Vector Piezoelectric Response and Ferroelectric Domain Formation in Hf0.5Zr0.5O2 Films

The piezoelectric response in polycrystalline films of doped ferroelectric HfO2 has been explored so far; however, the lack of texture in most of the studied films prevents its full understanding. By selecting the appropriate substrate orientation, the ferroelectric orthorhombic phase ratio and crystallographic orientation can be modified in epitaxial films. We exploit this possibility to get further insight into the ferroelectric hafnium oxide piezoelectric response. While characterizing in-plane and out-of-plane piezoelectric responses, it is observed that their magnitude is mainly ruled by the presence of the orthorhombic phase and the polar axis of the polarization along the probing direction. It is also found for the as-grown state that along the out-of-plane direction a single ferroelectric domain is formed, and instead the in-plane response reveals a rich domain structure with a domain size of approximate to 10-30 nm. By characterizing the in-plane piezoelectric response, it is observed that it is anisotropic if the specific orientation, (110), of the SrTiO3 substrate is used. We propose that an out-of-plane single domain is formed due to the presence of an imprint electric field, whereas in-plane domains are formed by non-purely electrostatic interactions as revealed by their relatively large size. Besides, the small but sizeable in-plane anisotropic response is found to result from the in-plane crystallographic configuration, ultimately determined by the selected substrate.