Hexagonal to monoclinic phase transformation in Eu2O3 thin films grown on GaN (0001)

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 ⟨112¯0⟩ ǁ Eu2O3 ⟨112¯0⟩ for the hexagonal phase, and GaN (0001) ǁ Eu2O3 (201¯), GaN ⟨112¯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.