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The discovery of ferroelectricity in HfO2-based materials in 2011 provided new research directions and opportunities. In particular, for atomic layer deposited Hf0.5Zr0.5O2 (HZO) films, it is possible to obtain homogenous thin films with satisfactory ferroelectric properties at a low thermal budget process. Based on experiment demonstrations over the past 10 years, it is well known that HZO films show excellent ferroelectricity when sandwiched between TiN top and bottom electrodes. This work reports a comprehensive study on the effect of TiN top and bottom electrodes on the ferroelectric properties of HZO thin films (10 nm). Investigations showed that during HZO crystallization, the TiN bottom electrode promoted ferroelectric phase formation (by oxygen scavenging) and the TiN top electrode inhibited non-ferroelectric phase formation (by stress-induced crystallization). In addition, it was confirmed that the TiN top and bottom electrodes acted as a barrier layer to hydrogen diffusion into the HZO thin film during annealing in a hydrogen-containing atmosphere. These features make the TiN electrodes a useful strategy for improving and preserving the ferroelectric properties of HZO thin films for next-generation memory applications.

The chemical, physical, and electrical properties of the atomic layer deposited Hf0.5Zr0.5O2 thin films using tetrakis(ethylmethylamino) (TEMA) and tetrakis(dimethylamino) (TDMA) precursors are compared. The ligand of the metal-organic precursors strongly affects the residual C concentration, grain size, and the resulting ferroelectric properties. Depositing Hf0.5Zr0.5O2 films with the TDMA precursors results in lower C concentration and slightly larger grain size. These findings are beneficial to grow more ferroelectric-phase-dominant film, which mitigates its wake-up effect. From the wake-up test of the TDMA-Hf0.5Zr0.5O2 film with a 2.8 MV/cm cycling field, the adverse wake-up effect was well suppressed up to 105 cycles, with a reasonably high double remanent polarization value of ~40 μC/cm2. The film also showed reliable switching up to 109 cycles with the 2.5 MV/cm cycling field without involving the wake-up effect but with the typical fatigue behavior.

Conventional resistive crossbar array for in‐memory computing suffers from high static current/power, serious IR drop, and sneak paths. In contrast, the “capacitive” crossbar array that harnesses transient current and charge transfer is gaining attention as it 1) only consumes dynamic power, 2) has no DC sneak paths and avoids severe IR drop (thus, selector‐free), and 3) can be fabricated on top of complementary metal–oxide–semiconductor (CMOS) circuits for 3D‐stacking. For the first time, ferroelectric Hf0.5Zr0.5O2 (HZO) capacitive crossbar arrays are experimentally demonstrated. Asymmetry of the HZO electrode interfaces leads to small‐signal capacitance on/off ratio >110% that can achieve read‐disturb‐free operation. The vector matrix multiplication (VMM) experiments are conducted on the fabricated capacitive crossbar array, showing a linear weighted sum versus numbers of input or on‐state weight. The array‐level VMM operation could maintain weight pattern reprogramming after 1) thousands of 1 ms/3 V pulses and 2) an extrapolated 10‐year retention at 85 °C. Array‐level circuit simulation at 22 nm node shows the energy consumption of a capacitive crossbar array is 20–200× lower than the resistive crossbar array counterpart. Moreover, analog‐shift‐and‐add circuits are designed for multibit weight summation, achieving 16.6% less area and 26.9% lower energy consumption than digital‐shift‐and‐add circuits. Proposed capacitive crossbar array based on ferroelectric HZO layer consumes low power and avoids serious IR drop across wires due to transient current and charge transfer mechanism with the nature of the capacitor structure. Moreover, 3D‐stacking with selector‐free scheme is possible in the capacitive crossbar array. Finally, it is free from read‐disturb as readout occurs at DC 0 V.

Recently, HfO2‐based ferroelectric thin films have attracted widespread interest in developing next‐generation nonvolatile memories. To form a metastable ferroelectric orthorhombic phase in HfO2, a post‐annealing process is typically necessary. However, the microscopic mechanism underlying the effect of annealing temperature on ferroelectric domain nucleation and growth is still obscure, despite its importance in optimizing the operation speed of HfO2‐based devices. In this study, the ferroelectric properties and polarization switching of Hf0.5Zr0.5O2 thin films annealed at different temperatures (550–700 °C) are systematically investigated. Evidently, the crystal structure, remnant polarization, and dielectric constant monotonically change with annealing temperature. However, microscopic piezoresponse force microscopy images as well as macroscopic switching current measurements reveal non‐monotonic changes in the polarization switching speed with annealing temperature. This intriguing behavior is ascribed to the difference in the ferroelectric‐domain nucleation process induced by the amount of oxygen vacancies in the Hf0.5Zr0.5O2 thin films annealed at different temperatures. This work demonstrates that controlling the defect concentration of ferroelectric HfO2 by tuning the post‐annealing process is critical for optimizing device performance, particularly polarization switching speed. The ferroelectric properties and polarization switching of Hf0.5Zr0.5O2 thin films annealed at various temperatures are systematically investigated. Piezoresponse force microscopy, as well as switching current measurements, revealed non‐monotonic changes in the polarization switching speed concerning the annealing temperature. These variations are attributed to differences in the ferroelectric‐domain nucleation process induced by different defect levels in the films.

Ferroelectric Hf0.5Zr0.5O2 (HZO) thin films have attracted wide attention in terms of potential applications of nonvolatile ferroelectric memories. However, the effect of strain on the resistance switching characteristics of the ferroelectric HZO thin‐film memristors has not been fully studied so far. In this work, the strain effects on the HZO thin‐film memristors are investigated. HZO films with different resistance properties are also prepared by controlling the value of oxygen pressure. Based on the testing results, it is proposed that the resistance switching behavior of HZO films may be caused by the joint participation between ferroelectricity and oxygen vacancy migration. The study also found that HZO films can successfully simulate learning behavior similar to the human brain. The applied pulses with a width of tens of nanoseconds timescale are beneficial to realize fast learning and computing. These results provide a fundamental and deep insight on HZO‐based ferroelectric semiconductor oxide thin‐film memristors and their potential applications in next‐generation artificial electronic synaptic devices. In this article, the strain effects on the Hf0.5Zr0.5O2(HZO) thin‐film memristors were investigated. Interestingly, it is proposed that the resistance switching behavior of HZO films may be caused by the joint participation between ferroelectricity and oxygen vacancy migration. The study also found that HZO films can successfully simulate learning behavior similar to the human brain.

Highlights Integration of perovskite BaTiO 3 with epitaxial fluorite Hf 0.5 Zr 0.5 O 2 is demonstrated. Polarization up to 15 μC cm −2 , leakage current densities below 10 –6 A cm −2 , endurance up to 10 8 cycles, and switching times shorter than 50 ns are achieved. Remote optical switching of the polarization is demonstrated, and it is shown to be controlled by the thickness of the BaTiO 3 capping layer. Optical switching of ferroelectric polarization is of interest for wireless and energy-efficient control of logic states. So far, this phenomenon has been widely demonstrated only in ferroelectric perovskites, while studies on other emerging ferroelectrics remain limited. In this regard, the paradigmatic example of a technologically relevant ferroelectric material is HfO 2 . However, HfO 2 has a very wide bandgap, limiting light absorption. So far, the proposed strategies to enhance light absorption in HfO 2 -based systems are detrimental to ferroelectric properties, i.e., bandgap lowering or on-purpose defect introduction, which reduce switchable polarization and increase the presence of leakage currents. Here, we show that good ferroelectric properties, i.e., sizeable polarization (up to 15 μC cm −2 ), low leakage current (under 10 –6 A cm −2 ), high endurance (up to 10 8 cycles) and fast switching (< 50 ns), can be achieved in epitaxial Hf 0.5 Zr 0.5 O 2 films through an alternative strategy, BaTiO 3 capping. While ferroelectric properties are remarkable, we demonstrate that the presence of BaTiO 3 allows light absorption and the concomitant electric field generation, as supported by density functional theory calculations, which enables optical switching of polarization in Hf 0.5 Zr 0.5 O 2 under 405 nm illumination. It is observed that optical switching is more efficient in films with thicker BaTiO 3 capping layer. The high polarizability of BaTiO 3 contributes to minimizing degradation in the ferroelectric response of the system. The results presented here indicate that appropriate designs can be followed to obtain optical switching of polarization in ferroelectric HfO 2 while preserving main functional properties.

Since the first report of ferroelectric HfO2 in 2011, researchers are making rapid progress in the understanding of both material properties and applications. Due to its compatibility with complementary metal oxide semiconductor, high coercivity voltage and the fact that ultrathin films remain ferroelectric, it is developed for applications in non‐volatile memories for data storage in different polarization states. As the most representative hafnium‐based ferroelectric materials, Hf0.5Zr0.5O2 has received a great deal of attention due to its various of outstanding properties. Magnetron sputtering is a promising method for the preparation of ferroelectric HfO2 films. This paper reviews recent developments in preparing Hf0.5Zr0.5O2 ferroelectric films and memories. Meanwhile, due to the many advantages of sputtering, such as higher throughputs, low cost and no carbon contamination, this review mainly focused on the preparation of Hf0.5Zr0.5O2 ferroelectric thin films by sputtering and explored its working mechanism and optimization strategy. In addition, the factors affecting the reliability of the memories, the mechanism of action, the solution ideas are introduced. These provide the basis for the design and optimization of Hf0.5Zr0.5O2 ferroelectric films and memories. This review focuses on the device structure and working principle of hafnium oxide based ferroelectric memories, the preparation strategy of hafnium oxide based thin films and the method of optimizing ferroelectric memories performance. The effect of magnetron sputtering on ferroelectric properties of Hf0.5Zr0.5O2 thin films and its optimization strategy are introduced.

In this study, the double remnant polarization (2Pr) is enhanced from ≈2 to 25 µC cm−2 at a low applied voltage of ±2 V (or from 10 to 35 µC cm−2 at a voltage of ±4 V) by decreasing the WNx interfacial capping layer (ICL) thickness from 6 to 2 nm in a novel Ru/WNx ICL/Hf0.5Zr0.5O2(HZO)/TiN structure after annealing at 400 °C in a furnace. This occurs because of the higher orthorhombic (o) plus rhombohedral (r) phases (>70%), which is analyzed by geometrical phase analysis (GPA) of high‐resolution transmission electron microscope (HRTEM) images. An optimized 2 nm WNx ICL memory capacitor shows a low coercive field (Ec) of 1.27 MV cm−1 and long endurance of > 109 cycles (remaining 2Pr value of 13.5 µC cm−2) under a low field stress of ±2 MV cm−1 and 0.1 µs hold pulse width (or ≈1.67 MHz). Even this long endurance of > 109 cycles is obtained by applying a higher stress of ±2 MV cm−1, 1 MHz, or 100 kHz. Under ±3 MV cm−1 stress, the mechanism is caused by m‐phase growth from both the HZO/TiN bottom electrode (BE) and WNx ICL/HZO interfaces, which is evidenced by HRTEM images after 2 × 107 cycles for the first time. An optimized 2 nm WNx ICL at the Ru/HfxZr1‐xO2 interface after low 400 °C in a furnace shows high polarization of 25 µC cm−2 and endurance of >109 cycles under low field. A fatigue mechanism, owing to the m‐phase that starts to grow from both the bottom and top interfaces, is evidenced through high‐resolution transmission electron microscope after the endurance cycles.

In this work, extremely thin silicon-on-insulator field effective transistors (ETSOI FETs) are fabricated with an ultra-thin 3 nm ferroelectric (FE) hafnium zirconium oxides (Hf0.5Zr0.5O2) layer. Furthermore, the subthreshold characteristics of the devices with double gate modulation are investigated extensively. Contributing to the advantages of the back-gate voltage coupling effects, the minimum subthreshold swing (SS) value of a 40 nm ETSOI device could be adjusted from the initial 80.8–50 mV/dec, which shows ultra-steep SS characteristics. To illustrate this electrical character, a simple analytical model based on the transient Miller model is demonstrated. This work shows the feasibility of FE ETSOI FET for ultra-low-power applications with dynamic threshold adjustment.