Regulating phase transformation kinetics via redox reaction in ferroelectric Ge-doped HfO2

HfO2 ferroelectricity is promising due to CMOS compatibility and various exceptional properties compared to conventional ferroelectricity. However, the mechanism for stabilizing the ferroelectric phase is still controversial. In this study, it is found that the redox reaction at high temperature strongly influences the phase transformation kinetics during the cooling process of Ge-doped HfO2 and alters the ferroelectric phase ratio at room temperature. The transformation to the non-ferroelectric phase becomes so fast during cooling in the oxidized sample that the transformation is unavoidable even in fast cooling and deteriorates ferroelectricity. It is further revealed that while the high-temperature redox reaction itself is reversible, the transformation to the non-ferroelectric phase during cooling is an irreversible process, which dominates ferroelectricity. These results help understand ferroelectric phase formation in doped HfO2 and elaborate the fabrication process of advanced ferroelectric devices.