Defect Engineering in A 2 B O 4 Thin Films via Surface‐Reconstructed LaSrAlO 4 Substrates

Ruddlesden–Popper oxides ( A 2 B O 4 ) have attracted significant attention regarding their potential application in novel electronic and energy devices. However, practical uses of A 2 B O 4 thin films have been limited by extended defects such as out‐of‐phase boundaries (OPBs). OPBs disrupt the layered structure of A 2 B O 4 , which restricts functionality. OPBs are ubiquitous in A 2 B O 4 thin films but inhomogeneous interfaces make them difficult to suppress. Here, OPBs in A 2 B O 4 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‐ x Sr x CuO 4 ( x = 0.15) are grown on surface‐reconstructed LaSrAlO 4 substrates, which are terminated with self‐limited perovskite double layers. To date, La 2‐ x Sr x CuO 4 thin films are grown on LaSrAlO 4 substrates with mixed‐termination and exhibit multiple interfacial structures resulting in many OPBs. In contrast, La 2‐ x Sr x CuO 4 thin films grown on surface‐reconstructed LaSrAlO 4 substrates energetically favor only one interfacial structure, thus inhibiting OPB formation. OPB‐suppressed La 2‐ x Sr x CuO 4 thin films exhibit significantly enhanced superconducting properties compared with OPB‐containing La 2‐ x Sr x CuO 4 thin films. Defect engineering in A 2 B O 4 thin films will allow for the elimination of various types of defects in other complex oxides and facilitate next‐generation quantum device applications.