Evolution of ferroelectric HfO₂ in ultrathin region down to 3 nm

The ferroelectric properties of ultrathin Y-doped HfO₂ films were investigated. Ferroelectricity was demonstrated experimentally in 3 nm-thick Y-doped HfO₂ via direct detection of displacement currents during polarization switching. The dependence on the HfO₂ thickness within the 30 to 3 nm range revealed that the ferroelectric properties decrease rapidly below a critical thickness. In the ultrathin HfO₂ region, methods such as higher Y doping or metal capping annealing were required to further stabilize the ferroelectric phase. These methods could be used to enhance the switchable polarization (P_sw) to 35 μC/cm² in 5 nm- and 10 μC/cm² in 3 nm-thick Y-doped HfO₂. This paper indicates that HfO₂ ferroelectricity is scalable even in the ultrathin region.