Crystal Phase Distribution and Ferroelectricity in Ultrathin HfO₂-ZrO₂ Bilayers

(Hf,Zr)O-2 ultrathin films are used as ferroelectric layers in emerging digital logic and nonvolatile memory devices. The ferroelectric properties of (Hf,Zr)O-2 can be improved by interface engineering, such as the formation of nanolaminates with distinct HfO2 and ZrO2 layers. Herein, the ferroelectric performance of HfO2-ZrO2 ultrathin bilayer devices is shown to depend on the stacking order of HfO2 and ZrO2, which affects the quantity of the noncentrosymmetric orthorhombic Pca2(1) crystal phase. By combining X-ray diffraction with a novel extended X-ray absorption fine structure (EXAFS) analysis procedure, the orthorhombic, tetragonal, and monoclinic phase fractions are quantified for bilayers composed of 3 nm HfO2 and 3 nm ZrO2. A significantly larger orthorhombic ZrO2 phase fraction is found when ZrO2 has an unconstrained surface during annealing, whereas the presence of a ZrO2 interface with the substrate results in a substantial tetragonal ZrO2 phase fraction and a 2.4x smaller remanent polarization. HfO2 is found to be less susceptible than ZrO2 to crystal phase templating. The methods presented herein enable mechanistic studies of ferroelectric wake-up, fatigue, and processing effects in (Hf,Zr)O-2 films, accelerating the development of electronic devices that rely on ferroelectric oxides.