Ferroelectric Tunnel Junction Memory Research Articles

Monolithic three-dimensional hafnia-based artificial nerve system

Artificial nerve systems that mimic the tactile perception ability of human skin could be adopted in the realization of devices for a hyperconnected society, including prosthetic devices, interfaces for virtual reality, and smart sensors. The system consists of a pressure sensor, memory, and signal processing circuit, which possess the capability of generating sensory neuron-like signals. Although technological advances have been made in semiconductor-based memory and transistors over the decades, sensor technology is still relying on micro-electromechanical systems, which are hard to realize miniaturization/high-density and high-performance technology. Here, we demonstrate a monolithic three-dimensional integrated artificial nerve on the basis of ferroelectric HfxZr1-xO2 for use in the core element of neuromorphic memory and a self-powered piezoelectric pressure sensor. The morphotropic phase boundary (MPB) state of hafnia film was formed between orthorhombic and tetragonal phases for high piezoelectric performance. The MPB-based sensor was integrated with self-rectifying ferroelectric tunnel junction memory and silicon-based ring oscillator. We realized a frequency-modulated pulse-like synaptic signal with pressure detection ability over a wide range of 1–50 kPa and a good sensitivity of 0.35 mV/kPa. Also, we demonstrate the real-time frequency-modulated operation of the artificial nerve system by adjusting the functional properties of the hafnia films.

Effects of Process-Induced Defects on Polarization Switching in Ferroelectric Tunneling Junction Memory

The scaling effects of ferroelectric tunnel junction (FTJ) memory are investigated by evaluating remanent polarization ( ${P} _{\mathrm {r}}$ ), coercive field ( ${E} _{\mathrm {c}}$ ), and polarization switching speed. Through the analysis on dc tunneling currents and frequency responses of ${E} _{\mathrm {c}}$ and ${P} _{\mathrm {r}}$ for FTJs with various sizes, it is found that polarization switching mechanism (domain nucleation limit) is not changed by the width reduction. However, by the length scaling, domain wall spreading-limited switching becomes dominant by process-induced damaged edge regions, leading to the larger ${P}_{\mathrm {r}}$ and the faster switching speed.

Understanding tunneling electroresistance effect through potential profile in Pt/Hf0.5Zr0.5O2/TiN ferroelectric tunnel junction memory

Hafnia ferroelectric materials have gained prominence as promising materials for advanced memory applications due to their high scalability and full-complementary metal oxide semiconductor compatibility. In this paper, we present a comprehensive study on the electrical properties of Pt/Hf0.5Zr0.5O2/TiN asymmetric ferroelectric tunnel junction (FTJ) devices. The ferroelectric behavior of 4- and 5-nm Hf0.5Zr0.5O2 (HZO) thin films was confirmed by using piezoresponse force microscopy and conductive-atomic force microscopy. The typical current–voltage characteristics of the FTJ devices with two resistance states due to the tunneling electroresistance (TER) effect have been analyzed using a direct tunneling model based on the Wentzel-Kramers-Brillouin approximation. Further, we have proposed a method to extract the effective mass of the HZO thin film by numerical analysis using the MOS leakage current model. Finally, a dependence of the TER on the HZO thickness is analyzed to realize a high TER ratio.

Characterization and Modeling of Co/BaTiO3/SrRuO3 Ferroelectric Tunnel Junction Memory by Capacitance–Voltage (${C}$ –${V}$ ), Current–Voltage (${I}$ –${V}$ ), and High- Frequency Measurements

Ferroelectric tunnel junction (FTJ) is a promising low power nonvolatile memory for beyond-CMOS applications. In this paper, Co/BaTiO3/SrRuO3 FTJs are fabricated, simulated, and their electrical characteristics are measured by capacitance–voltage ( ${C}$ – ${V}$ ), current–voltage ( ${I}$ – ${V}$ ), and high-frequency S-parameter measurements. Ferroelectricity of the ultrathin BaTiO3 (~3.2 nm) film is first verified by a butterfly shaped hysteresis ${C}$ – ${V}$ profile, from which saturation polarizations are calculated in the ON state and OFF state as ${P}_{ \mathrm{ON}}=+ {20}\,\,\mu {C} / {\text{cm}}^{ {2}}$ and ${P}_{ \mathrm{OFF}}=- {25}\,\,\mu {C}/{\text{cm}}^{ {2}}$ , respectively. The numerical simulation of the FTJ device is performed using experimental electronic band parameters of the heterostructure. The device structure was optimized for ON/OFF ratio, ${I}_{ \mathrm{ON}} / {I}_{ \mathrm{OFF}}= {75}$ for a depolarization field, ${E}_{{dpol}} . The calculated tunnel currents of $ {250}\,\,{nm}\times {250}\,\,{nm}$ FTJ in ON state ( ${I}_{ \mathrm{ON}}= {250}\times {250}^{- {250}}\,\,{A}$ ) and OFF state ( ${I}_{{OFF}}={{1.75}\times {250}}^{{-9}}\,\,{A}$ ) match with experimental values. From the S-parameters of RF measurements, the high-frequency small-signal device model of the FTJ in ON state ( ${C}_{ \mathrm{ON}}= {250}\,\,{fF},{R}_{ \mathrm{ON}}={0.8}\,\,{M}\Omega$ ) and OFF state ( ${C}_{ \mathrm{OFF}}={ {250}}\,\,{fF},{R}_{ \mathrm{OFF}}= {250}\,\,{M}\Omega$ ) is developed for a frequency range from 50 MHz to 5 GHz. The extracted capacitor model of Co/BaTiO3/SrRuO3 FTJ is useful for high-frequency simulation of benchmark FTJ circuits.

(Invited) Technology Breakthrough by Ferroelectric HfO2 for Low Power Logic and Memory Applications

In today’s highly information-oriented society, more and more electronic devices are needed for high-end servers in cloud and sensor node devices in edge. Because of the power constraint, highly energy-efficient computing is required, which new transistor and memory technologies enable [1]. As a general guideline for energy-efficient computing, higher Ion/Ioff at lower supply voltage is obtained by using steep subthreshold slope (SS) transistor, and normally-off operation is performed without much overhead by using non-volatile memory. Those logic and memory technologies must be developed at low cost. Recent discovery of ferroelectric (FE) HfO2 [2] will realize low-cost and energy-efficient logic and memory devices. In this paper, the recent status of negative capacitance FET (NCFET), non-volatile SRAM (NV-SRAM), and FE tunnel junction memory (FTJ), all of which are based on FE-HfO2, will be overviewed. NCFET is the transistor which has ferroelectric gate insulator and uses negative capacitance to amplify surface potential. It has been regarded as one of the most promising steep SS transistors because of (1) high Ion/Ioff ratio with high Ion, (2) CMOS process compatibility, and (3) minimum circuit modification, with the use of FE-HfO2 at low cost. Device design guideline revealed that NCFET can operate at as low as 0.2V supply voltage [3]. Moreover, NCFET gate stack material can fit to advanced CMOS process with replacement gate FinFET [4] and Nanowire [5]. In recent years, there were encouraging experimental demonstrations of steep SS in long and short channel devices. Operation speed of NCFET may be limited by the dynamics of atomic displacement and domain dynamics [3,6], which is one of the central topics. NC effect is regarded as an additive performance booster and expected to synthesize with novel channel material and other steep SS transistors such as TFET [7]. IoT sensor node devices are expected to operate in an intermittent mode. The active rate of a transistor is much lower than clock frequency in IoT device. In order to suppress standby leakage power, especially in SRAM, it is important to bring the transistors in sleep mode as much as possible [1]. Normally-off computing is the idea that stores the current state data in NV memory and restore the data after wake-up without much latency and power overhead. NV-SRAM has been revisited by using FE-HfO2 which realizes good scalability as well as cost effective process [8]. NV-SRAM has been recently designed, fabricated by integrating FE-HfO2 capacitor in back-end process, and characterized. Store, power-off, and recall operation are successfully demonstrated. New AI algorithms such as deep learning have made big impacts in business, however, it requires enormous amount of computing power. In order to use such algorithm in edge devices, energy efficiency should be greatly improved and hardware implementation of such algorithm will be highly required. Recently, artificial neural network (ANN) has been realized by using synaptic device such as resistive change RAM (ReRAM) as nonvolatile analog memory. FE memory is also the alternative for synaptic device. FTJ is an emerging memory cell which utilizes polarization switching to change on-sate and off-state tunneling electroresistance (TER) . FTJ has been recently demonstrated by using FE-HfO2 with as high TER as >30 and multi-level cell operation [9]. Linearity and reliability should be further examined for ANN applications.

Enhanced Non-Linearity Factor in Ferroelectric Tunnel Junction Based on HfO2 Heterojunction

Ferroelectric polarization, related to charge conduction, has shown some intriguing properties in ferroelectric tunnel junction as it has higher on/off ratio, lower power consumption and shorter process time. Conventional ferroelectrics have various problems when it is based on perovskite structure material, such as poor Si-compatibility, lead pollution and small energy bandgap. Recently, both ferroelectricity and anti-ferroelectricity have been reported in doped HfO2 thin film (<30 nm). The ferroelectricity and anti-ferroelectricity HfO2 thin films can be extremely thin with good Si-compatibility and large bandgap (>5 eV). Anti-ferroelectric/ferroelectric HfO2 heterojunction can enhance fatigue property and non-linearity of single ferroelectric tunnel junction. The space charge can be induced due to the stress in ferroelectric layer or ferroelectric/metal interface in 90° domain switching while anti-ferroelectric lowers the stress in the 180° domain switching. The HfO2 heterojunction can have non-linearity as it has higher resistance state under the threshold voltage for polarization switching. This structure with good fatigue property and non-linearity factor would contribute to the enhancement of ferroelectric tunnel junction memory device. This research was supported by the MOTIE (Ministry of Trade, Industry & Energy (#10080643) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.

Ferroelectric Tunnel Junction Memory Devices made from Monolayers of Vinylidene Fluoride Oligomers

Conductance bistability in Pt/Si–vinylidene fluoride (VDF) oligomer–Au ferroelectric tunnel junction devices is demonstrated. I–V and C–V measurements reveal bistable conductance switching within biasing voltage of ±0.5V with stable memory retention up to 1 × 104 cycles. The memory element exhibits a low switching voltage of ±1.0 V, coincides with the coercive field of the ferroelectric VDF oligomer monolayer films in use.