Direct Observation of Domain Motion Synchronized with Resistive

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Direct Observation of Domain Motion Synchronized with Resistive Switching in Multiferroic Thin Films Ji Hye Lee, Chansoo Yoon, Sangik Lee, Young Heon Kim, and Bae Ho Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12756 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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ACS Applied Materials & Interfaces

Direct Observation of Domain Motion Synchronized with Resistive Switching in Multiferroic Thin Films

Ji Hye Lee1, Chansoo Yoon1, Sangik Lee1, Young Heon Kim2, and Bae Ho Park1*

1Division

of Quantum Phase and Devices, Department of Physics, Konkuk University, Seoul 143-701, Korea

2Korea

Research Institute of Standards and Science, Daejeon 305-304, Korea

KEYWORDS: multiferroic, BiFeO3, domain dynamics, resistive switching, conduction mechanism, piezoresponse force microscopy

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ABSTRACT

The room-temperature resistive switching characteristics of ferroelectric, ferroelastic, and multiferroic materials are promising for application in non-volatile memory devices. These resistive switching characteristics can be accompanied by a change in the ferroic order parameters via applied external electric and magnetic excitations. However, the dynamic evolution of the order parameters between two electrodes, which is synchronized with resistive switching, has rarely been investigated. In this study, for the first time, we directly monitor the ferroelectric/ferroelastic domain switching dynamics between two electrodes in multiferroic BiFeO3 (BFO) planar devices, which cause resistive switching, using piezoresponse force microscopy. It is demonstrated that the geometrical relationship between the ferroelectric domain and electrode in BFO planar capacitors with only 71° domain walls significantly affects both the ferroelectric domain dynamics and the resistive switching. The direct observation of domain dynamics relevant to resistive switching in planar devices may pave the way to a controllable combination of ferroelectric characteristics and resistive switching in multiferroic materials.

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INTRODUCTION Resistive switching in oxide materials has been widely studied because of its potential application to electrically accessible next-generation non-volatile memory with high speed and density.1–7 In particular, resistive switching in ferroelectric BiFeO3 (BFO) has been focused on because intrinsic ferroelectric domain switching without mediated-defect migration is thought to cause the resistive switching behavior.5 The high on-state current in semiconducting BFO, unlike in other insulating ferroelectric materials, eliminates the requirement for an ultrathin and low-yield tunneling ferroelectric film for nanoscale memory devices.5 In addition, the resistance of multiferroic BFO depends on both the external electric and magnetic fields. This leads to multibit storage resistive switching devices compared to conventional oxide and/or perovskite resistive switching memories, which are operated solely by external electric fields. Ferroelectric BFO has three different types (71°, 109°, 180°) of domain walls (DWs) that are named according to the angles of their ferroelectric polarization switching behavior.8 In the last decade, its electric, magnetic, and even photovoltaic properties have been controlled by domain engineering using different substrates.9–15 In ferroelectrics, changes in the domain configuration accompanied by the switching of properties have been widely investigated in vertical capacitor structures using piezoresponse force microscopy (PFM), resulting in domain patterns at different states16–18 of the switching process and different positions on the capacitors.19 Recently, resistance tuning between ON and OFF states has been reported in BaTiO3- and BFO-based ferroelectric tunnel junctions with regard to domain configuration changes.20,21 However, in these capacitor structures where the top electrodes hindered direct observation of the domain evolution, it was not possible to achieve a change in depth profile of the ferroelectric domains between two electrodes, 3



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which might be relevant to the resistive switching.22,23 To overcome this difficulty and explore the ferroelectric domain configurations through the capacitor between two electrodes, planar BFO capacitor structures were studied using PFM.24,25 However, simultaneous measurements of resistive switching and ferroelectric domain changes in a planar BFO capacitor have not been performed yet. In this work, by fabricating planar capacitor structures using the BFO thin films, we directly monitor the time evolution of in-plane (IP) ferroelectric/ferroelastic domains between two electrodes, which is synchronized with resistive switching, using PFM. BFO thin films with only 71° DWs are selectively studied because of their simple and periodic behavior of stripe-like IP domain configuration. The BFO planar capacitors with Pt electrodes perpendicular to the DWs exhibit both repetitive domain reversal and resistive switching. In contrast, the BFO planar capacitors with Pt electrodes parallel to the DWs exhibit strong domain pinning and concurrently lose their bi-stable resistance states directly after the first voltage sweep. It is first demonstrated that the domain-electrode geometry significantly affects both the ferroelectric domain dynamics and non-volatile resistive memory switching process.

RESULTS AND DISCUSSION Using the pulsed laser deposition method, 180-nm (001)-oriented BFO thin films were grown epitaxially on (110)-oriented TbScO3 (TSO) substrates. We chose a 180-nm thickness to avoid the problem of leakage and obtain high crystalline quality. Details concerning the sample growth and structure are presented in the Experimental section and Supporting Information (SI) part of this paper. The as-grown thin films displayed a single contrast in the out-of-plane (OP) phase images obtained using PFM, which indicated that the samples were initially self-polarized towards one direction. However, the IP PFM phase images 4


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revealed striped structures of the domains. To identify the type of DWs, we first obtained an IP PFM phase image in a certain area; then, we rotated the sample by 90° and obtained a 90°-rotated IP PFM phase image of the same area (see SI, Figure S1). The 90°-rotated IP PFM phase image showed only one contrast; therefore, we concluded that our BFO thin film on the TSO substrate consisted of mainly 71° DWs. We fabricated BFO planar devices in which rectangular Pt electrodes were running parallel to the DWs (PLDWs) and perpendicular to the DWs (PPDWs), as shown in Figure 1. The as-fabricated BFO planar devices exhibited resistive switching behaviors in the current–voltage (I–V) graphs, as observed in Figure S5. For both the PPDW and PLDW geometries, the resistance state changes between a low-resistance state (LRS) and highresistance state (HRS) during the first DC voltage sweep in both the positive and negative bias branches: LRS -> HRS -> LRS -> HRS switching occurs during 0 V -> 50 V -> 0 V -> -50 V -> 0 V voltage sweep. In both cases, the depolarization field may yield HRS current minima at non-zero DC voltages where the net electric field approaches zero. Although conductive DWs of BFO films were reported in a previous report,9 the conductivity of DWs in our PPDW- and PLDW-geometry devices negligibly affected their initial I-V curves which showed similar current levels. Directly after these hysteretic I–V curves were obtained, we obtained IP PFM phase images and observed that the domain configurations of both devices were completely changed (see SI, Figure S6). The IP domain switching behavior of BFO has been reported by several researchers. Shafer et al. reported the 71° domain switching behavior of BFO/DyScO3 (DSO) (110)c films between IP SrRuO3 (SRO) bottom electrodes.24 Balke et al. reported the IP switching behaviors of BFO/SrTiO3 (STO) (110)c thin films, where the switching only occurred between two domain variants.25 In the leaky BFO thin films, the IP 5



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71° and 109° domain switching behavior of BFO/STO (001) was investigated using local photoconductive measurements by Lee et al.26 Johann et al. reported the stability of 71° stripe domains during repeated electrical pulse cycling for the IP configuration of BFO/DSO (110)c and OP configuration of BFO/SRO/DSO (110)c.27 Zou et al. reported the origin of fatigue behavior and the role of Schottky barriers in BFO/STO (001) thin films with IP 71° and 109° domains.28,29 Based on these results, we can expect that the different changes in domain configurations of our devices with PPDW and PLDW geometries after the first I-V sweep may induce dissimilar transport behaviors. Several planar devices were tested on the BFO thin films. All the BFO planar devices with PPDW and PLDW geometries initially exhibit hysteretic resistive switching behaviors similar to those in Figures 1b and 1c, respectively. The second cycle of the I-V sweep (Figure 1d), obtained from the BFO planar device with PPDW geometry, exhibits nearly the same hysteretic resistive switching behavior as that of its first I–V cycle (Figure 1b). However, the second I–V cycle (Figure 1e) of the PLDW-geometry BFO device is significantly different from the first I–V cycle (Figure 1c): in the negative bias branch of the second I–V cycle, the hysteretic resistive switching behavior completely disappears. Therefore, we need to compare the domain dynamics of BFO planar devices with PPDW and PLDW geometries, which may be related to the resistive switching behaviors. After each one-directional I–V sweeping process in the positive and negative bias branches, we obtained IP PFM images where we could monitor the degree of change in the ferroelectric/ferroelastic domains. Figure 2 shows the evolution of the domain configuration in the IP PFM image obtained after each one-directional unipolar I–V sweeping process in the PPDW-geometry BFO device. We collected I–V data from 0 V to +75 V, which are represented by the black scatter line in Figure 1b, and then obtained an IP PFM phase image, as shown in the left panel of Figure 2a. Shortly after this process, we rotated 6


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the sample by 90° and obtained a 90°-rotated IP PFM phase image of the same area, as presented in the right panel of Figure 2a. The dark and bright contrasts in the PFM phase images represent the opposite components of the IP polarization in BFO along the perpendicular axis to the cantilever. In our PPDW-geometry BFO device (Figure 2), the domain switching follows the typical polarization switching process of ferroelectrics, which occurs through nucleation and subsequent growth of the switched domain.30 The proportion of bright domains starts to increase after the first positive bias sweep from 0 V to +75 V in Figure 2a. Next, the switched bright domains exhibit lateral growth under the second positive bias sweep from +75 V to 0 V (Figure 2b). The first positive bias sweeping process consists of an increase in voltage from 0 V to + 75 V with a ramping rate of 6 V/s and abrupt voltage decrease to 0 V. The second positive bias sweeping process consists of an abrupt voltage increase to +75 V and decrease from + 75 V to 0 V with a ramping rate of 6 V/s. Therefore, the lateral growth of the domain observed after the second positive bias sweeping process results in one more positive bias sweep between 0 V and + 75 V after the first positive bias sweep. Conversely, directly after the first negative bias sweep from 0 V to -75 V, the proportion of these bright domains decreases (Figure 2c). This result directly implies domain switching and growth induced by the applied external voltage. Furthermore, the bright domains only remain in a very small area close to the interface between the BFO and left Pt electrode after the second negative bias sweep from -75 V to 0 V (Figure 2d). These residual domains, which might act as pinning centers, could hinder the full lateral domain growth and complete domain switching process.22,31 These pinned domains near the electrode remain even after applying 1000 consecutive pulses with a width of 100 ms and amplitude of ±75 V (see SI, Figure S8).

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We repeated the same switching and probing processes shown in Figures 2e–2h. The bright domains grow again after the positive bias sweep from 0 V to +75 V (Figure 2e) and reach the cathode after successive positive bias sweeping from +75 V to 0 V (Figure 2f). Under the negative bias sweeping, the proportion of dark domains appears to increase while that of the bright domains decreases (Figures 2g and 2h). Therefore, reversible nucleation and growth of the domains are observed during the second I–V cycle similar to that during the first cycle. This repetitive domain switching process can explain the similar resistive switching behaviors obtained in the first and second I–V cycles for the PPDWgeometry BFO device observed in Figures 1b and 1d. The 90°-rotated image of Figure 2a exhibits nearly one contrast and is not significantly different from the initially obtained image (see SI, Figure S1). However, as the domain switching process progresses, stripe domains parallel to the electrodes appear throughout all the voltage sweeping processes in Figures 2b–2h except in the small pinned areas near the left Pt electrode in the 90°-rotated image. Note that throughout this entire voltage sweeping process, there were no changes in the OP phase image, implying that 180° ferroelectric polarization switching was not induced by the electric field formed in this BFO planar device. Reversible polarization switching of the PPDW-geometry device was maintained even after applying 1000 consecutive pulses (see SI, Figure S8). In contrast to the PPDW-geometry BFO planar device (Figure 2), there are considerable pinned domains in the PLDW-geometry BFO planar device after I–V sweeping, as observed in Figure 3. The pinned domains with global DWs and saw-tooth edge patterns are mainly formed along the interface between the BFO and lower Pt electrode (anode) in the PLDWgeometry BFO device even after the second positive bias sweep from +75 V to 0 V (Figure 3b). Interestingly, once the pinned domains form, they do not reversibly switch back by applying the opposite DC bias, as shown in Figures 3c–3h. 8


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Zou et al. recently reported similar domain pinning behaviors in a BFO planar device with PLDW geometry.28,29 They explained that such domain pinning from charge injection yielded macroscopic fatigue in the thin films, and fatigue resistance was improved by using low work function metals or oxide electrodes.29 Conversely, Baek et al. reported that oxygen vacancies contributed to domain pinning and led to the fatigue problem in BFO.32 The domain pinning, possibly arising from the redistribution of oxygen vacancies and charge injection, in our BFO planar device with PLDW geometry also causes fatigue in BFO and thus the abrupt disappearance of hysteretic behavior in the negative bias branch of the second I– V cycle in Figure 1e. In both the PPDW- and PLDW-geometry BFO devices with mainly 71° DWs on TSO substrates, the I–V characteristics show asymmetric diode-like behaviors. These behaviors are quite different from the symmetric I–V curves of the BFO planar device with mixed DWs on a (001)-oriented STO substrate (see SI, Figure S9). Contrary to the previous report on BFO/STO25,28 there were no domain pinning regions in the IP PFM images of our BFO on the STO substrate (see SI, Figure S11) even after voltage sweeping. However, pinned domain areas are created near the electrode after the first voltage sweep in BFO on the TSO substrate with both PPDW and PLDW geometries. To understand the origin of the different domain switching behaviors synchronized with the transport characteristics in PPDW- and PLDW-geometry BFO planar devices, we fitted the I–V data in Figure 1 using well-known conduction mechanisms. These include the bulklimited Poole–Frenkel (PF), bulk-controlled space-charge-limited-current (SCLC), and interface-controlled Schottky emission mechanisms (Details are in SI, Section I.). The I–V data of PPDW- and PLDW-geometry devices seem to follow similar trends of change. The I– V data in the positive voltage regimes of Figure 1 were fitted using power laws for Ln (I) vs. Ln (|V|), which correspond to the dominant SCLC mechanisms in Figure 4. Our PPDW- and 9



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PLDW-geometry planar devices exhibit asymmetric I–V data even though they have symmetric electrode configurations, indicating that the diode-like behavior is regulated by the bulk characteristic of BFO as well as that of the interface. There are several regions with different slopes in the Ln (I) vs. Ln (|V|) graphs in Figure 4. The SCLC mechanism can be divided into three separate power-law relationships: Ohmic (I∝Vn, n=1), trap-filled limited (I∝Vn, n>2), and trap-free limited (I∝Vn, n=2). Conduction in the positive LRS is governed by the Ohmic relation at low voltage and by the trap-filled limited SCLC at high voltage, while that in the positive HRS follows the trap-filled limited SCLC. However, in the negative voltage regimes, the LRS I–V data are well fitted with the Schottky emission model in Figure 5 rather than with the SCLC model. In the negative LRS during the first switching of the PPDW-geometry device (PPDWs 1 of Figure 5a), there are two regimes with barrier heights of 0.83 and 0.58 eV. Similar barrier height reduction can be observed in the second switching of the PPDW-geometry device (PPDWs 2). In addition, in the negative LRS of PLDWs 1, there are two regimes. The Schottky barrier is approximately 0.83 eV in the low-voltage region (0 V to -4 V) and decreases to 0.58 eV in the high-voltage region. In the negative LRS during the second switching of the PLDWgeometry device (PLDWs 2), the Schottky barrier changes from 0.81 to 0.59 eV at -4 V (Figure 5b). Therefore, overall, the conduction follows the bulk-controlled mechanism in the positive voltage regimes and an interface-controlled mechanism in the negative voltage regimes.33,34 However, there are voltage regimes whose data are not fitted with either the SCLC or Schottky mechanism (negative HRS of both geometries). It is expected that in these regimes, the carrier transport might be affected by defect migration rather than the Schottky barrier or trapped charges. Because oxygen vacancies can be considered as possible defects in our 10


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BFO films, we estimated the mobility of oxygen vacancies by combining the Arrhenius law and the Nernst–Einstein relation.35,36 From the temperature dependent I-V data of our BFO films sandwiched between the top Pt and bottom SrRuO3 electrodes, which showed an increase in the current with temperature,37 we could determine the activation energy of the oxygen vacancies as approximately 0.26 eV. We could then calculate the mobility of oxygen vacancies to be approximately 10-12 cm2/V⋅s. The estimated mobility value of the oxygen vacancies is comparable to those (10-12 – 10-14 cm2/V⋅s) reported for the oxide materials.35,38,39 Therefore, the variation of the carrier transport behaviors in the first and second voltage sweeps of both geometries can be explained by both bulk and interface effects, in which the migration of defects, such as mobile positively charged oxygen vacancies, also plays a significant role. During the first positive voltage sweep in both the PPDW- and PLDWgeometry devices, free carriers induced by shallow impurities and defects can be present in BFO and lead to the Ohmic-like behavior in the low-voltage regime. As the voltage increases, the injected carrier density exceeds the free carrier density, and the SCLC associated with deep trap centers becomes dominant. In addition, in the voltage regimes where only a part of deep traps is filled, there is a fast increase in current with voltage, resulting in a powerlaw dependence with n > 2. This region is the so-called trap-filled limited region, and ionic defects such as oxygen vacancies can create the relevant deep-trap energy levels in the band gap. However, it appears that the applied voltage is not sufficient to induce trap-free SCLC in our case. In the positive HRS of both geometries, the trap-filled SCLC mechanism still operates. Conversely, in the PPDW geometry during the first negative voltage sweep (0 V to -75 V), the Schottky barrier is approximately 0.83 eV in the low-voltage region (0 V to -4 V) and decreases to 0.58 eV in the high-voltage region. The positive oxygen vacancies start to move 11



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toward the left BFO/Pt interface, where the domains are switched to dark contrasts in Figure 2c, leading to a decrease in the Schottky barrier height. During the second negative voltage sweep in Figure 2d (-75 V to 0 V), these oxygen vacancies further accumulate at the left BFO/Pt interface, and the domain only exhibits lateral growth rather than nucleation at the interface. Therefore, the negative HRS obtained during the second negative voltage sweep does not fit with any well-known transport mechanism, as mentioned above. Similar transport behavior is observed for the PLDW geometry. The Schottky barrier is also reduced from 0.83 to 0.58 eV during the first negative voltage sweep in Figure 5. In addition, the I–V curve does not fit with any well-known mechanism in the first negative HRS and appears to be strongly affected by oxygen vacancy migration. Because all types of DWs in BFO are conducting, defects such as oxygen vacancies moving through DWs play important roles for the system.9–11 In the PLDW geometry, positively charged oxygen vacancies can accumulate along the global DWs with saw-tooth edge patterns, as observed in Figure 3b, and can provide an electrostatic driving force to hinder the coherent domain switching, resulting in local domain pinning.26

CONCLUSION Using BFO planar devices, we were able to simultaneously observe both the ferroelectric domain dynamics and the resistive memory switching. It was demonstrated that the geometrical relationship between the ferroelectric domain and electrode in BFO planar capacitors with only 71° DWs affected polarization switching and domain pinning near the interface between multiferroic BFO and metal electrodes. In addition, the resistive switching behaviors were further affected. Therefore, explicitly understanding the effect of domain dynamics on electrical properties may increase the electronic and magnetic application of multiferroic materials. 12


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EXPERIMENTAL SECTION Sample Fabrication: The BFO thin films were epitaxially grown on TSO (110) and STO (001) substrates at 650 °C in an oxygen atmosphere of 0.15 mbar by ablating a BFO ceramic target with 10% Bi excess. A KrF excimer laser (λ = 248 nm) was used with an energy fluence of approximately 1 Jcm-2. Square Pt electrodes (30 × 30 μm2) were fabricated using electron beam lithography, sputtering, and lift-off processes. Structural Characterization: The epitaxial growth of the BFO thin films was verified using X-ray diffraction (XRD) and transmission electron microscopy (see SI, Figures S3 and S4). Electrical Characterization: The domain configuration was investigated using PFM (XE100 atomic force microscope, Park Systems) with a conductive Pt-coated atomic force microscopy (AFM) tip in contact mode. Macroscopic I–V data were obtained using an HP 4156B semiconductor parameter analyzer.

ASSOCIATED CONTENT Supporting Information Structural and electrical properties of the thin films, pulse experiment results, resistance retention result, and electronic conduction mechanism of the device. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes 13



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The authors declare no competing financial interest.

Author Contributions J.H.L. and B.H.P. planned the projects and designed the experiments; J.H.L performed film growth, XRD measurements, PFM and transport measurements, and data analysis; C.Y. and S.L. assisted in metal evaporation; J.H.J. and C.S.Y. assisted in transport measurement and data analysis; Y.H.K. performed transmission electron microscopy measurements; J.H.L. and B.H.P. interpreted the results; All authors participated in discussions and writing the manuscript.

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korea government (MSIP) (No. 2013R1A3A2042120, 2015001938, and 2011-0030229) and Nano⋅Material Technology Development Program through the NRF funded by the MSIP (No. 2016M3A7B4909668).

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(2) Gruverman, A.; Wu, D.; Wang, Y.; Jang, H. W.; Flokman, C. M.; Zhuravlev, M. Ye.; Felker, D.; Rzchowski, M.; Eom, C.-B.; Tsymbal, E. Y. Tunneling Electroresistance Effect in Ferroelectric Tunnel Junctions at the Nanoscale. Nano Lett. 2009, 9, 35393543. (3) Pantel, D.; Goetze, S.; Hesse, D.; Alexe, M. Room-temperature Ferroelectric Resistive Switching in Ultrathin Pb(Zr0.2Ti0.8)O3 Films. ACS Nano, 2011, 5, 60326038. (4) Rana. A.; Lu, H.; Bogle, K.; Zhang, Q.; Vasudevan, R.; Thakara, V. Gruverman, A.; Ogale, S.; Valanoor, N. Scaling Behavior of Resistive Switching in Epitaxial Bismuth Ferrite Heterostructures. Adv. Funct. Mater. 2014, 24, 3962-3969. (5) Jiang, A. Q.; Wang, C.; Jin, K. J.; Liu, X. B.; Scott, J. F.; Hwang, C. S.; Tang, T. A.; Lu, H. B.; Yang, G. Z.; A Resistive Memory in Semiconducting BiFeO3 Thin-film Capacitors. Adv. Mater. 2011, 23, 1277-1281. (6) Pantel, D.; Goetz, S.; Hesse, D.; Alexe, M. Reversible Electrical Switching of Spin Polarization in Multiferroic Tunnel Junctions. Nat. Mater, 2012, 11, 289-293. (7) Hong, S. H.; Choi, T.; Jeon, J. H.; Kim, Y.; Lee. H.; Joo, H.-Y.; Hwang, I.; Kim, J.-S.; Kang, S.-O.; Kalinin, S. V.; Park, B. H. Large Resistive Switching in Ferroelectric BiFeO3 Nano-island Based Switchable Diodes. Adv. Mater. 2013, 25, 2339-2343. (8) Zavaliche, F.; Yang, S. Y.; Zhao, T.; Chu, Y. H.; Cruz, M. P.; Eom, C. B.; Ramesh, R. Multiferroic BiFeO3 Films: Domain Structure and Polarization Dynamics. Phase Transitions, 2006, 79, 991-1017.

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Figure 1. Changes in resistive switching behaviors of BFO planar devices with PPDW and PLDW geometries. (a) A schematic diagram of planar devices with PLDW and PPDW geometries. (b, c) First and (d, e) second cycles of I–V sweep obtained from BFO planar devices with (b, d) PPDW and (c, e) PLDW geometries.

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Figure 2. Domain dynamics in a BFO planar device with PPDW geometry. IP PFM phase (left panel) and 90°-rotated IP PFM phase (right panel) images of a PPDW-geometry BFO device, which were obtained after DC bias sweeping of (a) 0 V -> +75 V, (b) +75 V -> 0 V, (c) 0 V -> 75 V, (d) -75 V -> 0 V, (e) 0 V -> +75 V, (f) +75 V -> 0 V, (g) 0 V -> -75 V, and (h) -75 V -> 0 V. The dark and bright contrasts represent the opposite components of the IP polarization along the perpendicular axis to the cantilever.

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Figure 3. Domain dynamics in a BFO planar device with PLDW geometry. IP PFM phase (left panel) and 90°-rotated IP PFM phase (right panel) images of a PLDW-geometry BFO device, which were obtained after DC bias sweeping of (a) 0 V -> +75 V, (b) +75 V -> 0 V, (c) 0 V -> 75 V, (d) -75 V -> 0 V, (e) 0 V -> +75 V, (f) +75 V -> 0 V, (g) 0 V -> -75 V, and (h) -75 V -> 0 V.

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Figure 4. Ln(I) vs. Ln(|V|) relationships obtained from first and second I–V hysteresis curves of (a) PPDW- and (b) PLDW-geometry devices in Figure 1.

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Figure 5. Schottky conduction model fitting results in negative LRS of both (a) PPDW- and (b) PLDW-geometry devices.

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Table Of Contents (TOC)

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Direct Observation of Domain Motion Synchronized with Resistive

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