Electronic Structure and Band Alignment at an Epitaxial Spinel/Perovskite Heterojunction

The electronic properties of solid–solid interfaces play critical roles in a variety of technological applications. Recent advances of film epitaxy and characterization techniques have demonstrated a wealth of exotic phenomena at interfaces of oxide materials, which are critically dependent on the alignment of their energy bands across the interface. Here we report a combined photoemission and electrical investigation of the electronic structures across a prototypical spinel/perovskite heterojunction. Energy-level band alignment at an epitaxial Co₃O₄/SrTiO₃(001) heterointerface indicates a chemically abrupt, type I heterojunction without detectable band bending at both the film and substrate. The unexpected band alignment for this typical p-type semiconductor on SrTiO₃ is attributed to its intrinsic d–d interband excitation, which significantly narrows the fundamental band gap between the top of the valence band and the bottom of the conduction band. The formation of the type I heterojunction with a flat-band state results in a simultaneous confinement of both electrons and holes inside the Co₃O₄ layer, thus rendering the epitaxial Co₃O₄/SrTiO₃(001) heterostructure to be a very promising material for high-efficiency luminescence and optoelectronic device applications