Crystal and Electronic Structure of Oxygen Vacancy Stabilized Rhombohedral Hafnium Oxide

Hafnium oxide is an outstanding candidate for next-generation nonvolatile memory solutions such as OxRAM (oxide-based resistive memory) and FeR A M (ferroelectric random access memory). A key parameter for OxRAM is t h e controlled oxygen deficiency in HfO2-x which eventually is associated with structural changes. Here, we expand the v i e w on the recently identified (semi-)conducting low-temperature pseudocubic phase of reduced hafnium oxide by further X-ray diffraction analysis and density functional theory (DFT) simulation and reveal its rhombohedral nature. By performing total energy and electronic structure calculations, we investigate phase stabi l i t y and band structure modifications in the presence of oxygen vacancies. With increasing oxygen vacancy concentration, the material transforms from the well-known monoclinic structure to a (pseudocubic) polar rhombohedral r-HfO2-x structure. The DFT analysis shows that r-HfO2-x is not merely epitaxy-induced but may exist as a relaxed compound . Furthermore, the electronic structure of r-HfO2-x as determined by X-ray photoelectron spectroscopy and UV/Vis spectroscopy corresponds very well with the DFT-based prediction of a conducting defect band. The existence of a substoichiometric (semi-)conducting phase of HfO2-x is obviously an important ingredient to understand the mechanism of resistive switching in hafnium-oxide-based OxRAM.