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

Domain structures with ferroelectric and ferroelastic characteristics of the epitaxial thin films of the polar orthorhombic phase of 7 mol % YO_1.5-doped HfO₂ (YHO) were investigated by aberration-corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (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 HfO₂ thin films.