No‐Heating Deposition of 1‐μm‐Thick Y‐Doped HfO₂ Ferroelectric Films with Good Ferroelectric and Piezoelectric Properties by Radio Frequency Magnetron Sputtering Method

Y‐doped HfO₂ ferroelectric films of ≈1 μm thick are deposited without heating by a radio frequency magnetron sputtering method. {100}‐oriented epitaxial films with orthorhombic phase are grown on (100)ITO//(100)YSZ substrates without heating. Their crystal structure is almost unchanged after postheat treatment at 800 °C. Ferroelectricity is confirmed for the no‐heating‐deposited films by polarization−electric field (P−E) curves, and their remanent polarization (P_r) and coercive fields are 12 μC cm⁻² and 1.5 MV cm⁻¹, respectively. These values are also almost unchanged after the postheat treatment. However, the postheat‐treated films show a lower breakdown electric field compared to the as‐deposited films without heat treatment. Approximately 1 μm‐thick films are also prepared without heating on (111)ITO/(111)Pt/TiO_x/SiO₂/(100)Si and (111)Pt/TiO_x/SiO₂/(100)Si substrates. Almost pure orthorhombic/tetragonal phase is deposited on both substrates without heating. The P_r value of the film on the (111)ITO/(111)Pt/TiO_x/SiO₂/(100)Si substrate is about 1.5 times larger than that on the (111)Pt/TiO_x/SiO₂/(100)Si substrate due to the better crystallinity of the film lattice‐matched with the underlying ITO layer. The effective piezoelectric constant (d_33,f) of the film deposited on the (111)ITO/(111)Pt/TiO_x/SiO₂/(100)Si substrate without heating is estimated to be 4 pm V⁻¹. The present low process temperature leads us to expect novel applications, especially for low‐heat‐resistance applications.