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Y-doped HfO2 films with thicknesses of 150−1000 nm were prepared on Pt/TiO x /SiO2/Si substrates by the sputtering method and subsequent heat treatment at 800 °C. XRD analysis showed that the films consisted of an almost pure orthorhombic/tetragonal phase. Hysteresis loops originating from the ferroelectricity were observed in the polarization−electric field relationship; the remnant polarization and coercive field were about 12 μC cm−2 and 1.2 MV cm−1, respectively. Piezoelectricity was also confirmed from the strain−electric field curves for 1 μm thick films, and the apparent piezoelectric coefficient, d 33,f, near 0 MV cm−1 was estimated to be about 2.5 pm V−1. Taking account of the relatively low dielectric constant of about 23, the piezoelectric responses from 1 μm thick films prepared by the sputtering method are useful for piezoelectric microelectromechanical system applications, especially for sensor applications, since the performance of such applications is proportional not only to the piezoelectric response but also to the inverse of the relative dielectric constant.

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 (Pr) 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/TiOx/SiO2/(100)Si and (111)Pt/TiOx/SiO2/(100)Si substrates. Almost pure orthorhombic/tetragonal phase is deposited on both substrates without heating. The Pr value of the film on the (111)ITO/(111)Pt/TiOx/SiO2/(100)Si substrate is about 1.5 times larger than that on the (111)Pt/TiOx/SiO2/(100)Si substrate due to the better crystallinity of the film lattice‐matched with the underlying ITO layer. The effective piezoelectric constant (d33,f) of the film deposited on the (111)ITO/(111)Pt/TiOx/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.

Electro-optic (EO) modulators for silicon photonics using CMOS-compatible materials and processes are in great demand. In this study, epitaxial (100)-undoped HfO2 and Y-doped HfO2 thin films were fabricated on Sn-doped In2O3/yttria-stabilized zirconia(100) substrates at room temperature via magnetron sputtering. EO measurement of the Y-HfO2 film using modulation ellipsometry showed that the phase was changed by 180° after application of positive and negative poling biases, and the modulation amplitude increased linearly with increasing AC electric field, indicating a linear EO effect based on ferroelectricity. The observed results indicate that ferroelectric HfO2-based films are viable candidates for CMOS-compatible EO devices.

Recently, we reported linear electro-optic (EO) effects in (100)-epitaxial yttrium-doped hafnium dioxide (Y-HfO 2 ) ferroelectric thin films. In this study, we have investigated the influence of orientation on the EO effect in Y-HfO 2 thin-film. (111)-epitaxial undoped HfO 2 and Y-HfO 2 films were deposited on Sn-doped In 2 O 3 /yttria-stabilized zirconia (111) substrates at room temperature through radiofrequency magnetron sputtering. Although the undoped HfO 2 film showed typical paraelectric characteristics, ferroelectricity was observed in the (111)-Y-HfO 2 film. Remnant polarization in the (111)-Y-HfO 2 film was higher than that in the (100)-Y-HfO 2 film. The (111)-Y-HfO 2 film exhibited a linear EO effect based on ferroelectricity, which is consistent with that of the (100)-Y-HfO 2 film. The average EO coefficient r c of the (111)-Y-HfO 2 film was 0.67 pm V −1 , which is higher than that of the (100)-Y-HfO 2 film. This result is reasonable considering the difference in remnant polarization between the (100)-Y-HfO 2 and (111)-Y-HfO 2 films.