High‐quality single‐crystal Nb and Ta films formed by an ultrahigh vacuum arc method

High‐quality epitaxial Nb thin films (thickness, 2000 A), characterized by flat and clean surfaces, high superconducting critical temperature (max.Tc, 9.44 K), large resistivity ratio (RR, resistivity at room temperature divided by resistivity at 10 K, max.RR, 44), and very small quantities of grains of different orientation, have been formed on sapphire‐A and sapphire‐C substrates at a relatively low substrate temperature (350–530 °C) using an ultrahigh vacuum arc method (4×10−7– 4×10−6 Pa). Structures and orientations of the films deposited on MgO(100), sapphire‐A, and sapphire‐C substrates are investigated by means of several techniques and they depend on symmetry properties of the substrates. 200‐A‐thick Nb films deposited on the MgO and the sapphire‐C substrates showed good crystallinity and planarity. High‐quality single‐crystal films of Ta (thickness, 90 A) were formed on single‐crystal Nb films (7000 A) obtained with the sapphire‐A plane.