Diverse Pathways to Superconductivity in Transparent Indium Tin Oxide films

To pursue the long coherence length and lossless interaction of phonons with superconducting qubits, transparent superconductors have been proposed as promising materials. Superconductivity in transparent and conductive oxide films can be induced through various different methods resulting in oxygen-deficient structures. In this work, we realize the superconducting state by wet electrochemical intercalation (reduction) of the either indium oxide (InO_x) or stoichiometric tin-doped (5-10 at%) indium oxide (ITO) film surfaces. Dry thermal annealing in a hydrogen atmosphere, topotactic reduction using alkali hydrides, or deposition of an oxygen reducing agents such as alkali-metal films (Li, Na, Mg, etc.) induce surface superconductivity without an externally applied potential. These treatments generate two-dimensional superconductivity localized at the surface of the host oxide film. In contrast, the deposition of InO_x and ITO films under low oxygen partial pressure conditions produces superconductivity throughout the film thickness, resulting in three-dimensional superconductivity. Controlled engineering of oxygen deficiency in metal-oxide host structures – both during deposition and through subsequent surface treatment via patterned openings - paves the way for variety of promising quantum applications.