Domain orientation relationship of orthorhombic and coexisting monoclinic phases of YO_1.5-doped HfO₂ epitaxial thin films

Abstract

Domain structures with ferroelectric and ferroelastic characteristics of the epitaxial thin films of the polar orthorhombic phase of 7 mol % YO 1.5doped HfO 2 (YHO) were investigated by aberration-corrected scanning transmission electron microscopy (STEM). The high-angle annular darkfield (HAADF)-STEM images of the YHO thin film reveal that the individual domain size is approximately 10 nm. Multislice simulation demonstrated that STEM imaging can detect individual domains on the nanoscale avoiding superposition with other domains in the projection direction. The STEM analysis of the domain structure of the polar orthorhombic phase elucidated that the orientation relationship between neighboring domains was affected by two factors: (i) tilt or twist relationship between the longest b-axis in the domains, and (ii) parallel or perpendicular relationship between the rotation axis and the longest b-axis. Corresponding to the longest b-axis orientation, two types of domain walls (DWs) exist: the strained DWs and the less strained ones. The latter should move more easily with less strain, and are of the tilt type with the longest b-axis parallel to the domain rotation axis. STEM imaging also reveals the coexistence of a nonpolar monoclinic phase, which deforms the orthorhombic phase around the monoclinic phase. This factor should also diminish the DW mobility. We propose that the formation of the monoclinic phase should be inhibited and that the longest b-axis should be oriented in the in-plane direction for improving the ferroelectric properties. The two-dimensional tensile stress is expected to be preferred for preparing a-/c-oriented YHO epitaxial thin films. The elastic constraint plays an important role for these purposes for controlling the ferroelectric properties of HfO 2 thin films.

Epitaxial Ferroelectric HfO2 Films: Growth, Properties, and Devices

“…This was experimentally observed by XRD in YHO films on YSZ(001) . Local analysis by transmission electron microscopy (TEM) revealed a lateral size of the crystallites of around 10 nm and that grain boundaries were along {100} or {110} planes . Shimizu et al observed that films on indium–tin oxide (ITO) buffered YSZ(001), presenting only b - and c -oriented domains ( c -axis was considered the polar axis), had large polarization of around 30 μC/cm 2 .…”

“…34 Local analysis by transmission electron microscopy (TEM) revealed a lateral size of the crystallites of around 10 nm, and that grain boundaries were along {100} or {110} planes. 35 Shimizu et al 36 observed that films on indium-tin oxide (ITO) buffered YSZ(001), presenting only b-and c-oriented domains (c-axis was considered the polar axis), had large polarization of around 30 µC/cm 2 . This suggested that ferroelectric switching should imply ferroelastic domain switching, which was later confirmed experimentally by micro-diffraction, focusing a synchrotron radiation beam on poled and pristine capacitors (Figure 2) where vanishing of (0l0) reflections was observed after junction cycling indicating a polarization rotation from in-plane to out of plane.…”

Large‐Scale Hf0.5Zr0.5O2 Membranes with Robust Ferroelectricity

“…This was unexpected because domain walls with angles of 90° and 180° only were predicted and observed in previous studies of hafnia‐based compounds. [ 35,36 ] Figure 4i,j shows two types of coherent interfaces between o‐HZO and m‐HZO phases, and the coherent interface shown in Figure 4i is reported for the first time. Phase transformation between the m‐phase and o‐phase most likely occurs at this coherent interface, which could be important for exploring the origins of wake‐up and fatigue processes.…”

Data-Driven Analysis of High-Resolution Hyperspectral Image Data Sets through Nanoscale Capacitance–Voltage Measurements to Visualize Ferroelectric Domain Dynamics

“…In contrast, at thicknesses below 20 nm, the film becomes thinner than the typical domain size. In such cases, domains are rarely stacked in the thickness direction. − A previous PFM study on PZT films 400 nm thick indicated that the configurations of the domain walls near the grain boundary and far from the grain boundary are different, causing the difference in piezoelectric response nonlinearity, while such a mechanism is unlikely to occur in ultrathin films.…”