Defect Engineering in A₂BO₄ Thin Films via Surface‐Reconstructed LaSrAlO₄ Substrates

Abstract Ruddlesden–Popper oxides (A₂BO₄) have attracted significant attention regarding their potential application in novel electronic and energy devices. However, practical uses of A₂BO₄ thin films have been limited by extended defects such as out‐of‐phase boundaries (OPBs). OPBs disrupt the layered structure of A₂BO₄, which restricts functionality. OPBs are ubiquitous in A₂BO₄ thin films but inhomogeneous interfaces make them difficult to suppress. Here, OPBs in A₂BO₄ thin films are suppressed using a novel method to control the substrate surface termination. To demonstrate the technique, epitaxial thin films of cuprate superconductor La_2‐xSr_xCuO₄ (x = 0.15) are grown on surface‐reconstructed LaSrAlO₄ substrates, which are terminated with self‐limited perovskite double layers. To date, La_2‐xSr_xCuO₄ thin films are grown on LaSrAlO₄ substrates with mixed‐termination and exhibit multiple interfacial structures resulting in many OPBs. In contrast, La_2‐xSr_xCuO₄ thin films grown on surface‐reconstructed LaSrAlO₄ substrates energetically favor only one interfacial structure, thus inhibiting OPB formation. OPB‐suppressed La_2‐xSr_xCuO₄ thin films exhibit significantly enhanced superconducting properties compared with OPB‐containing La_2‐xSr_xCuO₄ thin films. Defect engineering in A₂BO₄ thin films will allow for the elimination of various types of defects in other complex oxides and facilitate next‐generation quantum device applications.