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 (T_c) was measured at 13.8 K, and the upper critical field extrapolated to zero temperature reached 22 T, resulting in a coherence length (ξ) of 3.8 nm. The penetration depth (λ) 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 °C, resulting in a reduction of disorder at the nanoscale and an increase in T_c to 14.2 K. Despite this enhancement, the coherence length ξ (0) remained at approximately 3.8 nm, with no appreciable changes in the λ 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.