5-nm-Thick ferroelectric Y-doped HfO2 was intensively studied. The thickness dependence of ferroelectric properties indicates that stable ferroelectric characteristics are maintained down to 5-nm-thick by taking care of doping and capping effects. Furthermore, the cycling performance shows no wake-up behavior, no obvious degradation after 10 8 cycles. These results not only enable us to use ferroelectric HfO 2 for practical application, but also point out intrinsic properties in ultrathin ferroelectric HfO 2 film from materials science point of view.