Shinnosuke Yasuoka

Impacts of post-heat treatment in a H2-contained (97% Ar and 3% H2) atmosphere on the switching kinetics of (001)-oriented AlScN capacitors are investigated. Fully saturated polarization can be obtained for all capacitors at frequencies of 50 Hz–50 kHz. In contrast to other ferroelectric materials, the domain wall movement behavior does not change after the heat treatment with frequency. The switching behavior follows the Kolmogorov–Avrami–Ishibashi (KAI) model, and the dimension of domain growth, n, values are ∼1–2 for all films before and after the H2-contained heat treatment. The domain nucleation and lateral domain growth in both the electric field- and time-dependent domain dynamics visualized by piezoresponse force microscopy are consistent with the KAI model. The activation energy increases slightly for the film after the heat treatment. This highly resilient switching behavior brings the AlScN films one step closer to realizing the next generation of nonvolatile memory technologies.

The wurtzite-structure ternary alloys of (Al,Ga,Sc)N films with a high Sc/(Al+Ga+Sc) ratio are successfully deposited by a reactive magnetron sputtering method. The breakthrough in ScN solid-solution limit in AlN with wurtzite structure [up to Sc/(Al + Ga + Sc) = 0.47] is achieved, and its wurtzite-phase diagram region is expanded by adding a relatively small amount of Ga [Ga/(Al+Ga+Sc) < 0.1] into the (Al,Sc)N film. The c- and a-axes lattice constants depend strongly on Ga/(Al+Ga+Sc) and Sc/(Al+Ga+Sc) ratios, respectively, and the Sc/(Al+Ga+Sc) ratio primarily determines the internal structure showing a crystal anisotropy parameter, u parameter. The dielectric constant increases with the increase of Sc/(Al+Ga+Sc) ratio. In addition, the coercive field significantly decreases from 5.8 to 1.8 MV/cm with an increasing Sc/(Al+Ga+Sc) ratio from 0.33 to 0.47, while polarization slightly reduces with the increased u parameter. This result provides a new way to expand the content of the ScN solid solution in the wurtzite film to further decrease the coercive field for the memory device applications.

The changes in the crystal structure and ferroelectric properties of (Al0.8Sc0.2)N films sandwiched between Pt and TiN electrodes were investigated by subjecting the films to post-heat-treatment at various temperatures up to 600 °C in both H2 and Ar gases. The remanent polarization underwent slight change, whereas the coercive field strengthened by approximately 9% as a result of the post-heat-treatment up to 600 °C irrespective of the atmosphere and electrode material. This change is much smaller than that reported for ferroelectric (Hf,Zr)O2 films as well as for Pb(Zr,Ti)O3 and SrBi2Ta2O9 films for a wide temperature range from 400 to 600 °C and is almost independent of the Pt and TiN electrodes. The high stability of (Al,Sc)N films with both Pt and TiN electrodes under H2 atmosphere is highly promising to stabilize the properties through the device fabrication process.

The effect of composition and film thickness on the ferroelectric and piezoelectric properties of (Hf_1−x Ce _x )O₂ films deposited without substrate heating was investigated. (Hf_1−x Ce _x )O₂ films with various x values (x = 0.07–0.27) and thickness (150–880 nm) were deposited via RF magnetron sputtering on indium tin oxide (ITO)/Pt/TiO _x /SiO₂/(100)Si substrates. The crystalline phases of the films were observed by X-ray diffraction. The measurements of electrical properties revealed ferroelectric phases in the x range of 0.11–0.21. The film with x = 0.16 exhibited the maximum remanent polarization (Pr) of 15 μC cm⁻², as well as the highest effective piezoelectric coefficient. In addition, the ferroelectric and structural properties remained almost unchanged with increasing film thickness. Therefore, the no-heating deposition of ferroelectric (Hf_1−x Ce _x )O₂ films and their phase stability with respect to thickness were demonstrated in this study. This work provides a pathway for the deposition of ferroelectric (Hf_1−x Ce _x )O₂ films on flexible, wearable sensors.

Ferroelectric (Al, Sc)N thin films have the potential for use in low-power memory applications. This study demonstrates the thickness scalability of ferroelectricity down to an approximately 20 nm-thick (Al0.8Sc0.2)N film sandwiched between microfabricable TiN electrodes. The impact of the deposition gas atmosphere during the sputtering process and the top electrode materials on the crystal structures and ferroelectric properties was investigated for 20–30 nm-thick (Al0.8Sc0.2)N thin films deposited on Si substrates covered with a TiN layer by radio frequency magnetron sputtering. The deposition atmosphere did not strongly affect the crystal structures of the 30 nm-thick (Al0.8Sc0.2)N films but significantly affected their ferroelectric properties. The leakage current density decreased for films deposited under pure N2 gas compared to the films deposited under a gas mixture of 0.67Ar + 0.33N2. The ferroelectric properties of 20 nm-thick (Al0.8Sc0.2)N films were changed by the top electrode materials; both the switching electric field and its maximum applicable electric field increased for the TiN top electrodes compared with the Pt top electrodes, improving the saturation characteristics of the remnant polarization (Pr) against the applied electric field. Consequently, the 20 nm-thick (Al0.8Sc0.2)N film sandwiched between the microfabricable TiN top and bottom electrodes showed ferroelectricity without noticeable degradation with decreasing film thickness; the film maintained large Pr values of over 100 μC/cm2 in the temperature range from room temperature to 150 °C. The present data open the door to scalable ferroelectric random-access memories using almost thickness-degradation-free thin (Al, Sc)N films with microfabricable TiN electrodes.

In this study, the switching kinetics in a (001)-out-of-plane-oriented (Al0.8Sc0.2)N thin film were systematically investigated as a function of temperature and frequency. The switching behavior followed the Kolmogorov–Avrami–Ishibashi model within the temperature and write-pulse width ranges of 300–473 K and 10 Hz–600 kHz, respectively. The switching speed increased linearly with the electric field, even at high electric fields. The frequency dependence of the coercive field (Ec) evaluated at room temperature revealed that the scaling regime of the domain-wall propagation did not transition to the flow regime, and the switching velocity of the domain increased linearly with the electric field even at high frequencies up to 600 kHz. The invariant domain-wall motion regime implies the potential of using (Al,Sc)N films for high-frequency applications. In addition, a formula for estimating the coercive field of an (Al,Sc)N film was derived by fitting the parameters obtained from the study of switching behavior. The formula indicated that the switching electric field tends to become insensitive to frequency as temperature increases. Moreover, the frequency dependency of the field was minimal even at room temperature. This is a useful index for estimating the drive voltage at the operating frequency and temperature.

The ferroelectricity of wurtzite, Sc _x Ga_1−x N (x = 0.35–0.44), was demonstrated in a metal–ferroelectric–metal capacitor. The remanent polarization (P _r) obtained from positive-up negative-down measurements was high, exceeding 120 μC cm⁻². The coercive field (E _c) of Sc_0.44Ga_0.56N was approximately 3.6 MV cm⁻¹ at 300 K, which decreased to 3 MV cm⁻¹ at 473 K. We observed that regardless of the host material, the Sc concentration governed the E _c value in Sc _x Ga_1−x N and Sc _x Al_1−x N, suggesting that the polarization switching started in the wurtzite unit cells containing Sc. Additionally, the E _c of Sc _x Ga_1−x N was lower than that of Sc _x Al_1−x N when P _r was equivalent.

It is recently found that wurtzite aluminum nitride exhibits ferroelectricity by alloying with scandium. Because its remanent polarization (P_r) is large, Sc_xAl_1−xN has attracted much attention. Gallium nitride with similar structure and properties is also expected to show ferroelectricity. Herein, Sc_xGa_1−xN was prepared on a silicon substrate at 673 K using the sputtering method, and its ferroelectricity was investigated. Sc_0.41Ga_0.59N exhibited ferroelectricity before dielectric breakdown. P_r of this film, as evaluated via positive-up-negative-down (PUND) measurement, was around 120 μC/cm², which can reduce the influence of the leakage current. This was comparable to the value of Sc_xAl_1−xN with the same internal parameter u, which is approximately estimated from the lattice constants. Moreover, in the PUND measurement, the measurement temperature dependence of E_c was observed, which was 4.3 MV/cm at 300 K and reduced to 3.2 MV/cm at 473 K.