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The electronic structure of Eu sesquioxide (Eu2O3) presents a significant challenge to the electronic structure theory due to the presence of correlated Eu semicore 4f electrons. The bandgap values do not agree between computational methods, and even experimentally, there are discrepancies between reports. Eu2O3 was grown epitaxially in a thin film form on n-type GaN (0001) by molecular beam epitaxy. The film was analyzed using UV and x-ray photoemission spectroscopies as well as inverse photoelectron spectroscopy in order to characterize both occupied and unoccupied states. Signatures of Eu2+ are detected after annealing in UHV or after exposure to air, which can be removed by subsequent O2 annealing. The sample reduction is shown to strongly affect the electronic structure. The bandgap of 4.3 eV, electron affinity of 2.2 eV, and band alignment to the substrate with a valence band offset of 0.2 eV for a stoichiometric Eu2O3 film were extracted from the measurements of the occupied and unoccupied electronic states. The electronic structure is interpreted in view of recent theoretical models, and the energy band alignment across the Eu2O3/GaN interface is discussed.

The high-pressure hexagonal phase of Eu2O3 has been grown epitaxially on C-plane GaN (0001) by molecular beam epitaxy. A structural phase transition from the hexagonal to the monoclinic phase is observed with increasing film thickness by ex-situ X-ray diffraction. The critical thickness for the structural transition is between 2 and 6 nm. The observed epitaxial relationships between the substrate and the film are GaN (0001) ǁ Eu2O3 (0001), GaN ⟨11 2¯0⟩ ǁ Eu2O3 ⟨11 2¯0⟩ for the hexagonal phase, and GaN (0001) ǁ Eu2O3 (20 1¯), GaN ⟨11 2¯0⟩ ǁ Eu2O3 [020] with six rotational domains for the monoclinic phase. The (0.8 ± 0.2) eV conduction band offset and bulk dielectric constant of ∼14 makes Eu2O3 a possible gate dielectric for a GaN-based field effect transistor.

Using density functional theory and model Hamiltonian analysis, we investigate the localized states induced by an oxygen vacancy in rutile TiO2. We identify two classes of localized states—the hybrid and the polaron. The hybrid state is caused by the orbital overlap between three Ti atoms next to a vacancy and is mainly derived from the Ti eg orbitals. The polaron state is caused by the local lattice distortion and is mainly composed of one particular t2g orbital from a single Ti atom. The first principles calculation shows that the polaron state is energetically favored, and the tight-binding analysis reveals the underlying connection between the bulk band structure and the orbital character of the polaron. The magnetic coupling between two nearby polaron states is found to be ferromagnetic. Using this picture, we analyze the results of recent theoretical calculations and experiments and discuss the connection to vacancies in SrTiO3.