Yubao Li

Here, we report the fabrication of an ultrathin ZrO2/Hf0.5Zr0.5O2-based ferroelectric tunnel junction (FTJ) in a crossbar structure, which exhibits both a large memory window and high current reading, two essential criteria to meet in the practical integration of hafnia-based FTJ memory chips. Giant tunnel electroresistance ratios of over 107 are obtained, attributable to the superior ferroelectric stack of FTJs. The ON-state read current density is roughly inversely proportional to the critical dimension of the crossbar FTJ, with over 600 A/cm2 reached with a 3.5 μm device and 4000 A/cm2 with a submicrometer-sized one, which projects a promising trend in lateral scaling. Under 10 ns single-pulse programming, the FTJs are capable of multiple distinct stable resistance states and can survive up to 7.3 × 106 bipolar switching cycles with a stable memory window of over 10 000. Crossbar FTJs also demonstrate superb synaptic behaviors, including nearly symmetrical and linear long-term potentiation/depression, indicating their great potential in neuromorphic computing. An in-series threshold switching selector is further integrated with a crossbar FTJ to form a one-selector-one-resistor memory, which maintains a large memory window and high current density, paving the way for realizing high-density integration of FTJ crossbar arrays. This work demonstrates the coexistence of a large switching window and high current reading in scaled FTJs with ultrathin fluorite-structured ferroelectrics through back-end-of-line-compatible processing, foreseeing a bright pathway toward the mass production of hafnia-based FTJ memory.

Here, we report a ferroelectric tunnel junction (FTJ) based on ultrathin Hf0.5Zr0.5O2 (HZO) that can be electrically recovered from memory fatigue to reach significantly prolongated endurance. A high remanent polarization (Pr) of 19 μC/cm2 is obtained in 3 nm-thick HZO ferroelectric films, and its robust ferroelectricity can be recovered by employing a unipolar negative voltage recovery pulse technique, leading to significantly-prolongated endurance up to 3.7 × 109 cycles and 4.53 × 1010 cycles under bipolar square-waveform field cycling at 105 and 106 Hz, respectively. The fabricated W/WOx/HZO/Pt FTJs exhibited giant tunnel electroresistance ratios of up to 5.8 × 105, attributable to the superb HZO ferroelectrics and the incorporation of sputtered highly conductive WOx interfacial layer. By implementing a similar single-pulse recovery to release the pinned oxygen vacancy defects from the interfaces, the FTJs delivered superior cycling endurance of 108 cycles with a large memory window over 13 000 under 10 ns single-pulse programming, demonstrating the great potential of ultrathin HZO based FTJs as a promising nonvolatile memory. This work would foresee a bright future and pave the way for the practical applications of ultrathin HZO-based FTJ memory.

Ferroelectric tunnel junction (FTJ) based on hafnia/zirconia-based thin films has emerged as a promising class of nonvolatile memory, owing to their fast switching speeds, low power consumption, and full compatibility with complementary metal-oxide-semiconductor technology. Here, we report the fabrication of a high-performance FTJ based on 3.5 nm-thick Zr0.75Hf0.25O2 (ZHO) ferroelectrics with a remanent polarization of 12 μC/cm2. The fabricated TiN/WOx/ZHO/Pt FTJs exhibited large ON/OFF ratios (>1000) and good endurance of over 106 cycles, attributable to the robust ferroelectric properties and stability of electrodes. Micrometer-sized FTJ devices in crossbar structure delivered ultra-high read current densities of up to 520 A/cm2, and, importantly, the current density increases with shrinking FTJ, being attributed to a higher edge/corner effect from smaller electrodes. In addition, crossbar FTJ showed reliable synaptic phenomena including long-term potentiation/depression, paired-pulse facilitation/depression, and spike timing-dependent plasticity. This work demonstrates the excellent scalability of zirconia-based ferroelectric memory and their capability to emulate biological synapses.

Here, we report a high-performance ferroelectric tunnel junction (FTJ) built with thermal atomic layer deposition-processed 2.9 nm-thick Hf0.5Zr0.5O2 (HZO) and a 0.6 nm-thick ZrO2 capping layer atop by implementing a post-deposition-annealing at 400 °C. Ultrathin ZrO2/HZO ferroelectrics (FE) exhibit a high remnant polarization (Pr) of 18.3 μC/cm2 and superior endurance and retention, resulting from the formation of a high-ratio orthorhombic-phase in FE with the aid of a ZrO2 capping layer. The FTJ in the W/ZrO2/HZO/Pt structure delivers giant tunnel electroresistance ratios of up to 5.6 × 106, in attribution to the robust polarization of ultrathin FE and the formation of a WOx interfacial layer. Under 10 ns single-pulse programming, FTJ is capable of multiple distinct stable resistance states and can survive up to 4.3 × 106 bipolar switching cycles with a large memory window of over 3000. The results demonstrate a viable and simple processing for HZO-based BEOL-compatible FTJ, which would be of significance for practical application in integrating HZO-based embedded memory.

Two-dimensional transition metal dichalcogenides have attracted considerable attention in electrocatalytic hydrogen evolution due to their unique layered structures and tunable electronic properties. However, prior research has predominantly focused on the intrinsic catalytic activity of planar few-layer structures, which offer limited exposure of edge-active sites due to their restricted two-dimensional geometry. Moreover, van der Waals interactions between layers impose substantial barriers to electron transport, significantly hindering charge transfer efficiency. To overcome these limitations, this study presents the innovative synthesis of high-quality single-screw WS₂ with a 5° dislocation angle via physical vapor deposition. Second harmonic generation measurements revealed a pronounced asymmetric polarization response, while the selected area electron diffractionand atomic force microscopy elucidated the material’s distinctive screwed dislocation configuration. In contrast to planar monolayer WS₂, the conical/screw-structured WS₂—formed through screw-dislocation-mediated growth—exhibits a higher density of exposed edge-active catalytic sites and enhanced electron transport capabilities. Electrochemical performance tests revealed that in an alkaline medium, the screwed WS₂ nanosheets exhibited an overpotential of 310 mV at a current density of −10 mA/cm², with a Tafel slope of 204 mV/dec. Additionally, under a current density of 18 mA/cm², the screwed WS₂ can sustain this current density for at least 30 h. These findings offer valuable insights into the design of low-cost, high-efficiency, non-precious metal catalysts for hydrogen evolution reactions.

Ferroelectric tunnel junction (FTJ) with tunable tunnel electroresistance is promising for emerging nonvolatile memory applications. In this work, 6 nm-thick Hf-doped ZrO2 ferroelectrics with Zr : Hf = 3 : 1 (ZHO), exhibiting a high remanent polarization of 30 μC/cm2, was prepared and further used to build Pt/ZHO/Al2O3/W FTJ devices with adding 1 nm-thick Al2O3 dielectric layer to reduce the leakage. The FTJ delivered superior performance with a tunneling electroresistance ratio of over 7000, outperforming previously reported other FTJ devices based on hafnia/zirconia ferroelectrics. Under 100 ns single-pulse writing, the FTJ exhibited multiple stable states, good retention over 104 s, and switching endurance exceeding 5 × 104 cycles. Additionally, it delivered a relatively high read current density of 8 A/cm2 at 0.2 V. The results demonstrate that the ZHO/Al2O3 composite structure can effectively alter the tunneling barrier height and increase tunneling current, resulting in a large ON/OFF ratio. The results underscore a great potential of ZHO ferroelectrics in the future development of high-performance nonvolatile memory technologies.

SF ₆ plasma etching was developed as a contact surface pre-treatment to achieve efficient n-doping in multilayer WSe ₂ nanosheets and build lateral homojunctions exhibiting superb photovoltaic properties and self-powered wide-spectrum photodetection.