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

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Abstract
Highly pure single‐crystal Nb films (thickness, ∼2000 Å) with high superconducting transition temperatures Tc 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 Tc 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.