Anomalous Tunneling in Carbon/Alkane/TiO₂/Gold Molecular Electronic Junctions: Energy Level Alignment at the Metal/Semiconductor Interface

Carbon/TiO₂/gold electronic junctions show slightly asymmetric electronic behavior, with higher current observed in current density (J)/voltage (V) curves when carbon is biased negative with respect to the gold top contact. When a 1-nm-thick alkane film is deposited between the carbon and TiO₂, resulting in a carbon/alkane/TiO₂/gold junction, the current increases significantly for negative bias and decreases for positive bias, thus creating a much less symmetric J/V response. Similar results were obtained when SiO₂ was substituted for the alkane layer, but Al₂O₃ did not produce the effect. The observation that, by the addition of an insulating material between carbon and TiO₂, the junction becomes more conductive is unexpected and counterintuitive. Kelvin probe measurements revealed that while the apparent work function of the pyrolyzed photoresist film electrode is modulated by surface dipoles of different surface-bound molecular layers, the anomalous effect is independent of the direction of the surface dipole. We propose that by using a nanometer-thick film with a low dielectric constant as an insertion layer, most of the applied potential is dropped across this thin film, thus permitting alignment between the carbon Fermi level and the TiO₂ conduction band. Provided that the alkane layer is sufficiently thin, electrons can directly tunnel from carbon to the TiO₂ conduction band. Therefore, the electron injection barrier at the carbon/TiO₂ interface is effectively reduced by this energy-level alignment, resulting in an increased current when carbon is biased negative. The modulation of injection barriers by a low-κ molecular layer should be generally applicable to a variety of materials used in micro- and nanoelectronic fabrication.