Nucleation of body-centered-cubic tantalum films with a thin niobium underlayer

Abstract

We discuss the structural and electrical properties of high-quality Ta films prepared by ion beam sputter deposition. The Ta films grow in two different crystal structures, body-centered-cubic (bcc) or tetragonal (β-Ta), depending on the substrate preparation and sputtering conditions. Ta films deposited on a thin (>0.3 nm) Nb underlayer grow in the bcc crystal structure with properties approaching those of clean bulk polycrystalline material. The bcc-Ta films have a superconducting transition temperature of 4.3 K and a low-temperature (10 K) resistivity ρ∼6 μΩ cm. Ta films deposited without a Nb underlayer on Si substrates always grow in the tetragonal (β-Ta) structure. The β-Ta films do not superconduct above 1 K and have a high resistivity ρ∼150 μΩ cm. X-ray diffraction and transmission electron microscope studies of both Ta structures are presented. Both bcc-Ta and β-Ta films are deposited on room-temperature substrates. This allows either type of film to be easily patterned by standard photoresist liftoff methods.