Editorial: Advanced characterization methods for HfO2/ZrO2-based ferroelectrics

Abstract

Ferroelectric HfO2 and ZrO2-based materials are unconventional ferroelectrics compared to historically dominant perovskite-based ferroelectrics. These differences from conventional perovskite ferroelectrics are distinctly seen in HfO2-ZrO2’s fluorite-based structure that exhibits a rich polymorphism of competing crystal phases, enhanced ferroelectric behavior when scaling film thicknesses down to 10 nm and below, and a one order of magnitude lower relative permittivity and higher coercive field. Both the complex interplay of the fluorite crystal phases, as well as the intrinsic material and/or ferroelectric properties associated with them, have made ferroelectric HfO2-ZrO2-based ferroelectrics simultaneously challenging and interesting to characterize. Due in strong part to the nanoscale thicknesses of these fluorite-based ferroelectric films, the device behavior is an inseparable combination of the ferroelectric film properties (i.e., structure, crystalline orientation, grain size) and the material stack structure that encapsulates it into a two or three terminal device (i.e., interfaces, dielectric layers, electrode materials). Each article comprising this Research Topic on advanced characterization methods for HfO2/ZrO2-based ferroelectric illustrates different ways in which the intrinsic material properties or the ferroelectric film’s interaction with the device stack can be characterized to gain physical insight into this unconventional ferroelectric material system. Surface energy effects and grain size are often attributed to the predominant stabilization of either the non-polar tetragonal, monoclinic, or the polar orthorhombic phases in polycrystalline HfO2–ZrO2 films. It is generally observed that the polar orthorhombic phase is stabilized somewhere between the non-polar tetragonal and monoclinic phase boundaries, and that these non-polar phases are very sensitive to grain size in both HfO2 and ZrO2. Since the preferred crystal phase-dependence on grain size can be sensitive to stoichiometry (for instance, in Hf1xZrxO2 or Si-doped HfO2), it can be anticipated that there may be a complex interplay between grain-size engineering and ferroelectric film composition. In “Effect of Al2O3 interlayers on the microstructure and electrical response of ferroelectric doped HfO2 thin films” by Lederer et al., the grain-size and composition dependence of ferroelectric Hf1-xZrxO2 and Si-doped HfO2 is investigated in detail in which 1, 2, and 3 Al2O3 interlayers are used to control grain size while adjusting the stoichiometry of the fluorite-structured ferroelectric films. Using a combination of structural techniques, such as grazing-incidence X-ray diffraction and Kikuchi diffraction, as OPEN ACCESS