Origin of Ferroelectric Phase in Undoped HfO2 Films Deposited by Sputtering

Thin film metal–insulator–metal capacitors with undoped HfO2 as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The influence of film thickness and annealing temperature is characterized by electrical and structural methods. After annealing, the films show distinct ferroelectric properties. Grazing incidence X‐ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thicknesses 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 film during deposition and annealing is correlated to the phase formation process. Oxygen vacancies stabilize ferroelectric phase in HfO2. Undoped sputtered HfO2 shows good ferroelectric properties over a wide thickness range. Changing the oxygen ambient during deposition changes the amount of oxygen vacancies and influences the nucleation during deposition. During deposition, the formed tetragonal phase nuclei stabilize after annealing the metastable ferroelectric orthorhombic phase.