is essential for manifestation of the transition. Cross-sectional transmission electron microscopy analysis indicated diffusion of Fe through the oxide barrier and accumulation of Fe at the SiO2∕Si interface. The band bending at the interface resulting from Fermi level pinning due to interface states and the formation of (Fe+∕++B−)0∕+ pairs at the SiO2∕Si interface may be responsible for the observed transition.