Orientation control of phase transition and ferroelectricity in Al-doped HfO₂ thin films

Binary ferroelectric materials such as hafnium oxide have been intensively studied, which are expected to exhibit robust ferroelectricity comparable to the perovskite-based ferroelectrics at nanoscale. Here, using the combination of X-ray diffraction (XRD) associate with Transmission Electron Microscope (TEM), we reveal the epitaxial growth of Al-doped HfO₂ possessing various phase structures on different oriented SrTiO₃ substrates. The well-oriented films show different ferroelectricity indicated by piezoresponse force microscopy (PFM). The film grown on the (111) SrTiO₃ substrate shows the largest electromechanical effect. These results, with the analyses of structural characterizations, demonstrate the orientation control of phase transitions and ferroelectricity in Al-doped HfO₂ thin films.