Band offset and gap tuning of tetragonal CuO−SrTiO3 heterojunctions

In this work we analyze the electronic structure at the junction between a ${\mathrm{SrTiO}}_{3}$ (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 direct proof of a band gap increase in the tetragonal CuO layer (1.57 eV) with respect to the thicker monoclinic CuO layer (1.35 eV). This phenomenon is further discussed in light of cluster calculations and density functional theory $+U$ simulations. Finally, we report the full experimental band junction diagram, showing a staggered configuration suitable for charge-separation applications, such as photovoltaics and photocatalysis; this configuration is observed up to very low ($<3$ nm) film thickness due to the gap broadening effect.