Room‐Temperature Possible Current‐Induced Transition in Ca ₂ RuO ₄ Thin Films Grown Through Intercalation‐Like Cation Diffusion in the A ₂ BO ₄ Ruddlesden–Popper Structure

Cation deficiency tuning is a central issue in thin-film epitaxy of functional metal oxides, as it is typically more difficult than anion deficiency tuning, as anions can be readily supplied from gas sources. Here, highly effective internal deficiency compensation of Ru cations is demonstrated for Ca₂RuO₄ epitaxial films based on diffusive transfer of metal cations in the A₂BO₄ Ruddlesden-Popper lattice from solid-phase cation sources. Through detailed structural characterization of Ca₂RuO₄/LaAlO₃ (001) thin films grown with external cation sources by solid-phase epitaxy, the occurrence of intercalation-like, interstitial diffusion of La cations (from the substrates) in the A₂BO₄ structure is revealed, and that of Ru cations is also suggested. Relying on the interstitial-type diffusion, an optimized Ru deficiency compensation method, which does not induce the formation of Ca_n ₊₁Ru_nO₃ _n ₊₁ Ruddlesden-Popper impurity phases with higher n, is proposed for Ca₂RuO₄ epitaxial films. In the Ca₂RuO₄/LaAlO₃ (001) thin films grown with Ru deficiency compensation, record-high resistivity values (10²-10^-1 Ω cm) and a large (more than 200 K) increase in the temperature range of the nonlinear transport properties are demonstrated by transport measurements, demonstrating the possible advantages of this method in the control of the current-induced quantum phase transition of Ca₂RuO₄.