BiFe1-xCrxO3 ferroelectric tunnel junctions for neuromorphic systems

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BiFe CrO ferroelectric tunnel junctions for neuromorphic systems Gitanjali Kolhatkar, Bernhard Mittermeier, Yoandris González, Fabian Ambriz Vargas, Marco Weismueller, Andranik Sarkissian, Carlos Gomez-Yanez, Reji Thomas, Christina Schindler, and Andreas Ruediger ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00111 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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ACS Applied Electronic Materials

BiFe1-xCrxO3 ferroelectric tunnel junctions for neuromorphic systems Gitanjali Kolhatkar,1* Bernhard Mittermeier,1,2 Yoandris González,1 Fabian Ambriz-Vargas,1 Marco Weismueller,2 Andranik Sarkissian,3 Carlos Gomez-Yanez,4 Reji Thomas,5 Christina Schindler, 2 Andreas Ruediger1* 1

Institut National de la recherche scientifique, centre Énergie, Matériaux, Télécommunications, 1650 Boulevard

Lionel-Boulet, Varennes, Québec, J3X 1S2, Canada. 2

Munich University of Applied Sciences, Department of Applied Sciences and Mechatronics, Lothstrasse 34, 80335

Munich, Germany 3

Plasmionique Inc, 9092 Rimouski, J4X 2S3, Brossard, Québec, Canada

4

Departamento de Ingeniería en Metalurgia y Materiales-Instituto Politécnico Nacional, Zacatenco, 07738,

México. 5

Division of Research and Development, Lovely Professional University, Jalandhar- Delhi G.T. Road, Phagwara,

Punjab 144411, India *

Corresponding authors e-mail: [email protected]; [email protected]

ABSTRACT We report on the fabrication of ferroelectric tunnel junctions of BiFe0.45Cr0.55O3 as tunneling barrier, Nb-doped (111) SrTiO3 as bottom electrode, and platinum as top electrode. BiFeO3 is a generic multiferroic material with perspectives for multiferroic tunnel junctions, with chromium being introduced to shift and enhance the magnetic ordering from canted magnetization to ferrimagnetism. We deposit the ferroelectric films by radio-frequency magnetron sputtering, an 1 ACS Paragon Plus Environment

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industry-compatible synthesis method. After confirming the BiFe1-xCrxO3 film composition and its partial crystallinity, we found possible indications of ferroelectricity through piezoresponse force microscopy. X-ray photoelectron spectroscopy together with optical band measurements provide the electronic band profile of the Nb:SrTiO3/BiFe1-xCrxO3/Pt structures. Pulsed electrical characterization reveals resistive switching with very high fatigue resistance (>106 cycles) consistent with direct tunneling across a trapezoidal barrier for a surface fraction of the film. These results make BiFe1-xCrxO3 a promising candidate for ferroelectric tunnel junctions in particular as they are able to operate as artificial synapses for neuromorphic circuit tiles as evidenced by spiketiming-dependent-plasticity. KEYWORDS: Ferroelectric Tunnel Junctions, Neuromorphic systems, BiFe1-xCrxO3, Tunneling electroresistance effect, Electronic band alignment, Endurance, Spike-timing-dependentplasticity.

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1. INTRODUCTION After dominating the semiconductor memory market for the past four decades due to their simple structure (1 transistor - 1 capacitor) and fast write/read access (10 ns/10 ns), dynamic random access memories (DRAM) have now reached their scalability limit.1,2 Amongst the strongest contenders to replace DRAM technology are ferroelectric tunnel junctions (FTJs).3,4 These memory devices have the advantages of being nonvolatile and highly scalable, in addition to displaying a high switching speed (10 ns)5, a high endurance (4×106 cycles)6, and operate at low switching energies. Their structure is relatively simple (1 transistor - 1 resistor) similar to PCRAM, where one cell resistor is composed of an ultra-thin (a few nanometers) ferroelectric layer deposited between two conductive electrodes.7,8 FTJs exploit resistive switching, a reversible process that consists of toggling between a high resistance state (HRS) and a low resistance state (LRS) via bipolar voltages to the device.9–12 These two states correspond to the logical “0” and “1” in binary code directly related to the orientation of ferroelectric domains. In these devices, the ferroelectric layer acts as a potential barrier,3,13–15 through which the electrons have a finite probability to tunnel quantum mechanically, an effect known as the tunneling electroresistance (TER) effect. However, for thicker films, thermionic emission can be exploited as the dominant charge transport mechanism, again with similar bi-stable resistance states.16 Among the potential candidates for FTJs, bismuth ferrite (BiFeO3) is a multiferroic material that has the advantage of presenting electric and magnetic ordering at room temperature, making it suitable for non-volatile memories.17–19 With its rhombohedrally distorted perovskite crystal structure (a=b=c=5.63 Å and α=β=γ=59.4°), BiFeO3 belongs to the R3c symmetry group and presents a spontaneous polarization along the pseudo-cubic [111] axis.20 Its ferrolectricity (Curie temperature ~1100 K21) is due to the dangling bonds of the stereochemically active Bi3+ lone 3 ACS Paragon Plus Environment


pairs,22 while its antiferromagnetism (Néel temperature ~640 K21) originates from G-type ordering.23 In addition, neutron diffraction revealed a cycloidal antiferromagnetic order in the material.24,25 This material also presents magnetoelectric26 and photovoltaic27 properties, the latter making it appropriate for energy harvesting applications. The multiferroic nature of BiFeO3 makes it promising for multiferroic FTJs, in which four logical states (due to electrical and magnetic dipoles) can be exploited to encode information.3 Consequently, FTJs based on BiFeO3 as the potential barrier were reported in the past.28,29 Moreover, to enhance the material’s magnetic properties, Cr3+ ions can be incorporated in the crystal.30 This improvement of the magnetoelectricity, occurring through Cr3+ and Fe3+ interactions i.e. ferrimagnetic ordering,31 is an important parameter in the development of multiferroic tunnel junctions.3 The introduction of Cr3+ in the BiFeO3 crystal has led to the development of double-perovskite Bi2FeCrO6, a material that combines enhanced ferroelectric and ferromagnetic properties to promising photovoltaic properties.32–34 In previous work, we reported on FTJs using BiFe1-xCrxO3 and thermionic emission as the dominant charge transport mechanism for the LRS, demonstrating the potential of this material.35 Here, we fabricate BiFe0.45Cr0.55O3 ferroelectric tunnel junctions using niobium (Nb) doped (111) oriented strontium titanate (SrTiO3) and platinum (Pt) as the bottom and top electrodes, respectively. While BiFe1-xCrxO3 was mainly synthesized by pulsed laser deposition,31 chemical solution deposition36 or through microwave-assisted hydrothermal synthesis,35,37 we here exploit the more industry favored radio-frequency (RF) magnetron sputtering. After reconstructing the electronic band profile of our structures, we demonstrate the resistive switching in our devices through electrical characterizations. By modeling our results with Brinkman’s direct tunneling current model, we confirm the presence of a TER effect in our memory cells. Recently, resistively


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switching memory has attracted great attention due to possible applications in neuromorphic computing.38,39 It could be crucial for a power-efficient implementation of machine learning methods by using resistive memory cells as artificial synapses in artificial neural networks. For this purpose, spike-timing-dependent-plasticity (STDP) measurements have been exploited to investigate the devices’ potential for such applications.40–42 Using this method, we show synaptic learning behavior in our FTJs. 2. METHOD Material synthesis. The films are deposited by on-axis RF magnetron sputtering on conductive (111) Nb:SrTiO3 substrates. Prior to the depositions, the substrates are prepared following a method described elsewhere to ensure a atomically flat substrate surface displaying 0.22 nm step height with uniform surface termination.43 The sputtering tool consists of a table top multi-targets SPT310 computer controlled system from Plasmionique Inc. Before the deposition, the chamber is pumped down, with a dry pumping station, to a base pressure of ~10-6 Torr. The target is then sputter cleaned for 15 min by pre-sputtering to minimize contaminations and to ensure its homogeneity. During the deposition, the substrate temperature is kept at 600°C using a heated substrate holder. A sputtering pressure of 25 mTorr is employed and the sputtering medium consists of Ar and O2 mixture (O2:Ar ratio of 1:1 sccm). A power of 20 W (~5 W/cm2) is applied on the commercially available, 1inch diameter, 50%-50% BiFeO3-BiCrO3 target. For the electrical characterizations, ~45 nm thick Pt top electrodes with a diameter of ~300 µm are deposited at room temperature on the films’ surface by the same rf-magnetron sputtering unit with a 1 inch diameter Pt target and a sputtering pressure of 6.0 mTorr. Nanoscale characterization. The surface topography is imaged by atomic force microscopy (AFM), with an AIST-NT Smart SPM system operating in tapping mode (0.5 Hz scan rate, 5 ACS Paragon Plus Environment


Nanosensor AFM probe with a 10 nm radius Si tip). For the piezoresponse force microscopy (PFM), an AC voltage is applied through a conductive ~30 nm radius Pt-Ir coated Si-tip acting as a mobile top electrode and placed in contact with the film surface, while a conductive bottom substrate (Nb-SrTiO3) is employed as bottom electrode. The maps are acquired by applying a negative electrical bias potential first, followed by a positive one over different areas of the sample. Electrical characterisation. The current-voltage (I-V) and fatigue behavior of the Nb:SrTiO3/BiFe1-xCrxO3/Pt memory cells are measured with a Keysight B2091A Precision Source/Measure Unit (SMU). Two straight 12 µm radius tungsten measuring probes are carefully connected to the top electrode and, through a scratched surface and silver paste, with the bottom electrode. With a sweep limit of -6V to +6V and 0.025 V increments, the I-V curves are measured. For the fatigue tests, pulsed switching measurements are performed with write/erase pulses with an amplitude of 7V/-7V and a width of 10 ms. The fact that we do not observe a dielectric breakdown of the structure is attributed to the depletion layer in the Nb:SrTiO3 electrode for positive bias and the possible presence of an interfacial capacitance at the platinum interface due to the fact that the vacuum had to be broken for the deposition of the top electrodes through a shadow mask, both of which act as a voltage divider. For reading, 0.8 V is fixed as the base write/erase pulses (see the Supporting Information, section 1). In order to assess the suitability of the cells for neuromorphic applications we perform spiketiming-dependent-plasticity (STDP) measurements with an ArC ONE ™ hardware platform. We use identical pre- and post-synaptic voltage spikes for a simple model of biological action potentials with a positive triangular pulse and a negative triangular tail.44 The pulse has a maximum amplitude of 4 V and a width of 167 µs and the tail features a peak voltage of -2 V with a length of 833 µs. 6 ACS Paragon Plus Environment

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Structural and chemical characterization. The X-ray diffraction measurements (XRD) are performed in θ-2θ using a Panalytical X’Pert Pro Diffractometer with a 1.5418 Ǻ Cu Kα source. The samples’ chemical composition is determined by X-ray photoelectron spectroscopy (XPS) with a VG Escalab 220iXL operating with an Al Kα source (15 kV, 20 mA) in normal emission geometry. For the general survey, a pass energy of 100 eV is employed, with 20 eV for the highresolution scans. The reported binding energies are all referenced to the 285.0 eV adventitious C 1s hydrocarbon peak, and the Au 4f5/2 (87.0 eV) and 4f7/2 (84.0 eV) lines are used to calibrate the energy scale, with an uncertainty of 0.05 eV.

3. RESULTS AND DISCUSSION Figure 1(a) presents a typical topography image of BiFe0.45Cr0.55O3 films. Atomic step-terraces features, corresponding to the SrTiO3 substrate, are still observable. These terraces are covered with small grains, that could be due to favorable nucleation sites created by the high substrate stepdensity, in general the surface morphology of the as-deposited film suggests a Stranski-Krastanov growth mode which is characterized by the combination of the layer-by-layer and island growth modes.45,46 A root-mean-square (rms) roughness of ~0.63 nm was determined for a single terrace. XRD measurements (Figure 1(b)) presents the (222) BiFe1−xCrxO3 and (222) SrTiO3 diffraction peaks at 2θ~86°.47 The substrate peak depicts a doublet due to the Kα1 and Kα2 parasitic signatures. This diffraction pattern attests to the partial crystallinity and oriented growth of the BiFe1−xCrxO3 films along the (111)-crystallographic direction.



Figure 1. Typical (a) 1 µm × 1µm topography image and (b) XRD pattern of BiFe1−xCrxO3 films deposited on a (111) SrTiO3 substrate by RF magnetron sputtering. The films’ composition and stoichiometry are investigated by XPS (Figure 2). Figure 2(a) presents the general survey in the 1200-0 eV range. It shows that the film surface contains Fe, Cr, Bi, O and C. Figure 2(b)-(d) depict high resolution scans in the energy range of Bi 4f, Fe 2p, and Cr 2p, respectively. They indicate that Bi 4f is composed of four peaks linked to the Bi-O bond, with two broad peaks located at 163.20±0.05 eV (Bi 4f5/2) and 157.90±0.05 eV (Bi 4f7/2), and two satellite peaks at 164.60±0.05 eV and 159.20±0.05 eV. The Fe 2p consists of two main Fe-O peaks at 710.50±0.05 eV (Fe 2p3/2) and 723.30±0.05 eV (Fe 2p1/2), while Cr 2p presents two main Cr-O peaks at 587.90±0.05 eV (Cr 2p1/2), and 578.55±0.05 eV (Cr 2p3/2). Based on the peak area of each element, the chemical composition of the as-deposited film, and a Fe/Cr ratio of ~0.8 are determined. The concentration of Cr in the film is higher than that of Fe because of its higher sputtering yield.


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Figure 2. Typical XPS spectra acquired on a BiFe1−xCrxO3 film displaying (a) the general survey, (b) Bi 4f, (c) Fe 2p, and (d) Cr 2p. The black dotted lines correspond to the background curves obtained with the Shirley method. To investigate the ferroelectric properties and the resistive switching, the films are deposited for 4 min on a conductive (111) Nb (0.5 wt %) doped SrTiO3 substrate with the experimental conditions mentioned before. Using X-ray reflectometry (XRR) measurements, the film thickness



is determined to be 6.5 nm (see Supporting Information section 2). PFM measurements, illustrated in Figure 3, were exploited to study the films’ ferroelectricity. In this image, the dark regions are negatively poled (green square), while the brighter regions are positively poled (inner blue square). While the weak contrast thereby obtained could have different origins,48 it could indicate a poor ferroelectric retention due to the high concentration of Cr in the films.37 The remaining portion of the map is in its pristine state and reveals a preferential spontaneous downward polarization.

Figure 3. Typical PFM phase image acquiered on a 6.5 nm thick BiFe1−xCrxO3 film. Using our BiFe1−xCrxO3 films, we fabricate a series of devices by depositing Pt top electrodes (TE) on the films surface, while the conductive substrate acts as a bottom electrode (BE). As a side note, we need to break the vacuum in order to apply the shadow mask, a process step, which has been demonstrated to possibly introduce unintended interface states.7 Prior to the electrical measurements, we reconstructed the band alignment and electronic band profile with XPS measurements (Figure 4) in order to determine the dominant charge transport mechanism in our devices.49


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Figure 4. XPS spectra obtained on (a) a Pt reference substrate, (b) BiFe1−xCrxO3/Pt, (c) BiFe1−xCrxO3 reference film, (d) a Nb-SrTiO3 reference substrate, and (e) BiFe1−xCrxO3/SrTiO3. The valence band offset (VBO) at the Nb-SrTiO3/BiFe1-xCrxO3 interface is described by the valence band maxima (VBM) of the BiFe1-xCrxO3 and the Nb-SrTiO3 substrate according to 𝑉𝐵𝑂1 = (𝐸𝐵𝑖4𝑓7/2 − 𝑉𝐵𝑀)𝐵𝐹𝐶𝑟𝑂 − (𝐸𝑆𝑟3𝑑5/2 − 𝑉𝐵𝑀)𝑆𝑇𝑂 − (𝐸𝐵𝑖4𝑓7/2 − 𝐸𝑆𝑟3𝑑5/2 )𝐵𝐹𝐶𝑟𝑂/𝑆𝑇𝑂 . (1) The VBO at the BiFe1-xCrxO3/Pt interface is calculated according to 𝑉𝐵𝑂2 = (𝐸𝐵𝑖4𝑓7/2 − 𝐸𝑃𝑡4𝑓7/2 )𝐵𝐹𝐶𝑟𝑂/𝑃𝑡 + (𝐸𝑃𝑡4𝑓7/2 − 𝐸𝐹 )𝑃𝑡 − (𝐸𝐵𝑖4𝑓7/2 − 𝑉𝐵𝑀)𝐵𝐹𝐶𝑟𝑂 .

(2)

Using the Kraut method, we calculate the VBM values.50 The energy differences are determined as described in Figure 4. Using nominally pure Nb-doped (111) SrTiO3 and



polycrystalline Si/SiO2/Al2O3/Pt substrates, respectively, we determine that

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(ESr3d5/2-

VBM)STO=130.82±0.05 eV, and (EPt4f7/2-EF)Pt=70.50±0.05 eV. Furthermore, by characterizing BiFe1-xCrxO3 films deposited on Nb-doped (111) SrTiO3 and polycrystalline Si/SiO2/Al2O3/Pt substrates, respectively, the energy differences (EBi4f7/2-VBM)BFCrO=157.27±0.05 eV, (EBi4f7/2ESr3d5/2)BFCrO/STO=26.04±0.05 eV, and (EBi4f7/2-EPt4f7/2)BFCrO/Pt=87.71±0.05 eV are calculated. VBO1=0.41±0.05 eV, and VBO2= 0.94±0.05 eV are obtained by incorporating the above values into Eq. (1) and (2), respectively. Finally, the electric potential step heights (φ) are determined to be 2.39±0.07 eV at the Nb:SrTiO3/BiFe1-xCrxO3 interface, and 1.86±0.07 eV at the BiFe1-xCrxO3/Pt interface knowing 𝜑 = 𝐸𝑔 − 𝑉𝐵𝑂,

(3)

where Eg is the bandgap (2.8 eV51). The reconstructed electronic band profile is presented in Figure 5. This configuration corresponds to the BiFe1-xCrxO3 film in the spontaneous downward polarization state.


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Figure 5. Electronic band profile of the Nb:SrTiO3/BiFe1-xCrxO3/Pt structure for a spontaneous downward polarization. The electrical behavior of our devices is investigated to confirm the presence of resistive switching. A typical I-V curve obtained on the Nb:SrTiO3/BiFe1-xCrxO3/Pt structure is depicted in Figure 6(a). In these measurements, a negative voltage is applied to the top electrode. A hysteretic behavior, with a high and a low resistance state is observed, attesting to the presence of resistive switching in our devices. The HRS is linked to the upward ferroelectric polarization, toward TE while the



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LRS corresponds to the downward ferroelectric polarization, toward BE. An average RHRS/RLRS ratio of ~4 is obtained at a voltage of 0.8V.

Figure 6. (a) Typical I-V behavior recorded on a Nb:SrTiO3/BiFe1-xCrxO3/Pt device presenting a high and a low resistance state, and (b) fit in the low voltage range using the direct tunneling model (red) for the LRS (black dots) and HRS (green squares). The shape of the I-V curve suggests a TER effect. To confirm that the resistive switching observed here is indeed due to the TER effect, we compared the experimental results in the low voltage range (-0.2 V to +0.2 V) with a model that describes tunneling across a trapezoidal barrier, as illustrated in Figure 6(b). This voltage range was chosen as direct tunneling is predominant in this region.15 The theoretical model we use, known as the Brinkman model, describes the tunneling current density J such as52

4𝑒𝑚

𝐽 = − (9𝜋2 ћ3 ) (

3 3 𝑒𝑉 𝑒𝑉 2 2 1 1 𝑒𝑉 𝑒𝑉 𝛼2 (𝑉)[(𝜑2 − )2 −(𝜑1 + )2 ]2 2 2

exp⁡{𝛼(𝑉)[(𝜑2 − )2 −(𝜑1 + )2 ]}

1

3

𝑒𝑉 2

1

𝑒𝑉 2 𝑒𝑉

) × sinh⁡{2 𝛼(𝑉) [(𝜑2 − 2 ) − (𝜑1 + 2 ) ] 2 },

(4)

where e is the electron charge, m, the electron mass (9.11×10-31 kg)52, ћ is the reduced Planck constant, 𝛼(𝑣) = [4𝑑(2𝑚)1⁄2 ⁄[3ћ (𝜑1 + 𝑒𝑉 − 𝜑2 )] and d is the BiFexCr1-xO3 layer thickness. 14 ACS Paragon Plus Environment

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Using Eq. (4) and the potential barrier values for the spontaneous downward polarization obtained by XPS, (φ1=2.39 eV and φ2=1.86 eV), the theoretical I-V curve for the LRS is plotted and fitted to the experimental curve by varying only a scaling factor.52,53 Then, using the same scaling factor, presumed to be the same for both polarization states, the theoretical I-V curve is fitted to the HRS by changing the potential barrier heights. As a result, potential barriers of φ1=3.41 eV and φ2=1.83 eV are obtained for the HRS. As shown in Figure 6(b), an excellent agreement is reached between the theoretical and experimental curves for both the LRS and HRS. This analysis reveals potential barrier changes Δφ1 and Δφ2 of 1.02 eV and 0.03 eV respectively. Indeed, in an FTJ, the electronic band profile varies with the polarization orientation. A downward polarization (towards BE, LRS), will induce negative screening charges building up in the bottom electrode at the interface with the BiFexCr1-xO3 film. As a result, the conduction band will bend downward, causing a decrease of the potential barrier height and allowing the current to flow (Figure 7(a)). Moreover, for an upward polarization (towards TE, HRS), positive screening charges will build up at the SrTiO3/BiFexCr1-xO3 interface. The conduction band will then bend upward, and the potential profile will increase, preventing the current flow, Figure 7(b).3,4,54 Furthermore, XRR reveals that our BiFexCr1-xO3 film has an average thickness of ~6.5 nm, a value too high for tunneling to occur. Yet, XRR gives the average thickness over a relatively wide area, and does not take into account the inhomogeneity of the film. The AFM results indicate that the film surface is composed of grains, attesting to its inhomogeneity.



Figure 7. Schematic of the band profile of the Nb:SrTiO3/BiFe1-xCrxO3/Pt device in (a) the LRS and (b) the HRS. To evaluate the endurance of the devices, we recorded the resistances of the LRS and HRS on a single electrode at a read voltage of 0.8 V over 1.6×106 cycles using pulsed measurements. Figure 8(a) presents the evolution of the high and low resistances at a read voltage of 0.8 V. We can see that the resistances do not change significantly, attesting to the high endurance of our devices, a detailed analysis of the underlying mechanisms for the observed changes is under way. Moreover, we perform I-V measurements on 14 different memory cells to study the reproducibility, Figure 8(b). Resistive switching is observed on all the cells. The variations in the RHRS/RLRS ratio are attributed to inhomogeneities of the surface.


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Figure 8. Resistance value for the HRS (red circles) and LRS (black squares) at a read voltage of 0.8 V (a) over 1.6×106 cycles, and (b) over 14 different memory cells. Ferroelectric switching consists in a two-step process, where the nucleation occurs first, followed by lateral domain growth.55,56 The lateral domain wall mobility is governed by a creep phenomenon. Disorder in the material result in microscopic potential energy changes that impinge domain wall motion.57 One common source of pinning in ferroelectric material is oxygen vacancies.55,58 During fatigue measurements, oxygen vacancies will form and cluster and therefore result in pinning,59 ultimately leading to a loss of reversible polarization and thus resistance ratio between LRS and HRS. Resistively switching memory capable of multiple resistance levels have demonstrated their potential as artificial synapses in brain-like artificial neural networks for efficient, brain-inspired computation processes.38 Artificial synapses have to show learning behavior: external stimulation with voltage spikes leads to a gradual change of their resistance. To that end, memory cells must have the ability to access intermediate resistance states between HRS and LRS. For FTJs this can be enabled by a multi-domain switching process, where a gradual polarization switching of 17 ACS Paragon Plus Environment


multiple domains between bottom and top electrode results in a gradual change of resistance.28,56,60 The learning behavior of a synapse can be investigated by spike-timing-dependent-plasticity (STDP), which has been detected in hippocampal cells of rats as well as in artificial resistively switching memory devices.61,62 A STDP measurement mimics the voltage stimulation of two neurons to the synapse interfacing them. Both the pre- and post-synaptic neuron send voltage spikes which reach the synapse with different polarization. If the time difference (Δt) between preand post-synaptic spike is zero, they cancel each other out and do not affect the memory cell. If the pre-synaptic spike precedes the post-synaptic spike (positive Δt), the positive tail of the presynaptic spike superimposes with the positive pulse of the post-synaptic spike and together exceed the threshold voltage necessary to switch the cell (see inset of Figure 9).


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Figure 9. STDP measurements on the Nb:SrTiO3/BiFe1-xCrxO3/Pt device. The insets show how the pre- and post-synaptic voltage spikes superimpose to a cumulative voltage. For appropriate Δt, this cumulative voltage can exceed the voltage threshold (dotted line) necessary for a gradual resistance change of the cell.

4. CONCLUSIONS We have fabricated FTJs with an ultrathin ferroelectric BiFexCr1-xO3 layer as the tunneling barrier by RF magnetron sputtering. The surface topography consists of atomic step-terraces and some granular features. XRD measurements confirmed crystallinity of the films. With (111) oriented



Nb-SrTiO3 as bottom electrode and Pt as top electrode, we have fabricated FTJs. Electrical measurements have confirmed the presence of resistive switching with an RHRS/RLRS ratio of ~4 at 0.8 V. After reconstructing the electronic band profile and correlating it with the Brinkman model, we have identified direct tunneling as the dominant charge transport mechanism. Potential barrier values of φ1=2.39 eV and φ2=1.86 eV and φ1=3.41 eV and φ2=1.83 eV have been obtained for the LRS and HRS, respectively. While XRR revealed an average thickness of ~6.5 nm, tunneling is considered to occur in statistically distributed regions of lower thickness. By performing endurance and reproducibility studies, we were able to switch the devices 1.6×106 times, on 14 different electrodes. Through STPD measurements, we have shown that our FTJs synaptic learning behavior, making them are suitable for neuromorphic computing, consistently with previously reported work.40–42 Supporting Information. Schematic of the pulses used for the fatigue measurements; Thickness measurements by XRR. ACKNOWLEDGEMENTS G.K. is grateful for an FRQNT postdoctoral scholarship, and F. A.-V., for the financial support of CONACyT (National Council of Science and Technology-Mexico) as well as an individual FRQNT MELS PBEEE 1M scholarship. A.R. acknowledges funding from and NSERC discovery grant (RGPIN-2014-05024) and two NSERC strategic partnership grants (506289-2017; 50695317). B.M. and C.S. gratefully acknowledge financial support from Bayerische Forschungsallianz, project 15.312. REFERENCES (1)

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BiFe1-xCrxO3 ferroelectric tunnel junctions for neuromorphic systems

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