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Probing the ionic and electrochemical phenomena during resistive switching of NiO thin films Wanheng Lu, Juanxiu Xiao, Lai Mun Wong, Shijie Wang, and Kaiyang Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16188 • Publication Date (Web): 09 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Applied Materials & Interfaces

Probing the ionic and electrochemical phenomena during resistive switching of NiO thin films 1

1

2

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1,

Wanheng Lu, Juanxiu Xiao, Lai-Mun Wong, Shijie Wang and Kaiyang Zeng, *

1. Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117576 2. Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR), Fusionopolis Way, Innovis #08-03, Singapore 138634

KEYWORDS: scanning probe microscopy, electrochemical strain microscopy, conductive AFM, first order reversal curve, Kelvin probe force microscopy, filament, interface

ABSTRACT: Ionic transport and electrochemical reactions underpin the functionality of the memory devices. NiO, as a promising transition metal oxide for developing ReRAM, has been extensively explored in the terms of the resistive switching. However, there is limited experimental evidence to visualize the ionic processes of the NiO under the external electrical field. In addition, the correlation between the ionic processes and the resistive switching has not been established. In order to close this gap and also to determine the role of the ionic processes in resistive switching of the NiO, in this study, a series of SPM techniques including ESM, cAFM, KPFM, and a newly developed FORC-IV, are employed to measure the ESM response, 1 ACS Paragon Plus Environment

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the resistive switching performance, the work function, and the ionic dynamics of NiO, respectively. The results in this work have clearly visualized the ionic transport and electrochemical reactions of NiO when subjected to the electrical field. It has found that the ionic processes and the resistive switching are accompanied with each other. Furthermore, it is found that the electrochemical reactions play a determinative role in the resistive switching of the NiO, and this electrochemically induced resistive switching performance can be explained by an integrated mechanism that has combined the filamentary and the interfacial effects underlying resistive switching. In addition to providing a better understanding of the resistive switching of NiO, this work also provides effective methods to probe the ionic processes and to correlate these ionic processes to the performance of functional materials.

1.

Introduction Recent rapid advances in information technology necessitate the development of the high-

speed and high-density memory devices. In terms of speed and density, the conventional Sibased flash memories have been significantly improved, but they are approaching their physical and technical limits as continuously scaling down the devices. To break through these limits, many alternative systems have been proposed, including the ferroelectric random access memory (FeRAM),1-2 magnetoresistive RAM (MRAM),3 phase-change RAM (PRAM),4 and resistive switching random access memory (ReRAM).5-6 Among those devices, ReRAM has attracted great attention due to its simple structure, high scalable cross-point and the possibility for multilevel stacking structures.7 The operation of ReRAM is based on the electrical field induced resistance change in numbers of materials such as transition metal oxides.

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Nickel Oxide (NiO), a binary transition metal oxide, has been extensively studied as one of the most promising resistive switching materials.8-10 The resistance state of NiO can be switched by using voltage with the same polarity or with different polarities, that is, both unipolar and bipolar resistive switching were observed in NiO.11-15 A number of mechanisms have been proposed to understand the resistive switching of NiO, and the oxygen-based ionic motions have been considered to play a critical role.16-17 For example, the oxygen-based ionic phenomena, such as the electrically or thermally driven migration of oxygen ions, or the electrochemical reactions involving oxygen vacancies or Ni vacancies, were associated with the formation and rupture of conductive filaments in NiO.18 In addition, it has been suggested that there were possible correlations between the resistive switching, the ionic processes and the electrochemical reactions.6,19 However, few studies have experimentally shown the ionic and electrochemical processes during the resistive switching of NiO, and the correlation between these processes and the resistive switching has not been established yet, which will hinder the better understanding and thus optimization of the resistive switching performance of NiO based memory devices. To overcome this challenge, more studies are needed in order to obtain: i) direct evidence of the ionic processes during the resistive switching processes, and ii) determining the role of the ionic and electrochemical processes during the resistive switching processes. To characterize the ionic and electrochemical phenomena in various materials,20 Electrochemical Strain Microscopy (ESM), one of the Scanning Probe Microscopy (SPM) based techniques, is developed in recent years to probe the nanoscale ionic and electrochemical phenomena in oxide thin films21 and battery materials.22-23 In principle, ESM is based on the detection of surface deformation (or surface strain) that is induced by the ionic dynamics. Thus, ESM allows the exploration of the ionic phenomena in the absence of conductive current and the 3 ACS Paragon Plus Environment


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decoupling the ionic phenomena from the electric property (e.g. resistive switching).24 In addition, the application of another SPM based technique, conductive AFM (c-AFM), in the first order reversal curve method (denoted as FORC-IV), can also provide insights into the local ionic dynamics.25 This study aims to experimentally verify the ionic or electrochemical phenomena during the resistive switching and further to determine the role of these phenomena to the resistive switching of the NiO by employing multiple-mode SPM techniques. Specifically, the ionic or electrochemical process under the electrical field is detected by ESM and FORC-IV measurements; the resistive switching behavior of NiO is measured by c-AFM; the Schottky interface between the Pt-coated tip and NiO is confirmed by Kelvin Probe Force Microscopy (KPFM). Results in this work have shown an agreement between the ESM responses, resistive switching performance, and the FORC-IV evolution. It clearly demonstrates the correlation between the resistive switching and ionic/electrochemical phenomena. Furthermore, by performing similar experiments in different gas environments, an electrochemically induced resistive switching of NiO is observed, indicating the determinative role of the ionic and electrochemical phenomena in the resistive switching of NiO. Finally, this electrochemically induced resistive switching are explained by an integrated mechanism combining with the filamentary and interfacial effects.

2.

MATERIALS AND METHODS The nickel oxide (NiO) thin film samples were deposited on a commercial Pt-coated silicon

substrate (Addison Engineering Inc. CA, USA) by the Pulsed Laser Deposition (PLD) method with NiO target (99.90% purity). The parameters of the deposition were: 1) the laser frequency and energy were 20 Hz and 300 mJ, respectively; 2) the substrate temperature was maintained at 4 ACS Paragon Plus Environment

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350℃; 3) the oxygen partial pressure was 1.6 x 10-6 Torr; and 4) the deposition duration was 10 mins so that the thin film thickness was estimated to be 12 nm. The crystal structure of the asprepared NiO thin film was confirmed as a cubic structure by X-Ray Diffraction (XRD) (Figure S1, Supporting Information, SI). In this study, Electrochemical Strain Microscopy (ESM) technique was used to characterize the ionic and electrochemical processes of the NiO thin films; conductive AFM (c-AFM) technique was applied to measure the resistive switching behavior of the NiO thin film samples; Kelvin Probe Force Microscopy (KPFM) was used to measure the work function of NiO thin films. All SPM studies including ESM, c-AFM, and KPFM, were performed on a commercial SPM system (MPF-3D, Asylum Research, USA) with conductive Pt-coated Si tips (AC240TM, Olympus, Japan, with a nominal spring constant of 2 N/m and an average tip radius of 15 nm). To enhance the signal-to-noise ratio and to minimize the topographic cross-talk during the ESM measurements, DART-ESM (Dual AC Resonance Tracking ESM) technique was applied in the experiments.26 The data collected from DART-ESM measurements were then fitted by the damped simple harmonic oscillator (DSHO) model to acquire images of height, amplitude, phase, Q-factor and resonance frequency. The height image shows the sample topography; the amplitude image is related to the surface deformation that is induced by the electric field due to the intrinsic link between the ionic concentration and the materials’ molar volume; whereas the phase images indicates the angles between the dielectric dipoles and the direction of the electric field, and both the amplitude and phase images together can provide information about the ionic or electrochemical activity underneath the ESM tip; the Q-factor and resonance frequency images can reveal the change of mechanical properties due to the application of the electric field.26-27 Two modes of ESM techniques, i.e., imaging ESM and voltage spectroscopic ESM, 5 ACS Paragon Plus Environment


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were used in this study. For the imaging ESM, an AC voltage was used to detect the ESM responses over the sample surface; while for the voltage spectroscopic ESM, the responses were measured as a function of a slowly (~0.1 Hz) changing voltage waveform. During the c-AFM measurement, a series of resistors were used to limit the maximum current to 10 nA to avoid the breakdown of the NiO thin film samples. In addition, the c-AFM measurement in the first order reversal curve method (the FORC-IV measurement) was used to probe the ionic/electrochemical process during the resistive switching, of which details can be found in the results section. During the c-AFM measurements, the voltage is applied to the Pt electrode on the bottom of the samples with the Pt-coated tip (top electrode) being grounded. This is different from that used for other SPM measurements such as ESM and KPFM, where the voltage is applied to the SPM tip with the bottom electrode being grounded. For a clear description, in this paper, the voltage applied to the bottom electrode is defined as the sample voltage, while it is defined as the tip voltage when the voltage is applied to the tip. Note that the directions of the electrical field are opposite when applying the sample voltage or tip voltage, for example, the positive (negative) sample voltage corresponds to the negative (positive) tip voltage. In this study, a closed gas cell with slowly-flowing argon gas (with O2 < 0.02 ppm and H2O < 0.01 ppm) or synthetic air (with H2O < 5 ppm) was used for the environment control. To ensure the control over the moisture or oxygen, the required gas was introduced into the closed cell for at least 2 hours before SPM measurements and it continued flowing during the measurement process.


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3.

RESULTS AND DISCUSSION

3.1 Ionic processes of the NiO thin film induced by external electrical field In this work, ESM is used to detect the deformation of the NiO thin film under the external electrical field. Before the ESM imaging, a dc bias of 6 V is applied through a conductive tip on an 8×8 grid over an area of 3×3 µm2. Over the same area, DART-ESM imaging is then conducted to detect the surface variations induced by the dc bias.

Figure 1. The ESM images of NiO after applying a 6 V dc bias on an 8 × 8 grid over 3 × 3µm2 area in the ambient air environment (~ 50 – 60% moisture at room temperature): (a) surface topography; (b) resonance amplitude; (c) line section profile along the line in (b); (d) resonance phase; (e) resonance Q-factor; and (f) resonance frequency. Figure 1(a) shows a high-resolution topographic image of NiO thin films after the application of the dc bias, from which no topographic changes between the bias-written points 7 ACS Paragon Plus Environment


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and the pristine area can be observed. This suggests the stability of the surface structure under the external electric field. In addition, this topographic image also indicates the homogeneity over the sample surface. Figure 1(b) shows the resonance amplitude at the bias-written points is apparently larger than that at the pristine area, with the increment of about 10 pm [Figure 1(c)]. This indicates that the sample has expanded upon the application of the 6 V dc bias. Figure 1(d) shows the bias-induced phase contrast is about 70°, which is less than the 180° phase angle for typical piezo-/ferro-electrical materials. This is expected as NiO is not a piezo-/ferroelectric material. The bias induced variations of Q-factor and resonance frequency can be observed in Figure 1(e) and Figure 1(f), indicating the change of the dissipative property and the contact stiffness at the tip-sample junctions, respectively.27 The variations of resonance frequency induced by the dc bias [e.g. the row and column marked by the white circles in Figure 1(f)] are not significant as those in the Q-factor image, suggesting that the changes of contact stiffness are not significant as those of the dissipative property. The surface topography and property may be rebuilt due to the ESM data collection, which may impact the topography and the ESM contrasts shown in Figure 1. However, this can be excluded by comparing ESM images before [Figure S2, Supporting Information, SI] and after applying dc bias (Figure 1), in which can confirm that the changes in ESM responses are induced by the dc bias. This also suggests the grid pattern used in Figure 1 is an effective method to observe the bias induced variations.


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Figure 2. (a) The bias waveform in the spectroscopic ESM measurement; After each voltage pulse, an ac bias is still active to detect the evolution of the electrochemical deformation; (b) ESM amplitude loop (A); (c) ESM phase loop ( ); and (d) the calculated ESM responses [ ∗ ] as function of the applied voltages. The voltage spectroscopic ESM is used to measure the ESM responses as a function of a slowly changing voltage. Figure 2(a) shows the bias waveform applied to the tip during the voltage spectroscopic ESM, which consists of a set of amplitude-modulated pulses (~25 ms) by a slowly (0.1 Hz) changing triangular waveform. An ac bias is applied during and after each bias pulse to detect the corresponding ESM responses that are denoted as the bias-on and bias-off ESM responses, respectively. To avoid the electrostatic effects due to the tip-bias, the bias-off ESM responses are used here to discuss the ESM responses towards the slowly changing voltage. These bias-off ESM responses, including ESM amplitude, phase, and calculated ESM loops 9 ACS Paragon Plus Environment


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responses (base on × cos , where R is ESM loop response, A is ESM amplitude and is ESM phase), are shown in Figures 2(b)-(d), respectively. A hysteretic ESM response loop can be seen in Figure 2(d). According to the mechanisms of the formation of the ESM hysteretic loop,23 the hysteretic response may not appear in a system with limited ionic species, but it may emerge in a system of which the number of ionic species is large or increased due to the electrochemical reactions or ionic diffusion. Hence, the hysteretic responses shown in Figure 2 indicate the presence of large number of ionic species in NiO thin film which may be induced by ionic distribution or the electrochemical reactions when applying an external bias to NiO. In principle, ESM is based on the detection of the bias-induced surface deformation in the materials, which is very similar to the operation principle of PFM (Piezoresponse Force Microscopy) with the major difference on the mechanisms of the bias induced electromechanical coupling. The surface deformation measured by ESM is the electrochemical deformation induced by the ionic diffusion and the related molar volume change; while the deformation measured by PFM is the electromechanical strain due to the piezoelectric response.28 Therefore, it needs to carefully distinguish the bias-induced electromechanical coupling mechanism underlying the ESM (or PFM) responses. Note that the XRD pattern (Figure S1, Support Information: SI) has confirmed a cubic structure of NiO, and this centrosymmetric structure can exclude the piezoelectric/ferroelectric effects underlying the strong ESM responses observed in Figure 1 and Figure 2. In addition to the piezo-/ferroelectric effects, the ESM response may stem from other reasons such as deformation potential effect, electrostatic and flexoelectric effect.29-32 The deformation potential effect and the flexoelectric effect are associated with the fast response of the ionic system to the change of the local electrochemical potential and field gradients, which can contribute to the high-frequency ESM response as Figure 1. However, the slow frequency 10 ACS Paragon Plus Environment

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ESM response in Figure 2 indicates the slow ionic motions. Further discussion (presented in Supporting Information) demonstrates that the electrostatic effects can be neglected in this study. Therefore, the ESM responses shown in Figure 1 and Figure 2 strongly indicate the presence of the ionic processes upon the application of the dc bias.

3.2

Origins of the bias-induced ionic processes of NiO When subjecting to an electrical field, oxides may experience the redistribution of the ions

and even the initiation of electrochemical reactions, consequently altering the ionic concentration and hence the ESM response. In order to gain a detailed insight into the bias-induced ionic processes in the NiO thin film samples, similar experiments as those shown in Figure 1 are conducted in a closed cell with the slowly-flowing argon gas or synthetic air. The ESM responses after the application of 6 V dc bias in the argon and the moisture-free air (the synthetic air) are shown in Figure 3. It is found there are no bias-induced ESM responses in the argon gas and only very weak responses in the synthetic air, and this suggests that the presence of the moisture at the tip-sample junction can significantly enhance the ESM responses, whereas the absence of the oxygen and water molecules can completely suppress the ESM responses.



Figure 3. The ESM images of NiO after applying a 6 V dc bias on an 8 × 8 grid over 3 × 3 µm2 area in argon and synthetic air ambient. (a) – (e) in argon gas; (f) – (j) in synthetic air. Prior to the discussion of the ionic processes underlying the environment dependent ESM responses, the nature of NiO should be considered. NiO is a typical p-type binary metal oxide due to the nature of the Ni-deficient. This means that the electrical transport of NiO is dominated by hole carriers which are generated along with the nickel vacancies.17 As nickel vacancies are less mobile than that of the oxygen vacancies,11,

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the migration of oxygen vacancies are

considered to account for the variation of the nickel vacancies and thus the conductivity change of NiO.34 When a positive dc bias is applied to the sample surface through the ESM tip, oxygen vacancies will drift away from the ESM tip, finally decreasing the molar volume of materials and thus the ESM amplitude in the vicinity of the tip-sample junction.35 This is consistent with the observation of the ESM amplitude decrease in the synthetic air [Figure 3(g)], suggesting that the weak ESM responses observed in the synthetic air may be originated from the redistribution of oxygen vacancies driven by the electrical field. But for the case in the ambient air, the ESM amplitude does increase after applying positive dc bias [Figure 1(b)], which indicates that, in the ambient air, more ionic processes, rather than only ionic diffusion, have occurred underneath the 12 ACS Paragon Plus Environment

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ESM tip. These ionic processes may include the electrochemical reactions with the water molecule as an electrolyte-like medium. When the voltage is high enough, the oxygen vacancy can be generated underneath the ESM tip as the following reaction: 1 → 2 + + .. 1 2 where .. represents oxygen vacancy. In the ambient air, a moisture meniscus can be presented at the tip-sample junction.36 This moisture meniscus can act as the electrolyte, together with two electrodes (the oxide surface and the Pt-coated SPM tip), to facilitate the electrochemical reactions at the tip-sample junction. The charge and mass conservation of the electrochemical reactions requires the coupling between the cathodic and anodic processes,37 and thus cathodic processes are needed to consume the extra electrons generated in the anodic process shown in reaction (1). In the ambient environment, the possible cathodic processes may be the reduction of water molecules: 2 + 4 + → 4 2 The generated from the reaction (2) can combine with the metal cations on the sample surface such as to contribute the ESM amplitude increase in the ambient air environment. Due to the absence of water molecules, the electrochemical reactions are limited in the synthetic air, hence the ESM responses are very weak. As discussed earlier, these weak ESM responses may come from the oxygen vacancy drift or the oxygen absorption. The ESM responses are not visible in the argon gas, which is most likely due to the absence of electrochemical reactions and oxygen absorption.



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3.3 Correlation between the ionic processes and the resistive switching of NiO The above results have experimentally evidenced that the application of bias to NiO in the ambient air can result in the striking change of ESM responses and these ESM responses are related to ionic processes (especially, electrochemical reactions involving moisture). Furthermore, upon the application of the bias, the resistance (conductivity) of NiO can be switched. In most cases, this resistance switch occurs in the ambient air. Therefore, it is reasonable to assume that the ionic processes and resistive switching processes may coexist under the biased SPM tips. To examine this correlation, the conductive AFM (c-AFM) measurements together with the First Order Reversal Curve method (denoted as FORC-IV) are applied to study the ionic processes during the resistive switching processes of the NiO. The FORC-IV measurements are conducted on a 10×10 grid over the area of 3×3 µm2, and the FORC-IV curve averaged over these 100 points is used to show the onset and the dynamic of the ionic/electrochemical processes during the resistive switching of NiO thin film samples (Figure 4).


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Figure 4. (a) The bias waveform for the FORC-IV measurement; (b) The averaged FORC-IV curves as a function of the bias; (c) The FORC-IV loop area as a function of the bias amplitude. During the FORC-IV measurement, a bias waveform that consists of a sequence of triangular pulses with increasing amplitude [Figure 4(a)] is applied to the sample surface, and the current through the tip-surface junction is measured as a function of the applied bias wave. When the applied voltage is low, the current may not be able to change the chemical state of the system and thus the I-V curve is non-hysteretic. When the applied voltage is high, the current can alter the chemical or physical state of the systems, and even can initiate the ionic processes, such as transport of mobile ionic species, or surface electrochemical reactions. Commonly these processes are associated with the significant kinetic or thermodynamic hysteresis, consequently changing the I-V curve from non-hysteretic to hysteretic. Thus, the emergence of the hysteretic IV curve and the evolution of the hysteretic loop can demonstrate the onset and the dynamics of 15 ACS Paragon Plus Environment


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the ionic processes during the resistive switching, respectively. The onset and the dynamics can be determined from the turning point and the slope of the loop opening vs bias amplitude curve from the FORC-IV tests.25 Figure 4(b) shows the measured FORC-IV curves, and the loop shape is found to be dependent on the bias amplitude. This loop evolution as the bias amplitude can be seen from the curve in Figure 4(c), in which plots the loop opening (or loop area) as the function of the bias amplitude. The curve is flat until the bias amplitude increases to 6 V and it becomes steep after 6 V, indicating that the ionic transport or the surface electrochemical reactions may be activated at around 6 V. Note that after application of 6 V, the ESM responses have been detected in the ambient air and these responses are originated from electrochemical reactions involving the water molecules as an electrolyte (Figure 1). Therefore, the turning point of the loop opening-bias amplitude curve [Figure 4(c)] represents the onset of the surface electrochemical reactions during the resistive switching. The electrochemical reactions and the consequent ESM responses are found to be severely limited in the moisture-free environment (Figure 3). To further explore the correlation between the electrochemical reactions and the resistive switching of the NiO, c-AFM measurements are conducted in the controlled atmospheric environments, i.e., argon gas or synthetic air. The average I-V curves (over 10×10 points) obtained in various environments are shown in Figure 5. The I-V curve in the ambient air well demonstrates the resistive switching behavior of the NiO thin film sample, but this resistive switching behavior disappears when the experiments are conducted in either argon gas or synthetic air, these results confirm the correlation between the electrochemical reactions and the resistive switching. In addition, these results also indicate that the resistive switching of NiO requires the presence of the electrochemical reactions in the ambient air. Hence, it is termed as the electrochemical reaction induced resistive switching. 16 ACS Paragon Plus Environment

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Figure 5. The averaged I-V curves of NiO in ambient air, synthetic air and argon gas. To better present the correlation between electrochemical reactions and resistive switching, SPM scans are performed in a box-poling pattern where an area is first poled with one voltage [e.g., +5 V tip bias in the yellow area in Figure 6(a)], followed by poling in a smaller area with a voltage of opposite polarity [e.g., -5 V tip bias in the purple area in Figure 6(a)], and finally a larger area [e.g., the blue area in Figure 6(a)] including both poled areas as well as the pristine area is scanned by several SPM modes: first ESM and KPFM modes, and then c-AFM mode, to image the variations triggered by the poling processes. Figures 6(b)-6(e) show the voltage induced variations in height, current, ESM amplitude, and contact potential difference (VCPD), respectively. 17 ACS Paragon Plus Environment


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It is found that, when the surface is poled with the tip bias of 5 V, the current decreases [Figure 6(c)], the ESM response increases [Figure 6(d)], and VCPD increases [Figure 6(e) and 6(f)], it should be noted that the increased VCPD corresponds to the decreased work function of the NiO thin film (see next section). The opposite effects are found when the surface is poled with a tip bias of -5 V, i.e., current increases [Figure 6(c)], ESM response decreases [Figure 6(d)], and VCPD decreases which is corresponding to the increased work function of the NiO thin film. The current image [Figure 6(c)] is consistent with the resistive switching curve in the ambient air showed in Figure 5, i.e., when poling with 5 V sample bias (corresponding to the tip bias of -5 V), the current is larger than that of the region poled with -5 V sample bias (corresponding to 5 V tip bias). It also demonstrates the non-volatility of resistive switching behavior, that is, the switched state can be held after removing the poling voltage. The ESM responses [Figure 6(d)] suggests that different ionic/electrochemical processes have been initiated by the voltages with different polarities. It should note that all images in Figure 6 are acquired by scanning the same area with an identical SPM tip. Except that the surface topography is not affected by the poling voltage, other images show obvious variations between the poled and unpoled regions as well as the areas poled by different voltages. These variations strongly suggest the correlation between the resistive switching and the ionic/electrochemical processes as well as the work function changes, which will be also discussed in the next section.


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Figure 6. (a) The schematic of SPM poling and scanning in a box pattern, where the blue area is the pristine film, and the yellow and purple areas are poled with tip biases of 5 V and -5 V, respectively; by scanning over the same area with the SPM system in different modes, the surface properties variations can be compared between the pristine state and the poled states. (b) The height image obtained by AFM. (c) The current image obtained by c-AFM. (d) The ESM amplitude obtained by ESM. (e) The VCPD (associating to the work function change of NiO) image obtained by KPFM. (f) The line profile of the VCPD along the dashed line in Figure 6(e), indicating the changes of the VCPD of the pristine film, +5V poled region and -5V poled regions, respectively.

3.4 Mechanisms underlying the electrochemical reactions induced resistive switching of NiO In this work, the prepared NiO thin films show the resistive switching behavior that is induced by the electrochemical reactions in the vicinity of the tip-sample junctions in the ambient air. The resistive switching of NiO is widely considered to be associated with the 19 ACS Paragon Plus Environment


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conductive filaments mechanism, that is, the changes of the resistance states are the consequences of the formation and rupture of the conductive filaments between two electrodes,7 and these conductive paths are the conductive Ni-deficient phases that can be generated by the oxygen ion (oxygen vacancy) migration.17 However, this mechanism cannot explain the variations of the I-V curves in the ambient air, synthetic air, and argon gas (Figure 5), suggesting that other additional mechanisms may contribute to the observed resistive switching. On the other hand, as shown in Figure 5, the resistive switching behavior is not symmetric between the negative and positive sample biases, and the area enclosed by the I-V loop under the positive sample bias is larger than that at negative sample bias, indicating that the interface effect is active for the resistive switching process in the NiO thin film samples. As the work function of NiO is reported to be larger than that of Pt, it is likely to form a Schottky interface similar to that between a p-type semiconductor and metal, and this can be confirmed by the contact potential difference (VCPD) measured by Kelvin Probe Force Microscopy (KPFM). Figure 6(e) and Figure 6(f) clearly demonstrate the VCPD variations over the NiO surface regions where different biases are applied. The region outside the yellow area is the pristine NiO thin film without applying any bias, where the average VCPD is negative. According to

V"#$

∅&'()* − ∅,'&-./012.(/3 3 e

where ∅6789: and ∅;76?@A=8>B are the work function of metal (here, it is Pt) and semiconductor (here, it is NiO).38 Therefore, this negative VCPD can confirm that the work function of NiO is larger than that of Pt. Hence a Schottky interface between a NiO (p-type semiconductor) and Pt (metal) is formed, where a negative tip bias or a positive sample bias is 20 ACS Paragon Plus Environment

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equivalent to a forward bias. For the poled areas, the VCPD increases in the area poled by +5V tip bias [the yellow region in Figure 6(a)], and it decreases in the area poled by -5V tip bias [the purple region in Figure 6(a)]. Given that the work function of Pt (tip) remains constant during the experiments, according to Equation (3), the increase of VCPD indicates the decrease of the work function of the NiO thin films; and similarly, decreasing VCPD means the increase of the work function of the NiO thin film. Hence, 5 V tip bias (corresponding to -5 V sample bias in c-AFM measurements) can lead to the decrease of the work function of NiO. Note that Figure 6(e) also suggests that the presence of the electrostatics effects that may contribute to ESM signals, and the discussion of the effects of the electrostatics on the ESM measurements is presented in Supporting Information (SI: page S-8). Figures 7(a)-(b) schematically show the band diagrams of the metal and p-type semiconductor before and after contacting each other. The work function of p-type semiconductor (∅C ) is larger than that of the metal (∅D ), and the work function difference (∅C − ∅D ) is the potential barrier for the carrier (hole) drift. Lowering the potential barrier or narrowing the depletion layer will decrease the contact resistance at the metal-semiconductor junction, facilitating the carrier transport between the metal and semiconductor.7 The KPFM result [Figure 6(e)] has confirmed that applying positive tip bias (corresponding to negative sample bias in c-AFM measurements) can lead to the decrease of the work function of NiO and thus lower the potential barrier for carrier drift in the Schottky interface between the Pt and NiO.



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Figure 7. The schematically band diagrams of a metal and a p-type semiconductor (a) before contact; (b) after contact; and (c) the schematic diagram of the resistive switching processes in NiO film, and the number 1-4 corresponds to the same numbers in Figure 5. Based on above results, we propose a mechanism integrating both the filamentary and interfacial effects to explain the electrochemically induced resistive switching for NiO thin films. Figure 7(c) schematically shows this integrated mechanism, where the number 1-4 corresponds to the voltage swap step during the I-V curve measurement showed in Figure 5. Note that, the positive (negative) sample bias for the c-AFM corresponds to the negative (positive) tip bias for other SPM measurements. When a positive sample voltage is applied to the sample through the bottom electrode, the direction of the electrical field is upward and the oxygen vacancy will drift along the electrical field. As the oxygen vacancies migrate towards the top electrode, Ni-deficient NiO starts to form from the bottom electrode. Because that the decrease of the oxygen vacancy will increase the 22 ACS Paragon Plus Environment

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concentration of the main carriers (holes) in the NiO, the conductivity of Ni-deficient NiO is larger than the pristine NiO, and the Ni-deficient NiO is considered as the conductive paths (or filaments) in the NiO. Also as the migration of oxygen vacancies, a concentration gradient of oxygen vacancy is likely to form to against the driving of the electrical field, which may make the conductive filaments exhibit a conical-like shape of which the base area is gradually decreased from the bottom to the top.39 At the top of the filaments, i.e., near the tip-sample junction, it is possible to develop Ni-relatively sufficient phases with a lower conductivity, denoted as Ni-NiO in the following discussion. The oxygen may adsorb and integrate into the tip-sample junction to reduce oxygen vacancies and thus to improve the conductivity. Subsequently, the conductive paths can be formed and the current is sharply increased [Figure 5, the inner box in Figure 6 and step 1 in Figure 7(c)]. Note that the positive sample bias corresponds to the negative tip bias which is equivalent to a forward bias for the Schottky interface between tip and NiO, where the interfacial effect can be neglected. As the current passes through the conductive filaments, Joule heat is inevitably generated with the current, and the filaments can be melted at the position near the top of the conductive filaments due to the small geometrical size of the top of the filament [step 2 in Figure 7(c)]. When the voltage is reversed, the oxygen vacancies drift downward to restore the filaments, but the interfacial effect becomes significant since the negative sample bias is a reverse bias for the Schottky interface between tip and NiO, where the current will be rectified. It is reported that the work function of NiO can be reduced due to the hydroxylation of the oxide surface.40 Reaction (2) has indicated the hydroxylation of NiO under the positive tip bias (corresponding to the negative sample bias) in the ambient air, and Figure 6(e) has confirmed the decrease of the work function (corresponding to larger VCPD value in the yellow area) under this positive tip bias. The 23 ACS Paragon Plus Environment


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decrease of the work function will lower the potential barrier and alleviate the interfacial effect when applying the negative sample bias. Finally, the conductive paths between the top and bottom electrodes can be formed again to increase the current [step 3 in Figure 5 and Figure 7(c)]. This may also explain why the electrochemical reactions in ambient air can induce the resistive switching of NiO, but no resistive switching has been observed in argon gas and synthetic air. At the end of the process, the filaments rupture under the Joule heating effect, transforming the system to a high resistance state [step 4 in Figure 7(c)]. Although most mechanisms of the resistive switching of NiO are considered to be filamentbased, the interface-based resistive switching has been reported in NiO.41-42 In some cases, both the filament-based and the interface-based mechanisms are reported to coexist,43-44 and transform between each other.45 The filament and interface integrated mechanism has also been proposed in the TiO2 resistive switching system.46

4.

CONCLUSIONS In this study, the ionic processes induced by the external electrical field, including the ionic

motions and the electrochemical reactions in NiO thin film, have been studied by using the Electrochemical Strain Microscopy (ESM) in both the imaging mode and the voltage spectroscopic mode. Furthermore, it has been proved, by the ESM measurements in different gas environments, that the electrochemical reactions can be limited in the moisture-free environments due to the conservation of charge and the requirement for the coupling the anodic and cathodic reactions. On the other hand, c-AFM and the FORC-IV measurements have shown the resistive switching behavior in NiO thin films and the coupling of the ionic dynamic and the switching processes, respectively. This resistive switching behavior is also found to be induced by the electrochemical reactions since it occurs in the ambient air condition but disappears in the 24 ACS Paragon Plus Environment

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moisture-free environments, which can be explained by an integrated mechanism involving the filamentary and interfacial effects. By combining variously advanced scanning probe microscopy techniques with the controlled environments, this work has verified a determinative role of the electrochemical reactions in the resistive switching of NiO. This work also implies that the moisture content (or the humidity) may impact the intensity of the ESM responses, as well as the resistive switching performance of NiO thin film. A humidity control setup is needed to attach to the SPM measurements to quantitatively study on this relationship between the humidity and ESM responses as well as the resistive switching.

AUTHOR INFORMATION * Corresponding author: Dr.K.Y.Zeng, E-mail: [email protected]; Tel: (+65) 6516 6627; Fax: (+65) 6779 1459.

SUPPORTING INFORMATION XRD pattern, ESM images before applying dc biases, demonstration of the model used to visualize the ionic movements during the resistive switching, and discussion of effects of the electrostatics on ESM measurements are presented in Supporting Information.

ACKNOWLEDGEMENT This work is supported by Ministry of Education (Singapore) through National University of Singapore under the Academic Research Funding (AcRF) (R-265-000-495-112). One of the authors (WHL) would also like to thank the postgraduate scholarship provided by National University of Singapore. 25 ACS Paragon Plus Environment


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