Fabrication of niobium titanium nitride thin films with high superconducting transition temperatures and short penetration lengths

We report a systematic study of the superconducting and normal state properties of reactively sputtered Nb_0.62Ti_0.38N thin films deposited on thermally oxidized Si wafers. The superconducting transition temperature (T_c) was found to increase from 12 K for films prepared on unheated substrates to over 16 K for films prepared on substrates maintained at 450°C. A Nb buffer layer was found to improve T_c by ∼0.5 K for growths at lower substrate temperatures. The films fabricated at 450°C have an amply smooth surface (1.5±0.25 nm root mean square roughness), a sufficiently high T_c, and sufficiently small penetration depth (200±20 nm at 10 K) to be useful as ground planes and electrodes for current-generation 10 K rapid single-flux quantum circuit technology.