Electron channeling in TiO₂coated Cu layers

Abstract

Electron transport in metal conductors with ∼5–30 nm width is dominated by surface scattering. In situ transport measurements as a function of surface chemistry demonstrate that the primary parameter determining the surface scattering specularity is the localized surface density of states at the Fermi level N(Ef). In particular, the measured sheet resistance of epitaxial Cu(001) layers with thickness dCu = 9–25 nm increases when coated with dTi = 0.1–4.0 monolayers (MLs) of Ti, but decreases again during exposure to 37 Pa of O2. These resistivity changes are a function of dCu and dTi and are due to a transition from partially specular electron scattering at the Cu surface to completely diffuse scattering at the Cu–Ti interface, and the recovery of surface specularity as the Ti is oxidized. X-ray reflectivity and photoelectron spectroscopy indicate the formation of a 0.47 ± 0.03 nm thick Cu2O surface layer on top of the TiO2–Cu2O during air exposure, while density functional calculations of TiOx cap layers as a function of x = 0–2 and dTi = 0.25–1.0 ML show a reduction of N(Ef) by up to a factor of four. This reduction is proposed to be the key cause for the recovery of surface specularity and results in electron confinement and channeling in the Cu layer upon Ti oxidation. Transport measurements at 293 and 77 K confirm the channeling and demonstrate the potential for high-conductivity metal nanowires by quantifying the surface specularity parameter p = 0.67 ± 0.05, 0.00 ± 0.05, and 0.35 ± 0.05 at the Cu–vacuum, Cu–Ti, and Cu–TiO2 interfaces.