High T c superconducting NbN films deposited at room temperature

Abstract

Thin films of niobium nitride with superconducting transition temperature (Tc ) of 15.7 K have been deposited on a variety of amorphous as well as crystalline substrates including glass, glazed ceramic, fused quartz, and sapphire, maintained at room temperature, by dc reactive magnetron sputtering in a mixture of Ar and N2 gases. The effects of the deposition conditions, particularly the carrier gas pressure and composition, on the film crystal structure, orientation, and resistivity have been studied in an effort to maximize the superconducting transition temperature. A study of the variation of nitrogen consumption with nitrogen injection pressures for constant background argon pressures is conducted and found to be an absolute indicator of the NbN formation systematics. Initially, the consumption increases linearly with the injection pressure but beyond a certain threshold, it shows a distinct drop. The desired high Tc NbN with B1 crystal structure is formed in the vicinity of this turning point of the reactive gas consumption-injection characteristic. High Tc films possess B1 (fcc, NaCl-type) crystal structure as revealed by their x-ray diffraction patterns. An initial increase in the injection pressure of the reactive gas (N2) results in a remarkable increase in the (111) diffraction line intensity along with an increase in the film Tc. This trend continues up to the turning point of consumption-injection characteristic, beyond which the crystal structure distorts into the substoichiometric tetragonal phase with a consequent reduction in the transition temperature. A general protocol for studying the formation systematics of transition metal nitrides has thus emerged.