Wei Zhang

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.

Atomic-scale two-dimensional (2D) ferroelectrics are garnering immense research interest for powering next-generation high-performance nanoelectronics. Herein, we report the discovery of robust room-temperature out-of-plane ferroelectricity in thin 2D β-Bi2SeO5 films, with the ferroelectric response persisting even down to the monolayer limit. The ferroelectricity of a 10 nm thick β-Bi2SeO5 thin film was also unambiguously revealed by ferroelectric analysis using a ferroelectric analyzer, achieving an out-of-plane polarization of 10 μC/cm2. Furthermore, a pronounced in-plane ferroelectricity and photoelectric effect were also observed, along with a strong coupling between them. First-principles density functional theory calculations identify the displacement and distortion of [SeO3] tetrahedra under external electric fields as the primary origin of the observed out-of-plane and in-plane ferroelectric polarization in β-Bi2SeO5 films. These findings open up new possibilities for using β-Bi2SeO5 thin films in future nanoscale electronic and quantum devices.

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.

Atomic-scale two-dimensional (2D) ferroelectrics are of great interest for advancing next-generation high-performance electronics. Here, we report the discovery of room-temperature (RT) ferroelectricity in ultrathin 2D Bi₂O₂Se (BOS) films grown by molecular beam epitaxy. These films exhibit degenerate semiconducting characteristics through a unique self-modulated doping mechanism featuring in-plane electron confinement and a high RT mobility of 311 cm² V^-1 s^-1. Remarkably, robust out-of-plane ferroelectricity persists down to the single-unit-cell limit. Through atomic-scale characterization, we reveal that the ferroelectric polarization stems from electric-field-modulated dipole ordering associated with Se atom displacements. This study confirms the ferroelectricity of ultrathin BOS films, thereby opening new avenues for the development of innovative nanoscale devices and the engineering of quantum materials.

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.

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.

Recently discovered ferroelectricity in fluorite-structure ZrO₂ thin film has attracted increasing and intense interest due to its lower crystallization temperature and higher content in nature in comparison to hafnium oxide. Here, the effect of HfO₂ interfacial layer on the ferroelectric properties of ZrO₂ thin films is investigated systematically by designing four types of interfacial structures. It is revealed that the ferroelectric orthorhombic phase, remanent polarization, and endurance can be improved in ZrO₂ thin film by inserting both a top- and bottom-HfO₂ interfacial layer. A maximal ferroelectric remanent polarization (2P _r) of 53.4 μC cm⁻² and an optimal endurance performance of 3 × 10⁷ field cycles under frequency of 100 kHz are achieved in Pt/HfO₂/ZrO₂/HfO₂/Pt capacitors, with ferroelectric stacks being crystallized at 450 °C via post-deposition annealing method. X-ray photoelectron spectroscopy analysis confirms that the HfO₂ bottom-layer plays a very important role in the formation of a higher ratio o-phase, thus enhancing the ferroelectricity. These results suggest that designing appropriate interfaces would help achieve excellent ferroelectric properties in ZrO₂ films.