Large thermal hysteresis of ferroelectric transition in HfO2-based ferroelectric films

The transition between the dielectric tetragonal and ferroelectric orthorhombic phases in 7%Y doped HfO2 and Hf0.5Zr0.5O2 films with various orientations and film thicknesses was investigated by high-temperature x-ray diffraction. All films demonstrate a different phase transition temperature on heating and cooling with thermal hysteresis with a gap of ΔT. This result clearly shows that the phase transition of the ferroelectric HfO2-based film is first order. The ΔT value of 40–210 °C in HfO2-based films is larger than that of other ferroelectric materials but similar to that of martensitic materials with large lattice deformation. This implies that the ferroelectric phase transition of HfO2-based films involves large lattice deformation. Moreover, we show that ΔT is changed by the size and composition effects. Our results are a step toward elucidating the mechanism of phase transition in ferroelectric HfO2-based films.