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 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.

Persistence of superconductivity in niobium ultrathin films grown on R-plane sapphire

“…The sapphire surface annealing that is performed prior to niobium growth has been proven critical to improve the epitaxy 3,10 . Atomic Force Microscopy analysis performed prior to growth clearly shows vicinal surfaces with atomically flat terraces separated by straight steps edges 7 of 0.3 nm height (Fig.…”

“…Having the highest critical temperature among elemental superconductors, niobium is an ubiquitous material for superconducting thin films and its performance is known to increase when epitaxy conditions can be reached. Hetero-epitaxy of niobium is best 3, 4 performed on sapphire substrates since both their lattice parameters and thermal dilatation coefficients match rather well 3 . Most combined structural and transport studies have involved A-plane (1000) oriented sapphire 5 as the growing substrate.…”

Effect of substrate temperature on the microstructure of thin niobium films

“…The epitaxial relationship between sapphire and niobium has been reported by several authors [3][4][5][6] [ 14].…”

“…The electrical transport porperties and the superconducting transition temperature T c depend strongly on substrate orientation and film thickness D. An influence of Ts on the resistivity of Nb films has been reported for Ts = 450-750 °C [6]. For thin Nb films (D < 40 A) on sapphire (1T02), strained-layer epitaxy was observed when the substrate was cleaned by ion bombardment prior to the deposition [7].…”

“…The preparation of thin niobium films on sapphire is a subject of continuing interest for the study of superconductivity and epitaxial growth [1][2][3][4][5][6][7]. The electrical transport porperties and the superconducting transition temperature T c depend strongly on substrate orientation and film thickness D. An influence of Ts on the resistivity of Nb films has been reported for Ts = 450-750 °C [6].…”