LAUSR

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 La2‐xSrxCuO4 (x = 0.15) are grown on surface‐reconstructed LaSrAlO4 substrates, which are terminated with self‐limited perovskite double layers. To date, La2‐xSrxCuO4 thin films are grown on LaSrAlO4 substrates with mixed‐termination and exhibit multiple interfacial structures resulting in many OPBs. In contrast, La2‐xSrxCuO4 thin films grown on surface‐reconstructed LaSrAlO4 substrates energetically favor only one interfacial structure, thus inhibiting OPB formation. OPB‐suppressed La2‐xSrxCuO4 thin films exhibit significantly enhanced superconducting properties compared with OPB‐containing La2‐xSrxCuO4 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.