Penetration depth in dirty superconducting NbTiN thin films grown at room temperature

Abstract

We present a study on the superconducting properties of 500 nm thick NbTiN films grown by reactive co-sputtering on silicon substrates at room temperature. The samples exhibit a chemical composition with Nb (50 at.%) and Ti (50 at.%), revealing a polycrystalline structure characterized by columnar growth and an average lateral grain size of approximately 40 nm. The superconducting critical temperature (Tc) was measured at 13.8 K, and the upper critical field extrapolated to zero temperature reached 22 T, resulting in a coherence length (xi) of 3.8 nm. The penetration depth (lambda) was determined through local magnetic force microscopy measurements conducted at temperatures of 4.25 and 6 K. The obtained values were 400 (15) nm at 4.25 K and 430 (15) nm at 6 K. Extrapolating these measurements to zero temperature, we obtained an estimated value of 380 nm. A comparison was made with samples that underwent thermal annealing at 700 degrees C, resulting in a reduction of disorder at the nanoscale and an increase in Tc to 14.2 K. Despite this enhancement, the coherence length xi (0) remained at approximately 3.8 nm, with no appreciable changes in the lambda values. Our findings contribute to understanding fundamental superconducting parameters in nitride thin films, with potential applications ranging from resonant accelerator cavities to Josephson junctions and radiation detectors.