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Ferroelectric materials are used in building non-volatile memories due to the existence of spontaneous electric polarization of which direction could be reversed by applying an electric field. This functionality represents a classic application of these materials as a binary system, but new challenges are brought by the evolution of nowadays computation needs (as increased speed or increased amount of stored data). In this way, the new paradigms of computing appeared and they address all classes of materials used in these technologies. 
In this study, we experimentally prove that multilayered ferroelectric structures present a different switching behavior than simple ferroelectric thin films. By using alternating layers of ferroelectric and isolator materials architectures we can have multiple step-like polarization switching. This behavior could be used to separately access more than 2 polarization states, which presents good time stability. In this way, we could obtain 2n different states, where n is the number of isolator layers. Experimentally, we obtained structures with 4 and 8 polarization states. Moreover, these different states could be differentiated by different capacitive values. The process of measuring the capacitance is based on applying a low voltage signal and does not determine a reversing/changing of polarization, being in this way a non-destructive reading method. The way the system could pass by different polarization states enables the algorithm to be performed for simple binary computation functions such as OR/NOR and AND/NAND. This proves that we can have data processing and storing the result on the same device, which represents a parallel computing system based on a ferroelectric memcapacitor. The current circuit architecture based on classic logic gates can be simplified and optimized using a multilayered ferroelectric capacitor, leading also to significantly reduced power consumption and reduced cost of manufacturing.


Materials Challenges for Memory

Ferroelectric based multilayered thin films structures for multi-bit memory and memcomputing

non-destructive reading;

multi-bit memory

memcomputing

memcapacitor

ferroelectric


![](https://assets.underline.io/uploads/markdown_image/1/image/2e10c515564e9ec3287ee4c664011c20.png)

The energy cost of storing and moving data is enormous, and is growing at such a rapid pace that within a few years the power demand may overwhelm some developed nations. Non-volatile memory (NVM) and neuromorphic computing are enabling critical advances in this area, but there are many scientific and technological challenges. This conference will cover the materials challenges of different memory technologies, from the fundamental processes and mechanisms, to industry perspectives on materials specifications and manufacturing.

poster

One of the most promising contenders for ultralow-energy electronic devices is resistive switching memory (RRAM). It delivers sustainably-scalable ‘neuromorphic’ computing, potentially capable of reducing energy consumption in IT by > 50% [1]. Currently, a poor understanding of the nanoscale kinetics of the switching mechanisms is preventing its commercial utilization. RRAMs are mostly studied by electron microscopy, which is destructive, invasive, and under drastically different conditions, so is not sufficient for finding true understandings [2]. 

We develop a novel non-destructive fully-optical technique to observe in-situ and in real-time switching in memristive materials under ambient conditions, otherwise invisible to conventional electron microscopy techniques [3-4]. The formation of conductive paths in a memristive material is captured within a tightly-confined plasmonic hotspot, which greatly enhances optical signals coming from a hotspot only [4]. This allows the optical measurements to be intimately linked to the electrical and material properties of the switching channel, hence accessing memristive dynamics directly. So far, we have characterized memristive switching in RRAMS based on SiO2 (electrochemical metallization mechanism (ECM)) [3] and SrTiO3 (valence change mechanism (VCM)) [4] .

For SiO2-based ECM cells, we consistently observed a change of the optical spectra only upon switching, which we correlated with SET and RESET cycles inside the memristive material in a plasmonic hotspot. This resulted in the first in-situ optical observations of the nanoscale kinetics of RRAMs under ambient conditions. Our findings indicated that the filament nucleation position can differ from cycle to cycle in the SiO2 based RRAMs [3].

The investiagtions of the SrTiO3 based RRAMs have demonstrated the method’s ability to track oxygen vacancies motion and identify the breaking mechanisms in valence change mechanism (VCM) cells. Our numerical framework couples each spectral resonance peak to the drift of oxygen vacancies and to the formation of an O2 reservoir at the STO/TiO2 interface. The reservoir might be the cause of the device breakdown upon cycling [4].

Our future plans involve investigating switching in h-BN and HfO2 based memristors.


References
[1] Miller, D. A. B., (2009) IEEE Proceedings, 97, 7, 1166.
[2] Tappertzhofen, S.; Di Martino, G.; Hofmann, S., (2019) Physica Status Solidi (a), 217, 6, 190058.
[3] Di Martino, G. et al, (2016) Small, 12, 10, 1334.
[4] Di Martino, G. et al, (2020) Nature Electronics, 3, 687.

Real-Time In-Situ Optical Tracking of Memrisitive Switching

HfOx-based resistive random access memories (RRAMs) have attracted much attention for future non-volatile memory (NVM) to realize a neuromorphic system. They demonstrated successful CMOS compatibility, good stability, and low power consumption. At room temperature and low temperature (77 K) or cryogenic temperature (4 K), HfOx-based RRAM exhibited their potential for low power memory devices. However, there are several problems to be solved to apply HfOx-based RRAM for low temperatures. For operating the devices at low temperature, a vacuum environment is needed to protect from contamination. But due to oxygen scarcity in the vacuum environment, reduction of oxygen vacancies in HfOx film can be disturbed, then redox-based HfOx RRAM characteristics are degraded. Thus, the buffer layer which plays a role as an oxygen reservoir is important in a vacuum environment.
In this work, the device performance dependence on the buffer layer thickness in a vacuum environment was studied. The titanium thin film was used as a buffer layer because of its oxygen affinity and its thickness was varied from 30 nm to 150 nm to examine the effect of oxygen gettering ability in a vacuum environment. As a result, devices show a decrement in switching voltages in a vacuum environment and thicker buffer layer, because the high concentration of oxygen vacancies makes leakier HfOx film which needs less energy for switching.


Influence of buffer layer thickness on the performance of HfOx-based RRAM at vacuum environment

In this paper, pentacene-based organic floating-gate memory characteristics utilizing N-doped LaB6 metal and LaBxNy insulating layer were investigated. The pentacene-based floating-gate memory was fabricated using N-doped LaB6 metal and LaBxNy insulator stacked layer with annealing of 200oC. The memory window (MW) of 0.77 V and 10 years retention were successfully observed with low program/erase voltage and time of ±4 V/10 ms.

Pentacene-based organic floating-gate memory utilizing N-doped LaB6 metal and LaBxNy insulating layers for flexible device applications

Charge transport across the interfaces in complex oxides attracts a lot of attention because it allows the creating of novel functionalities. The movable domain walls in epitaxial BiFeO3 films possess enhanced conductivity that can be used for reading out in ferroelectric-based memories [1]. Chemical solution deposition of BiFeO3 thin films is one of the most commercially available techniques to produce large-scale low-cost coatings for application in memory devices. In this work, complementary piezoresponse force microscopy, conductive atomic force microscopy, and electron backscattered diffraction methods are used to study the local phase distribution, morphology, piezoelectric response, and leakage current of the polycrystalline BiFeO3 thin films. The relation between the polarization, strain, and conductivity in sol-gel BiFeO3 films with special emphasis on grain boundaries as natural interfaces in polycrystalline ferroelectrics is investigated [2]. It was demonstrated that an interaction between the polarization and grain boundaries at elevated temperature leads to the formation of the highly conductive grain boundaries with the electrical conductivity about two orders higher than in the bulk. At room temperature, the conductive traces of the charged defects at the position of the grain boundaries remain and do not change upon polarization switching. These collective states provide further insight into the physics of complex oxide ferroelectrics and strongly affect their practical applications because reveal an additional mechanism of the leakage current in such systems.

The research was made possible by Russian Science Foundation (Grant 19-72-10076). The equipment of the Ural Center for Shared Use “Modern nanotechnology” UrFU was used.

[1] J. Seidel et al., “Conduction at domain walls in oxide multiferroics,” Nat. Mater., vol. 8, no. 3, pp. 229–234, Mar. 2009.
[2] D. Alikin et al., “Strain-polarization coupling mechanism of enhanced conductivity at the grain boundaries in BiFeO3 thin films,” Appl. Mater. Today, vol. 20, p. 100740, Sep. 2020.

Local electromechanical properties of the polycrystalline BiFeO3 thin films: collective polarization and transport phenomena

Ultrafast and energy-efficient writing by spin-orbit torque (SOT) prevents device degradation within thermally stable magnetic random-access memory (MRAM) with stable perpendicular magnetic anisotropy (PMA) by separating the read and write paths. However, orthogonality between spin polarization and PMA impedes its application at industrial level because of non-deterministic switching. Previously proposed approaches to break this orthogonality includes structural deformation and external magnetic field control. However, all of those approaches increase the fabrication complexity, are highly sensitive to the SOT current duration and magnitude, or increase the switching energy. By exploiting the precessional nature of the field-like SOT component, we propose toggle PMA SOT-MRAM that achieves field-free and energy-efficient switching with a simple structure, and is robust to variations in the SOT current magnitude and duration. 
The proposed three-terminal structure consists of a magnetic tunnel junction (MTJ), heavy metal, and compensating ferromagnet. A unidirectional SOT current applied through the heavy metal stabilizes the MTJ free ferromagnet magnetization at an excited steady state after crossing the hard axis plane. Switching off the excitation relaxes the magnetization to the nearest easy axis, thus completing a toggle cycle. The next excitory pulse toggles the magnetization back to its initial state. The deformation-free structure provides efficient data read-out and can also be leveraged for directional writing through a simple XOR between the stored and incoming bits. Robust MRAM writing with greater than 50% tolerance to variations in the excitory SOT current magnitude has been demonstrated through micromagnetic simulation. Moreover, room temperature simulations indicate that this switching is robust to thermal noise. The proposed device therefore provides a promising solution for compact, highly-efficient, deformation-free and robust SOT-MRAM with PMA.

Field-Free Toggle Mode Spin-Orbit Torque Driven MRAM with Perpendicular Magnetic Anisotropy

Exchange Bias (EB), based on the concept of hysteresis (MH) loop shift, investigated almost 70 years back, is one of the most studied phenomenon. The shift in the MH loop has garnered a good amount of interest due to the huge range of applications in spintronic devices. Vertical magnetization shift (VMS) of the MH loops has gained attention recently in exchange bias (EB) regime. But no concrete results have been found yet. We present a novel counter-intuitive approach to realizing the vertical MH loop shift in soft/hard heterostructure. A thin film of 13 unit cells (t= 5nm) of SrRuO3 (SRO) is grown epitaxially by pulsed laser deposition on SrTiO3 (STO) (100) substrate. On top of that, a polycrystalline Ni80Fe20 (Py) thin film of t=2nm is grown by DC/RF sputtering. We observe a clear and significant vertical MH loop shift at 2K after cooling from room temperature (300K) under a bias field (i.e. field cooled-FC) (Figure. a). There is a positive (negative) shift of the MH loop on applying a positive (negative) bias field of around 1T. Essentially, there is no vertical shift for zero field cooling (ZFC). This was only observed when the MH loop is measured with low field range (±500mT). For higher field range (e.g. ±7T) MH measurement, no loop shift is found for both ZFC and FC measurements. There is a threshold to field tracing range (< SRO’s saturation field) below which we can observe a vertical shift phenomenon only. The vertical shifts are observed only below 100K as well which is nearby to the Curie temperature of SRO (TC~160K) below which SRO becomes ferromagnetic. The reason of the shift is also ascribed to the anisotropy play of the two layers. Surprisingly, the coupling between Py and SRO here is exchange spring (ES) type rather than EB type, but simultaneously exhibiting strong EB like shift but in vertical direction. Moreover, the micromagnetic OOMMF simulations are performed, which are in broad agreement with the experimental results (Figure. b). We simulate for the same ratio of thicknesses of the two layers to manifest a generalized ground. The computational results with modelling serve as a verification of the experimental findings qualitatively and quantitatively. We make use of ES tenets to effectuate the novel phenomena of vertical shift. The vertical shift due to the ES interactions between the soft and the hard layer expands the territory of ES applications.

Bias field mediated vertical hysteresis loop shift in Ni80Fe20/SrRuO3 heterostructure for novel memory switching

We report the first results of the growth of nickel and permalloy (Ni80Fe20) circular nanodot arrays with a dot diameter of 300 nm using Talbot lithography and electrochemical deposition. Samples with different thicknesses (50 nm to 400 nm) and, thus, aspect ratios were prepared by the variation of deposition times. The magnetization reversal and the associated magnetic configuration were probed by vibrating sample magnetometry (VSM) and magnetic phase diagram was plotted for both nickel and permalloy nanodot arrays. We observe a transition from a uniform magnetisation in the inplane direction to a vortex state followed by a uniform magnetisation to the out of plane direction, with an increase in thickness of the dots. The magnetic phase diagram in dependence on the dot geometry is in good agreement with micromagnetic simulations and for sputtered structures. We also probe the magnetic vortex states in permalloy nanodot arrays using Kerr microscopy and further aim to magneto-ionically control the magnetic states in our dot arrays.

Magnetic vortex and single domain states in transition metal nanodot arrays prepared by Talbot lithography and electrodeposition

It is an urgent need to design the next generation non-volatile electronic memory with higher density, and resistive random access memory (ReRAM) based on the mechanism of resistive switching is a promising candidate. Here, we study the resistive switching mechanism of Pr0.7Ca0.3MnO3 with strong correlation effects, which has a clockwise bipolar resistive switching property without forming process. When give a positive voltage on the surface of Pr0.7Ca0.3MnO3, it can change from low resistive state (LRS) to high resistive state (HRS), when apply negative voltage, it transforms from HRS into LRS. We also find the transition between LRS and HRS is due to the field induced redox reaction which can change (increase or decrease) the concentration of oxygen vacancies in Pr0.7Ca0.3MnO3. The increase (decrease) of oxygen vacancies in Pr0.7Ca0.3MnO3 can result in the increase (decrease) of electron filling on Mn 3d orbit. And finally lead to a change of Mott band gap in Pr0.7Ca0.3MnO3 with a resistive switching.

A study of Mott-transition based resistive switching in Pr0.7Ca0.3MnO3 film

We developed a novel method to directly integrated few-layer BP which was exfoliated in melamine aqueous by liquid-phase exfoliation (LPE), and then adding cyanuric molecules in the solution, resulting in self-assembling melamine-cyanurate (MCA) by the networks of hydrogen bonds together with embedded few-layer BP sheets. It is noteworthy that we used melamine to facilitate exfoliation of BP bulk in the water under ultrasonication, where high crystalline few-layer BP sheets can be obtained within a half-hour. The as-exfoliated BP sheets presented stable and well dispersed in the aqueous solution. Afterward, we integrated mixed precipitation to a two-terminal RRAM-based memristor as an active layer, exhibiting analog synaptic behaviors under both operations of DC sweeping and AC pulses as emulating artificial synapse.

Analog synaptic behavior of memristor composed of few-layer black phosphorus embedded in supramolecular

Electric-field control of magnetism is a fundamentally different approach towards the implementation of ultralow heat-dissipation memories operated at the nanoscale as it does not involve the use of electric currents and thus minimizes Joule heating effect (resistive heating). Here, we investigate the effects of voltage on arrays of highly porous iron oxide microdisks (8 µm in diameter), obtained combining sol-gel with lithography and wet chemical etching processes, that are virtually non-magnetic in the as-prepared state (OFF state). Electrolyte-gating is then employed to induce a tunable ferromagnetic response in these disks at room temperature, thus accomplishing an OFF-ON switching of magnetism. The changes in magnetic properties are attributed to magnetoelectrically-driven oxygen ion exchange with an anhydrous liquid electrolyte, which is enhanced due to nanoporosity. This causes partial reduction of the various iron oxide phases present in pristine samples to metallic Fe. The effect can be considerably reversed by applying voltage of opposite polarity. These results are appealing for diverse technological applications that require the use of patterned structures with easily tunable magnetic properties, such as magnetic micro-electro-mechanical systems, microfluidic and lab-on-a-chip platforms for biomedical therapies and, ultimately, energy-efficient magnetic memories or neuromorphic computing.

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