  In this work we analyze the electronic structure at the junction between a
SrTiO3 (001) single crystal and a thin tetragonal CuO layer, grown by off-axis
RF-sputtering. A detailed characterization of the film growth, based on atomic
force microscopy and X-ray photoelectron diffraction measurements, demonstrates
the epitaxial growth. We report several markers of a thickness-dependent
modification of the film gap, found on both Cu 2p and valence band spectra;
through spectroscopic ellipsometry analysis, we provide a direct proof of a
band gap increase of the tetragonal CuO layer (1.57 eV) with respect to the
thicker monoclinic CuO layer (1.35 eV). This phenomenon is further discussed in
the light of cluster calculations and DFT+U simulations. Finally, we report the
full experimental band junction diagram, showing a staggered configuration
suitable to charge-separation applications, such as photovoltaics and
photocatalisys; this configuration is observed up to very low (<3 nm) film
thickness due to the gap broadening effect.
