Effect of Ce/(Hf + Ce) ratio on ferroelectric and piezoelectric properties and effect of film thickness on ferroelectricity for thick CeO₂–HfO₂ films deposited by sputtering method without substrate heating

Abstract

The dependence of ferroelectric properties on composition and film thickness was investigated for CeO₂–HfO₂ films with various ratios x = Ce/(Hf + Ce), deposited on polyethylene terephthalate coated with an amorphous indium tin oxide layer (amorphous ITO/PET) substrates without substrate heating. Additionally, the dependence of composition on piezoelectric properties was examined. X-ray diffraction (XRD) analysis showed that the films consisted of a tetragonal phase (T.) and/or orthorhombic (O.) phases with (111) and {100} out-of-plane orientations for the films with thicknesses ranging from 440 to 910 nm at x = 0.23. In addition, the remnant polarization (P_r) increased from 6.0 to 11.7 µC/cm² with the increase in the film thickness. To investigate the composition dependence, CeO₂–HfO₂ films with a thickness of 800–1000 nm and x values of 0.17, 0.18, 0.23, and 0.28 were prepared. XRD showed (111) and {100} orientations in all films. Clear P-E hysteresis loops and switching currents were observed for x = 0.18 and 0.23, with P_r exceeding 10.0 µC/cm², even in films deposited on amorphous ITO/PET without substrate heating. Moreover, these films exhibited piezoelectric properties consistent with P-E loops, showing their potential for flexible piezoelectric applications.