Physics, Structures, and Applications of Fluorite-Structured Ferroelectric Tunnel Junctions.

The interest in ferroelectric tunnel junctions (FTJ) has been revitalized by the discovery of ferroelectricity in fluorite-structured oxides such as HfO2 and ZrO2 . In terms of thickness scaling, CMOS compatibility, and 3D integration, these fluorite-structured FTJs provide a number of benefits over conventional perovskite-based FTJs. Here, recent developments involving all FTJ devices with fluorite structures are examined. The transport mechanism of fluorite-structured FTJs is explored and contrasted with perovskite-based FTJs and other 2-terminal resistive switching devices starting with the operation principle and essential parameters of the tunneling electroresistance effect. The applications of FTJs, such as neuromorphic devices, logic-in-memory, and physically unclonable function, are then discussed, along with several structural approaches to fluorite-structure FTJs. Finally, the materials and device integration difficulties related to fluorite-structure FTJ devices are reviewed. The purpose of this review is to outline the theories, physics, fabrication processes, applications, and current difficulties associated with fluorite-structure FTJs while also describing potential future possibilities for optimization.

Monolithically Integrated Complementary Ferroelectric FET XNOR Synapse for the Binary Neural Network.

Neuromorphic computing, which mimics the structure and principles of the human brain, has the potential to facilitate the hardware implementation of next-generation artificial intelligence systems and process large amounts of data with very low power consumption. Among them, the XNOR synapse-based Binary Neural Network (BNN) has been attracting attention due to its compact neural network parameter size and low hardware cost. The previous XNOR synapse has drawbacks, such as a trade-off between cell density and accuracy. In this work, we show nonvolatile XNOR synapses with high density and accuracy using a monolithically stacked complementary ferroelectric field-effect transistor (C-FeFET) composed of a p-type Si MFMIS-FeFET at the bottom and a 3D stackable n-type Al:IZTO MFS-FeTFT, achieving 60F2 per cell (2C-FeFET). For adjusting the threshold voltage and improving the switching speed (100 ns) of n-type ferroelectric TFT, we employed a dual-gate configuration and a unique operation scheme, making it comparable to those of Si-based FeFETs. We performed array-level simulation with a 512 × 512 subarray size and a 3-bit flash ADC, demonstrating that the image recognition accuracies using the MNIST and CIFAR-10 data sets were increased by 3.17 and 14.07%, respectively, in comparison to other nonvolatile XNOR synapses. In addition, we performed system-level analysis on a 512 × 512 XNOR C-FeFET, exhibiting an outstanding throughput of 717.37 GOPS and an energy efficiency of 196.7 TOPS/W. We expect that our approach would contribute to the high-density memory systems, logic-in-memory technology, and hardware implementation of neural networks.

Flexible Ferroelectric Hafnia-Based Synaptic Transistor by Focused-Microwave Annealing.

Hafnia-based ferroelectric memory devices with excellent ferroelectricity, low power consumption, and fast operation speed have attracted considerable interest with the ever-growing desire for nonvolatile memory in flexible electronics. However, hafnia films are required to perform a high temperature (>500 °C) annealing process for crystallization into the ferroelectric orthorhombic phase. It can hinder the integration of hafnia ferroelectric films on flexible substrates including plastic and polymer, which are not endurable at high temperatures above 300 °C. Here, we propose the extremely low-temperature (∼250 °C) process for crystallization of Hf0.5Zr0.5O2 (HZO) thin films by applying a focused-microwave induced annealing method. HZO thin films on a flexible mica substrate exhibits robust remnant polarization (2Pr ∼ 50 µC/cm2), which is negligibly changed under bending tests. In addition, the electrical characteristics of a HZO capacitor on the mica substrate were evaluated, and ferroelectric thin film transistors (Fe-TFTs), using a HZO gate insulator, were fabricated on mica substrates for flexible synapse applications. Symmetric potentiation and depression characteristics are successfully demonstrated in the Fe-TFT memory devices, and the synaptic devices result in high recognition accuracy of 91.44%. The low-temperature annealing method used in this work are promising for forming hafnia-based Fe-TFT memory devices as a building block on a flexible platform.

Selector-less Ferroelectric Tunnel Junctions by Stress Engineering and an Imprinting Effect for High-Density Cross-Point Synapse Arrays.

In the quest for highly scalable and three-dimensional (3D) stackable memory components, ferroelectric tunnel junction (FTJ) crossbar architectures are promising technologies for nonvolatile logic and neuromorphic computing. Most FTJs, however, require additional nonlinear devices to suppress sneak-path current, limiting large-scale arrays in practical applications. Moreover, the giant tunneling electroresistance (TER) remains challenging due to their inherent weak polarization. Here, we present that the employment of a diffusion barrier layer as well as a bottom metal electrode having a significantly low thermal expansion coefficient has been identified as an important way to enhance the strain, stabilize the ferroelectricity, and manage the leakage current in ultrathin hafnia film, achieving a high TER of 100, negligible resistance changes even up to 108 cycles, and a high switching speed of a few tens of nanoseconds. Also, we demonstrate that the usage of an imprinting effect in a ferroelectric capacitor induced by an ionized oxygen vacancy near the electrode results in highly asymmetric current-voltage characteristics with a rectifying ratio of 1000. Notably, the proposed FTJ exhibits a high density array size (>4k) with a securing read margin of 10%. These findings provide a guideline for the design of high-performance and selector-free FTJ devices for large-scale crossbar arrays in neuromorphic applications.

Excellent Reliability and High-Speed Antiferroelectric HfZrO2 Tunnel Junction by a High-Pressure Annealing Process and Built-In Bias Engineering.

Hafnia-based ferroelectric tunnel junctions (FTJs) have great potential for use in logic in nonvolatile memory because of their complementary metal-oxide-semiconductor process compatibility, low power consumption, high scalability, and nondestructive readout. However, typically, ferroelectrics have a depolarization field, resulting in poor endurance owing to the early dielectric breakdown. Herein, an outstandingly reliable and high-speed antiferroelectric HfZrO tunnel junction (AFTJ) is probed to understand whether it is a promising candidate for next-generation nonvolatile memory applications. High-reliability AFTJ can be explained by less charge injection due to the low depolarized field. The formation of two stable nonvolatile states, even with antiferroelectric materials, is possible if asymmetric work function electrodes and fixed oxide charges are employed, generating a built-in bias and shifting the polarization-voltage curve. In addition, via high-pressure annealing, a critical voltage that determines the transition from the t-phase to the o-phase is effectively reduced (22%). The AFTJ shows a higher endurance property (>109 cycles) and faster switching speed (<30 ns) than FTJ. Hence, it is proposed that with the help of internal bias modulation and high-pressure annealing, AFTJs can be employed in next-generation memory devices.