Surface termination effect of SrTiO₃ substrate on ultrathin SrRuO₃

A uniform 1-unit-cell-high step on the SrTiO3 (STO) substrate is a prerequisite for growing high-quality epitaxial oxide heterostructures. However, it is inevitable that defects induced by mixed substrate-surface termination exist at the interface, significantly impacting the properties of ultrathin films. In this study, we microscopically identify the origin for the lateral inhomogeneity in the growth of ultrathin SrRuO3 films due to the step effects of SrTiO3 (001). By using atomic-resolved scanning transmission electron microscopy, we observe two distinct types of step propagation along the [011] and [0 (1) over bar 1] crystallographic direction in SrTiO3-SrRuO3 heterostructures, respectively. In particular, the type-II [0 (1) over bar 1] step results in lateral discontinuity of monolayer SrRuO3 and originates from the SrO-terminated regions along the TiO2-terminated step edge. Such an induced lateral discontinuity should be responsible for the distinct electronic and magnetic properties of monolayer SrRuO3. Our findings underscore the critical importance of using single-termination STO substrate to achieve high-quality termination-selective films and to unveil the intrinsic properties of epitaxial films in the atomic limit.