High-quality single-crystal Nb films and influences of substrates on the epitaxial growth

Abstract

Highly pure single‐crystal Nb films (thickness, ∼2000 Å) with high superconducting transition temperatures T_c of ∼9.3 K and high resistance ratios R₃₀₀/R₁₀ up to ∼200 are successfully grown epitaxially on single‐crystal sapphire (α‐Al₂O₃) and MgO substrates at ∼500–∼700 °C by using an electron‐beam evaporation technique. The most high‐quality single‐crystal Nb film (with the maximum T_c of 9.45 K and the maximum R₃₀₀/R₁₀ of 199) is obtained on a sapphire (11̄02) substrate, which has a thermal expansion coefficient very close to that of Nb as well as a small lattice misfit to Nb. The quality of obtained single‐crystal Nb films is found to be lowered by the deposition of the films on substrates with thermal expansion coefficients different far from that of Nb because of the production of internal strains and lattice defects in the films cooled down.