横田 幸恵

Abstract The compositional dependence of phase-formation behavior in fluorite-type ferroelectric films of the binary HfO ₂ – ZrO ₂ system was systematically evaluated while considering the size effect associated with film thickness. The crystalline phase of the CSD-derived films (1- x ) HfO ₂ - x ZrO ₂ films ( x = 0 - 1.00) generally exhibited a transition from the monoclinic fluorite (m-phase) to orthorhombic and / or tetragonal phases (o-/t-phase) with increasing the ZrO ₂ content. However, the phase boundary between m-phase and o-/t-phases shifted toward the ZrO ₂ -rich region as the film thickness decreased. The composition exhibiting enhanced ferroelectricity also shifted toward the ZrO ₂ -rich side, coincident with the phase boundary: the 40 nm-thick films showed the maximum remnant polarization at x = 0.75, which is noticeably different from the commonly reported optimum composition of x = 0.50 (i.e., Hf0.5Zr0.5O ₂ ) for ultrathin (~9 nm) films.