  To achieve high quality epitaxial thin films and heterostructures of
transition metal oxides with atomically controlled interfaces, one critical
requirement is the use of atomically flat single terminated oxide substrates
since the atomic arrangements and the reaction chemistry of the topmost surface
layer of substrates determine the growth and consequent properties of the
overlying films. Achieving the atomically flat and chemically single terminated
surface state of commercially available substrates, however, requires judicious
efforts because the surface of as-received substrates is of chemically mixed
nature and also often polar. In this review, we summarize the surface treatment
procedures to accomplish atomically flat surfaces with single terminating layer
for various metal oxide substrates. We particularly focus on the substrates
with lattice constant ranging from 4.00 to 3.70 angstrom, as the lattice
constant of most perovskite materials falls into this range. For materials
outside the range, one can utilize the substrates to induce compressive or
tensile strain on the films and explore new states not available in bulk. The
substrates covered in this review, which have been chosen with commercial
availability and, most importantly, experimental practicality as a criterion,
are KTaO3, REScO3 (RE = Rare-earth elements), SrTiO3,
La0.18Sr0.82Al0.59Ta0.41O3 (LSAT), NdGaO3, LaAlO3, SrLaAlO4, and YAlO3.
Analyzing all the established procedures, we conclude that atomically flat
surfaces with selective A- or B-site single termination would be obtained for
most commercially available oxide substrates. We further note that this topmost
surface layer selectivity would provide an additional degree of freedom in
searching for unforeseen emergent phenomena and functional applications in
epitaxial oxide thin films and heterostructures with atomically controlled
interfaces.
