Investigation of the tunnel barrier in Nb-based junctions prepared by sputtering and electron beam evaporation

Nb-based tunnel junctions prepared by DC magnetron sputtering and electron beam evaporation are compared. Tunnel barriers consisting of Nb oxide, Al oxide and Si oxide are analyzed by anodization, Auger electron, and inelastic electron tunneling spectroscopy. Results obtained from Nb₂-Si oxide/Si-Nb₁ and Pb/Bi-Si oxide/Si-Nb₁ structures are presented. Nb and Si were deposited by electron beam evaporation and Pb/Bi from tantalum boats. Silicium oxide has a relatively low dielectric constant (∈SiO₂ ≃4, ∈SiO≃6) and is thus of interest for tunnel barriers. The anodization profile of Nb₂-Si oxide Si-Nb₁ layers for Si thickness up to 10 nm are discussed. Pb/Bi-Si oxide/Si-Nb₁ junctions showed reasonably good quality Si thickness between 0.5 and 1 nm. The presence of Si in the barrier is confirmed by the inelastic electron tunnel spectrum (d² I/dV² as a function of junction bias voltage V) which shows the longitudinal and transversal acoustic phonons of Si