Isaku Kanno

In this study, (Mg,Mn)-codoped ZnO (Zn _0.97−x (Mg _x Mn _0.03 )O) epitaxial thin films were fabricated on (111)Pt/(0001) sapphire substrates by RF magnetron sputtering. Structural analysis revealed that the wurtzite structure with c -axis orientation was maintained over the Mg composition range of x = 0.05–0.53. The relative permittivity ( ϵ _r ) remained nearly constant at 13–16 regardless of Mg composition. In contrast, Mn incorporation effectively improved the insulating properties of the films, resulting in a low dielectric loss (tan δ ) of less than 0.02 over the entire Mg composition range. Ferroelectric characterization demonstrated well-saturated P – E hysteresis loops with coercive electric fields ( E _c ) of 2.9–3.3 MV cm ⁻¹ and remanent polarizations ( P _r ) of 80–120 μC cm ⁻² . E _c tended to increase with increasing Mg concentration, whereas P _r decreased as the Mg concentration increased. Current density–electric field ( J – E ) measurements revealed that the leakage current density remained below 10 ⁻⁵ A cm ⁻² and decreased with increasing Mg concentration.

In this study, we investigated the composition and thickness dependence of ferroelectricity in c-axis-oriented Zn1−x(Ce,Mn)xO (Zn(Ce,Mn)O) epitaxial thin films co-doped with Ce and Mn in an approximately 1:1 ratio. With increasing (Ce,Mn) doping concentration, the c-axis length decreased while the a-axis length increased, and the c/a ratio decreased to 1.581 with 7% (Ce,Mn) doping. Meanwhile, (Ce,Mn) doping reduced leakage current and dielectric loss, confirming the contribution of Mn doping. In the Zn(Ce,Mn)O thin films doped with 7%–15% (Ce,Mn), clear ferroelectricity was confirmed. The 7% and 11% (Ce,Mn)-doped Zn(Ce,Mn)O thin films exhibited high remanent polarization (Pr) exceeding 80 μC/cm2, while the coercive electric field (Ec) was approximately 2.5 MV/cm, independent of the doping concentration. Notably, a high Pr exceeding 100 μC/cm2 was also observed in the ZnMnO epitaxial thin films without Ce doping, and Pr decreased with decreasing Ce concentration. For the 11% (Ce,Mn)-doped Zn(Ce,Mn)O thin film, the thickness dependence of ferroelectricity was investigated. A decrease in Pr was observed for film thicknesses below 50 nm. However, ferroelectricity with Pr of 15 μC/cm2 was confirmed even at a film thickness of 30 nm. At this thickness, the polarization switching voltage can be significantly reduced to approximately 9 V. The ferroelectric nature of the 30 nm-thick Zn(Ce,Mn)O thin films was also confirmed by observing polarization switching via piezoresponse force microscopy.

Wurtzite ferroelectric films with stable ferroelectric phases exhibit significantly larger remanent polarization (Pr) compared to other materials, positing them as promising candidates for next-generation non-volatile memories. However, their intrinsically high coercive field (Ec) remains a critical challenge for low-power device operation, necessitating a comprehensive understanding of its underlying origin. In this work, the frequency- and time-dependent switching behavior in (Ce,Mn)-substituted ZnO [Zn0.89(Ce,Mn)0.11O] thin films is systematically investigated, and compared with (Al0.8Sc0.2)N films. The switching process follows the Kolmogorov–Avrami–Ishibashi model, consistent with the behavior observed in (Al0.8Sc0.2)N, and Ec is found to be determined by the activation field (Ea). Through pulse switching and PUND measurements across a broad temperature range, Zn0.89(Ce,Mn)0.11O shows much lower Ea and Ec than (Al0.8Sc0.2)N, while maintaining stable Pr. The origin of Ea is found to correlate with the nucleation energy barrier and threshold field independent of temperature. Furthermore, a comprehensive map of Ec as a function of time and temperature is established, offering critical insight into the operational window for ferroelectric memory devices. These findings provide a deeper understanding of the switching physics in wurtzite ferroelectrics and suggest strategies for tailoring Ec.

In this study, in-situ X-ray absorption fine structure (XAFS) measurements were conducted on amorphous Li₃PO₄-doped V₂O₅ (VO-LPO) cathode in an all-solid-state thin-film lithium-ion battery to observe the behavior of Li ions during charge/discharge. The thin-film lithium-ion battery was fabricated by sputtering deposition and consisted of an amorphous multilayer structure with VO-LPO, LiPON, and Si as cathode, electrolyte, and anode, respectively. In X-ray absorption near edge structure (XANES) measurements, the energy shift of the Vanadium K-edge indicated that the valence of V changed from 4.64 to 5.11 in the cathode during charge/discharge processes. This result indicates that Li ions were removed and inserted into the amorphous VO-LPO cathode layer. The pre-edge peak intensity also increased during charging and decreased during discharging. Additionally, extended X-ray absorption fine structure (EXAFS) measurements indicated that the V-O bond distance shifted during charge/discharge process. From these measurements, we clearly observed that the coordination symmetry and structure around V in the amorphous VO-LPO cathode changed with the charge/discharge reaction.

In this study, an electrical generator was designed for a wave energy converter in which the electricity is generated via horizontal axis-oriented Darrieus turbines installed in the naturally oscillating water near the breaking wave zone. The design goal is to achieve an average output power of >10 kW at 180 rpm, with safe underwater installation and waterproof construction. To verify the expected performance, a scaled-down generator was built with identical configurations, with the stack length reduced from 400 mm to 60 mm, and the expected output power reduced to >1 kW at 180 rpm under test conditions. The evaluation results showed that the desired characteristics were achieved with an output power of 1356 W at 180 rpm and an efficiency of 80.1%. Further, our height-adjustable platform ensures high efficiency at both low and high tide. The variable stack length in our WEC design allows the output power to be adapted to the specific location conditions. Once the experimental tests with the small-scale turbines are accomplished, we plan to conduct field tests with our full-scale wave energy converter.