Atomic Structure of Domain and Interphase Boundaries in Ferroelectric HfO2

Though ferroelectric HfO2 thin films are now well characterized, little is currently known about their grain substructure. In particular, the formation of domain and phase boundaries requires investigation to better understand phase stabilization, switching, and phase interconversion. Here, scanning transmission electron microscopy is applied to investigate the atomic structure of boundaries in these materials. It is found that orthorhombic/orthorhombic domain walls and coherent orthorhombic/monoclinic interphase boundaries form throughout individual grains. The results inform how interphase boundaries can impose strain conditions that may be key to phase stabilization. Moreover, the atomic structure near interphase boundary walls suggests potential for their mobility under bias, which has been speculated to occur in perovskite morphotropic phase boundary systems by mechanisms similar to domain boundary motion. Grain substructure of polycrystalline, ferroelectric HfO2 thin films is investigated with electron microscopy. Orthorhombic and monoclinic phases are found to coexist within single grains, and commonly forming coherent interphase boundaries. These complex structures have implications for phase stability and electric field cycling behavior, which are discussed.