Structural evolution of Re (0001) thin films grown on Nb (110) surfaces by molecular beam epitaxy

Abstract: The heteroepitaxial growth of Re (0001) films on Nb (110) surfaces has been investigated. Nb/Re bilayers were grown on A-plane sapphire -- alpha-Al2O3 (11-20) -- by molecular beam epitaxy. While Re grew with a (0001) surface, the in-plane epitaxial relationship with the underlying Nb could be best described as a combination of Kurdjumov-Sachs and Nishiyama-Wassermann orientations. This relationship was true regardless of Re film thickness. However, an evolution of the surface morphology with increasing Re thickness was observed, indicative of a Stranski-Krastanov growth mode. Re (0001) layers less than 15 nm thick were atomically smooth, with a typical rms roughness of less than 0.5 nm, while thicker films showed granular surface structures. And despite the presence of a substantial lattice misfit, the Re layer strain diminished rapidly and the Re lattice was fully relaxed by about 20 nm. The strain-free and atomically smooth surface of thin Re overlayers on Nb is ideal for the subsequent epitaxial growth of ultra-thin oxide tunnel barriers. Utilizing bcc/hcp (or bcc/fcc) heteroepitaxial pairs in advanced multi-layer stacks may enable the growth of all-epitaxial superconductor/insulator/superconductor trilayers for Josephson junction-based devices and circuits.