Surface morphology and nucleation of infinite-layer (Sr,Ca)CuO2 firms on atomically flat SrTiO3 (100) substrates studied by atomic force microscopy

The surface morphology and nucleation of molecular beam epitaxy-grown 'infinite-layer' structure, (Sr,Ca)CuO2 films on atomically flat SrTiO3 are studied by reflection high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). The SrTiO3 (100) surfaces were prepared by an ultra-high vacuum (UHV) annealing and displayed regular arrays of 0.4 nm steps spaced by atomically flat terraces of about 400 nm. The terraces exhibited smooth edges which were parallel to the [110] substrate direction. AFM images of only a two unit cell thick (0.66 nm) (Sr,Ca)CuO2 layers were recorded. Edge roughness of the substrate terraces was found to be affected by the growth process. Atomically flat (Sr,Ca)CuO, islands with the vertical steps of multiple heights of one unit cell were identified. The obtained AFM and RHEED data strongly suggested the island to step-growth mode of 'infinite-layer', (Sr,Ca)CuO2 films grown on atomically smooth (100) SrTiO3 surfaces. Different surface morphology of thicker (Sr,Ca)CuO2 films grown with other Sr/Ca nominal compositions were also investigated by AFM.