Atomically flat SrLaAlO₄ substrates for superconductor film growth

Epitaxial growth of superconducting La_2-xSr_xCuO₄ (LSCO) films with low surface roughness requires an atomically-flat and singly-terminated substrate with lattice constants that are well-matched to that of bulk LSCO. While growing LSCO on SrTiO₃ (STO) substrates imposes 3% tensile strain and lowers T_c, growing on SrLaAlO₄ (SLAO) imposes 0.5% compressive strain and increases T_c to 49K, compared to a bulk T_c of 25K. Preparing SLAO is challenging because its Ruddlesden-Popper structure allows for termination at multiple planes, making it difficult to achieve consistent step heights and identify the termination. We experiment with annealing techniques to obtain a precise procedure for preparing high-quality SLAO substrates. We compare the surface quality of samples annealed under three different lanthanum sources: a LaAlO₃ crystal, La₂O₃ slurry, and dry La₂O₃ powder. We use atomic force microscopy to characterize the topography of our samples, as well as X-ray diffraction to study the crystallinity. We find that annealing SLAO in a cation-rich environment provided by the La₂O₃ slurry yields half-unit-cell height terraces most optimal for LSCO film growth. We further use scanning transmission electron microscopy to compare the LSCO growth structure on LSAO with that of STO.