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Unravelling Resistive Switching Mechanism in ZnO NW Arrays: The Role of the Polycrystalline Base Layer Gianluca Milano, Samuele Porro, Md Younus Ali, Katarzyna Bejtka, Stefano Bianco, Federico Beccaria, Alessandro Chiolerio, Candido Fabrizio Pirri, and Carlo Ricciardi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09978 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry

Unravelling Resistive Switching Mechanism in ZnO NW Arrays: The Role of the Polycrystalline Base Layer Gianluca Milano†§*, Samuele Porro†, Md Y. Ali†, Katarzyna Bejtka§, Stefano Bianco†, Federico Beccaria‡, Alessandro Chiolerio§, Candido F. Pirri†§ and Carlo Ricciardi†* †

Department of Applied Science and Technology, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy.

§

Center for Sustainable Future Technologies, Istituto Italiano di Tecnologia, C.so Trento 21, 10129 Torino, Italy.

‡

Department of Physics, Università degli Studi di Torino, Via Pietro Giuria 1, 10125 Torino, Italy.

ABSTRACT The physical mechanism involved in resistive switching phenomena occurring in devices based on ZnO nanowire (NW) arrays may vary considerably, also depending on the structure of the switching layer. In particular, it is shown here that the formation of a ZnO base layer between the metallic catalyst substrate and the NW, which is typical of CVD grown ZnO NW arrays, should

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not be neglected when explaining the switching physical mechanism. The structural and electrical properties of this layer are investigated after the mechanical removal of NWs. Electrical measurements were performed in presence of NWs and after their removal, showing that the base alone exhibits resistive switching properties. The proposed switching mechanism is based on the creation/rupture of an oxygen vacancies conductive path along grain boundaries of the polycrystalline base. The creation of the filament is facilitated by the high concentration of vacancies at the grain boundaries that are oriented perpendicularly to the electrodes, as a direct consequence of the ZnO growth along the c-axis of the wurtzite lattice.

INTRODUCTION The need for fast and scalable memory technologies has dramatically increased in recent years with the great growth of digital data. Among new memories, the memristor has attracted great attention due to low power, high speed, high density and low production cost.1,2 These characteristics make this kind of devices suitable for numerous applications, such as logic devices3, next-generation memories4 and neuromorphic systems.5 Even if the realization of the ideal memristor postulated by L. Chua6 in 1971 is far from being achieved, the ideal concept of memristor is associated to resistive switching (RS) devices that can similarly fulfil the requirements for highly scalable and low consuming non-volatile memories. Two-terminal RS devices can retain an internal state that depends on the history of applied current and voltage. The latest technological achievements demonstrate that metal-insulator-metal (MIM) structures show this behaviour.7 Resistive switching phenomena, observed already in 1960s8,9, have been recently investigated in various materials, such as organic layers10, perovskite oxides11, carbonbased systems12,13 and transition metal oxides (TMOs). 14,15 These phenomena can be interpreted


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as a reversible soft breakdown of the material due to the formation of a conductive path (filament) between the electrodes. In the case of electrochemically active electrodes, such as Cu or Ag, the conductive filament can be formed by electromigration of metal ions.16 Instead, when an injection of metal cation does not occur in the active materials, the resistive switching in metal oxides was widely attributed to the migration of oxygen ion related defects.17 The two mechanisms are exploited for the realization of Electrochemical Metallization Memories (ECM) and Valence Change Memories (VCM), respectively. Recently, great attention was devoted to the investigation of resistive switching in nanostructures. Indeed, resistive switching phenomena were observed in different types of quasione dimensional nanostructures, such as single metal-oxide nanowires (NWs), heterostructured NWs and core-shell NW structures.18 Moreover, these phenomena were observed in different nanostructured materials such as cobalt oxide NWs19, copper oxide NWs20, nickel oxide NWs21, titanium dioxide nanotube arrays22 and carbon nanotubes.23 In virtue of the high surface area-tovolume ratio of these structures, new physical mechanisms of multistate resistive switching can be observed exploiting surface effects.24 Furthermore, nanostructures are good candidates as building blocks for nanotechnology and nanoscience.25,26 Indeed, using a bottom-up approach, arrays of these structures can be exploited for the realization of a wide range of electronic devices.27,28 The wide bandgap (3.37 eV at room temperature

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chemical and thermal stability and biocompatibility make ZnO one of the most promising nanostructured materials. In addition, ZnO exhibits resistive switching behaviour that was already observed in thin films grown by sputtering, atomic layer deposition (ALD), pulsed layer deposition (PLD) and sol-gel methods.31 Furthermore, ZnO nanostructures can be synthesized in


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various forms by large-scale and low-cost production methods.32 The resistive switching behaviour typical of ZnO thin films was recently reported also in ZnO NW and nanorods (NR) arrays33–39 and in single NWs.40,41 However, in the case of nanostructured ZnO there is still a lack of understanding of the physical mechanism involved in resistive switching, and the localization of the conductive filament in these complex systems remains a challenge. In single NWs, in presence of an electrochemically active electrode, the resistive switching was explained by the formation/rupture of a filament on the NW surface40,41 or by the migration of metal ions that can act as dopants on the NW surface.42 Instead, when using NW arrays, RS may occur following different mechanisms. In this study, we proposed an innovative understanding of the switching mechanism responsible for resistive switching in ZnO NW arrays. Resistive switching behavior was investigated in ZnO NW arrays grown by low pressure Chemical Vapor Deposition (LP-CVD) on a Pt substrate that is used as both catalyst (during the growth process) and the bottom electrode during electrical measurements. As a consequence of the LP-CVD growth of NWs, a thin layer of ZnO was formed between NWs and the Pt substrate.43 Here it is demonstrated for the first time that the presence of a base layer can play a substantial role and must not be neglected in order to understand the physical mechanism responsible for the resistive switching behavior in ZnO NW arrays.

EXPERIMENTAL SECTION Synthesis ZnO NWs were synthesized in a quartz tube furnace by LP-CVD, using the catalytic properties of Pt without the deposition of any seed layer, differently from previous works.44,45 A substrate


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of Pt (100 nm)/Ta (20 nm) was realized by sputtering on a SiO2 (190 nm)/Si wafer. The Pt layer was deposited without breaking vacuum after the deposition of Ta that acted as an adhesion layer. The substrate was then placed into the quartz tube on an alumina boat, surrounded by Zn foil (purity 99.99%) that was used as Zn source. The LP-CVD was performed at 650°C for 20 min, using 300 sccm of Ar as carrier gas and 150 sccm of O2 as gas precursor. The pressure was fixed to 1.4 Torr during the process. In order to characterize and study the effects of the ZnO base on the switching mechanism, NWs were mechanically removed by sonication in ethanol for 60 min (power 135 W, frequency 59 kHz). Characterization The chemical composition of as-grown ZnO NWs was studied by means of X-ray photoelectron spectroscopy (XPS), using a Kα source with energy of 1486.6 eV and an Ar ion gun for depth profile measurements. The morphology of the grown structures was investigated by field emission scanning electron microscopy (FESEM; Zeiss Merlin). The elemental mapping analysis was conducted in a Zeiss Merlin FESEM equipped with an EDX Oxford X-act with a 102 mm SDD detector. The EDX elemental map was acquired using an electron beam at 20 kV as excitation source. The structural characterization of nanostructures was assessed by Raman spectroscopy (excitation wavelength of 514 nm) with a Renishaw inVia Reflex micro-Raman spectrophotometer equipped with a cooled charge-coupled device camera and by X-ray diffraction (XRD). Transmission Electron Microscopy (TEM) was performed with a FEI Tecnai F20ST equipped with a field emission gun (FEG) operating at 200 kV. Specimen cross-section for TEM was prepared by using a Focused Ion Beam (FIB) system (Zeiss Dual Beam Auriga) operated at 30 kV. A final cleaning step using a FIB voltage of 2 kV was also performed in order to achieve the final thickness and minimize the damage on the sample. In order to measure the


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electrical properties of NWs and the base, Pt and Cu top electrodes with diameter of ~1 mm and thickness of 200 nm (measured on a flat surface) were deposited through a shadow mask by sputtering and thermal evaporation, respectively. The Pt catalytic substrate was used as the bottom electrode, after granting access by etching the ZnO layer from a portion of the sample, using a solution of diluted HCl. The I-V measurements, endurance and retention tests of devices were performed at room temperature in air using a Keithley 4200 semiconductor parameter analyzer. During the endurance measurements in sweeping mode, the bottom electrode was connected to ground while a sweep voltage was applied to the top electrode. RESULTS AND DISCUSSION Morphological and compositional characterization The morphology of the as grown ZnO NWs is shown in Figure 1 (a) and (b). FESEM images show high density, vertically aligned and hexagonally shaped ZnO NWs. Statistical analysis of

Figure 1. FESEM images of ZnO NWs and base; (a) top view and (b) cross section of ZnO NWs after the growth process by CVD with 300 sccm of Ar and 150 sccm of O2; the inset in (a) shows ACS Paragon Plus Environment a detail of a NW tip; (c) top view and (d) cross section of ZnO base after the sonication process.

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NW dimensions, measured during FESEM analysis, revealed an asymmetric distribution of lengths and diameters, and a representative estimation of these parameters is represented by their median values that are 808 nm and 72 nm, respectively (details are reported in Supplementary information S1). In particular, cross section images show the presence of the ZnO base layer between the Pt bottom layer and the NWs. The thickness of this base layer was estimated to be (201 ± 61) nm. During the growth process, the metal catalyst serves as nucleation sites to form ZnO, as a consequence of Zn deposition over the Pt surface and the following oxidation by the oxygen flux. Furthermore, the absence of Pt at the top of NWs, confirmed by high resolution FESEM analysis on the NW tip (inset in Figure 1 (a)), indicated a Vapor-Solid (VS) growth of NWs that is the result of a self-organization of ZnO atoms along a preferential orientation.46 The sonication process allowed to remove entirely and uniformly the NWs, as reported in Figure 1 (c) and (d), leaving a continuous polycrystalline base layer with grain size ranging from 100 to 250 nm, as revealed by SEM analyses. Moreover, the Pt bottom electrode did not exhibit significant dewetting and remained continuous after the high temperature process (details in Supplementary information S2). The composition of the as-grown ZnO nanostructures was investigated by means of XPS that revealed the presence of Zn and O without other relevant contaminations (details in Supplementary information S3). A depth profile XPS study on NW arrays revealed an atomic concentration ratio of nearly 1:1 for Zn and O along the whole ZnO structure, including the base (Figure 2). This suggests that the NWs and the base layer do not present significant differences in stoichiometry. However, despite their stoichiometry, ZnO nanostructures are well known to be intrinsically n-type doped, as a consequence of the presence of various impurities and intrinsic


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defects that act as deep or shallow donors.29,47–49 In particular, it is well known that predominant defects in ZnO are zinc and oxygen vacancies, because of their low formation energy.50

Figure 2. XPS depth profile of as grown ZnO NWs and base on Pt (100 nm)/Ta (20 nm)/SiO2 (190 nm) substrate.

Structural characterization Raman spectroscopy was used to investigate the crystal properties of ZnO structures grown on Pt substrate. Raman spectra of ZnO NWs and base are presented in Figure 3 (a) and (b) and show the typical features of ZnO with wurtzite crystal structure (w-ZnO), which are briefly commented in the following. In both cases the high frequency E2 mode is located at 436 cm-1 and dominates the spectra, indicating that the samples have high crystal quality (FWHM < 10 cm-1).51 This non-polar mode involves mainly the motion of the O sublattice. Furthermore, oxygendominated modes relative to the A1 and E1 symmetries can be identified in the spectra. These


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polar modes are split into LO and TO modes and four peaks can be identified. The slight asymmetry of the E2(high) peak is linked to a weak shoulder peak in the low frequency direction, which correspond to the E1(TO) mode, while the A1(TO) mode is observed at 378 cm-1 in both ZnO NWs and base. In addition, in both spectra an LO band is observed at 576 cm-1, which is the consequence of the A1(LO) and E1(LO) modes with close wavenumbers. The LO band is usually associated to the presence of intrinsic defects, such as oxygen vacancies or Zn interstitial.52 The asymmetry of this peak is linked to the presence of B1(high) silent modes of w-ZnO, that can be observed as a consequence of disorder-activated Raman scattering (DARS) induced by defects or impurities that change the lattice symmetry.53,54 Furthermore, second order peaks can be identified in Raman spectra. Peaks relative to the E2(high)-E2(low) difference mode are observed at 329 cm-1 and 330 cm-1 for ZnO NW arrays and for the base, respectively. In addition, two broad bands are observed in the range 1000-1250 cm-1, as a consequence of 2LO phonon scattering processes associated with A1 symmetry. Raman spectroscopy reveals that both vertically aligned NWs and base have similar high crystal quality. Moreover, no evident lattice mismatch between the ZnO base and NW arrays was found, as confirmed by the fact that both samples displayed the same peak positions (details are reported in Supplementary information S4). Typical XRD diffractograms of NWs and base are presented in Figure 3 (c) and (d), respectively. In both cases, peaks can be indexed to the hexagonal phase of ZnO with P63mc symmetry. Strong peaks are located at 2θ equal to 34.54° and 34.52° for the vertically aligned NWs and for the base, respectively. These peaks are the consequence of the diffraction of (002) plane of w-ZnO. The comparison of XRD peak intensities with ZnO powder diffraction pattern (from standard data file JCPDS No. 89-0511) revealed that both vertically aligned ZnO NWs and


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grains forming the base are highly oriented in the [002] c-axis direction. This suggests, according also to FESEM images, that NWs are the prolongation of the base grains. However, XRD suggested that the structure of the base is slightly more disordered than that of vertically aligned NWs, showing a more pronounced [103] orientation. (details of XRD analysis are reported in Supplementary information S5).

Figure 3. Fitted Raman spectra of (a) ZnO NW arrays and (b) ZnO base; black circles are background subtracted data while the cumulative fit is presented in red; single components of the fit are presented in blue. The insets show the fit of the second order Raman peaks in the range 950-1250 cm-1; XRD diffractograms (intensity in log scale) of (c) ZnO NW arrays and (d) ZnO base on Pt/Ta/SiO2/Si substrate.


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Electrical characterization First, symmetric Pt electrodes were considered in devices prepared with ZnO NWs and after their removal. Considering ZnO NWs, FESEM analysis on a cross section using the backscattered electrons detector, reported in Figure 4 (a), showed that sputtered Pt can penetrate into the ZnO nanostructured array and deposit along the borders of NWs, which result coated down to the base. High magnification FESEM image of the NWs base (reported in Supplementary information S6) confirmed that Pt deposition can reach the ZnO base layer. This observation is supported also by the EDX elemental map that clearly shows the presence of Pt along NWs, reaching the ZnO base (Figure 4 (b)-(e)). This is a consequence of a non-tightly

Figure 4. (a) Backscattered electrons FESEM image of a cross section of ZnO NWs covered by the Pt top electrode deposited by sputtering; (b) EDX elemental mapping results; (c) Pt map, (d) Zn map and (e) O map.


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packed ZnO NWs distribution that allows the bottom region to be exposed to the sputtered Pt during deposition of the top electrode. Indeed, a prevalence of Zn and O is observed in the sectioned ZnO base. The presence of the base layer, which on one hand warrants avoiding direct contact between top and bottom electrodes, on the other hand can have an active role during the switching process. The resistive switching behavior of the device and the switching mechanism were evaluated by electrical measurements. The electrical characterization was performed in voltage sweep mode. Firstly, a conductive channel was created through electroforming, which can be considered a soft breakdown of the active oxide material. This was obtained by applying several voltage sweeps to the top electrode of pristine devices (details in Supplementary information S7). To avoid a permanent breakdown of the device by Joule heating, a current compliance (CC) was imposed by the experimental setup to control the maximum current into the device during the SET transition. Figures 5 (a) and (c) show the two-terminal I-V characteristics of Pt/ZnO/Pt devices in presence of NWs and the base only, respectively. During the voltage sweep in negative polarity, the device turns to the high resistance state (HRS) in correspondence of VRESET where a RESET process occurs. Conversely, a SET process occurs when the applied voltage is swept over a positive threshold value (VSET) and the current dramatically increases, turning the device into the low resistance state (LRS). In both cases a CC of 11 mA was imposed during the positive voltage sweep where a SET transition occurs, while compliance current was not applied during the negative voltage sweep (RESET process). The role of CC is to prevent an irreversible breakdown of the device, due to Joule heating, during the abrupt decreasing of resistance in the SET transition. In both cases, a bipolar resistive switching characterized by comparable VSET, VRESET and IRESET currents was observed. However, ZnO NW arrays showed a more resistive HRS than


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devices with ZnO base only. To further investigate the conduction mechanism, the data presented in Figure 5 (a) and (c) were re-plotted in full logarithmic scale (see Supplementary Information S8). The I-V characteristic in LRS of both ZnO NWs and the base exhibited an Ohmic conduction behavior, which is ascribable to the formation of a conductive filament. In HRS, even if the electrical transport followed an Ohmic conduction at low voltages, non-linear effects were observed at high voltages (V> 0.5V) for both ZnO NWs and the base. This conduction mechanism can be better described by trap-controlled space charge limited conduction (SCLC).55 The differences in conduction mechanism of LRS and HRS suggested that the conductivity of the ON state should be confined rather than homogeneously distributed.14

Figure 5. Two terminal I-V characterization of Pt/ZnO/Pt devices; IV characteristics and endurance of the as grown NWs (a and b) and of the base (c and d). The arrows denote the sweeping direction of voltage.


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To evaluate the stability of the devices, endurance properties were tested showing stability for about 70 cycles. Results for NWs and the base are presented in Figure 5 (b) and (d), respectively. The decrease of the HRS/LRS window observed for ZnO NW arrays after about 60 cycles is mainly attributed to the decrease of the HRS state. This behavior can be ascribed to the formation of strong conductive channels that make difficult a complete RESET process. Moreover, it is important to notice that lower values of CC applied to the device do not allow to achieve stable resistive switching (details in Supplementary Information S9). A comparison between the resistive switching performances of our devices with literature is reported in Supplementary information S10. Even if a similar conduction mechanism was observed in ZnO NWs and base in both LRS and HRS, the HRS/LRS ratio was reduced after the removal of NWs. This is mainly attributable to the decrement in resistance of the HRS, as can be observed from the comparison of Figure 5 (b) and (d). In addition, no substantial differences in switching parameters between ZnO NWs and base were observed (VSET, VRESET and IRESET values as function of cycles are reported in Supplementary information S11 for both devices). Switching mechanism in the polycrystalline base Our results show that the polycrystalline base plays a crucial role in the switching mechanism. Since the top electrode can reach the base even in presence of NWs, and established that the base alone exhibits a resistive switching behaviour, the switching mechanism in ZnO NW arrays can be located at the base layer. This hypothesis is corroborated by electrical measurements that revealed similar characteristics for the conduction mechanism in LRS and HRS and similar switching parameters for ZnO NWs and base, as previously discussed. In this scenario, it is possible to understand the decreasing of the HRS/LRS ratio after NWs removal. In both devices, the ON state can be attributed to a confined conduction through a filament located in the base


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layer and for this reason the values of LRS after the removal of NWs are not strongly affected. In case of the OFF state, where the whole electrode is reasonably involved in a more delocalized conduction, the values of HRS decreased after NWs removal. Thus, even in presence of NWs, the physical mechanism of switching should be investigated considering the columnar-grained structure of the base layer. Indeed, grain boundaries (GBs) in insulating polycrystalline materials are well known to be electrically conductive regions, while the grain center is non-conductive and generally not involved in the switching mechanism, as previously reported by Muenstermann et al.56 in SrTiO3 films and by Moriyama et al.57 in NiO films. This is due to the high concentration of defects at GBs regions that locally influence the conductivity. As previously discussed, ZnO is typically rich of oxygen vacancies (ܸை •• , ܸை •). During electroforming, when a positive bias voltage is applied to the top electrode, these positively charged ionic defects are expected to move towards the grounded electrode and assemble, forming a conductive channel at GBs, where a faster defect motion is enabled.36,58 It is important to notice that, since oxygen vacancies are not defined as chemical species, the migration of these defects should be attributed to the migration of negatively charged oxygen ions that move in the opposite direction (towards the positively biased TE during electroforming). In this case, GBs can act as oxygen vacancies reservoir, facilitating the formation of localized filaments. When an opposite polarity is applied, the rupture of the filament occurs in the thinnest region due to Joule heating. Indeed, the high temperature increases the ion defects mobility. Thus, ionic defects (oxygen vacancies) start to migrate towards the negative biased electrode opening a depleted gap ∆x in the conductive filament in correspondence of VRESET59. As a consequence, the device is turned to the HRS. After the creation of the conductive channel, the annihilation/formation of the filament, which reasonably occurs only in the thinnest region of the conductive path, gives rise to


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the bipolar RS behavior. According to the numerical model of formation/rupture of conductive filaments in metal-oxide based resistive switching devices developed by Larentis et al.59, the abrupt SET transition has to be attributed to a self-acceleration of ions migration. This is due to the progressively enhanced electric field and temperature at the conductive filament tip that occur during the reduction of the gap ∆x. A schematic representation of the resistive switching mechanism along defect-rich GBs of the ZnO polycrystalline base is presented in Figure 6.

Figure 6. Schematic diagram of bipolar resistive switching in the ZnO polycrystalline base through the formation/rupture of a filament along grain boundaries; (a) pristine state, (b) conductive path of oxygen vacancies in the LRS and c) HRS.

It is worth noticing that a SET process can be forced in both polarities, suggesting that the measured devices are initially almost symmetric (details are reported in Supplementary information S12). However, an asymmetry of the oxygen vacancies concentration is introduced


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after forming. Thus, positively charged vacancies tend to accumulate near the negative charged electrode. Moreover, it can be noticed that the resistance states are non-volatile at room temperature. Indeed, resistance states are preserved when applying a voltage below the threshold voltage and thus the driving force is not sufficient for oxygen vacancies to drift. The stability of the conductive path in the ZnO base is demonstrated by retention tests, where current is measured applying a stress voltage of 0.1 V for 5 ms every 10 s, after the device programming to LRS and HRS. The device showed a stability of at least 104 s at room temperature (Figure 7).

Figure 7. Retention test on ZnO polycrystalline base with symmetric electrodes of Pt. Test was performed at room temperature, applying a stress voltage of 0.1 V for 5 ms every 10 s.


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In the present case, the high orientation of the base grains can naturally promote the creation of current paths of oxygen vacancies perpendicular to electrodes. Indeed, the high orientation of GBs perpendicularly to the electrodes can be clearly seen in cross-sectional TEM measurements (Figure 8 (a)) that show the columnar structure of the ZnO base layer. The high-resolution TEM (HRTEM) image (Figure 8 (b)) shows a typical interface between two grains in the base sample. The clear lattice fringes demonstrate the high crystallinity of both grains. The image insets in Figure 8 (b) are the Fast Fourier Transform (FFT) patterns derived from the HRTEM image.

Figure 8. (a) Bright Field TEM cross-section image of ZnO base sandwiched between top (TE) and bottom (BE) electrodes, arrows indicate the grain boundaries (GBs); (b) HRTEM image of interface between two grains, with the corresponding FFT patterns.


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As discussed by Chang et al.60 in ZnO thin films obtained by sputtering, oriented GBs can facilitate the creation of straight (rather than branched) localized conductive paths, consequently bringing a narrow dispersion of resistance states and low forming electric fields. In our case, we observed high variability of the pristine state. In addition, it was found that in some electrodes a conductive path was directly formed after the growth process, without need of an electroforming step. In those cases, we found the device in a pristine low resistance state, due to the conductive path at GBs, and the RESET to HRS occurred before the SET during I-V cycling. This observation underlines the importance of the pre-existing oxygen vacancies in as-grown ZnO.61 As previously discussed, the coarse grain size of the polycrystalline base is an intrinsic consequence of the high temperature growth process. In particular, the limited number of GBs can potentially improve the electrical stability of devices, reducing the randomization of the formation/rupture of the conductive filament , as observed by Zhuge et al.62 in ZnO sputtered films. In addition, resistive switching was observed for a ZnO base layer thickness up to ~ 320 nm. This thickness is considerably higher than ZnO thin film usually realized by other techniques

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, suggesting that in our case the high crystallinity and grain boundaries orientation

can facilitate resistive switching even in case of higher ZnO thickness (a detailed discussion is reported in Supplementary information S13). Defects-rich GBs can also act as a facilitating path for diffusion of an electrochemically active metal, as previously reported by Zhuge et al..62 To further prove the proposed mechanism, Cu top electrodes were realized on the ZnO base. Indeed, Cu is well known for its high diffusivity and ability to form a conductive path between the two electrodes. Therefore, ZnO base was sandwiched between an electrochemically active electrode (Cu) and an electrochemically inert counter electrode (Pt). The application of a positive voltage to the Cu electrode induced the


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anodic dissolution of Cu and drift of Cu2+ ions across the ZnO layer, under the action of a high electric field (when 1 V is applied between the two electrodes, the magnitude of electric field across the base is 5·106 V/m). When Cu2+ ions reached the inert electrode surface, electrocrystallization and reduction to Cu occurred, giving rise to a metallic bridge growing in the direction of the active electrode.16 However, ion migration and oxide defects in the material are both reasonably involved in the formation of conductive filaments , which can be the result of an intermix of Cu and oxide vacancies, as previously discussed by Celano et. al..63 As a probable result of the competition between the two effects, the stability of asymmetric devices was strongly affected in the present case. Furthermore, the endurance was dramatically reduced, as can be noticed in the I-V characteristics of Cu/ZnO base/Pt presented in Figure 9. It was observed that different devices were easily formed applying different polarities to Cu top electrodes, meaning that Cu2+ ions did not dominate the switching mechanism. Therefore, it can be inferred that oxygen vacancies play a crucial role in the switching mechanism, even in case of electrochemically active electrodes of Cu.

Figure 9. Two terminal I-V characterization of Cu/ZnO base/Pt devices; The SET process occurred at both (a) positive and (b) negative polarities applied to different Cu top electrodes. The insets show the endurance of devices. ACS Paragon Plus Environment

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CONCLUSIONS This work investigates the switching mechanism of memristive devices based on ZnO NW arrays grown by LPCVD. Because of the CVD process, a ZnO thin layer is typically formed between NWs and the substrate. This “base” layer is polycrystalline, with a prevalence of [002] orientation, and was found to be exposed to the evaporation of the metallic top electrodes during devices fabrication, together with the NW arrays. Therefore, the possible active role of the base layer should be considered when explaining the physical mechanism of resistive switching in ZnO NW arrays. The switching mechanisms and performances of devices made with ZnO NW arrays plus ZnO base were thus compared with devices made with ZnO base only. Considering Pt inert electrodes, the proposed switching mechanism of the ZnO polycrystalline base layer was ascribed to the formation/rupture of oxygen vacancies filaments along grain boundaries. Furthermore, the high orientation of grain boundaries can promote the formation of straight and localized filaments, improving the switching stability. The stability of devices was strongly reduced when using an electrochemically active electrode (Cu), as a probable consequence of the competition between electrochemical metallization mechanism and the oxygen vacancies dominated filament formation.

AUTHOR INFORMATION Corresponding Author *Gianluca Milano, e-mail: [email protected] *Carlo Ricciardi, e-mail: [email protected]


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Author Contributions G.M, S.P. and C.R designed the experiment. G.M and M.Y.A performed CVD, device fabrication and electrical characterization. K.B. performed electron microscopy analyses. S.B. performed X-ray diffraction analysis. G.M., S.P. and C.R. analyzed the data and wrote the manuscript. All authors participated in the discussion of results and revision of the manuscript. ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website. ZnO NWS diameter and length analysis; Pt bottom electrode after the ZnO NW growth at high temperature; XPS survey; Raman spectra analysis of ZnO NW arrays and base; XRD data analysis; High magnification FESEM image of Pt coated ZnO base; Details of the forming process; A comparison of resistive switching performances with literature; Full logarithmic plots of I-V cycles; Additional information on electrical characterizations; Additional I-V characteristics; Resistive switching and ZnO base thickness; ACKNOWLEDGMENT The support by M. Raimondo in helping with FESEM measurements and EDX analysis, by M. Laurenti and D. Perrone in helping with sputtering deposition and fabrication of devices, and by S. Guastella for performing XPS measurements is gratefully acknowledged. The authors declare no competing financial interest.


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Unravelling Resistive Switching Mechanism in ZnO ... - ACS Publications

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