Synergistic enhancement of critical temperature and normal resistance by dual doping in disordered aluminum thin films

We report the electrical transport properties of aluminum thin films grown by reactive RF sputtering in a mixed oxygen-nitrogen atmosphere. The films were deposited at room temperature on oxidized silicon substrates, using a fixed low oxygen concentration and varying nitrogen content. Compared to single-dopant cases reported in the literature, our results suggest a synergistic effect in which nitrogen enhances the superconducting critical temperature (T_c), while oxygen increases the normal-state resistivity. This combined effect is most prominent in the high-resistivity range (1–5 mΩ cm), where the typical dome-like dependence of T_c on normal-state resistivity exhibits values over 0.5 K higher than those of similarly resistive films obtained with only oxygen or nitrogen using sputtering. The corresponding sheet kinetic inductance was estimated under standard BCS assumptions and reaches several hundred pH/sq, consistent with values reported in high-resistivity aluminum films. This dual-doping strategy enables the fabrication of aluminum films that combine enhanced superconductivity with robust normal-state resistance using a simple, scalable deposition method at room temperature. Although the internal structure of the films was not resolved at the nanoscale, their electrical and morphological characteristics fall within the regime typically associated with granular aluminum. These findings demonstrate that controlled disorder engineering through reactive sputtering provides a versatile route to tailor superconducting properties and may extend the operational window of aluminum-based materials for quantum devices, resonators, and microwave kinetic inductance detectors.