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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.

Materials Challenges for Memory

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

pulsed laser deposition

2d material

memristive

neuromorphic computing


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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.

poster

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


A multi-code investigation of promising spin Hall materials

The metallic to semiconducting resistivity ratios and the transition temperature sharpness of annealed atomic layer deposited (ALD) VO2 films on amorphous silicon dioxide are found to depend on grain size in a closely similar way to films from previous sputtered or pulse laser deposited work. This occurs because the dominance of grain growth and compaction processes leads to a common scaling of properties with grain size. Density functional simulations find that V-V dimerizations allow grain boundaries to remain semiconducting but with reduced band gap, while others create metallic grain boundaries, reducing the resistivity ratio for smaller grains in both cases.

Microstructure Scaling of Metal-Insulator Transition properties of Atomic Layer Deposited VO2 films

Single-Molecule Magnets (SMM) and Single Ion Magnets (SIM) it are perspective materials for dense data storage. One molecule can contain 1 bit of information. Control over splitting of spin levels in SMM and SIM are usually controlled by external magnetic field tuned to satisfy conditions of effective thermally activated overcoming crystalline field barrier for magnetization reversal. We propose composite material containing known in advance SIM powder with magnetically hard microparticles (MPs) as an effective media changing to external magnetic field by magnetically controllable residual demagnetizing field of MPs.
In the SIM+MPs composite, we have found peaks of magnetic AC susceptibility on its frequency dependences in the absence of external magnetic field, while no peaks have been found for SIM and MPs powders measured separately in similar conditions. We have found the dependence of AC susceptibility of SIM on remnant magnetization of the MPs. Significance of the obtained results is the development of a very simple experimental method eliminating necessity of permanent magnet and external field to
reveal spin transitions in SIM. Ferromagnetic inclusions incorporated in SIM powder create high enough local residual demagnetizing field able to control over splitting of spin levels in small groups of SIM complexes. 
The work was supported by the Russian Foundation for Basic Research grant № 20-33-90256.

Magnetic memory of composite based on single ion magnet biased by dipolar field of microparticles

We obtain orthorhombic HfO2 ferroelectric phase induced by Ge component in 3-layers of control HfO2/floating gate of Ge NPs in HfO2/tunnel HfO2/p-Si.
3-layer structures are deposited by magnetron sputtering (MS) of HfO2/Ge-HfO2/HfO2 layers stack on Si wafer and nanostructured by RTA.1–3 As-deposited Ge-HfO2 intermediate (int.) layer is rich in Ge (≈60% Ge). Morphology, structure, composition is investigated by HRTEM, HAADF-STEM, EDX, SAED, GI-XRD. Polarization studies are carried out by P–V hysteresis loops measurements, PFM and DFT. Memory properties are evidenced by C–V hysteresis loops.
We demonstrate the HfO2 orthorhombic phase formation in floating gate with extension in adjacent layers (control – ctr. & tunnel – tun. HfO2). Total Pr reaches ~7 µC/cm2 - very high for the 3-layer structures having minimum thickness of 35 nm.
These memory structures have very high memory window (ΔV) of C–V hysteresis due to both contributions of Ge NPs as charge storage nodes & ferroelectric HfO2.
1. Nanotechnology 30, 365604 (2019) 2. Nanotechnology 28, 175707 (2017)
3. Nanotechnology 31, 495207 (2020) 4. Appl. Phys. Lett. 113, 213106 (2018)

Ferroelectric orthorhombic HfO2 phase in 3-layer memory structures of control HfO2 /floating gate of Ge nanoparticles in HfO2 /tunnel HfO2 on Si wafers

Resistive random access memory (RRAM) is a promising candidate for the memory, logic and neuromorphic device exhibiting advantage in power consumption. Tantalum oxide is an attractive candidate, as the material exhibits rich and complex electrically resistive switching behaviors. It varies based on the film composition, the structure (e.g., crystallinity), and the mode of electrical operation. 
In this study, we focused on the resistive switching behavior of tantalum oxide through the atomic layer deposition (ALD) using tantalum ethoxide and water depending on the ALD deposition cycle. The stable resistive switching properties was obtained with TaOx film in 250 ALD cycle. Film thickness was measured by using surface profiler. X-ray photoelectron spectroscopic analysis was carried out to analyze the chemical state of the films. The electrical performance was obtained via current-voltage measurement at room temperature. Through these analyses, we aimed to demonstrate the relation between the film thickness and the resistive switching behavior of tantalum oxide thin films. 


A study on the nonvolatile memory characteristics of tantalum oxide thin films formed by atomic layer deposition

Tin oxide (SnO2) has received significant attention in both industries and academia due to its attractive features such as wide bandgap (3.6 eV), n-type semiconductivity, and surface chemistry, and useful for several applications such as transparent electronics, electrochemical storage devices, sensors and catalysis. It is also attractive for memristor or resistive random access memory (RRAM) devices due to its tunable defect density. Although some reports are present on it, reports that show analog switching characteristics and cyclic stability are very scarce in the literature. Here, in this work, we demonstrate Pt/SnO2/Pt RRAM devices that show more than 10 resistive states (or analog switching characteristics), good cyclic stability (>100 cycles) and retention measurements (>1000 seconds). Moreover, these devices do not require any compliance current (compliance-free) during set and reset, and require very low voltages to operate (≤2 V). Furthermore, these devices show pulse width dependence characteristics, which gives the flexibility of programming and erasing with different voltage pulses. Therefore, these low power, compliance-free SnO2 RRAM devices with analog switching characteristics can pave the way for new window of applications such as portable gasistors and neuromorphic computing devices.

Compliance-free, analog switching characteristics of SnO2 based memristors

The rapid development of memory devices has brought the areal density to approach 10 Tb/in2 (i.e., nearly 244 bits or 44 independent memory units in the area of one square inch), corresponding to an average size around 10 mm2 (or 33 mm2) for one memory unit. Thanks to the interfacial lattice coupling and switchable electric polarization, the artificially-designed perovskite oxide multilayers are capable to exhibit magnetically-working units with the characteristic length of only a few nanometers. For examples, the 12-unit-cells nonmagnetic SrTiO3 (or PbTiO3) spacers can make the 9-unit-cells ferromagnetic (La,Sr)MnO3 layers switch independently in a multilayer heterostructure, of which the functional unit is only 21 unit cells or around 8 nm. Also, the paramagentic CaRuO3 spacer can continuously change the bond angle in the adjacent ferromagnetic (La,Ca)MnO3 layer, resulting in multiple tunable coercivities in a coherently-grown multilayer. More importantly, our recent data show that the thickness of those spacers can be further reduced to 3~4 unit cells (1.2-1.6 nm), while the independence of each magnetic layer is still maintained. Our results demonstrate the promising potentials of well-designed heterostructures in high-density memory devices.

Towards High Density Memory Devices Using Perovskite Oxide Heterointerfaces

The ever-increasing demand for data storage has driven the development of many cryogenic memory storage technologies. Among these, magnetic random-access memory (MRAM) in the form of magnetic tunnel junctions (MTJs) has emerged as a leading candidate. MTJs have proven successful as a means of data storage, but the large resistance of the tunnel barrier prevents this structure from integration with other low-temperature technologies such as superconducting computing.

Use of a low-resistance barrier, such as in a GMR device using current perpendicular to plane geometry (CPP), has potential to solve this issue, however now we are presented with the opposite problem of having too low resistance. Here we present a way to model this problem and thereby provide a solution to find a ‘Goldilocks’ zone for the conductivity ratio of each layer that allows for a reliable and meaningful voltage readout.

Furthermore, we propose a series of materials with a tunable conductivity that is dependent on the initial growth parameters to replace the commonly used transition metals in this endeavor. The rare-Earth nitride (REN) series are ferromagnetic semiconductors with massive tunability and show promise for integration with existing superconducting technologies.

Cryogenic memory storage using tunable ferromagnetic semiconductors

The field of magnetic random-access memory (MRAM) has captivated immense recognition recently given its capability to provide non-volatility along with accelerated read and write performance. Conventional MRAM devices based on spin-orbit torque (SOT)1, spin-transfer torque (STT)2, or current-driven methods employ magnetization switching using external current. The current-based methods accompany thermal issues and thereby surges the power dissipation. An attractive alternative that has emanated lately is a conception of straintronic (STR) based MRAM devices to store information. To carry out magnetization switching in STR devices external small voltage is employed. Currently, in STR-based MRAM devices, the magnetization reversal without external bias magnetic field is not explored. Although external bias is not desired since it increases device size and cost and lessens thermal stability and retention time by lowering the energy barrier between two extremes stable states.
In this work, size-dependent in-plane magnetization switching of elliptical multiferroic magnets of FeGaB on single-crystal PMN-PT (011) piezoelectric are studied using the finite element method (FEM). To ensure minimum thickness (dmin) for multiferroic magnets in-plane induced energy well (∆E) is scaled by kbT, such that if ∆E/ kbT ≤ 70, STR based MRAM device is thermally unstable. To investigate maximum thickness corresponding to self-biasing (dsb), pre-stress equilibrium exchange and demagnetization energy of multiferroic magnets are examined against thickness. Large size magnets strive to diminish the energy by line up magnetization with magnet boundary that ultimately causes vortex state. Oppositely smaller size magnets strive to diminish the energy by line up magnetization along the easy axis (0o or 180o) of the multiferroic magnet. This examination is deciding after all such self-biasing would give superiority to future STR-based MRAM devices because it gets rid of an external magnetic bias field. Even though the thickness range of dmin≤d≤dsb gives an idea about the STR-MRAM device that is stable and self-bias without applying the external magnetic bias field, it is observed that maximum thickness (dmax) for in-plane 180o magnetization switching is limited by critical stress requirement. When external stress more than or equivalent to critical stress is generated using external voltage, in-plane 90o magnetization switching is observed (Fig 1(a)) from the pre-stress state (0o or 180o). The critical stress requirement for fixed thickness and increased cross-sectional area rises. During in-plane 90o magnetization switching exchange and stress anisotropy energies are observed almost negligible and the demagnetization energy rises since the magnetization is switched from the easy axis to the hard axis. To obtain in-plane 180o switching magnetic precession is used. As soon as the magnetization component along the x-axis reaches its maximum negative value, removal of external voltage causes another 90o switching (Fig 1(b)) since the multiferroic magnet makes an effort to reduce its total energy by falling to its nearest easy axis. In-plane fastest 180o magnetization switching time and associated external strain are probed. For fixed cross-sectional area, when the thickness is increased trade-off between the fastest switching time and associated switching energy is observed. For fixed thickness, when the cross-sectional area is increased faster switching time and lesser switching energy is observed. This work validates and verifies the STR-based MRAM device that abolishes the external magnetic bias field and achieves in-plane 180o magnetization switching.
Reference 
1C. Zhang, S. Fukami, H. Sato, F. Matsukura and H. Ohno, Appl. Phys. Lett. 107, 012401 (2015).
2C.F. Pai, L. Liu, Y. Li, H. W. Tseng, D. C. Ralph and R. A. Buhrman, Appl. Phys. Lett. 101, 122404 (2012).


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