Growth optimization of Ruddlesden–Popper nickelate high-temperature superconducting thin films

The discovery of ambient-pressure nickelate high-temperature superconductivity provides a new platform for probing the underlying superconducting mechanisms. However, the thermodynamic metastability of Ruddlesden-Popper nickelates Ln_n+1Ni_nO_3n+1 (Ln = lanthanide) presents significant challenges in achieving precise control over their structure and oxygen stoichiometry. This study establishes a systematic approach for growing phase-pure, high-quality Ln₃Ni₂O₇ thin films on LaAlO₃ and SrLaAlO₄ substrates using gigantic-oxidative atomic-layerby-layer epitaxy. The films grown under an ultrastrong oxidizing ozone atmosphere are superconducting without further post annealing. Specifically, the optimal Ln₃Ni₂O₇/SrLaAlO₄ superconducting film exhibitsan onset transition temperature (<i>T</i>_c,onset) of 50 K. Four critical factors governing the crystalline quality and superconducting properties of Ln₃Ni₂O₇ films are identified: 1) precise cation stoichiometric control suppresses secondary phase formation. In a Ni-rich sample (+7%), the thin film forms a Ln₄Ni₃O₁₀ secondary phase, and the <i>R-T</i> curve correspondingly exhibits metallic behavior. In contrast, a Ni-deficient sample forms a Ln₂NiO₄ secondary phase, with its <i>R-T</i> curve indicating insulating behavior over the entire temperature range. 2) Complete atomic layer-by-layer coverage minimizes stacking faults. Deviation from ideal monolayer coverage induces in-plane atomic number mismatch, whichdirectly triggers out-of-plane lattice collapse or uplift near bulkequilibrium positions. 3) Optimized interface reconstruction can improve the atomic arrangement at the interface. This can be achieved through methods such as annealing the SrLaAlO₄ substrate or pre-depositing a 0.5-unit-cell-thick Ln₂NiO₄-phase buffer layer, which enhances the energy difference between the Ln-site and Ni-site layers to promote proper stacking. 4) Accurate oxygen content regulation is essential for achieving a single superconducting transition and high <i>T</i>_c,onset. Although the under-oxidized sample demonstrates a relatively high <i>T</i>_c,onset (50 K), it displays a two-step superconducting transition. Conversely, the over-oxidized sample exhibits a reduced <i>T</i>_c,onset of 37 K and similarly manifests a two-step transition. These findings provide valuable insights for the layer-by-layer epitaxy growth of diverse oxide high-temperature superconducting films