T.S., M.M., I.S., and M.C. received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 780302 (project 3εFerro). M.H.P. was supported by Humboldt postdoctoral fellowship from Alexander von Humboldt Foundation and later by the Basic Science Research Program through an NRF (National Research Foundation of Korea) grant funded by the Ministry of Education (NRF-2018R1C1B5086580). P.D.L was funded by the German Ministry of Economic Affairs and Energy (BMWi) project (16IPCEI310). This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State Univ., which was supported by the State of North Carolina and the National Science Foundation (Award No. ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI). The authors thank Ilya Karpov for helpful discussions on the topics of oxygen vacancies and phase stability. The authors also thank Robin Materlik and Christopher Künneth for providing their simulation data and fruitful discussions.

Thin lm metal–insulator–metal capacitors with undoped HfO2 as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The in uence of lm thickness and annealing temperature is characterized by electrical and structural methods. After annealing, the lms show distinct ferroelectric properties. Grazing incidence X-ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thick- nesses in the 10–50 nm range and a negligible non-ferroelectric monoclinic phase fraction. Sputtering HfO2 with additional oxygen during the deposition decreases the remanent polarization. Overall, the impact of oxygen vacancies and interstitials in the HfO2 lm during deposition and annealing is correlated to the phase formation process.