Electrode Dependence of Local Electrical Properties of Chemical

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Electrode dependence of local electrical properties of Chemical Solution Deposition-derived BiFeO3 Thin Films Qi Zhang, Abhimanyu Rana, Xiaoyan Liu, and Nagarajan Valanoor ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00064 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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ACS Applied Electronic Materials

Electrode dependence of local electrical properties of Chemical Solution Depositionderived BiFeO3 Thin Films Qi Zhang1, Abhimanyu Rana1,2, Xiaoyan Liu3*, Nagarajan Valanoor1* 1

School of Materials Science and Engineering, University of New South Wales, Sydney, 2052, Australia

2 3

School of Engineering and Technology, BML Munjal University, Gurgaon 122413, India College of Metallurgy and Materials Engineering, Chongqing Key Laboratory of Nano/Micro Composites and Devices, Chongqing University of Science and Technology, Chongqing 401331, China

[email protected] [email protected]

Keywords: p-type bismuth ferrite thin films; electrical properties; chemical solution deposition, ultrathin lanthanum strontium manganite; ferroelectric

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Abstract The nanoscale electrical properties of chemical solution deposition(CSD)-derived BiFeO3 grown on pulsed laser ablated La0.67Sr0.33MnO3//SrTiO3 (001) thin film heterostructures are investigated using a host of scanning probe microscopy techniques, including electrostatic force microscopy(EFM), scanning Kelvin probe microscopy(SKPM), piezoresponse force microscopy (PFM), and conductive AFM(CAFM). EFM and SKPM confirm the p-type nature of the CSD derived BFO thin films as well as charge accumulation at the film surface after electrical bias. For BiFeO3 films of a fixed thickness (~35 nm), the local current-voltage (I-V) behavior obtained by CAFM is strongly dependent on the La0.67Sr0.33MnO3 bottom electrode thickness. BiFeO3 films on a 20 nm thick La0.67Sr0.33MnO3 demonstrate the typical switchable diode behavior governed by polarization orientation. However, when the thickness of La0.67Sr0.33MnO3 is reduced to less than 5 nm, the BiFeO3 films show only forward diode behaviors regardless of polarization orientation, when the applied bias is up to ±4 V. Higher sweep bias (i.e. ±8V) breaks down the diode, following which the BiFeO3 film shows strong resistive switching. The interface band structure for the ultra-thin LSMO case, which is very sensitive to accumulation/depletion of carriers at the BFO-LSMO interface, is suggested as the trigger for this resistive switching.


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1. Introduction Bismuth ferrite (BiFeO3, BFO) is a room-temperature1, 2 multiferroic material that exhibits exceptional functional properties, such as electric field and strain control of magnetism3, 4, anomalous photovoltaic effect5-7, surface photoelectrochemical activity8, 9 , domain wall conductivity

10-13

and resistive switching (RS)

14-21.

Such effects have been

reported for various forms of BFO, from bulk22 to nanostructures23, 24 and finally thin films of BFO17, 18, 25, 26. For example, BFO thin films have demonstrated high ON/OFF RS ratio and the ability to be integrated with silicon17, which makes them appealing for non-volatile memory devices as well as disruptive technologies such as neuromorphic computing27-30. Mechanisms that govern the electrical conduction properties (and RS behavior) in BFO thin films range from electron tunneling14 to oxygen vacancy migration16,

18, 19

to BFO/electrode interface controlled band bending under external

electric field14, 16. Most electrical behavior studies have focused on epitaxial thin films made by physical vapor deposition methods where the BFO is n-type and typically fabricated on a p-type oxide electrode14,

16, 17.

For such a configuration, a typical p-n

junction is formed at BFO/bottom electrode interface, which demonstrates the (now well-understood) diode-like resistive switching behavior 16. On the other hand, RS or conductivity properties investigations on chemical solution deposition (CSD)31, 32 derived epitaxial BFO thin films are not that abundant.33, 34. Recently we have shown it possible to synthesize high-quality epitaxial BFO films via CSD with robust ferroelectric properties on lanthanum strontium manganite (La0.67Sr0.33MnO3, LSMO)/ buffered (001) strontium titanite (SrTiO3, STO) substrates (where the LSMO layer was grown by pulsed laser deposition – PLD). These CSD-derived BFO thin films were fabricated using stoichiometric precursors (Bi:Fe=1:1). As a consequence they characteristically showed a phase-pure perovskite structure but with 3 ACS Paragon Plus Environment


high bismuth deficiency31 and hence a p-type electronic behavior35, 36. The work function of p-type BFO (>4.7 eV )35 is close to that of LSMO electrode (~4.8-4.9 eV) 37, 38. Due to this, the electrical behavior of CSD-derived BFO thin films on LSMO is expected to be distinct from those shown by the more common (typically PLD-grown) n-type BFO films. In addition, when pushed to the ultrathin regime, the carrier concentration of the LSMO reduces significantly39. This should have a strong effect on the BFO/LSMO band-bending and hence the charge depletion widths. For example, it was shown that polarizationinduced electric fields can induce significant electrostatic modulation of an underlying ultrathin LSMO electrode and further enhance tunneling electroresistance40. In the same vein, CSD-derived BFO thin films with ultrathin LSMO promise entirely unexplored nanoscale RS characteristics and ultimately, as a consequence, nanoelectronic device opportunities. Here we present a systematic study of the nanoscale (or local) electrical behavior of p-type CSD-derived BFO thin films as a function of an underlying LSMO bottom electrode thickness. We use a range of scanning probe techniques, namely, piezoresponse force microscope (PFM), electrostatic force microscopy (EFM), scanning Kelvin probe microscopy(SKPM), and conductive atomic force microscopy (CAFM). 35±5 nm (001)-oriented CSD-derived BFO thin films were deposited on 5 nm or 20 nm thick LSMO buffered (001) STO single crystalline substrates. In both cases we show the ferroelectric switching is easily confirmed using PFM techniques. EFM was then employed to identify the type of charges accumulated at the film surface of the switched domain regions. The EFM results were then applied in tandem with SKPM data to ascertain the p-type nature of the CSD derived BFO thin films. Following this CAFM was then employed to investigate the local RS behaviors. It was found that the BFO/20 nm 4 ACS Paragon Plus Environment

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LSMO//STO system demonstrates the well-known diode-like response, with the RS being strongly governed by the BFO polarization orientation16. However, BFO films on the ultra-thin LSMO show polarization-dependent conductivity opposite to the 20 nm LSMO case, but only up to a bias of ±4 V. Uniquely they demonstrate low conduction for the polarization down state regardless of the sweep bias polarity. However, a sharp RS change is observed when the sweep bias is increased to ~±8V. These observations are attributed to the ultrathin LSMO bottom electrode, a channel layer whose carrier density is easily influenced by the ferroelectric polarization and interface band structure. Once the RS is triggered, it persists with further cycling. Furthermore, the critical bias at which the sharp RS is observed as well as the asymmetry in the I-V curve dramatically reduces; this hints that once started, the RS is then sustained by a filamentary mechanism. Thus, we show that subtle changes in the bottom electrode have a significant influence on electrical properties and hence the RS behavior of the BFO thin films. 2. Experimental Section CSD-derived epitaxial BFO thin films were prepared on LSMO-buffered (001) STO or (001) lanthanum aluminate (LaAlO3, LAO) substrates. An amorphous film was first prepared by spin coating stochiometric precursor (the mole ratio of Bi to Fe equals to 1) on the substrate followed by drying in air. Later the film was further heated at a higher temperature to obtain a crystallized film with thickness of 35±5 nm. This process was conducted in oxygen atmosphere to prevent large volume of oxygen vacancies. However energy dispersive spectroscopy (EDS)-based elemental analysis confirmed a high bismuth deficiency level such that Bi:Fe ratio was approximately 0.8:1.031. Full details of the film preparation and crystallographic evolution can be found elsewhere31, 41. Suffice 5 ACS Paragon Plus Environment


to say that all films studied here were found to be phase-pure and epitaxial along (001) direction. Both LSMO thicknesses used in the study for the CSD-derived BFO films were fabricated by pulsed laser deposition (PLD, Neocera, US). In addition, a reference BFO/LSMO (001) heterostructure was also prepared by PLD. Topography, PFM, EFM, SKPM and CAFM were investigated using a commercial scanning probe microscope (Cypher, Asylum Research, US). Platinum coated conductive silicon cantilevers (BudgetSensors ElectricMulti75-G, Bulgaria) or a diamond coated silicon cantilever (Nanosensors, CDT-CONTR, Switzerland), were used for the above SPM mapping and I-V spectroscopy measurements. For the CAFM and I-V measurements, the bias was applied from the bottom LSMO electrode to PFM probe. To eliminate the crosstalk influence from topography on the EFM and SKPM data, these modes were operated using a Nap mode technique where the topography and EFM/SKPM images were captured via two scans. In the EFM mode, a DC voltage of +3 V was applied to the tip, which was raised 10 nm above the sample during scanning. For the KPFM, the probe was also 10 nm away above the surface and +1V AC bias was applied. This AC bias could result in the cantilever oscillation with the presence of surface charge, and hence a DC bias was added to the tip to nullify the electrostatic force between tip and sample. This DC bias was then used for the quantitative analyze of the sample charge (potential) details following the methods detailed in ref 42, 43. 2. Results and discussions Figure 1(a) shows the configuration of two BFO/LSMO//STO (001) thin films systems studied here along with their corresponding vertical PFM (VPFM) analysis. The images have a scan size of 1 µm × 1 µm. Larger scans (not shown) have similar results. Both films display relatively smooth surface with roughness of approximately 3 nm 6 ACS Paragon Plus Environment

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(Figure 1b). The VPFM amplitude (Figure 1c) and phase (Figure 1d) images reveal that both films are polydomain in their as-grown state, with strong amplitude signal and clear domain walls. The corresponding phase images reveal a 180° phase change between the yellow and purple contrasts. However, note that the domain density increases for the system with 5 nm LSMO thickness. We attribute this to an increased depolarization field (discussed later).

Figure 1 (a)Film configuration, (b)as-grown topography (scan size: 1 µm × 1 µm) of BFO/20 nm LSMO//STO (001) and BFO/5 nm LSMO//STO (001) thin films, and corresponding (c) PFM amplitude and (d)phase images

We then proceeded to switch the local polarization by applying a DC electric field to the conductive AFM tip (so-called writing process) and investigate its influence on the corresponding local electrical properties using SPM techniques, starting with piezoresponse (PFM), followed by an investigation into the nature of surface/screening charges (EFM), the surface potential (SKPM) and finally conduction (CAFM), as shown in Figure 2. Here a 3 µm × 3 µm square region was poled down for the BFO films on 20 nm 7 ACS Paragon Plus Environment


LSMO by applying an external DC bias -6V, and a 1 µm × 1 µm square inside this 3 µm × 3 µm region was switched upwards by +6V bias. In contrast, for the BFO films on 5 nm LSMO, the outer square box (3 µm × 3 µm) was switched upwards under external DC bias of +6V, and the inner box (1 µm × 1 µm) was poled down by -6V bias. Note for both PFM reading and writing, the bias was applied from bottom LSMO electrode to AFM probe. The yellow and purple contrasts in the vertical phase images denote upward and downward polarization, respectively. Thus under +6V, the polarization of the inner box (outer box) of BFO/20 nm LSMO//STO (BFO/5 nm LSMO//STO) is switched upwards. The contrary is true for an applied bias of -6V. For EFM and SKPM, the domains were switched via a tip instead of via bottom electrode, thus compared with PFM, the outer box was switched under -6V (poled up) and inner box was switched under +6V (poled down). Figure 2 is the result of a comprehensive SPM study that shows the VPFM -phase (a,e), EFM (b,f), SKPM (c,g) and CAFM (d,h) images after domain switching under DC bias. Similar to the VPFM phase images, EFM and SKPM images of the switched areas for the two films display opposite contrasts, indicating opposite type of charges accumulate on the surfaces of poled-up and poled-down areas respectively. During EFM and SKPM measurements, the tip was applied a positive bias and kept at 10 nm above the sample surface. In EFM technique, the phase image is a result of the tip response to the surface charge as a result of either repulsive or attractive force. A higher phase angle (bright contrast) suggests an attractive force and thus a negative surface charge in this case, while a lower phase angle (dark contrast) indicates a positive surface charge accumulation20. Thus, in Figure 2 (b) and (e), there are negative surface charges on poled-up regions and positive charges on poled-down domain surface. These charge 8 ACS Paragon Plus Environment

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polarities agree well with the expected surface screening charges20. Furthermore, the SKPM images give a quantitative potential difference between sample surface and probe, (discussed later in Figure 3). Even though both films show the same type of charge distribution after poling, the CAFM images of these two films display an opposite contrast. This indicates that the polarization dependent conductivity of the switched BFO thin films varies as a function of the bottom LSMO electrode thickness. When the LSMO thickness is 20 nm, the poleddown domains possess a higher conductivity than the poled-up domain regions, consistent with previous observations14. On the other hand, when the LSMO is reduced to 5 nm or thinner (e.g. see our data for BFO on 2 nm and 5 nm LSMO-coated LAO (001) substrates in the supporting documents Figure S1), an opposite trend is found (i.e. poled up domains show a higher conductivity). The CAFM images here were obtained by scanning under a +2V reading bias. Note that for both types of film, the as-grown domains have a low conductivity implying the observed differences do not originate from sample-to-sample variations.



Figure 2 PFM phase images (scan size: 5 µm × 5 µm) of (a) BFO/5 nm LSMO//STO (001) and (e) BFO/20 nm LSMO//STO (001) thin films after domain switching, and their corresponding, (b) and (f) EFM, (c) and (g) SKPM and (d) and (h) CAFM images. For PFM and CAFM images, the films are switched under +6V (pole up) and -6V(pole down) via bottom electrode. For EFM and SKPM measurement, the films are switched -6V (pole up) and +6V(pole down) via AFM probe. Yellow contrast in PFM phases images indicates upwards polarization and purple phases indicate downwards polarization. Although we focus in the main manuscript on BFO thin films prepared on LSMObuffered (001) STO, it is worthwhile to note that we performed the same studies on CSD BFO thin films prepared on (001) LSMO//LAO (Figure S1). These films have either the well-known tetragonal (T)-rhombohedral (R) mixed phase (5 nm electrode) or purely T phase (2 nm electrode thickness)44. This allowed the investigation of the influence of strain effects on the local electrical properties. Similar results were obtained for both the STO and LAO cases, indicating the behavior is not dependent on BFO strain state, topography or crystallographic phase. In addition, local PFM hysteresis loops were easily acquired regardless of the LSMO thickness, suggesting sufficient electrical connectivity for domain switching. The PFM loops in Figure S2 reveal that the coercive voltages for both type of film are approximately ±2V, similar to the CAFM reading bias 10 ACS Paragon Plus Environment

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(+2V). Thus, to eliminate the effect of transient switching currents, all CAFM images analyzed were after capturing 2 successive scans. A detailed understanding of why varying the LSMO thickness creates differences in the local electrical properties of these two systems requires unambiguous identification of the major carrier type in each of the films. Hence, we first compared the surface potential of CSD derived BFO thin film on 20 nm LSMO with the PLD derived counterpart via SKPM technique. The contrasts from SKPM images indicate the contact potential difference (CPD) between the tip and the sample surface. That is, VCPD = (Φsample – Φtip)/e, where Φsample and Φtip are the work functions of the tip and sample, respectively, and e is the elementary charge43. Note we used the same probe and repeated scans to guarantee the reliable results. As shown in Figure 3(a) and (b), surface potential mappings of two films after switching were obtained. For both films, outer boxes (3 µm × 3 µm) were switched under +6 V tip bias (poled down) and inner boxes (1 µm × 1 µm) were switched under -6 V tip bias (poled up). It is clear that both films show an increasing potential difference at poled down regions with respect to the as-grown states, and reduced potential difference at poled up regions. As mentioned previously, the EFM images confirm that the charge contrast detected for these measurements stem from the screening process. During poling, under a negative tip bias (poled up) the hole concentration at film surface increases. This results in an increase of the negative screening charges at surface after poling (since the tip is above the surface during SKPM measurement) and thus a decreasing potential of samples according to the SKPM measurement principle20. The opposite is then true for the poled-up regions. The SKPM line profiles (along the dashed lines in Figure 3(a) and (b)) of two films are compared in Figure 3(c). Although pure phase BFO films made by PLD is often 11 ACS Paragon Plus Environment


considered as a n-type material due to its high oxygen vacancies16 (when a bismuth rich target is used), many reports also claim they may present p-type conductivity due to the bismuth loss45, 46. First, the average potential of the PLD derived film is higher than CSD film (see line-profile in the as-grown regions), which means the work function of CSD film is higher than that of PLD film. Secondly, note that the surface potential of CSD film for the poled-up region dips significantly lower than the case for the PLD film. This is attributed to a higher hole accumulation at the poled-up surface regions for CSD film compared to the PLD film, resulting in a higher negative screening charge at surface and hence a stronger drop. On the other hand, the differences in the potential for the poled-down regions for the CSD vs. PLD film are not as strong. We attribute this to a lower concentration of oxygen vacancies in the CSD-derived BFO film. Nevertheless, the significant difference in the surface potential changes between poled-up and poled-down regions for the CSD film indicate its p-type like nature. Further, the potential differences between as-grown to pole up and as-grown to pole down are much larger in CSD than PLD, indicating more holes than oxygen vacancies in CSD than PLD. Following the SKPM results, and assuming a p-type nature of CSD derived BFO thin films, the respective band diagrams after poling are given in Figure 3 (d) and (e). During upwards poling, the holes are accumulated at the top ferroelectric/air interface and depleted at the bottom BFO/LSMO interface, respectively. This would lead to an upwards band bending at top surface and downwards band bending at BFO/LSMO interface, as shown in Figure 3(d). The opposite occurs for the poled down regions (Figure 3e).


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Figure 3 SKPM of (a) CSD derived and (b) PLD derived BFO thin films on 20 nm LSMO buffered STO(001) substrate; (c) SKPM profile of two samples along the dashed line in (a) and (b) . The band diagram of CSD derived BFO thin films in their (d) poled-up and (e) poled-down regions. Having confirmed the p-type nature of our CSD films, we are now in a position to understand the contrasting CAFM behaviors observed for the two CSD BFO heterostructures. We begin by comparing the local I-V loops acquired using the same CAFM tip for the BFO thin film on the 5 nm and 20 nm LSMO electrodes, respectively. Keeping in mind that local I-V and PFM loops strongly depend on topography or local chemical homogeneity, all I-V curves shown here are representative of numerous I-V datasets collected across different written squares from various parts of the film. As shown in Figure 4(a) and (c), both BFO/20 nm LSMO//STO and BFO/5 nm LSMO//STO films show highly asymmetric I-V curves for the switched regions. The blue (red) square 13 ACS Paragon Plus Environment


indicates the poled up (down) domains. To avoid complications from ferroelectric domain switching during I-V measurement, a sweep bias (from sample to tip) from -4V to +4V were used for poled-down domain regions, , while bias was swept from +4V to 4V for poled-up region. We begin with the case for BFO thin films on 20 nm LSMO//STO. A switchable diode behavior is observed, as shown in Figure 4(a). That is, in its poled-down regions, a forward diode with current up to 400 pA is measured at positive voltage while the current remains nearly zero under negative bias up to -4 V. In contrast, for the poled-up regions, higher current (conductivity) is only observed upon reaching increased negative bias( 4.7 eV) is close to the LSMO film (4.8 eV -4.9 eV) 37, 38. In this case, the depletion layer in BFO film could be thick and thus cause only a slight band-bending at interface. In particular, LSMO has a smaller band gap of approximately 0.7 eV while its large thickness provides high carrier density, which leads to the upwards but small band bending within LSMO.


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The expected band structure for each switched region is depicted in Figure 4(b), and this has been analyzed in detail using band structure alignment arguments14, 16, 40, 50, 51.

After poling, the trapped charges move to the interface under the external electric

field, and the built-in potential inside BFO induces electron depletion (or hole accumulation) inside LSMO at the BFO/LSMO interface. This band configuration (i.e. bending upwards) is such that under forward-bias conditions the band-bending further increases the barrier height, and thus no current is observed under positive voltage. On the other hand, the application of a negative bias at LSMO reduces the barrier width, allowing current to flow. However, the band-bending of poled-down BFO leads to electron accumulation (bending downwards) at the BFO/LSMO interface, and thus the opposite situation is true. For the case of the BFO on 5 nm LSMO, and contrary to the case of poled-up BFO on 20 nm LSMO, a strong forward diode behavior is observed for poled-up regions, with current values approaching 200 pA at +4V (Figure 4c). Under positive bias the poleddown region of the BFO/5 nm LSMO//STO system suggests only a weak current (less than 50 pA at +4V). These results are valid only for a sweep bias range within ±4V. We have previously found that the carrier density of the LSMO is reduced by 5 orders to 1015 cm-3 (compared to thick LSMO ~1020 cm-3) with the thickness scaling down from 20 nm to 5 nm52. This low carrier density could affect its initial work function53 and in turn change the Schottky barrier of BFO/LSMO interface. Since the accumulation/depletion of carriers is much stronger for a layer with a low carrier concentration, a larger upwards band bending occurs inside the ultra-thin LSMO at BFO/LSMO interface. This is necessary to align the Fermi level with BFO. In addition, the low carrier density of LSMO also reduces its screening length and leads to a larger



depolarization field inside the BFO film. This would cause the BFO to break into smaller domains, as confirmed previously in the PFM images Figure 1. As shown in Figure 4(d), in the poled-up regions, the barrier height and width are increased at the BFO/Pt interface and reduced at the BFO/LSMO interface, due to the movement of holes from the ultra-thin LSMO layer to BFO35 under external electric field. This is consistent with the SKPM results of Figure 3. Thus, current can easily flow in the forward direction from the LSMO to Pt, yielding a diode behavior opposite to that observed for the 20 nm LSMO case. On the other hand, a downwards polarization leads to large hole depletion and in turn to a downwards band-bending40. Therefore, a higher voltage is required to overcome the barrier under forward bias, and only a low current may be observed at positive bias within a similar voltage range (i.e. +4V). This explains that not only does the polarization state (as is observed typically), but also the electrode thickness, can be used to control local electrical performance of the CSD-derived BFO thin films.


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Figure 4 I-V curves of (a) BFO/20 nm LSMO//STO and (c) BFO/5 nm LSMO//STO thin films; The bias was applied from bottom electrode to probe. Band diagram of p-type BFO with 20 nm LSMO in (b) poled up regions and poled down regions; Band diagram of ptype BFO with 5 nm LSMO (d) poled up regions and poled down regions. Most intriguing is the effect of increasing sweep bias range. For example, when sweeping from -6V to +6V, the BFO/20 nm LSMO//STO sample still retains its diode behavior (Figure S5(a) in the supporting documents). On the other hand, a transition to sharp RS behavior is found for the BFO/5 nm LSMO//STO which we discuss further. Figure 5 shows this change from the forward diode-like asymmetric curve to a clear symmetric and sharp RS characteristic with increasing sweep bias and cycles in the BFO/5 nm LSMO//STO film. Although we show below the case for poled-up domains, we also found the same for poled-down domains given in (Figure S6). First, under the sweep bias of ±6V, an asymmetric resistive switching (for positive applied voltages) and diode behavior (for negative applied voltages) under 17 ACS Paragon Plus Environment


forward bias conditions. The sweep sequence is indicated by the arrows in Figure 5(a), The current increases gradually as the voltage increases to +6V (Step 1 and Step 2). The current then starts to reduce gradually when the bias is swept back to below +1V (Step 3). The schematics in Figure 5(c)-(d) show the change of interface band alignment with bias in each step. The red arrows suggest the polarization orientation and the yellow arrows indicate the relation between input voltage and output current. In the initial upwards poling state (Step 1), there are finite barrier heights (and widths) at both the BFO/LSMO and Pt/BFO interfaces, which could suppress currents under low bias voltages. Then upon increasing bias in the forward direction, electron tunneling occurs at the Pt/BFO interface, along with the diffusion of holes from LSMO valence band (VB) to BFO VB, resulting in a switching event (Step 2). With the following reverse sweep (but still in the positive bias regime; Step 3), hysteresis is now observed due to retention of these carriers at the interface resulting from the built-in potential in BFO.


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Figure 5 Resistive switching behavior of BFO/5 nm LSMO//STO at poled-up domains under ±6 V sweep bias (a) and ±8 V sweep bias (b); blue and red plots in (b) indicate 1st cycle and 4th cycle sweep, respectively. The bias was applied from bottom electrode to the AFM probe. (c)-(d) are the schematic descriptions for resistive switching step (1)-(3) in (a). When the bias increases to ±8V (Figure 5(b)), the transition to sharp RS can be seen. Now at maximum positive bias the current peaks dramatically to above 20 nA ( i.e. several times larger than low bias case). For the reverse sweep direction (under negative bias) the I-V shows large current spikes and over all the current values are significantly higher in comparison to the low bias measurement found in Figure 5 (a). Also, one observes an asymmetry in the I-V shape. Typically, asymmetry in the I-V curve is attributed to an interface-type-mechanism54. However, after a few cycles, the I-V behavior changes from asymmetric to symmetric behavior but with persistent sharp RS in both positive and negative halves of the I-V curve(within 4 cycles). Furthermore, the critical set bias at which the sharp RS is observed reduces with the cycling. This gradual shift to a symmetric nature coupled with the reduction in critical bias strongly hints that 19 ACS Paragon Plus Environment


the RS is no longer driven by purely an interface mechanism. Thus although the RS is triggered by the interface band structure of the ultra-thin LSMO, it is very likely that over repeated cycles the RS is now sustained by a filamentary mechanism55. This transition can be attributed to movement of the defects (vacancies) which self-align into a filamentary network after a few sweep cycles.

3. Conclusion In summary, we have used local VPFM, EFM, SKPM, CAFM and I-V spectroscopy measurements to investigate the electrical behavior of CSD-derived p-type BFO thin films on LSMO buffered STO substrates. When ultra-thin LSMO is used as the bottom electrode, BFO presents forward diode behaviors, regardless of polarization orientation, at low sweep voltages (i.e.± 4V). With the increase of sweep bias and sweeping cycles, BFO shows a transition towards strong resistive switching as a result of defect movement (and accumulation) and formation of filament. Thus, controlling the bottom electrode and the consequent resistive switching of BFO thin films provides an alternative route for ferroelectric-based resistive-switching memory devices. The approaches presented here could also be extended to other oxide heterostructure systems that may be attractive for a wide range of applications such as neuromorphic computing 27, 30, logic 28 and spintronics19, 56. Supporting Information Brief statement in nonsentence format listing the contents of the materials supplied as Supporting Information.


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Acknowledgements This work was supported by the National Natural Science Foundation of China (51472037)

and

Shenzhen

Science

and

Technology

Innovation

Committee

(JCYJ20170818160815002). The authors would like thank Steven Dunn for his useful comments, and would also like to acknowledge the support from the Australian Research Council (ARC) Discovery Projects, and the support from Australian National Fabrication Facility (ANFF, UNSW).



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