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With rapid development of information technology and artificial intelligence, optically controlled ferroelectric junctions are becoming promising candidates for data storage and logics [1]. We have recently reported that the resistance state of a BaTiO3 ferroelectric tunnel barrier can the switched from ON to OFF states by illumination with a suitable wavelength (blue laser, λ = 405 nm) if an imprint field (EIMP ≈ -2.8MV/cm) exists that unbalances the equilibrium polarization up/down domain configuration. The observation that smaller energy photons (red laser, λ = 648 nm) does not produce such effect strongly suggests that photocarriers play major role on polarization and resistance switching. Here we explore in greater detail the possible role of thermal effects and potential thermally induced suppression of polarization. A number of experiments, including the time-dependent response, its sensitivity to illumination power and recovery processes, have been performed. Data shows the prominence of electronic process (photocarriers and polarization screening) as the ultimate reason for light-induced resistive switching [2].

References
[1] Uchino, Kenji. Ferroelectric devices. CRC press, 2018.
[2] Long, Xiao, et al. &quot;Non-volatile optical switch of resistance in photoferroelectric tunnel junctions.&quot; Nature communications 12.1 (2021): 1-9.


Materials Challenges for Memory

Disentangling electronic and thermal contributions to the observed light-induced resistance switching in BaTiO3 ferroelectric tunnel junction

polarization screening

photo-ferroelectric

resistive switch


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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.

With rapid development of information technology and artificial intelligence, optically controlled ferroelectric junctions are becoming promising candidates for data storage and logics [1]. We have recently reported that the resistance state of a BaTiO3 ferroelectric tunnel barrier can the switched from ON to OFF states by illumination with a suitable wavelength (blue laser, λ = 405 nm) if an imprint field (EIMP ≈ -2.8MV/cm) exists that unbalances the equilibrium polarization up/down domain configuration. The observation that smaller energy photons (red laser, λ = 648 nm) does not produce such effect strongly suggests that photocarriers play major role on polarization and resistance switching. Here we explore in greater detail the possible role of thermal effects and potential thermally induced suppression of polarization. A number of experiments, including the time-dependent response, its sensitivity to illumination power and recovery processes, have been performed. Data shows the prominence of electronic process (photocarriers and polarization screening) as the ultimate reason for light-induced resistive switching [2].

References
[1] Uchino, Kenji. Ferroelectric devices. CRC press, 2018.
[2] Long, Xiao, et al. "Non-volatile optical switch of resistance in photoferroelectric tunnel junctions." Nature communications 12.1 (2021): 1-9.


poster

The atomic-scale interfaces of 3D transition-metals oxides are the fascinating playground for several atomic-scale interactions of spins. Novel physical phenomena such as magnetoelectric coupling, unconventional exchange bias (EB) coupling, etc. which originate at such interfaces have huge potential for next-generation nanoscale spintronic devices. Among them, EB is the most studied due to its importance for device applications such as high-density memory and sensor. Here, we demonstrate EB coupling (EB shift, HEB~40 Oe at 2 K) at the interface of epitaxially grown ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) – ferroelectric (FE) BaTiO3 (BTO) bilayer thin films on SrTiO3 (STO) single crystal substrates in absence of any conventional antiferromagnetic (AFM) material. Such EB is only observed when the thickness of LSMO is only a few unit cells (<10nm). Importantly, we find that the EB coupling follows training effect, bias field dependency, and temperature dependency. By X-ray magnetic circular dichroism (XMCD) measurement, it is observed that in such epitaxial heterostructure Ti of BTO poses magnetisation at 2K and can reversibly switch between two distinct magnetization states by switching external applied magnetic field. A remanent magnetization (MR) in Ti is also observed. The magnetization of Ti as well the EB vanishes at high temperature (300K) which confirms the EB coupling between Ti of BTO and Mn of LSMO at low temperature. This is an important step towards creating EB coupling between FM and FE materials without conventional AFM materials, which may enable a new class of multi-state memory devices by switching magnetization, FE polarisation and EB coupling in the same device.

Atomic-scale control of exchange bias at BaTiO3 - La0.67Sr0.33MnO3 interface for new multi-state memory

The last years have witnessed major advancements in all-electrical doping control on cuprates. In the vast majority of cases, the tuning of charge carrier density has been achieved via electric field effect by means of either a ferroelectric polarization or by using a dielectric or electrolyte gating. Unfortunately, these approaches are constrained to rather thin superconducting layers and require large electric fields in order to ensure sizable carrier modulations1,2. In this work, we focus on the investigation of oxygen doping in an extended region through current-stimulated oxygen migration in YBa2Cu3O7−δ superconducting bridges. The underlying methodology is rather simple and avoiding sophisticated nanofabrication process steps and complex electronics. A patterned multiterminal transport bridge configuration allows us to electrically assess the directional counterflow of oxygen atoms and vacancies. Importantly, the emerging propagating front of current-dependent doping δ is probed in situ by optical microscopy and scanning electron microscopy. The resulting imaging techniques, together with photo-induced conductivity and Raman scattering investigations reveal an inhomogeneous oxygen vacancy distribution with a controllable propagation speed permitting us to estimate the oxygen diffusivity. These findings provide direct evidence that the microscopic mechanism at play in electrical doping of cuprates involves diffusion of oxygen atoms with the applied current. The resulting fine control of the oxygen content would permit a systematic study of complex phase diagrams and the design of non-volatile memories.

Direct Visualization of Current-Stimulated Oxygen Migration in YBa 2 Cu 3 O 7−δ Thin Films

High temperature shape memory alloys (HTSMAs) has attracted a great deal of scientific interest for their ability to operate at a higher temperature above 800 K, for applications as actuators for aerospace and automotive components due to the reversible martensitic transformation. TiPt amongst other binary systems undergoes a reversible martensitic transformation from B2↔B19 at elevated temperatures. Previous studies showed that the TiPt alloy is mechanically unstable with the negative C′ (–32) and soft modes in the negative frequency of the phonon dispersion curves along the gamma region at 0 K. The supercell approach was used to substitute Ti with Hf on TiPt structure to evaluate their mechanical stability from elastic properties and the phonon dispersions curves. The elastic properties suggest that an increase in Hf concentration enhances the mechanical stability of ternary structures. The C' becomes positive and larger at 25 < x < 50, which suggests a reduced martensitic transformation at x ≥ 43.75. The Ti50–xHfxPt50 ternary becomes more ductile with the increase in Hf concentration, which suggests that Hf stabilizes the system at a higher concentration. The analyses of the vibrational properties of Ti50–xHfxPt50 structures with respect to phonon dispersion are also discussed.

First-Principle Study of TiPtHf High Temperature Shape Memory Alloys Applications

Despite being predominantly known for their extraordinary performance in photovoltaic application, hybrid organic-inorganic perovskites (OIPs) are also widely studied in terms of neuromorphic computing and artificial synapses. 
The possibility to mimic the behaviour of neurons with OIP-based memristive devices is attributed mainly to the creation and rupture of metallic and/or vacancy defect conducting filaments under external electric field and modulation of the Schottky barrier at the OIP/metal interface. It has already been shown, that OIPs can emulate simple synaptic learning rules, such as spike-timing dependent plasticity, spike-rate dependent plasticity, metaplasticity and memory consolidation.1,2 However, the unique optoelectronic properties of perovskites and their strong interaction with light create an opportunity to induce the neuromorphic effects with light as well.
Here, we present a memristive photoactive device based on a thin OIP layer sandwiched between Au and ITO electrodes that exhibits voltage- and light-induced synaptic effects. To induce the photo-synaptic effects, we have illuminated the sample with short (200 ms) light pulses. With no charge trapping layer, the device exhibits a gradual increase of photocurrent amplitude (short-term facilitation). However, upon the incorporation of charge trapping carbon nitride nanoparticles (CN-NPs) into the device, we have observed a much more complex photoresponse, with steep learning and slow photocurrent decrease (short-term depression). Such photocurrent generation dynamics suggest complex charge-trapping mechanisms at the Au/OIP and CN-NPs/OIP interfaces.
Our preliminary results show that the photocurrent generation dynamics can be controlled with the switching voltage as well, indicating that the incorporation of charge-trapping species might be useful in tailoring the photo-synaptic effects of light-sensitive OIP-based memristive devices.

Light-induced synaptic effects controlled by incorporation of charge-trapping layer into hybrid perovskite volatile memristor

Spintronics, based on spin dynamics and often intimately connected to effects driven by spin-orbit coupling (SOC), is relevant in technological applications over the last decades. Creating and detecting spin currents is of crucial importance within this field. One route is the direct and inverse spin Hall effect [1, 2]. While theoretically it is the spin Hall conductivity which is routinely calculated, experimentally it is often the spin accumulation at interfaces including all imperfections which is determining the observation [3]. Understanding this spin accumulation is critical and has been studied in theoretically and experimentally to varying degrees [4, 5]. In order to explore this complicated physics, evaluating the spin galvanic and spin Hall effect [6], we utilise the semi-classical Boltzmann equation based on a first-principle, relativistic Korringa-Kohn-Rostoker Green’s function method. Varying the host and impurity material in thin metallic films will allow us to establish trends across a wide range of materials. A particular focus will be on the influence of substitutional impurities on the spin accumulation and their positional dependence. As impurities will break space inversion symmetry this will give access to both symmetric and asymmetric contributions to the spin accumulation. Finally, as interfaces are of high relevance for practical applications we will investigate the key differences arising between surface or adatom impurities.
References
[1] J. E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999).
[2] T. Kimura et al., Phys. Rev. Lett. 98, 156601 (2007).
[3] S. Takahashi and S. Maekawa, Sci. Technol. Adv. Mater. 9, 014105 (2008).
[4] G. Géranton et al., Phys. Rev. B 93, 224420 (2016).
[5] O. Gueckstock et al., Advanced Materials 123, 2006281 (2021).
[6] F. Hellman et al., Rev. Mod. Phys. 89, 025006 (2017).

Spin accumulation in metallic thin films

The deposition of a thin elemental Al layer on the clean surface of bulk insulating SrTiO3 results in the formation of surface oxygen vacancies on the oxide. As a result a two dimensional electron system (2DES) with remarkable electronic properties is obtained. The electronic structure of such 2DES, and specially the Rashba spin splitting, has been shown to be directly related with the spin-to-charge conversion process that takes place in devices based on such system [1]. In this work we will present measurements of the electronic structure of the 2DES by means of angle-resolved photoemission spectroscopy (ARPES) that are in good agreement with our self-consistent tight binding supercell calculations. Moreover, we will show how we can tailor the electronic carrier density by controlling the Al layer thickness and thus the amount of oxygen vacancies induced in the surface. We demonstrate that it is possible to tune the electronic density in Al/STO to match that of the celebrated LAO/STO system and perform a comparative study of the electronic structure in both systems. In conclusion, we show how defects such as oxygen vacancies can be used as an effective tuning knob to tailor a desired electronic structure.
[1] D.C. Vaz, et al. Nature Materials 18, 1187 (2019).
We acknowledge financial support from Comunidad de Madrid (Atracción de Talento grant No. 2018-T1/IND-10521) and by MICINN PID2019-105238GA-I00.


An ARPES Perspective on the Electronic Structure of a Two Dimensional Electron System (2DES) in Metal/SrTiO3 interfaces.

FeRh alloy is mostly known for its phase transition from antiferromagnetic (AFM) to ferromagnetic (FM) near room temperature. This transition is not only fascinating for fundamental studies on magnetism and spintronics, but also has the potential to be widely used in technological applications such as heat-assisted magnetic recording media and thermal energy management. In this work we use optical pump-THz emission spectroscopy to study spin emission from FeRh over a wide range of temperatures (20 K – 420 K). When optically pumped by a femtosecond infrared pulse FeRh emits spin current pulses, which are collected by an adjacent Pt layer and converted into a picosecond electrical pulse via the inverse spin Hall effect. At low temperature, deep in the antiferromagnetic phase, the intensity of the emitted THz is comparable to that in the ferromagnetic phase. Our measurements link the origin of the emitted spin currents to the presence of superparamagnetic islands within the antiferromagnetic matrix.

Optical Pulse - THz emission from FeRh/Pt

Ferroelectric memories based on hafnium oxide are an attractive alternative to conventional memory technologies due to their scalability and energy efficiency. However, there are still many open questions regarding the optimal material stack and processing conditions for reliable device performance. Here we shed light on the impact of the sputtering process conditions, of the commonly used TiN top electrode, on the ferroelectric properties of Hf1-xZrxO2. We highlight the importance of tuning the texturing and composition of the top electrode in achieving high remanent polarization and improved endurance.

Impact on Ferroelectricity in Hf1-xZrxO2 by TiN Top Electrode Process Conditions

Von Neumann’s architecture, currently applied to electronic computing systems, is known to be at its optimization limits since CMOS technology cannot be further miniaturized without performance degradation. Neuromorphic computation is an effective alternative as it allows for power-efficient systems with high density, in memory computation and parallel data processing. A key component for this technology is the memristor, whose conductance can be altered upon application of an electric field. These devices have striking similarities to biological synapses, revealing many potential applications in neuromorphic engineering. 
To achieve a fully integrated system consisting of memristors and thin-film transistors (TFTs), both should share the same materials. Here, we study two amorphous oxide semiconductors (AOS) as resistive switching (RS) layers on memristors: indium-gallium-zinc oxide (IGZO) and zinctin oxide (ZTO) and the engineering of the top and bottom contacts. These two materials are widely applied in TFT driver circuits of active-matrix display technology, making them an ideal choice for memristors in neuromorphic system-on-panel solutions.
In the memristive devices we present, the main intrinsic donors within the AOS, commonly referred to as oxygen vacancies, are responsible for the RS, generally following the valence change mechanism (VCM). To explore self-rectification, an asymmetry in the device structure was established. Pt was chosen for the bottom electrode as it is inert and forms a Schottky barrier when in contact with ZTO and IGZO. For the top electrode, Ti/Au was used since Ti has a high oxygen affinity which leads to the removal of oxygen from the AOS layer, forming a highly conductive region at the interface.
We show these devices can be operated at two distinct modes. Device initialization in reverse polarity leads to an abrupt VCM-type counter-8-wise filamentary switching, whereas device initialization in the forward direction reveals an 8-wise area-dependent switching with resistance state ratios up to two orders of magnitude. The latter switching mode maintain the rectifying characteristic and allows analog control over resistance states which provides the means for subsequent high-density processing and storage in a neuromorphic system.[1]
However, Pt and Ti/Au are expensive and, usually, patterned by lift-off, which limits device downsizing, reproducibility, and yield, due to irregularly shaped electrode edges, causing electrical shortcuts. A noble metal-free strategy with easy integration reduces the overall system cost: Mo was employed as bottom and top electrodes which, being patterned by dry etching, improves reproducibility and yield. Here, we present an IGZO-based memristor with Mo electrodes ready for TFT integration tests. With this memristor structure, the current rectification was achieved by oxidizing the bottom Mo at the interface. Devices down to 0.5 μm2 were fabricated using conventional photolithography, with an extraordinary yield of 100%. We
successfully demonstrate the memristor’s synaptic behavior through potentiation and depression, short-term to long-term memory transition and simulation of the learning process.[2]
[1] Casa, N., Deuermeier, J., Martins, J., Carlos, E., Pereira, M., Martins, R., Fortunato, E., Kiazadeh, A., 2D Resistive Switching Based on Amorphous Zinc–Tin Oxide Schottky Diodes. Adv. Electron. Mater. 2020, 6, 1900958.
https://doi.org/10.1002/aelm.201900958
[2] Pereira, M., Deuermeier, J., Nogueira, R., Carvalho, P. A., Martins, R., Fortunato, E., Kiazadeh, A., Noble‐Metal‐Free Memristive Devices Based on IGZO for Neuromorphic Applications. Adv. Electron. Mater. 2020, 6, 2000242.
https://doi.org/10.1002/aelm.202000242

AOS-based memristive devices towards TFT integration: Materials and challenges

As the miniaturizing limit of the traditional memory quickly approaches, resistive random-access memory (RRAM) has shown strong promise owing to simple compositions, relatively fast switching, as well as good endurance and retention. The most commonly accepted resistive switching mechanism is filamentary switching. In filamentary switching, the change in resistance arises from the growth and rupture of localized conduction filaments, usually composed of oxygen vacancies, through the insulating layer. However, the irreversible electroforming process, which is often required to activate the filamentary RRAM devices, has become one major problem in achieving scaling, and low power consumption memory devices. Thus, an electroforming-free RRAM device is highly desired. By incorporating simple binary oxides (HfO2 and CeO2), which are both CMOS compatible, in the form of vertically aligned nanocomposites, improved properties for memristor applications are expected.

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