Ga2O3 is emerging as an interesting semiconductor for high-power electronics and solar-blind ultraviolet photodetectors because of its ultrawide bandgap and high breakdown field. To fully extend its applications in optoelectronics, it is highly desirable to fabricate a p–n heterojunction. In this work, we report detailed investigations on the epitaxial growth and interface properties of a p–n heterojunction consisting of wide bandgap NiO and β-phase Ga2O3. We show that the NiO(111) layer can be grown on β-Ga2O3(01) thin films, with an epitaxial relationship of NiO(111)||β-Ga2O3(01) and NiO{110}||β-Ga2O3(12). The p–n diode exhibits a large current rectification ratio of about 6 orders of magnitude at ±2.0 V. A detailed X-ray photoemission spectroscopy study reveals a “staggered” band alignment with valence band offsets of 2.1 eV. More interestingly, a large upward built-in potential of 1.1 eV for β-Ga2O3 is observed near the interface region. The valence band offset and large built-in potential formed at the heterointerface provide advantageous energetics for the separation and migration of photogenerated excitons, of particular interest for self-powered solar-blind ultraviolet photodetection.

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Delafossite CuFeO2 is a p-type oxide semiconductor with a band gap of ∼1.5 eV, which has attracted great interests for applications in solar energy harvesting and oxide electronics. However, there are still some discrepancies in the literature regarding its fundamental electronic structure and transport properties. In this paper, we use a synergistic combination of resonant photoemission spectroscopy and X-ray absorption spectroscopy to directly study the electronic structure of well-defined CuFeO2 epitaxial thin films. Our detailed study reveals that CuFeO2 has an indirect and d–d forbidden band gap of 1.5 eV. The top of the valence band (VB) of CuFeO2 mainly consists of occupied Fe 3d states hybridized with Cu 3d and O 2p, and the bottom of the conduction band (CB) is primarily made up of unoccupied Fe 3d states. The localized nature of the Fe 3d states at both CB and VB edges would limit the carrier mobility and the dynamics of photoexcited carriers. In addition, Mg doping at Fe sites in CuFeO2 increases the hole carrier concentration and leads to a gradual shift of the Fermi level toward the VB. These insights into its electronic structure are of fundamental importance for rational designing and improving the performance of CuFeO2 as photocatalysts.

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