In-situ atomic-level observation of reversible first-order transition in Hf_0.5Zr_0.5O ferroelectric film

In this work, we revealed the dynamic process of atomic structure transitions of ferroelectric (FE) Hf0.5 Zr0.5 O2 (HZO) film across Curie temperature { mathrm{T}}-{ mathrm{c}} in spherical aberration corrected transmission electron microscope (Cs-TEM) with in-situ controlled heating and cooling. We have the following new observations: (1) A first-order transition between polar orthorhombic (o-) and nonpolar tetragonal (t-) phases occurs as heating up to { mathrm{T}}-{ mathrm{c}}, while the reverse process is triggered by cooling with obvious thermal hysteresis ( Delta mathrm{T} approx 298{o} mathrm{C}); (2) Monoclinic (m-) phase contributes to the stability of o-structure by introducing local compressive stress; (3) The cooling t-to-o transition is identified as martensitic-like transformation, therefore increasing the phase change driving force would be helpful to FE enhancement. This work provides solid evidences on structure transitions near { mathrm{T}}-{ mathrm{c}} of HZO film at atomic-scale, laying foundation for understanding the physical nature of ferroelectricity in hafnium oxide materials.