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# Oxygen vacancy engineering of TaOₓ-based resistive memories by Zr doping for improved variability and synaptic behavior

To cite this article: João H Quintino Palhares et al 2021 *Nanotechnology* **32** 405202

View the [article online](#) for updates and enhancements.

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# Oxygen vacancy engineering of TaOₓ-based
resistive memories by Zr doping for
improved variability and synaptic behavior

João H Quintino Palhares<fnref n="1" />,<fnref n="2" />,* ¹D, Yann Beilliard<fnref n="2" />,<fnref n="3" />,<fnref n="4" /> ¹D, Fabien Alibart<fnref n="2" />,<fnref n="3" />,<fnref n="5" />,
Everton Bonturim<fnref n="6" /> ¹D, Daniel Z de Florio<fnref n="1" /> ¹D, Fabio C Fonseca<fnref n="7" /> ¹D,
Dominique Drouin<fnref n="2" />,<fnref n="3" />,<fnref n="4" />,* and Andre S Ferlauto<fnref n="1" />,* ¹D

E-mail: quij2205@usherbrooke.ca, andre.ferlauto@ufabc.edu.br and dominique.drouin@usherbrooke.ca

Received 21 April 2021

Accepted for publication 23 June 2021

Published 12 July 2021

<figure><img src="image_2.png" /></figure>

CrossMark

## Abstract

Resistive switching (RS) devices are promising forms of non-volatile memory. However, one of the biggest challenges for RS memory applications is the device-to-device (D2D) variability, which is related to the intrinsic stochastic formation and configuration of oxygen vacancy ($V_O$) conductive filaments (CFs). In order to reduce the D2D variability, control over the formation and configuration of oxygen vacancies is paramount. In this study, we report on the Zr doping of TaOₓ-based RS devices prepared by pulsed-laser deposition as an efficient means of reducing the $V_O$ formation energy and increasing the confinement of CFs, thus reducing D2D variability. Our findings were supported by XPS, spectroscopic ellipsometry and electronic transport analysis. Zr-doped films showed increased $V_O$ concentration and more localized $V_O$s, due to the interaction with Zr. DC and pulse mode electrical characterization showed that the D2D variability was decreased by a factor of seven, the resistance window was doubled, and a more gradual and monotonic long-term potentiation/ depression in pulse switching was achieved in forming-free Zr:TaOₓ devices, thus displaying promising performance for artificial synapse applications.

Supplementary material for this article is available online

Keywords: TaOₓ, oxygen vacancy engineering, memristor, variability, doping, resistive switching, synaptic behavior

(Some figures may appear in colour only in the online journal)

<figure><img src="image_3.png" /></figure>

<footnote n="1">¹CECS, Federal University of ABC, Santo André 09210-580, SP, Brazil</footnote>

<footnote n="2">²Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, Sherbrooke J1K 0A5, Canada</footnote>

<footnote n="3">³Laboratoire Nanotechnologies Nanosystèmes (LN2)—CNRS UMI-3463—3IT, CNRS, Sherbrooke J1K 0A5, Canada</footnote>

<footnote n="4">⁴Institut Quantique (IQ), Université de Sherbrooke, Sherbrooke J1K 2R1, Canada</footnote>

<footnote n="5">⁵Institute of Electronics, Microelectronics and Nanotechnology (IEMN), Université de Lille, F-59650, Villeneuve d'Ascq, France</footnote>

<footnote n="6">⁶Department of Chemistry, School of Engineering, Mackenzie Presbyterian University, 01302907, São Paulo, SP, Brazil</footnote>

<footnote n="7">⁷Nuclear and Energy Research Institute, IPEN-CNEN, São Paulo, 05508-000, Brazil</footnote>

<footnote>* Authors to whom any correspondence should be addressed.</footnote>
# 1. Introduction

Resistive switching (RS) memories based on oxide thin films are a promising technology for next-generation high-density non-volatile memory applications and the hardware implementation of artificial neural networks [1–4], due to their impressive switching operation performance in terms of speed, endurance, scaling and multistate resistance modulation [5–8]. However, both cycle-to-cycle (C2C) and device-to-device (D2D) variability in the switching characteristics still hinders the widespread use of this technology, as these reduce the average performance and generate cost increases due to the need for complex overhead circuits [5, 9, 10]. RS is generated by the creation and dissolution of conductive channels within an oxide layer in a capacitor-like structure, as a result of the nanoionic movement of oxygen vacancies ($V_O$) under the application of high electric fields [11, 12]. An initial operation step called a forming process is typically required in order to generate sufficient $V_{OS}$ to allow for the formation of conductive filaments (CFs). The stochastic nature of the forming and switching steps leads to slightly different CF configurations for each device, resulting in D2D and C2C variability [5, 10, 12–16]. Understanding and control of the formation, movement and arrangement of $V_{OS}$ in RS devices is therefore paramount [13, 17, 18].

Various strategies have been explored for the creation of excess oxygen vacancies in a controlled manner, in order to mitigate forming-induced variability [19–21]. The most common and direct method of $V_O$ generation is intrinsic doping, i.e. the use of low oxygen partial pressure during fabrication of the oxide layer, using methods such as reactive sputtering or pulsed-laser deposition (PLD). This approach has been particularly successful for tantalum oxide films [20–22]. Intrinsic doping has certain limitations, however, as it is difficult to control, relies on external parameters and is subject to instabilities in the processing conditions [17, 23–25]. A more reliable alternative is extrinsic doping, which consists of adding dopant atoms with a valence that is lower than the host metal cation; this leads to the formation of $V_{OS}$ in order to maintain electroneutrality [12, 17, 26, 27]. Extrinsic doping also helps to reduce the randomness of $V_O$ generation. Theoretical studies of $Ta_2O_5$ doped with Ti, Zr and Al have indeed shown that the formation of $V_{OS}$ is favored near the dopant species [26]. In these cases, charged $V_{OS}$ are compensated and attracted by p-type dopants, forming dopant-$V_O$ complexes. These complexes trap $V_{OS}$, and can interconnect to form CFs. A reduction in variability has been experimentally observed for extrinsically doped $TaO_x$ [28] and $HfO_2$ based [29, 30] devices. It has been claimed that implanted or embedded dopant elements (e.g. Gd, Al) in $HfO_2$ help to localize $V_O$ formation and lead to more confined CFs, resulting in significantly lower device variability. This interaction between the dopant species and $V_{OS}$ can change the switching dynamics and switching voltage operation [18], meaning that extrinsic doping can be leveraged to further optimize switching performance for specific applications, such as artificial synapses.

In this work, we report the fabrication of $TaO_x$-based memristors with reduced D2D variability and controllable pulse switching dynamics, thanks to doping of the active layer with Zr. For comparison purposes, Zr-doped ($Zr:TaO_x$) and pure $TaO_x$-based devices were prepared in an oxygen-poor atmosphere using PLD. Oxygen-poor conditions are used to suppress electroforming step and its effect on the active layer properties. The chemical compositions of both types of devices were investigated using x-ray photoelectron spectroscopy (XPS) analysis and spectroscopic ellipsometry (SE). RS investigations in DC and pulsed mode revealed that D2D variability was decreased by a factor of seven, the resistance window was doubled and more gradual and monotonic long-term potentiation and depression (LTP/LTD) in pulse switching was achieved in $Zr:TaO_x$ devices. By analyzing the device current density–electric field (J–E) curves using a hopping transport model, we demonstrate that $Zr:TaO_x$ devices have a higher $V_O$ concentration and a lower variability of this concentration between different devices.

# 2. Methods

Devices were fabricated on silicon substrates covered with 200 nm of $SiO_2$ (figure 1(a)). Sputtering was used to deposit tungsten (W) on the bottom (BE) and top electrodes. The latter were patterned to give 80 µm round pads, using UV photolithography and lift-off. $TaO_x$ and $Zr:TaO_x$ layers were deposited at room temperature via PLD (with a TSST system) using a $Ta_2O_5$ target and a $Ta_2O_5:20 mol\% ZrO_2$ target, respectively, at a target–substrate distance of 45 mm, with a laser fluence of 3 J cm⁻² and a repetition rate of 10 Hz under an oxygen flow of 2 sccm. Layer thickness was estimated to be 10 nm by using the growth rate as determined by SE measurements. Based on previous studies [22], we used an $O_2$ partial pressure of $2 \times 10^{-2}$ mbar to induce the formation of slightly sub-stoichiometric active layers. The generation of $V_{OS}$ by extrinsic and intrinsic doping was investigated using XPS (KRATOS Axis UltraDLD), SE (JA Woollam, model M-2000) and J–E conduction transport analysis of the devices. The electrical properties of the RS devices under DC and pulsed conditions were carried out with a Keithley S4200 semiconductor parameter analyzer and a Keithley 4225-PMU module. For all measurements, the BE was grounded and the signals were applied to the top electrodes.

# 3. Results and discussions

The thin films were analyzed using XPS and SE. The ratios (O/M) between oxygen and the sum of the metal atoms ($M = [Ta]$ for $TaO_x$ and $M = [Ta] + [Zr]$ for $Zr:TaO_x$) was determined by quantification of the Ta 4f, Zr 3d and O 1s peaks shown in figures 1(b)–(d). Due to the low $pO_2$ during deposition, $TaO_x$ and $Zr:TaO_x$ films had O/M ratios of 2.45 and 2.25, respectively. This is lower than the value of O/M = 2.5 corresponding to the oxygen concentration of the stoichiometric tantalum pentoxide ($Ta_2O_5$), thus indicating the presence of $V_O$ [20, 21]. This effect was also confirmed
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) Schematic of W/Zr:TaOx/W and W/TaOx/W RS devices and active layer characterization results: (b) XPS analysis of Ta 4f, (c) O 1s and (d) Zr 3d peaks for the Zr:TaOx and TaOx samples. (e) Absorption spectra of TaOx and Zr:TaOx obtained via SE.</figcaption></figure>

by the change in the XPS Ta 4f peak, which suggests the presence of Ta cations in lower valence states (Ta⁴⁺ and Ta³⁺) (figure 1(b)). The energy positions of the chemical components Ta⁵⁺, Ta³/⁴⁺ and O 2s were 26.3, 25.7 and 22 eV, respectively. For the Ta 4f components, the area ratio was fixed at 0.75 and the spin-orbit splitting at 1.9 eV [20, 31]. The presence of Ta in a lower oxidation state is expected to result from the partial reduction of tantalum oxide (intrinsic doping), as given by the following reaction in Kröger-Vink notation [32]

$$2\text{Ta}_{\text{Ta}}^{x} + \text{O}_{\text{O}}^{x} \rightarrow 2\text{Ta}_{\text{Ta}}' + \text{V}_{\text{O}}^{\ddagger} + \frac{1}{2}\text{O}_{2(\text{g})}. \quad (1)$$

As expected, the Zr-doped films are more sub-stoichiometric, since the presence of Zr⁴⁺ cations induces V₀ formation

according to the reaction

$$2\text{ZrO}_2 \xrightarrow{\text{Ta}_2\text{O}_5} 2\text{Zr}'_{\text{Ta}} + \text{V}^{\ddagger}_{\text{O}} + 4\text{O}_x^x. \quad (2)$$

Furthermore, as shown in figure 1(c), the O 1s peaks have an asymmetric line shape that can be deconvoluted into two components centered at 531.3 and 530.0 eV; these can be assigned to non-lattice (Onl) and lattice oxygen, respectively. The non-lattice oxygen peak can be associated with the presence of V₀ [33, 34]. The fact that Zr:TaOx films contain a larger percentage of non-lattice oxygen (26%) compared to TaOx films (20%) suggests that Zr⁴⁺ substitution indeed promotes V₀ formation. This is in agreement with results reported by Park et al [35] for KNBO films doped with Cu²⁺, and by Kim et al [36] for Si-doped tantalum oxide-based memristors. The XPS spectrum for the Zr 3d peak was deconvoluted into Zr 3d₃/₂ and Zr 3d₅/₂, centered at 182.4 and 184.7 eV respectively [37–39], corresponding to Zr⁴⁺ states (figure 1(d)). The nominal relative concentration of Zr with respect to the concentration of Ta atoms [Zr]/[Ta] was determined as 8%.

Figure 1(e) shows the optical absorption spectra obtained by fitting the ellipsometry data using a Tauc-Lorentz analytical function, which is widely used to describe the optical properties of amorphous and polycrystalline semiconductors and dielectric films [40, 41]. In this function, the imaginary part of the dielectric function is described by a Tauc expression for photon energies above the gap, and is forced to zero below the band gap. The overall interband transition in the higher range of photon energies analyzed here is described by a Lorentz oscillator. Although this function can be used to describe pure stoichiometric dielectric Ta₂O₅ films, absorption in the sub-gap region is observed for sub-stoichiometric films. This feature can be modeled using a broad Lorentz function. Sub-gap absorption has been observed previously in sub-stoichiometric TaOₓ films [42], and was associated with the presence of V₀s [43–45]. From figure 1(e), it can be seen that the optical responses of both TaOₓ and Zr:TaOₓ exhibit considerable sub-gap absorption, thus confirming the XPS results. It is interesting to note that the optical absorption intensity is similar for both samples, even though the Zr-doped films have a higher V₀ concentration. One possible explanation for this is that the dopant-V₀ interaction may suppress the optical activity of the V₀ due to changes in the charge [18, 44].

To gain insight into the effects of Zr doping on the characteristics and variability of RS, current- and DC voltage-controlled sweeps were used for SET and RESET operations, respectively. For each device, in the SET and RESET operation, the current compliance and the maximum operation voltage values, respectively, were defined to maximize the device resistance window while avoiding irreversible damage. These values slightly vary among devices due to an intrinsic variation in the threshold voltage and maximum resistance window of each device. Figure 2 shows RS characterizations of W/TaOₓ/W (figure 2(a)) and W/Zr:TaOₓ/W (figure 2(b)) devices. Up to five full RS cycles were performed on each device, and the resistance state was measured at 0.2 V. For
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Current-voltage (I-V) characteristics for the 10 devices tested, with a statistical analysis of the resistance states per device: (a) W/TaOx/W RS devices and (b) W/Zr:TaOx/W RS devices. To improve visibility, the switching curve for one random device is shown in color. The resistance was measured at 0.2 V. The resistance values for the Zr:TaOx devices exhibit improved D2D variability.</figcaption></figure>

<table><caption>**Table 1.** Comparison of key parameters of Zr:TaOx- and TaOx-based memristors</caption><thead><tr><th rowspan="2"></th><th colspan="2">Zr:TaO<sub>x</sub></th><th colspan="2">TaO<sub>x</sub></th></tr><tr><th>μ ± σ</th><th>CV (%)</th><th>μ ± σ</th><th>CV (%)</th></tr></thead><tbody><tr><td>LRS/HRS</td><td>21.8 ± 3.3</td><td>15.1</td><td>12.5 ± 7.6</td><td>60.8</td></tr><tr><td>R<sub>LRS</sub> (kΩ) @0.2 V</td><td>0.70 ± 0.07</td><td>10.0</td><td>3.7 ± 2.7</td><td>73.0</td></tr><tr><td>R<sub>HRS</sub> (kΩ) @0.2 V</td><td>14.5 ± 2.2</td><td>15.2</td><td>46.5 ± 49.6</td><td>106.7</td></tr><tr><td>J(A cm<sup>-2</sup>) @0.2 V - LRS</td><td>6.0 ± 0.7</td><td>11.6</td><td>1.4 ± 0.7</td><td>50.0</td></tr><tr><td>J(A cm<sup>-2</sup>) @0.2 V - HRS</td><td>0.30 ± 0.05</td><td>16.7</td><td>0.20 ± 0.10</td><td>50.0</td></tr><tr><td>V<sub>SET</sub> (V)</td><td>2.3 ± 0.4</td><td>17.4</td><td>1.7 ± 0.5</td><td>29.4</td></tr><tr><td>V<sub>RESET</sub> (V)</td><td>2.3 ± 0.5</td><td>21.7</td><td>2.0 ± 0.6</td><td>30.0</td></tr><tr><td>a (nm) - HRS</td><td>0.6 ± 0.1</td><td>16.7</td><td>0.7 ± 0.2</td><td>29.0</td></tr><tr><td>a (nm) - Pristine</td><td>1.0 ± 0.2</td><td>20.0</td><td>1.6 ± 0.7</td><td>43.8</td></tr><tr><td>[n<sub>VO</sub>] (10<sup>21</sup>/cm<sup>-3</sup>) - HRS</td><td>6.6 ± 3.4</td><td>51.5</td><td>5.5 ± 4.4</td><td>80.0</td></tr><tr><td>[n<sub>VO</sub>] (10<sup>21</sup>/cm<sup>-3</sup>) - Pristine</td><td>1.5 ± 1.4</td><td>93.3</td><td>1.1 ± 1.8</td><td>163.6</td></tr></tbody></table>

more RS cycles and performance information see supplementary material (available online at stacks.iop.org/NANO/32/405202/mmedia). In each RS plot, ten switching curves (the last RS cycle of each device) are given, corresponding to ten different devices with the same stack structure. The coefficient of variation (CV), defined as the standard deviation (σ) divided by the average value (μ) of a given device parameter, is used to account for the variability and dispersion. Table 1 summarizes the statistical data for main RS parameters. In the pristine state, most devices had resistance values ranging between the subsequent high resistance states (HRSs) and low resistance states (LRSs), and hence a forming process was not required. Few devices had initial resistance values higher than the HRS values, and these were switched with a voltage that was lower than in the subsequent SET and RESET operations. For both the Zr:TaOx and TaOx devices,

the forming-free behavior was due to the high vacancy concentrations resulting from both intrinsic and extrinsic Zr$^{4+}$ doping [20, 21]. The devices showed bipolar switching, with the SET operation occurring with a positive bias (V$_{SET}$) and the RESET operation with a negative bias (V$_{RESET}$) for all devices. The average values of V$_{RESET}$, V$_{SET}$ were slightly increased for Zr:TaOx devices (up to ~2.3 V as compared with ~2 V for TaOx), an effect that can be ascribed to the dopant-V$_{O}$ interaction that may lead to defect clustering and consequent change in defect mobility. In addition, the current density values (@0.2 V) were slightly higher for the Zr:TaOx devices, which can be explained by the increased V$_{O}$ concentration. C2C variability in LRS and HRS resistance was similar for both the Zr:TaOx and TaOx devices, with average values for the CV of around 18% and 14%, respectively (see the curves in the supporting information). On the other hand,
significant contrast was observed in the D2D variability, and especially in the resistance values in both the LRS and HRS states, in which the CV for the Zr:TaOx device in the LRS was reduced from 68% to 9% and in the HRS from 110% to 15% compared to TaOx (see table 1). Another important feature observed for the Zr:TaOx devices was an increase in the average resistance window, from 12.5 for the TaOx-based devices to 21.8 for the Zr-doped ones. A similar contrast was previously noted in a comparison of sub-stoichiometric to purely stoichiometric TaOx devices [20], and was attributed to CF confinement and an increase in carrier concentration, which results in higher current densities. This localized Joule effect gives rise to greater filament dissolution and thus a larger resistance window [20]. These results indicate that Zr doping helps to improve RS performance by lowering D2D variability.

To further evaluate the effects of doping on the transport mechanisms, we conducted a carrier transport analysis by applying a hopping model that has been used previously in studies of tantalum oxide-based devices [46–48]. The J–E curves for the devices in both the pristine state and the HRS were fitted using the following expression [47, 49, 50]

$$J = qanv \exp \left( \frac{qaE}{k_B T} - \frac{\emptyset_t}{k_B T} \right), \quad (3)$$

where a denotes the average distance between trap sites, $\emptyset_t$ is the trap energy, $k_B$ is the Boltzmann constant, q is the electronic charge, n is the carrier concentration and v is the attempt-to-escape frequency. As can be seen from figures 3(a)–(d), the I–V characteristics are well described by the hopping model (the fitted curves and parameters extracted for all devices are provided in the supplementary material). The values obtained for the average distance between traps for pristine Zr:TaOx- and TaOx-based devices were 1.0 ± 0.2 and 1.6 ± 0.7 nm, respectively (see table 1). The HRS trap distances were 0.6 ± 0.1 and 0.7 ± 0.2 nm for Zr:TaOx and TaOx devices, respectively. The trap concentration for each type of device can be estimated by calculating [51, 52]

$$[n_{vo}] = a^{-3}. \quad (4)$$

Pristine Zr:TaOx devices had a higher concentration of traps, with a value of (1.5 ± 1.4) × 10²¹ cm⁻³ compared with (1.1 ± 1.8) × 10²¹ cm⁻³ for TaOx devices. The same trend was observed for the HRS states: the Zr:TaOx-based device has a trap concentration of traps (6.6 ± 3.4) × 10²¹ cm⁻³ as compared to (3.6 ± 1.1) × 10²¹ cm⁻³ for TaOx.

The transport mechanism in TaOx films depends critically on the VO concentration. TaOx exhibits Fermi glass behavior, in which VO complexes are trap sites and transport occurs via electron hopping [53, 54]. An increase in VO concentration gives rise to a percolation process, and once the fraction of hopping sites exceeds a certain threshold, they form a continuous conductive path. The results in figure 3 indicate that the distances between traps for both Zr:TaOx and TaOx devices are in good agreement with reference values [44, 48, 54, 55]. The shorter distance in the Zr:TaOx-based devices explains the increased current density (@0.2 V). It is also important to emphasize that the variability of the devices

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Experimental J–E plot and hopping model with fitted lines and extracted hopping distances for pristine (a) TaOx and (b) Zr:TaOx devices, and subsequent LRSs and HRSs for (c) TaOx and (d) Zr:TaOx. Zr:TaOx-based devices have a lower distance between traps in all resistance states.</figcaption></figure>

is directly correlated with the variability in the distance between VOS or VO concentration, indicating that extrinsic doping is a more controllable way of changing the VO configuration.

In order to assess the synaptic-like behavior of the devices, the switching dynamics were evaluated using pulsed measurements. We investigated comparative potentiation and depression characteristics using same time interval for LTP and LTD. As shown in figure 4(a), a sequence of 200 pulses for each LTD and LTP process was performed using time intervals of 200 µs and 600 ns, respectively. The LTD voltage amplitudes used were 2.4 V and 3.2 V, and LTP voltages were 2.1 V and 3.0 V for the TaOx and Zr:TaOx devices, respectively. The values of the applied voltage for the TaOx and Zr:TaOx devices were different because the voltage for RS operation is higher for the latter, as previously discussed. However, even at a higher voltage amplitude, it was observed that Zr:TaOx had more gradual potentiation and depression dynamics in its operation at a similar LRS/HRS ratio, as shown in the normalized pulse switching curves in figures 4(b), (c). CV for both maximum and minimum conductance states are given for nine full LTD/LTP cycles. A non-monotonic increase in conductance can be seen in the first few potentiation pulses in the TaOx-based device, and it has been claimed that this is related to a competition between thermal- and field-driven effects on the movement of VOS and the consequent CF stability [56]. This feature is detrimental for synaptic applications where weight update dynamics should be monotonic. The LTP curve for Zr:TaOx devices does not exhibit this problem however. This improvement can be attributed to the additional energy needed to overcome the attractive interaction between VOS and Zr cations. Although extrinsic doping generates a higher vacancy concentration, the trapping of VOS by dopant-VO complexes also leads to a reduction in the overall VO kinetics. This tunability of the
<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a) Conductance values as a function of the number of pulses, plotted on a semi-log scale. Nine repeated pulsed-based LTD and LTP operations are shown, and synaptic weight emulation is demonstrated for the W/TaOx/W (green) and W/Zr:TaOx/W (orange) RS devices. (b), (c) Average conductance and standard deviation as a function of the number of pulses for nine full LTD/LTP cycles of TaOx and Zr:TaOx devices. The latter exhibit a more gradual, reproducible, and monotonic variation of the conductance. CV of maximum (Gmax) and minimum (Gmin) conductance states are shown for both devices.</figcaption></figure>

switching dynamics through extrinsic doping can be gen-
eralized to other p-type dopants, as previously mentioned by
Lübben *et al* [18]

Low C2C and D2D variability has been associated with a
more reproducible formation and homogenous distribution of
CFs [14, 57]. In this work, the decrease in D2D variability
cannot be attributed solely to the higher concentration of V<sub>O</sub>,
but is also related to the interaction between V<sub>OS</sub> and dopant
species, which promotes V<sub>O</sub> localization around the dopant
cations. This interaction has been well described for several
doped oxides, in both experimental and theoretical simulation
studies [26–28]. This localization may provide a guiding path
for the formation of CFs, resulting in a less random dis-
tribution of CFs along the active layer. Moreover, simulation
has shown that the presence of dopants reduces the V<sub>O</sub> for-
mation energy [26, 27]. It is interesting to note that Ambrogio
*et al* [58] have demonstrated that device variability is asso-
ciated with fluctuations in V<sub>O</sub> energy barrier for hopping
conduction. It can be argued that the presence of the dopant
reduces this fluctuation, thus pinning the V<sub>O</sub> energy to a fixed
value and reducing the variability. The V<sub>O</sub>-dopant interaction
is also reflected in the slower V<sub>O</sub> kinetics and the consequent
change in the pulsed switching dynamics.

In this study, we have demonstrated that doping can be
used to tune the V<sub>O</sub> content and consequently the defect trap
density and CF configuration in tantalum oxide-based mem-
ristor devices. It was shown that Zr doping (extrinsic doping)
and control over the partial pressure of oxygen during PLD
(intrinsic doping) can promote V<sub>O</sub> formation and lead to
forming-free devices. In addition, extrinsic doping proved to
be better in terms of controlling the V<sub>O</sub> configuration and
content. The unique effect of Zr doping is that it localizes V<sub>OS</sub>
and consequently reduces the D2D variability. It was also
used to tune the pulse switching dynamics for the devices.
This enhancement in device parameters in terms of switching
variability and dynamics reinforces the importance of
exploring vacancy engineering for the tuning of memristor
behavior for high-density memory applications and memris-
tor-based artificial synapses.

See the supplementary material for additional informa-
tion on target preparation, C2C variability, device perfor-
mance (endurance), hopping model fitted curves, retention
measurements and thin film topography using an atomic force
microscope.

Acknowledgments

This work was supported by Natural Sciences and Engi-
neering Research Council of Canada (NSERC). Financial
support of CNPq (INCT Carbon Nanomaterials), CNEN,
Center for Innovation on New Energies-CINE SHELL (ANP)
/ FAPESP 2017/11937-4, FAPESP (14/50279-4) and
Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior-Brasil (CAPES)-Finance Code 001, is acknowl-
edged. This research was undertaken thanks in part to funding
from the Canada First Research Excellence Fund. We would
like to acknowledge Abdelouadoud El Mesoudy and Well-
ington de Oliveira Avelino from 3IT for their help and
insightful discussions regarding device electrical character-
izations, and Sonia Blais from Plateforme de Recherche et
d'Analyse des Matériaux (PRAM) at the Université de
Sherbrooke for her support with XPS measurements. We also
acknowledge LCPNano at the Federal University of Minas
Gerais (UFMG) for providing access to the clean room
facility and for ellipsometry measurements.

Data availability statement

The data that support the findings of this study are available
upon reasonable request from the authors.

ORCID iDs

João H Quintino Palhares https://orcid.org/0000-0002-2820-1627

Yann Beilliard https://orcid.org/0000-0003-0311-8840
Everton Bonturim https://orcid.org/0000-0001-5302-9091

Daniel Z de Florio https://orcid.org/0000-0002-7291-1917

Fabio C Fonseca https://orcid.org/0000-0003-0708-2021

Andre S Ferlauto https://orcid.org/0000-0003-3056-7289

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