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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.

Materials Challenges for Memory

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

supramolecular

black phosphorus (bp)

two-dimensional (2d) materials

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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.

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.

Electric-field control of magnetism in arrays of nanoporous iron oxide microdisks

Memristors are metal/insulator/metal devices with high potential to be implemented as a new generation of non-volatile memories and novel new neuromorphic computing devices. These structures have the property of changing their resistance between different non-volatile states upon the application of external electrical stimuli, a process usually called resistive switching (RS). RS mechanisms are usually associated with the electromigration of defects such as oxygen vacancies (OV), either through the creation/disruption of conductive nanofilaments or by modulating the height of Schottky barriers, formed at metal/insulator interfaces. When the insulating oxide is ferroelectric, the resistive change at Schottky interfaces could be of electronic nature and associated to the switching of the direction of the ferroelectric polarization [1]. Ferroelectric memristors are considered to present a faster and more reliable response, as the physical mechanism does not require electromigration of defects. 
In this work [2] we study the memristive response of PZT and BaTiO3-based ferroelectric memristors. We find in both cases that RS is related to two competing effects acting on metal/ferroelectric Schottky interfaces: the switching of the ferroelectric polarization and OV electromigration. Moreover, we propose that both effects are entangled and the latter is mainly controlled by the depolarizing field, arising from an incomplete screening of ferroelectric bound charges. We model the experimental response with a modified version of the VEOV model [3] that accounts for the presence of ferroelectricity and satisfactorily reproduces several non-trivial aspects of the experimental response. Finally, we show that these systems display coexisting non-volatile and volatile memristive response, an issue that could be useful for the development of neuromorphic devices.

[1] P. W. M. Blom, R. M. Wolf, J. F. M. Cillessen, and M. P. C. M. Krijn, Phys. Rev. Lett. 73, 2107 (1994)
[2] C. Ferreyra, M. Rengifo, M.J. Sánchez, A. S. Everhardt, B. Noheda, and D. Rubi, Phys. Rev. Appl. 14, 044045 (2020)
[3] M. J. Rozenberg, M. J. Sánchez, R. Weht, C. Acha, F. Gomez-Marlasca, and P. Levy, Phys. Rev. B 81, 115101 (2010).

Coexistence of polarization and ionic electromigration effects in the memristive response of ferroelectric oxides

Resistive switching (RS) devices are considered promising among other emerging devices to meet the demand for future memory device applications. Various transition metal oxides have been investigated for their RS properties. Out of different binary transition metal oxides, WO3-x is attractive because of its promising switching result and compatibility with CMOS process technology. While RS properties depend strongly on the defect concentration in the oxides and metal/oxide interfaces, it has been found that in various metal oxides the RS characteristics are also strongly affected by atmospheric moisture content. In this work the RS characteristics of Au/WO3-x/Pt device and its influence by the atmospheric moisture has been investigated. The non-stoichiometry and the adsorbed water molecule on the tungsten oxide’s surface is confirmed by the X-ray photoelectron spectroscopy. The device exhibited rectification and stable bipolar RS characteristics without the need of any forming step. DC conduction mechanism investigated at different temperature suggested the switching is primarily dominated by the Schottky type Au/WO3-x interface. Current conduction through the active layer has been found to be dominated by Schottky emission at low electric field and Poole–Frenkel emission at high electric field. The reverse current has been found to increase significantly on subjecting the device to ambient moisture. Modification of the Schottky barrier due to defect redistribution and molecular dipole at the Au/WO3-x interface have been discussed as a possible mechanism of the observed RS modulation. The modification of Schottky barrier height with ambient moisture reveals an important challenge the WO3-x based interface type resistive switching material faces which must be considered for the devolvement of resistive switching based future memory devices.

Modulation of resistive switching characteristics of WO3-x based MIM structure by ambient moisture

Development of floating gate (FG) non-volatile memories (NVMs) based on nanocrystals (NCs) is of great importance for the continuous increasing requirements for highly efficient data storage.1 NCs by tuning of their size, density & interface quality with embedding oxide make possible the adjusting of device parameters. These NVMs use FG of either metal or semiconductor NCs in oxide as SiO2, HfO2, Al2O3.1–7. In particular, group IV-based NCs of Si, Ge, Sn are of high interest due to the compatibility with CMOS technology.3–8
Here we report on obtaining NVM capacitors with FG of SiGeSn NCs acting as charge storage nodes, being embedded in HfO2. The advantage of using SiGeSn NCs consists in tailoring NC compositions (up to Sn segregation limit), reducing thermal budget by using Sn for NCs formation and at same time obtaining NCs stability by alloying with Si.
We use configuration of trilayer (gate HfO2/FG of SiGeSn NCs in HfO2/tunnel HfO2) that we prepare on Si wafers.4 Trilayers are deposited by magnetron sputtering of HfO2/ SiGeSn-HfO2/HfO2 and annealed for nanostructuring in rapid thermal processor (520–530 oC). XTEM, XRD & Raman spectroscopy are used for complex characterization of layers composition, morphology & structure. NVM structures are capacitor-like, with Al top & bottom contacts. XTEM reveals formation of ~5 nm Ge-rich SiGeSn NCs in FG, with 5% Si content & diamond structure. 2 types of SiGeSn NCs are evidenced (also by Raman) in function of their position in FG. 1st type NCs are positioned inside FG (extending on the whole FG thickness), having a small Sn amount (2%), while 2nd type NCs more rich in Sn (15%) are positioned at FG interfaces with gate & tunnel layers. The SiGeSn NCs in FG produce good NVM performance by C–V hysteresis loops with ~1 V memory window.
1. Chem Rev 120, 3941 (2020) 2. Sci Adv 2, e1501101 (2016)
3. ACS Appl Electron Mater 1, 1852 (2019) 4. Appl Phys Lett 113, 213106 (2018) 
5. Appl Phys Lett 107, 093102 (2015) 6. Nanotechnology 30, 365604 (2019)
7. Appl Surf Sci 542, 148702 (2021) 8. Scripta Mater. 113, 135 (2016)

Trilayer non-volatile memory structures with floating gate of SiGeSn nanocrystals embedded in HfO2

Already used industrially in TFTs in OLED displays, amorphous In-Ga-Zn oxide (IGZO) is a particularly attractive material also for Resistive-random-access-memory (ReRAM) applications, thanks to its low oxygen vacancy migration barrier, size scalability and ease of fabrication at RT by sputtering. In traditional IGZO-based ReRAM, conducting filaments are stochastically formed (usually by a “forming” process) and ruptured to induce switching between high and low resistance states (HRS and LRS). The issues then lie in poor uniformity, unfavorable scaling, limited endurance and stability issues.
In this study, novel IGZO ReRAM devices are investigated with an interface-type mechanism. These devices overcome stability issues. Bipolar Resistive switching (BRS) is achieved by the modulation of Schottky barrier height, through oxygen vacancy migration at the electrode interface.

Interface-controlled Resistive Switching in a-InGaZnO Memristors

In most atomic force microscopy (AFM) based measurements, the electrostatic effect is a common phenomenon, which will affect the measured electromechanical response in contact-based dynamic force microscopy. The electrostatic contributions might result in a misinterpretation of the observed response. The development of piezoresponse force microscopy (PFM) has enabled researches to investigate ferroelectric characteristics on the nanoscale. PFM is not free of being affected by electrostatic effects and leads to misinterpretation. Therefore, it is crucial to explore the original of electrostatic effects in PFM testing. Nevertheless, the relationship between electrostatic effect and nano metal-semiconductor (M-S) junction is barely discussed. Here, we examine how to differentiate ferroelectric from non-ferroelectric of well-known ferroelectric (BiFeO3, BaTiO3), paraelectric (SrTiO3) and semiconducting (Si) systems via various PFM measurement characteristics. The nano M-S junction studied by characterizing the I-V response. In the end, we observed that for different materials should apply different DC voltages to compensate for the excess charge on the surface. The obtained results reveal that the nano M-S junction rules the electrostatic effect between the tip and the sample, and the DC voltage applied to compensate surface charging is related to the rectification behavior due to Schottky contact at the nano M-S junction.

The Surface Charge Effects of Ferroelectric and Non-Ferroelectric Materials in Piezoresponse Force Microscopy

Resistive random-access memory (RRAM) has been studied for next generation memory, due to volatile property of DRAM and long write and erase time (W/E time) of NAND flash. To date, many resistive switching effects have been observed in various materials, which is binary oxide (TiO2, NiO, Al2O3 etc.), metal doped PCMO (Pr1-xCaxMnO3 etc.), chalogenicle materials (GeSeTe etc.), metal doped perovskites (SrTiO3, SrZrO3 etc.) and etc. However, there is no clear evidence about resistive switching mechanism of ZnO thin film. Here, we study N, F codoped ZnO, which shows bipolar resistive switching. Performing micro spectroscopic studies on exposed surfaces allow you to track systematic electronic structural changes in both low and high resistance states. Atomic layer deposition (ALD) technology is a highly advanced deposition process for low temperature processing and high uniformity of films, which allow atomic level control of dopant. Through these dopant control, we can see the significant difference in electronic conductivity, due to the field-induced oxygen vacancies that cause the Mott metal-insulator transition on the surface. Our study can be the new way to explain mechanism of resistive switching of RRAM.

A study on the mechanism of resistive random-access memory for doped zinc oxide thin film by atomic layer deposition

Memristive devices based on two-dimensional layered materials have attracted a lot of attention for neuromorphic applications owing to their high downscaling potential, easy stacking due to vdW-forces and possible mechanical flexibility. Here, memristive switching is demonstrated in a vertical device structure based on layered Sb2Te3. For this, high quality epitaxial Sb2Te3 was grown by pulsed laser deposition directly on p-type Si (111) and Ag was utilized as top electrode metal. The device was characterized in detail by current-voltage measurements, X-ray diffraction and transmission electron microscopy. Analog programmability, stable retention and endurance with a memory window larger than one order of magnitude were observed. Additionally, the influence of Sb2Te3 layer thickness and further top electrode metals were studied. Overall, this work shows the high potential of layered Sb2Te3 for neuromorphic applications and offers a scalable method for integration into the existing Si platform.

Memristive behavior of layered epitaxial Sb2Te3 thin films grown on Si (111) by pulsed laser deposition

Oxide-based metal-insulator-metal structures are of special interest for future non-volatile resistive switching memory, also known as resistive random-access memory (ReRAM) [1]. In such cells, resistive switching in the oxide layer is widely believed to be caused by the electric‐field‐driven motion and internal redistribution of oxygen ions (or vacancies) with filament region, resulting in valence change on the cation sublattice [1]. There are very special oxide-material groups which undergo a reversible topotactic phase transition between the insulating brownmillerite and conducting perovskite structure upon the redox processes [2].
Recently, we introduced a topotactic phase changing brownmillerite SrMO2.5 (M = Co, Fe, etc.) as a novel material platform to harness exceptional oxygen ion transport properties for resistive switching memory devices for the first time [3,4]. Accordingly, we were able to demonstrate a promising resistive switching performance with high endurance (> 106 cycles), fast switching speed (10 ns), and high uniformity in key switching parameters [5,6]. By using X-ray absorption spectromicroscopy and in site TEM, we could prove that the resistance change is indeed caused by a reversible topotactic phase transition between an insulating SrFeO2.5 brownmillerite and a conductive SrFeO3 perovskite phase [7]. By engineering the ordered oxygen vacancy channels in brownmillerite structure, we further demonstrated complimentary resistive switching and synaptic memory behavior in the SrFeOx memristive devices [8]. 
Beyond resistive switching, our results demonstrated a clear path to tune the ionic controlled functionalities in the multiphase oxides by electrical bias at the nanoscale level, which is highly interesting in the field of magnetoelectric and spintronic device applications.

Reference 
[1] R. Waser, R. Dittmann, C. Staikov, K. Szot, Adv. Mater. 21 (2009) 2632–2663.
[2] H. Jeen, W.S. Choi, M.D. Biegalski, C.M. Folkman, I.C. Tung, D.D. Fong, J.W. Freeland, D. Shin, H. Ohta, M.F. Chisholm, H.N. Lee, Nat. Mater. 12 (2013) 1057–1063.
[3] O.T. Tambunan, et. al. Appl. Phys. Lett. 105 (2014) 063507.
[4] V.R. Nallagatla, C.U. Jung, Appl. Phys. Lett. 117 (2020).
[5] S.K. Acharya, R.V. Nallagatla, O. Togibasa, B.W. Lee, C. Liu, C.U. Jung, B.H. Park, J.Y. Park, Y. Cho, D.W. Kim, J. Jo, D.H. Kwon, M. Kim, C.S. Hwang, S.C. Chae, ACS Appl. Mater. Interfaces 8 (2016) 7902–7911.
[6] S.K. Acharya, J. Jo, N.V. Raveendra, U. Dash, M. Kim, H. Baik, S. Lee, B.H. Park, J.S. Lee, S.C. Chae, C.S. Hwang, C.U. Jung, Nanoscale 9 (2017) 10502–10510.
[7] V.R. Nallagatla, T. Heisig, C. Baeumer, V. Feyer, M. Jugovac, G. Zamborlini, C.M. Schneider, R. Waser, M. Kim, C.U. Jung, R. Dittmann, Adv. Mater. 31 (2019) 1903391.
[8] V. Raveendra Nallagatla, J. Kim, K. Lee, S. Chul Chae, C. Seong Hwang, C. Uk Jung, Cite This ACS Appl. Mater. Interfaces 12 (2020) 41740–41748.


Topotactic Metal-Insulator Phase Transition for Memristive Devices

The spin Hall effect (SHE) is a phenomenon with vast potential for application in spintronic memory devices. Spin Hall materials based on alloys show great promise due to the ability to tune the resistance and spin Hall conductivity as a function of composition. However, predicting and optimizing the total spin Hall conductivity can be challenging due to the complex interactions between intrinsic (band-related) and extrinsic (skew scattering) contributions to spin Hall conductivity. Over the years, different numerical methods have been optimised for distinct SHE mechanisms or configurations. Using a multi-code approach can provide us with a comprehensive framework to address and optimize all aspects of spin Hall conductivity in complex alloys.
Tungsten in its β phase has a particularly large spin Hall effect (SHE) and therefore has potential application in future low power, high endurance MRAM devices. In this study, we used the screened Korringa-Kohn-Rostocker (KKR) method [1] along with the linearized Boltzmann Transport equation, Quantum Espresso [2] and PAOFLOW [3] to reveal how dilute N or O impurities affect both the intrinsic and extrinsic SHE in the bcc (α) and β phase tungsten [4]. Our results, when examined in combination, suggest that the formation of grain boundaries in the doped system plays an important role in amplifying the SHE in β tungsten.
Further techniques such as the coherent potential approximation [5,6] enables us to treat not only dilute but concentrated alloys as well. With this expanded tool set, an even wider array of other spintronic materials, such as Pt based alloys will be explored.

[1] L. Peti et al, Philos. Mag. B 78, (1998) 449
[2] P. Gianozzi et al, J. Phys.: Condens. Matt. 21, (2009) 395502
[3] M. B. Nardelli et al, Comp. Mat. Sci. 143, (2018) 462-472
[4] O. L. W. McHugh, M. Gradhand, W.-F. Goh and D. A. Stewart, Phys. Rev. Mat. 4, (2020) 094404
[5] D. Ködderitzsch, H. Ebert, D. A. Rowlands, A. Ernst, New J. of Phys. 9(4), 2007, 81-81
[6] P. R. Tulip et al, Phys. Rev. B 77, 2008, 165116


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