Ferroelectric HfO2-based synaptic devices: recent trends and prospects

The polarization reversal in ferroelectric HfO 2 is adopted to store information in three distinct device classes – ferroelectric field effect transistors (FeFET), ferroelectric capacitors (FeCAP) and ferroelectric tunnel junctions (FTJ). Common to all three concepts is the adoption of a ferroelectric layer stack that acts either as gate dielectric in the FeFET or as the capacitor dielectric and tunneling barrier in the FeCAP or FTJ, respectively. A composite structure including an inevitably or purposefully formed dielectric layer is frequently adopted. In this work we report on the co-existence of all three memory concepts within one device structure and propose a 2T1C ferroelectric memory cell that allows the operation and comparative characterization of the trinity of ferroelectric memory devices.

Abstract

Silicon technology has advanced through the past four decades at exponential rate both in performance and productivity. Along with the miniaturization, the power demand grew also exponentially. New technologies are studied in order to develop switches that commute faster and, in the near future, we might see an increasing number of hybrid approaches to overcome the limitations CMOS performance. The ferroelectric thin layers have been studied for memory applications, one transistor memory cell, and recently ferroelectrics have been proposed by Salahuddin and Datta as dielectric materials in transistors in order to overcome the 60mV/dec limit of the subthreshold swing (SS) in silicon Metal Ferroelectric Semiconductor Field Effect Transistors, Fe-FET. The objective of this thesis is to study the negative capacitance effect of the ferroelectrics that are integrated into the gate stack of a MOS transistor. For this purpose different Ferroelectric Field Effect Transistors (Fe-FETs) have been designed, fabricated and characterized. Due to its remarkable characteristics, like low temperature deposition, medium-k and low leakage current, vinylidene fluoride trifluorethylene, P(VDF-TrFE), an organic ferroelectric copolymer, of 100 nm and 160 nm thickness was integrated into several MOS devices. Three devices were fabricated, a metal-ferroelectric-oxide-semiconductor transistor, a metal-ferroelectric-metal-oxide-metal capacitor and a metal-ferroelectric-metal-oxide-semiconductor transistor. The internal metal was introduced in-between the two dielectrics so that the potential at that point could be probed. For the first time, subthreshold slopes as low as 48mV/dec were obtained over more than three decades of drain current. The polarization was accurately extracted and the S-shape polarization and the negative slope could be observed. The polarization presented both hysteresis and negative slope polarization. A first-order model, that can accurately characterize a Fe-FET without the negative capacitance effect, was developed and validated with the measurements. Furthermore, a model for predicting the subthreshold slope improvement in case of the negative capacitance transistor, based on the Landau's ferroelectric parameters, was presented. Among others, two possible applications of the ferroelectric transistor are highlighted, one a temperature sensor using the metal-ferroelectric-metal-oxide-semiconductor device and the other one is a Schmitt trigger based on the metal-ferroelectric-oxide-semiconductor transistor.

Due to the voltage driven switching at low voltages combined with nonvolatility of the achieved polarization state, ferroelectric materials have a unique potential for low power nonvolatile electronic devices. The competitivity of such devices is hindered by compatibility issues of well-known ferroelectrics with established semiconductor technology. The discovery of ferroelectricity in hafnium oxide changed this situation. The natural application of nonvolatile devices is as a memory cell. Nonvolatile memory devices also built the basis for other applications like in-memory or neuromorphic computing. Three different basic ferroelectric devices can be constructed: ferroelectric capacitors, ferroelectric field effect transistors and ferroelectric tunneling junctions. In this article first the material science of the ferroelectricity in hafnium oxide will be summarized with a special focus on tailoring the switching characteristics towards different applications.The current status of nonvolatile ferroelectric memories then lays the ground for looking into applications like in-memory computing. Finally, a special focus will be given to showcase how the basic building blocks of spiking neural networks, the neuron and the synapse, can be realized and how they can be combined to realize neuromorphic computing systems. A summary, comparison with other technologies like resistive switching devices and an outlook completes the paper.

Since the discovery of CMOS-compatible and highly scalable ferroelectric HfO2, there has been a significant revival of interest in developing ferroelectric devices for high performance and energy-efficient embedded nonvolatile memories. Multiple ferroelectric memory devices are under investigation by harnessing the nonvolatile polarization states. These devices include the ferroelectric FET (FeFET), ferroelectric capacitor based random access memory (FeRAM), and ferroelectric tunnel junction (FTJ). Though the underlying memory storage mechanisms are the same in these devices, their memory sensing mechanisms are different. This difference leads to fundamentally different, and even opposite ferroelectric optimization directions. Given their different characteristics and individual advantages, it is likely that all these devices will co-exist to meet varying needs. Therefore, it is important to establish and compare the design guidelines for the three different ferroelectric memory devices. The design optimization will also be constrained by the reliability limit, which is critical to guarantee the success of the ferroelectric memory devices.

Filamentary and interface switching of CMOS-compatible Ta2O5 memristor for non-volatile memory and synaptic devices

The discovery of the ferroelectricity of HfO<sub>2</sub> in 2011 has opened up new avenues for the application of ferroelectric technology. With the stability of ferroelectricity in a few nm scales, HfO<sub>2</sub> has become a valuable material for the development of next-generation electronic memory devices. The unique structure of HfO<sub>2</sub> gives rise to various ferroelectric properties and behaviors that can be utilized in different types of devices such as ferroelectric field effect transistors (FeFETs), negative capacitance field effect transistors (NCFETs), ferroelectric tunnel junctions (FTJs), and ferroelectric capacitors (FeCAPs). In this review, we explore the potential of HfO<sub>2</sub> for the high-density storage of data and low-energy consumption in next-generation devices. We demonstrate the operating principles and strengths of HfO<sub>2</sub> in various device applications, shedding light on how this material can help address the electronics industry's current challenges.

Advanced non-volatile memory concepts such as the 1T1C ferroelectric (FE) random-access memory (FeRAM) and the 1T1C FE field-effect transistor (FeFET) can be realized by connecting a metal-ferroelectric-metal (MFM) capacitor placed in the back end of line (BEoL) of a microchip to the drain and gate contacts of a standard logic device, respectively. With the vertical distributed select devices in the front-end of line (FEoL) and the storage elements in the BEoL, both concepts increase the effective memory density of a microchip without introducing major changes in the FEoL fabrication technology. However, for advanced neuromorphic computing architectures, the 1T1C FeFET is the device of choice, since it provides non-destructive readout. The most promising material for the integration of FE non-volatile memory functionalities into the BEoL is Zr doped HfO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (HZO). It crystallizes at low temperatures in the orthorhombic phase (the one with FE properties) and with a polycrystalline structure. The latter is important to enable analogue like switching in synaptic devices. Herein, the above-mentioned memory concepts are introduced and key steps to optimize the HZO films for the BEoL integration and for the neuromorphic computing use case are described.

Ferroelectricity exhibits two stable states with permanent polarizations, making it promising for nonvolatile memory applications. Compared to conventional perovskite ferroelectrics, hafnium-based ferroelectric materials, such as Hf0.5Zr0.5O2 (HZO), attracts much attention due to their compatibility with CMOS technology [1]. Furthermore, with the high demand in high-performance computing and the emerging quantum computing, cryogenic memory technologies become more important due to the required much lower operational temperatures. In this work, we investigate the properties of HZO-based ferroelectric capacitors and characterize ferroelectric field-effect transistors (FeFETs) from 300 K to 4 K.A 10-nm HZO layer was deposited by atomic layer deposition (ALD) to serve as the ferroelectric layer in both capacitors and FeFETs (Fig. 1). For the ferroelectric capacitors, 75-nm TiN was deposited by sputtering for the bottom and top electrodes, which provides tensile stress to the HZO layer and facilitates the formation of the ferroelectric phase [2]. Similarly, in FeFETs, a 75-nm TiN is deposited as the gate metal. Subsequent rapid thermal annealing (RTA) steps were performed at 650 °C for 30 seconds in forming gas to form the ferroelectric orthorhombic phase. For the FeFETs, different gate stacks were deposited: a 10-nm HZO layer (MFS structure) or a combination of a 10-nm HZO layer and a 1-nm Al2O3 interlayer (IL) between the HZO and the Si substrate to improve the interface quality (MFIS structure).For ferroelectric capacitors, the polarization vs. voltage (P-V) results are measured at temperatures from 300 K to 4 K (Fig. 2(a)). The extracted remnant polarization (2Pr) and coercive voltage (2Vc) are shown in Fig. 2(b). 2Pr value decreases with a reducing temperature due to the reduced mobility of oxygen vacancies, leading to weaker polarizations in cryogenic environments [3]. 2Vc increases with a decreasing temperature, which means that a higher voltage is required to switch the polarization direction at lower temperatures [4]; that is, polarization switching becomes more challenging.MFS and MFIS FeFETs were also characterized at 300 K to 4 K. The Id-Vg results with various gate sweeping ranges of MFS and MFIS FeFETs at 300 K are shown in Fig. 3. We observed a competition between ferroelectric polarization switching (counter-clockwise Id-Vg hysteresis loops) and charge trapping effects (clockwise Id-Vg hysteresis loops). To further analysis, we define the required gate bias with zero hysteresis (Vsweep at ΔVth = 0) as the voltage where ferroelectricity starts to dominate. For MFIS FeFETs, the required sweep bias is larger than that of MFS FeFETs, which suggests that a higher applied voltage is needed to switch the dipoles in MFIS FeFETs. This is because a portion of the applied voltage drops across the IL, thereby decreasing the electric field in the ferroelectric layer. Furthermore, there’s an internal depolarization field induced by Al2O3 IL, which opposes the polarization in the ferroelectric layer and reduces its polarization [5]. As the scan range of gate bias becomes larger, MFS FeFETs exhibit stronger ferroelectricity (counter-clockwise hysteresis with a negative ΔVth) (Fig. 4(a)). As the temperature decreases, the required scan range of gate biases becomes much larger, which makes the HZO films more difficult to convert to ferroelectric phases. In contrast, MFIS FeFET shows counter-clockwise hysteresis only at 300 K (Fig. 4(b)), below which charge trapping effects dominate over polarization switching even under a sweep voltages up to 5 V. This trend is consistent with the results in ferroelectric capacitors, where higher voltages are required for polarization switching at lower temperatures. Acknowledgements This work was supported by the Powerchip Semiconductor Manufacturing Corporation (PSMC) of Taiwan (grant No: 113H1004-C15), the National Science and Technology Council (NSTC 113-2119-M-002-022-), and the Center of Data Intelligence: Technologies, Applications, and Systems, National Taiwan University (grant No: 113L900903). References Müller et al., IEDM, 10.8 (2013).Si Joon Kim et al., Appl. Phys. Lett. 111, 242901 (2017).Starschich et al., Appl. Phys. Lett. 108, 032903 (2016).Stephen Ducharme et al., Phys. Rev. Lett. 84, 175 (2000).Patrick D. Lomenzo et al., Non-Volatile Memory Technol. Symp., 1 (2019). Figure 1

Memory devices are an essential part of modern electronics. Efforts to move beyond the traditional “read” and “write” of digital information in volatile and non‐volatile memory devices are leading to the rapid growth of neuromorphic technology. There is a growing demand for memory devices with continuous memory states with various retention times and greater integration density with more energy‐efficient mechanisms. Two types of memory devices (i.e., non‐volatile digital memory and neuro‐synaptic devices) have been extensively investigated with emerging materials. Among numerous materials for such memory devices, in this review, the authors focus on 2D ferroelectric materials for promising memory and synaptic devices. Three types of memory devices based on 2D ferroelectric materials are classified and discussed here: 1) ferroelectric gating of semiconducting channels, 2) active ferroelectric channels, and 3) ferroelectric tunnel junction devices. It is known that atomically thin geometry competes with ferroelectricity, which can degrade the quality of the devices based on atomically thin ferroelectric materials. Various efforts to resolve the fundamental issue with emerging 2D ferroelectric materials and how they can be used as a critical element for memory and synaptic devices are surveyed.

Synaptic devices based neuromorphic computing applications in artificial intelligence

The recent discovery of ferroelectricity in pure ZrO2 has drawn much attention, but the information storage and processing performances of ferroelectric ZrO2-based nonvolatile devices remain open for further exploration. Here, a ZrO2 (∼8 nm)-based ferroelectric capacitor using RuO2 oxide electrodes is fabricated, and the ferroelectric orthorhombic phase evolution under electric field cycling is studied. A ferroelectric remnant polarization (2Pr) of >30 μC/cm2, leakage current density of ∼2.79 × 10-8 A/cm2 at 1 MV/cm, and estimated polarization retention of >10 years are achieved. When the ferroelectric capacitor is connected with a transistor, a memory window of ∼0.8 V and eight distinct states can be obtained in such a ferroelectric field-effect transistor (FeFET). Through the conductance manipulation of the FeFET, a high object image recognition accuracy of ∼93.32% is achieved on the basis of the CIFAR-10 dataset in the convolutional neural network (CNN) simulation, which is close to the result of ∼94.20% obtained by floating-point-based CNN software. These results demonstrate the potential of ferroelectric ZrO2 devices for nonvolatile memory and artificial neural network computing.

Electroluminescent synaptic devices with logic functions

9 - Novel photoelectroactive memories and neuromorphic devices based on nanomaterials

Neuromorphic computing systems are based on new architecture inspired by biological learning behavior and biological structure to overcome the shortcomings of the von Neumann system, such as high energy consumption and generation of excessive redundant data. However, common synaptic‐device designs are still need to carry an external power source, which is detrimental to both the miniaturization of smart devices and the further reduction of energy consumption. Therefore, self‐powered optoelectronic synapses hold great promise for achieving energy‐efficient artificial intelligence applications. Given the advantages of heterogeneous integration and ferroelectric performance, the 2D ferroelectric In2Se3 has become a typical candidate material for biological synaptic devices and neuromorphic computing. Although, various structures of ferroelectric synaptic devices are developed, most of which are controlled by optical or electrical pulses, hardly any reports discuss the operation in self‐powered mode. In this work, an In2Se3/MoS2 ferroelectric lateral heterojunction‐type optoelectronic synaptic device is fabricated to mimic biological synaptic functions in voltage mode or self‐driven mode. In addition, two simplified image pre‐processing methods based on device arrays are shown to improve the recognition accuracy of a back‐end neural network. The proposed approach suggests a new route for designing and applying high‐performance synaptic devices.