THz electrodynamics and superconducting energy scales of ZrN thin films

The terahertz properties of ZrN thin films grown with CMOS techniques on industry-standard 300mm silicon wafers are investigated in order to explore their superconducting behavior. The films have thicknesses ranging from 18 to 48nm, and their critical temperatures T c are between 5 and 7.3K. We probe the real and imaginary parts of the complex dynamical conductivity σ ̂ in the frequency range from 100 to 540GHz (0.4–2.2meV) and as a function of temperature. The experiments provide direct access to the low-energy electrodynamics and key material parameters such as the superconducting energy gap and superfluid density. Our findings indicate that ZrN is a weakly coupled BCS-type superconductor with a gap-to-T c ratio of ≈ 3.4 in the thick film limit. For thinner films, this coupling ratio increases up to 4.0, departing from the BCS prediction. The results establish large-scale ZrN thin films as a promising material for high-frequency superconducting applications.