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 Nb/sub 2/-Si oxide/Si-Nb/sub 1/ and Pb/Bi-Si oxide/Si-Nb/sub 1/ 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 ( in SiO/sub 2/ approximately=4, in SiO approximately=6) and is thus of interest for tunnel barriers. The anodization profile of Nb/sub 2/-Si oxide Si-Nb/sub 1/ layers for Si thickness up to 10 nm are discussed. Pb/Bi-Si oxide/Si-Nb/sub 1/ 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 (d/sup 2/I/dV/sup 2/ as a function of junction bias voltage V) which shows the longitudinal and transversal acoustic phonons of Si.