  A uniform one-unit-cell-high step on the SrTiO3 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 [0-11]crystallographic direction in SrTiO3-SrRuO3 heterostructures,
respectively. In particular, the type-II [0-11] 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.
