Unusual two-dimensional electrical charge transport at the surface of polycrystalline perovskite ultrathin films

Abstract

The discontinuity of the lattice at the surface leads to an inherent presence of dangling bonds and atomic coordinations that differ from that of the bulk. As a consequence of this effect, we report on a two-dimensional confinement of the charge transport at the surface of polycrystalline perovskite oxide. Studying the surface transport separately from the bulk effect was approached by the investigation of ultrathin and stress-free LaCoO3 films that are grown on amorphous and nonconducting substrates using pulsed-spray evaporation chemical vapor deposition. The electrical characterization demonstrates an intriguing surface localization of the charge carriers. This surface trapping, which is observed above room temperature, dominates the electrical transport up to a temperature that depends on the film thickness, e.g., up to 440 K for 8-nm-thick films. This high-temperature effect, which is attributed to the surface adsorption of oxygen, points at a largely ignored surface effect in the study of thin and ultrathin films of transition metal oxides. Desorbing surface oxygen was experimentally shown to disrupt the two-dimensional confinement of the charge transport.