Crystal Phase Distribution and Ferroelectricity in Ultrathin HfO2–ZrO2 Bilayers

Abstract

(Hf,Zr)O2 ultrathin films are used as ferroelectric layers in emerging digital logic and nonvolatile memory devices. The ferroelectric properties of (Hf,Zr)O2 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 Pca21 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 2.4× 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)O2 films, accelerating the development of electronic devices that rely on ferroelectric oxides.