Atomic Layer Deposition of Amorphous Niobium Carbide-Based Thin Film Superconductors

Abstract

Niobium carbide thin films were synthesized by atomic layer deposition (ALD) using trimethylaluminum (TMA), NbF5, and NbCl5 precursors. In situ quartz crystal microbalance (QCM) measurements performed at 200 and 290 °C revealed controlled, linear deposition with a high growth rate of 5.7 and 4.5 Å/cycle, respectively. The chemical composition, growth rate, structure, and electronic properties of the films were studied over the deposition temperature range 125–350 °C. Varying amounts of impurities, including amorphous carbon (a-C), AlF3, NbFx, and NbClx, were found in all samples. A strong growth temperature dependence of film composition, growth rate, and room temperature DC resistivity was observed. Increasing film density, decreasing total impurity concentration, and decreasing resistivity were observed as a function of increasing deposition temperature for films grown with either NbF5 or NbCl5. Superconducting quantum interference device (SQUID) magnetometry measurements down to 1.2 K revealed a superconducting transition at Tc = 1.8 K in a 75 nm thick film grown at 350 °C with TMA and NbF5. The superconducting critical temperature could be increased up to 3.8 K with additional use of NH3 during ALD film growth.