ビスマス系2次元強誘電酸化物を開発、超低電圧・高耐久強誘電トランジスタを実現（Novel Bismuth-Based 2D Ferroelectric Oxide Pushing the Limits of Ultra-Low Voltage and High Endurance in Ferroelectric Transistors）

Ferroelectric field-effect transistors (FeFETs) are promising candidates for next-generation embedded nonvolatile memories and computing-in-memory architectures. Wafer-scale production and scaling down the thickness of the ferroelectric layer is essential for achieving high-density and low-power devices. However, producing uniform ferroelectric materials at sub-5-nm dimensions across a wafer remains a major challenge, largely because of performance degradation at atomic scales and interface-induced depolarization effects.

Van der Waals (vdW) ferroelectrics present a potential solution. Benefiting from their dangling-bond–free and atomically smooth surfaces, they can maintain stable ferroelectricity even at atomically thin scales and integrate seamlessly with two-dimensional (2D) semiconductors. However, vdW ferroelectrics that simultaneously offer a high dielectric constant (κ), wide bandgaps, suitable coercive fields, and large remanent polarization remain scarce. Therefore, there is a clear and urgent need to realize wafer-scale production and seamless integration of ultrathin, uniform vdW ferroelectric films.

We found that a new high-κ vdW ferroelectric oxide, bismuth selenite (α-phase Bi₂SeO₅) retains stable ferroelectricity down to monolayer thickness and is immune to depolarization fields. Furthermore, we demonstrated the wafer-scale uniform synthesis and back-end-of-line (BEOL)–compatible monolithic integration of ultrathin α-phase Bi₂SeO₅ by controllably oxidizing 2D semiconductor Bi₂O₂Se below 400°C. Unlike previous methods such as transfer techniques or atomic layer deposition, our approach enables the formation of Bi₂SeO₅-Bi₂O₂Se vdW heterostructures with atomically smooth and coherent interfaces through precise oxidation, avoiding reliability degradation caused by charge trapping and trap generation at defective interfaces. Wafer-scale fabrication and comprehensive characterization of 2D Bi₂O₂Se-Bi₂SeO₅ FeFETs confirm the exceptional uniformity and device performance, exhibiting <5% device-to-device variation, a high on/off ratio of >10⁶, and a large memory window of up to 0.9 V at 1 V operation. Benefiting from the fatigue-resistant ferroelectricity of bismuth oxide (Bi₂O₂)–based ultrathin vdW structure and the high-quality native-oxide coherent interface, our FeFET demonstrated record-low 0.8-V operation with 20-ns write speed, achieving ultrahigh endurance that exceeded 1.5 × 10¹² cycles, meeting the strict reliability requirements of edge computing and computing-in-memory architectures.

We have established a wafer-scale and BEOL-compatible approach for the synthesis of uniform ultrathin vdW ferroelectric oxide and seamless integration of ferroelectric-semiconductor heterostructures. This effectively overcomes key limitations in conventional ferroelectric integration—such as critical thickness constraints, interface charge trapping, and poor large-area uniformity—enabling the realization of FeFETs with exceptional device-to-device consistency and high performance. We anticipate that this vdW ferroelectric platform holds great promise for addressing the intertwined challenges of scalability, reliability, and 3D integrability for post-Moore computing architectures.