Comprehensive Study on the Kinetic Formation of the Orthorhombic Ferroelectric Phase in Epitaxial Y-Doped Ferroelectric HfO2 Thin Films

The crystal structure and ferroelectric properties of 12- to 18 nm-thick epitaxial YO1.5-HfO2 films with 5–9% YO1.5 on (111)ITO//(111)YSZ substrates are investigated to clarify the formation mechanism of the ferroelectric phase. The ferroelectric orthorhombic phase can be obtained by transformation from the higher symmetric tetragonal phase by surmounting a relatively low energy barrier. The orthorhombic phase is obtained for 6% and 7% YO1.5-doped HfO2 films by heat treatment at 1000 °C. Although the 5% YO1.5-doped HfO2 film heat-treated at 1000 °C is in a monoclinic phase, the orthorhombic phase was increased by heat treatment at 1200 °C because the high temperature promotes the phase transition from the monoclinic phase in as-deposited films to the tetragonal phase. The 8% and 9% YO1.5-doped HfO2 films have a tetragonal structure without the transition to the orthorhombic phase. Nevertheless, the 8% YO1.5-doped HfO2 film exhibits ferroelectricity by polarization-electric field hysteresis measurement. A microarea X-ray diffraction study reveals that the electric-field-induced phase transition can take place in an 8% YO1.5-doped HfO2 film. The comprehensive study of high-temperature X-ray diffraction measurements implies that the tetragonal phase in 8% YO1.5-doped HfO2 is a supercooled state. Therefore, external stimulation, such as application of an electric field, induces the transition from the tetragonal to the orthorhombic phase. The supercooled tetragonal phase can also be reduced by a slower cooling rate. These results reveal that the formed phase in YO1.5-doped HfO2 epitaxial film is not governed by the simple difference in the formation energy; rather, the kinetics is more important for obtaining the ferroelectric orthorhombic phase.