Intrinsic Instability of Rhombohedral (Hf, Zr)O2 Phases and Endurance Crisis of Hf-based Ferroelectric Devices

Abstract Since the experimental report of rhombohedral HfO2 and Hf0.5Zr0.5O2 (HZO) phases on La0.7Sr0.3MnO3 (LSMO) substrate, they have attracted intensive attention as a new type of Hf-based ferroelectrics. However, ferroelectric devices with good endurance based on the rhombohedral phases (r-phases) have not been successfully fabricated so far. We find by first-principles calculations that the rhombohedral R3m and R3 structures have imaginary phonon modes and negative formation energies of point defects (O vacancies or interstitials), showing obviously that the r-phases are intrinsically unstable. The instability originates from the easy transformation from rhombohedral structures to non-rhombohedral structures, which has negligible or even no energy barrier, no matter at the strain-free state or under the compressive epitaxial strain from LSMO substrate. Non-rhombohedral phases such as ferroelectric orthorhombic and paraelectric monoclinic P21/m phase can coexist even if r-phases can be fabricated, and the easy transformation to paraelectric P21/m phase can cause quick fatigue and endurance crisis of devices. The experimentally observed monoclinic phase in HZO films grown on LSMO substrate should be attributed to the P21/m structure, which has the lowest-energy structure on LSMO substrate but was misidentified as the well-known P21/c structure. These results reveal the microscopic origin of the difficulty in fabricating r-phase HZO devices and provide a map for guiding the device endurance optimization through controlling epitaxial strain and suppressing defect formation.