Transition from the m Phase to o Phase in an HZO Thin Film Revealed by In Situ TEM

Understanding phase evolution in Hf_0.5Zr_0.5O₂ (HZO) under electrical stress is essential for clarifying the mechanism of formation of the ferroelectric phase, which remains under debate because of the lack of direct observation. In this work, the electrically induced phase evolution of HZO, from the m phase to the ferroelectric o-Pca2₁ phase via an intermediate o-Pbca phase, was observed by using in situ transmission electron microscopy. Notably, this transition is accompanied by a change from a low-resistance state to a high-resistance state, which is attributed to oxygen vacancy redistribution under DC biasing. Meanwhile, Joule heating is believed to play an important role in the formation of the o-Pbca phase. Furthermore, density functional theory calculations confirmed that oxygen vacancies reduce the energy barriers for the m-to-o-Pbca transition and subsequent ferroelectric phase formation. These findings can deepen our understanding of phase evolution in hafnium-based oxides and provide a foundation for the structural design and reliability analysis of hafnium-based devices.