DSpace at VNU: Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive Random-Access Memory

Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-016-4361-4 Ó 2016 The Minerals, Metals & Materials Society Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive Random-Access Memory THI KIEU HANH TA,1 KIM NGOC PHAM,1 THI BANG TAM DAO,1 DAI LAM TRAN,2 and BACH THANG PHAN1,3,4 1.—Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam 2.—Vietnam Academy of Science and Technology, Ha Noi, Viet Nam 3.—Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam 4.—e-mail: pbthang@hcmus.edu.vn The electrical conduction and associated resistance switching mechanism of top electrode/WO3/bottom electrode devices [top electrode (TE): Ag, Ti; bottom electrode (BE): Pt, fluorine-doped tin oxide] have been investigated The direction of switching and switching ability depended on both the top and bottom electrode material Multiple electrical conduction mechanisms control the leakage current of such switching devices, including trap-controlled spacecharge, ballistic, Ohmic, and Fowler–Nordheim tunneling effects The transition between electrical conduction states is also linked to the switching (SET–RESET) process This is the first report of ballistic conduction in research into resistive random-access memory The associated resistive switching mechanisms are also discussed Key words: Resistive switching, WO3 thin films, trap-controlled spacecharge limited conduction, Fowler–Nordheim tunneling, ballistic conduction INTRODUCTION In terms of nonvolatile memory, it is generally believed that transistor-based flash memory will approach the end of scaling within about a decade As a result, non-field-effect transistor (FET)-based devices and architectures will likely be needed to satisfy growing demand for high-performance memory and logic electronics applications.1 Recent research has demonstrated that nonvolatile resistance-switching resistive random-access memory (ReRAM) is a promising alternative to floating-gate technology beyond the 32-nm technology node In addition, transparent electronics is one of the most important emerging technologies for next-generation electronics systems Oxide-based ReRAM structures exploit the functionality of capacitor structures in which oxide materials, for example, perovskites (Cr-doped SrTiO3, Cr-doped SrZrO3, Pr0.7Ca0.3MnO3, etc.),1–8 chalcogenide materials (Received October 10, 2015; accepted January 16, 2016) (GeSbTe),9 transition-metal oxides (TMOs), and ordinary oxides (NiO, TiO2, CuOx, HfO2, ZrOx, ZnO, Cr2O3, WO3),10–23 are sandwiched between two metal electrodes Choosing a material that is compatible with complementary metal–oxide–semiconductor (CMOS) processes is currently a crucial challenge in ReRAM research Among the various materials used, TMOs have the major advantages of simple fabrication and compatibility with CMOS processes These TMOs are mostly transparent, with wide bandgap energies, making ReRAM a good candidate for enabling transparent memory.19 The switching phenomena in these material systems remain controversial There are many publications addressing different switching behaviors, even in the same system, making understanding even more difficult.20,21 Understanding the electrical conduction behavior of such materials is an important step in the design of applications using these materials Previously, we reported the correlation between the crystallinity and resistive switching behavior of sputtered WO3 thin films.16 In this study, we investigated the electrical Ta, Pham, Dao, Tran, and Phan Fig I–V curves of (a) Ag/WO3/Pt, (b) Ag/WO3/FTO, (c) Ti/WO3/Pt, and (d) Ti/WO3/FTO devices conduction of WO3 thin films using different electrode materials purposely chosen to improve understanding of the switching mechanism 0.02 V The bottom electrode was biased, while the top electrode was grounded EXPERIMENTAL PROCEDURES RESULTS AND DISCUSSION Ag, Ti, and tungsten oxide films were fabricated on fluorine-doped tin oxide (FTO) and Pt/Ti/SiO2/Si substrates using direct-current (DC) sputtering at room temperature using metallic Ag, Ti, and W targets Deposition of 300-nm-thick WO3 thin films was carried out under total pressure PAr+O2 of 10À3 Torr, 300°C, and mixture ratio of oxygen to argon gas (PO2/PAr+O2) fixed at 90% During the deposition of the 100-nm-thick top electrode (Ag or Ti) in argon environment at 10À3 Torr, a mask was used for top electrode patterning The crystalline phases of the thin films were characterized in h–2h mode using a D8 Advance (Bruker) x-ray diffractometer (XRD) with Cu Ka radiation (k = 0.154 nm) and by Fourier-transform infrared (FTIR) spectroscopy The surface morphologies of the films were obtained using field-emission scanning electron microscopy (FESEM) X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical state of the films These characterizations were reported previously.16 Current–voltage (I–V) measurements were carried out using a semiconductor characterization system (Keithley 4200 SCS) and probe station I–V curves were obtained in voltage sweep mode using a fi ±Vmax fi fi ±Vmax fi voltage profile, sweep speed in normal mode, and step voltage of Figure shows the I–V characteristics of top electrode/WO3/bottom electrode devices where the top electrode (TE) was Ag or Ti and the bottom electrode (BE) was Pt or FTO I–V hysteresis was observed for three devices, viz Ag/WO3/Pt, Ag/WO3/FTO, and Ti/WO3/FTO (Fig 1a, b and d), whereas no I–V hysteresis was observed for the other device, i.e., Ti/WO3/Pt (Fig 1c) In these switching devices, the direction of switching depended on both the top and bottom electrode material It was observed that, for WO3 devices with FTO bottom electrode, replacing the Ag top electrode with a Ti electrode caused a change of the switching direction, while for WO3 devices with Pt bottom electrode, replacing the Ag top electrode with a Ti electrode caused suppression of switching For both the Ag/WO3/Pt and Ag/WO3/FTO devices (Fig 1a and b), the initial high-resistance state (HRS) was changed to a low-resistance state (LRS) as negative bias (0 fi ÀVmax) was applied to the Pt (FTO) bottom electrode The device remained in the LRS as the negative bias was decreased and progressively changed to the HRS only on voltage sweeping in the positive voltage range (0 fi +Vmax) For the Ti/WO3/FTO device (Fig 1d), the initial high-resistance state (HRS) was changed to a low- Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive RandomAccess Memory Fig I–V curve of HRS of Ag/WO3/Pt device under fi À1.5 V process plotted in terms of: (a) SCLC, (b) BC, (c) SC, (d) PF, and (e) FN conduction mechanisms resistance state (LRS) by application of positive bias (0 fi +Vmax) to the FTO bottom electrode The device remained in the LRS as the positive bias was decreased and progressively changed to the HRS only on voltage sweeping in the negative voltage range (0 fi ÀVmax) To understand the switching mechanism in these devices, the dominant leakage processes involved in the Ag/WO3/Pt, Ag/WO3/FTO, and Ti/WO3/FTO devices were determined from the measured I–V data of the HRS and LRS in both polarity biases All the I–V curves were examined in terms of various potential leakage mechanisms: space-charge-limited conduction (SCLC), ballistic conduction (BC), interface-limited Schottky emission conduction (SC), interface-limited Fowler–Nordheim (FN) tunneling, and bulk-limited Poole–Frenkel (PF) emission.24–27 JSCLC ¼ JOhm ỵ JTFL ỵ JChild JOhm $ V JTrapfilled $ V m ðm > 2Þ ð1Þ JChild $ V JSC rffiffiffiffiffiffi 2q 3=2 E ; JBC $ d  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 Àq ub À qE=4per e0 A; $ T exp@ kB T ð3Þ JPF pffiffiffiffiffiffiffiffiffi 3=2 À4 2mà ub A ; $ E2 exp@ 3qhE ð2Þ ð4Þ Ta, Pham, Dao, Tran, and Phan Fig I–V curve of LRS of Ag/WO3/Pt device under À1.5 V fi process plotted in terms of: (a) SCLC, (b) BC, (c) SC, (d) PF, and (e) FN conduction mechanisms  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 Àq ut À qE=per e0 A; $ E exp@ kB T JFN ð5Þ where er is the relative dielectric constant, e0 is the permittivity of free space, d is the film thickness, and A is a constant; ub and ut are the height of the Schottky barrier and trap ionization energy, respectively Electrical Conduction Mechanisms in Switching Devices For the Ag/WO3/Pt device, the I–V curve obtained under the fi À1.5 V sweeping process (Fig 1a) showed that the leakage current of the HRS followed a nonlinear I–V dependence where one or more conduction processes may be involved The leakage current depended linearly on voltage (I–V), then increased steeply with voltage (I–V12), followed by an I–V1.5 dependence (Fig 2a) This behavior suggests trap-controlled SCLC as the dominant leakage process However, for SCLC (Eq 1), Ohmic conduction (OC) at low voltages is followed by a trap-filled limit region with I–Vm dependence (m > 2) and trap-free SCLC (Child’s law, I–V2) The measured I–V data therefore not completely fit with SCLC at high electric fields We carried out further analysis and found that the I–V1.5 dependence can be classified as ballistic conduction (Fig 2b, Eq 2) Therefore, SCLC is largely responsible for the leakage behavior of the Ag/WO3/Pt device at low electric fields whereas BC dominates at high electric fields It is also noticed that ballistic conduction is rarely reported in published ReRAM papers However, in this study, we found ballistic conduction To further determine whether Schottky barriers, PF emission, or interface-limited FN tunneling were largely involved, similar analyses were conducted Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive RandomAccess Memory Fig I–V curve of LRS of Ag/WO3/Pt device under fi +2 V process plotted in terms of: (a) SCLC, (b) BC, (c) SC, (d) PF, and (e, f) FN conduction mechanisms according to Eqs 3–5 For a linear fit with Eqs and 4, an appropriate er value is necessary to extract the optical dielectric permittivity of WO3 from the slopes of these plots The refractive index of WO3 is n % 2.1,28 thus an optical dielectric permittivity of er = n2 % 4.41 can be reasonably assumed From the Schottky and PF plots in Fig 2c and d, the er value differs greatly from the value of 4.41 for WO3, indicating that involvement of Schottky barriers or PF emission is highly unlikely Figure 2e shows a plot of ln J/E2 as a function of 1/E without a negative slope at high electric fields (marked by red rectangle), suggesting that interface-limited FN tunneling is not a dominant process Similar analyses were conducted for the sequence of the sweeping process, À1.5 V fi fi +2 V and +2 V fi 0, as shown in Figs 3, 4, and The I–V curve for the LRS obtained for the À1.5 V to sweeping process shows a linear dependence, a behavior corresponding to an Ohmic-conductionbased filament path (Fig 3a) The I–V curve for the LRS obtained for the fi +2 V sweeping process shows nonlinear dependence, with electrical conduction corresponding to an Ohmic-conductionbased filament path and FN tunneling, as shown in Fig 4a, e and f The I–V curve is well fit by FN tunneling conduction with a negative slope (marked in red rectangle in Fig 4e and f) It is well known that FN tunneling conduction is dominant in thin dielectric films at high electric field, where charge carriers are injected from the electrode to the insulator by tunneling through a high potential barrier In contrast to the abrupt change of current with voltage of the HRS under negative bias, the I–V curve for the HRS under positive bias (+2 V fi 0) showed nonlinear dependence, and its electrical conduction follows FN tunneling (Fig 5e) Ta, Pham, Dao, Tran, and Phan Fig I–V curve of HRS of Ag/WO3/Pt device under +2 V fi process plotted in terms of: (a) SCLC, (b) BC, (c) SC, (d) PF, and (e) FN conduction mechanisms In summary, the dominant electrical conduction mechanisms in the Ag/WO3/Pt device under the fi À1.5 V fi fi +2 V fi sweeping process are shown in Fig and also listed in Table I Similar analyses were carried out for the Ag/WO3/ FTO and Ti/WO3/FTO devices, whose electrical conduction mechanisms are shown in Figs and and listed in Table I All the switching devices showed a transition from SCLC to BC along with the HRS to LRS switching Note that SCLC implies trap levels within the bandgap Our previously reported XPS results show that the WO3 film contains defects such as nonlattice oxygen ions.16 Defects in the WO3 film could form trap sites in the bandgap below the conduction band, where injected charge carriers can be trapped After saturation of all such defect levels by injected carriers, additional excess charges appear in the conduction band, resulting in a sudden increase of current With increasingly negative applied voltage, the I–V curve was well fit by BC Ballistic transport refers to carrier transport without scattering Accordingly, the conduction mechanism changes from SCLC to BC and the resistance switches from HRS to LRS This analysis suggests that the mechanism of the SET process (HRS fi LRS) is due to the presence of scatteringfree regions in these switching devices For example, the change from SCLC in the HRS to BC and then Ohmic conduction in the LRS for both the Ag/ WO3/Pt and Ag/WO3/FTO devices can be ascribed to resistive switching controlled by metallic filaments, as described below For the Ti/WO3/FTO device, the electrical conduction of the LRS before the RESET (LRS fi HRS) process follows ballistic conduction instead ÀVmax fi HRS Fowler–Nordheim tunneling conduction HRS Fowler–Nordheim tunneling conduction HRS Fowler–Nordheim tunneling conduction LRS fi HRS Ohmic conduction fi Fowler–Nordheim tunneling conduction LRS LRS fi HRS Ohmic conduction Ohmic conduction fi Fowler–Nordheim tunneling conduction Sweeping process: fi +Vmax fi fi ÀVmax fi +Vmax fi 0 fi ÀVmax LRS LRS fi HRS Ballistic conduction Ballistic conduction fi Fowler–Nordheim tunneling conduction LRS Ohmic conduction fi +Vmax HRS fi LRS Trap-controlled space-charge-limited conduction fi ballistic conduction Ti/WO3/FTO Device Ag/WO3/FTO fi +Vmax 2Vmax fi 0 fi 2Vmax HRS fi LRS Trap-controlled space-charge-limited conduction fi ballistic conduction HRS fi LRS Trap-controlled space-charge-limited conduction fi ballistic conduction On the basis of the nature of the electrode, the direction of switching, and the analyzed electrical conduction mechanisms, it seems that the mechanism of resistive switching in both the Ag/WO3/Pt and Ag/WO3/FTO devices is controlled by electrochemical redox reactions.16,20 The resistive switching mechanism of the Ag/WO3/Pt (FTO) device is modeled in Fig and explained as follows: On application of a negative voltage to the Pt (FTO) Ag/WO3/Pt Resistance Switching Mechanism of Ag/WO3/ Pt and Ag/WO3/FTO Devices Device of Ohmic conduction as obtained for both the Ag/ WO3/Pt and Ag/WO3/FTO devices Therefore, the switching mechanism in the Ti/WO3/FTO device is controlled by another factor, as also discussed below After the RESET process, the electrical conduction in the HRS for all devices can be well described by FN tunneling These above-classified electrical conduction mechanisms help us to understand the resistive switching behavior of these switching devices Sweeping Process: fi 2Vmax fi fi +Vmax fi Fig Dominant electrical conduction mechanisms of Ag/WO3/Pt device under (a) negative bias (SCLC fi BC fi OC) and (b, c) positive bias (OC fi FN) Table I Dominant electrical conduction mechanisms in Ag/WO3/Pt, Ag/WO3/FTO, and Ti/WO3/FTO devices +Vmax fi Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive RandomAccess Memory Ta, Pham, Dao, Tran, and Phan Fig Dominant electrical conduction mechanisms of Ag/WO3/FTO device under (a) negative bias (SCLC fi BC fi OC) and (b, c) positive bias (OC fi FN) bottom electrode (positive voltage to the Ag top electrode), an electrochemical reaction occurs at the anode (Ag), which oxidizes Ag metal atoms to Ag ions These Ag+ ions start from the top interface and drift through the WO3 films to connect with the bottom electrode At the Pt (FTO) cathode, electrochemical reduction and electrocrystallization of Ag occur This electrocrystallization process results in formation of an Ag filament, which grows toward the Ag electrode As a result, Ag filaments grow and connect to the Ag top electrode, leading to HRS to LRS switching To RESET the cell, a positive voltage is applied to the Pt bottom electrode (negative switching voltage to the Ag top electrode), which leads to dissolution of the Ag filament, and LRS to HRS switching occurs In our previous publication,16 XPS analysis showed the presence of oxygen vacancies V2+ O in + ions have WO3 thin films As both V2+ O and Ag positive charge, they will drift in the same direction under the bias processes Such oxygen vacancies can affect the resistive switching of the WO3 thin films A postannealing process was carried out in air at 600°C for h to check the effect of these oxygen Fig Dominant electrical conduction mechanisms of Ti/WO3/FTO device under (a) positive bias (SCLC fi BC) and (b, c) negative bias (BC fi FN) vacancies; the results showed that both the postannealed Ag/WO3/Pt and Ag/WO3/FTO devices retained the same switching behavior as the asgrown devices Therefore, oxygen vacancies may be involved in the electrical conduction but not make a major contribution, if any, to the switching mechanism In addition, the Ag filament was confirmed through the replacement of Ag by Ti as top electrode The Ti/WO3/Pt device exhibited no resistive switching behavior It can again be concluded that Ag filament paths mediated by electrochemical redox reactions are responsible for resistive switching in the Ag/WO3/Pt and Ag/WO3/FTO devices Resistance Switching Mechanism of the Ti/WO3/FTO Device For the Ti/WO3/FTO device, the resistance switching mechanism is modeled in Fig 10 and explained as follows: Resistance switching involves back and forth drift of O2À ions through the bottom interface It is suggested that this occurs because the polycrystalline phase of the FTO electrode acts as a reservoir for defects such as V2+ O sites and, simultaneously, O2À ions The FTO electrode Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive RandomAccess Memory Fig Switching model for clockwise I–V hysteresis devices: Ag filament paths mediated by electrochemical redox reactions Fig 10 Switching model for anticlockwise I–V hysteresis devices: negative O2À ions migrate under polarity biases contains enough vacancy sites near the interface to readily provide resting sites for migrating O2À ions This implies that the FTO electrode can be treated as an O2À ion source The O2À ions move a short distance from the bulk oxide films, through the bottom interface, into the FTO bottom electrode during the positive bias process This movement of O2À ions is associated with formation of oxygen vacancies V2+ O in the bulk oxide films near the bottom interface; consequently, the SET process (HRS to LRS) occurs During the negative bias process, the O2À ions move back from the FTO bottom electrode to the bulk oxide films, eliminating oxygen vacancies V2+ O at the interface, resulting in the RESET process (LRS to HRS) It is also noted that the postannealed Ti/WO3/ FTO device did not show any switching behavior It can be concluded that oxygen ions are mainly involved in the switching mechanism of the asgrown Ti/WO3/FTO device CONCLUSIONS The electrical conduction and associated resistance switching mechanism of TE/WO3/BE devices (TE: Ag, Ti; BE: Pt, FTO) were investigated In the switching devices, the direction of switching depended on both the top and bottom electrode material It was observed that, for WO3 devices with FTO bottom electrode, replacing the Ag top electrode with a Ti electrode caused a change of the switching direction, while for WO3 devices with Pt bottom electrode, replacing the Ag top electrode with a Ti electrode caused suppression of switching Ta, Pham, Dao, Tran, and Phan For devices with the same switching direction, i.e., Ag/WO3/Pt and Ag/WO3/FTO devices, the resistance switching is due to Ag filament paths mediated by electrochemical redox reactions The governing electrical conduction mechanisms are FN tunneling for the HRS under positive bias, trap-controlled SCLC for the HRS and ballistic and Ohmic conduction for the LRS under negative bias, and Ohmic conduction before the RESET process under positive bias For the device exhibiting the opposite switching direction, i.e., Ti/WO3/FTO, the resistance switching involves back and forth drift of oxygen ions through the bottom interface The governing electrical conduction mechanisms are FN tunneling for the HRS under positive bias, trap-controlled SCLC for the HRS, and ballistic conduction for the LRS under both negative and positive biases Both the ballistic and Ohmic conduction mechanisms suggest that the SET process (HRS fi LRS) occurs due to the presence of scattering-free regions in the switching devices ACKNOWLEDGEMENTS This work is financially supported by Vietnam National University in Ho Chi Minh City under Grant HS2015-18-02 REFERENCES S.H Jo, Doctoral Thesis, The University of Michigan, 2010 S.Q Liu, N.J Wii, and A Ignatiev, Appl Phys Lett 76, 2749 (2000) Y Watanabe, J.G Bednorz, A Bietsch, Ch Gerber, D Widmer, A Beck, and S.J Wind, Appl Phys Lett 78, 3738 (2001) R Waser and M Aono, Nat Mater 6, 833 (2007) B.T Phan and J Lee, Appl Phys Lett 93, 222906 (2008) B.T Phan and J Lee, Appl Phys Lett 94, 232102 (2009) B.T Phan, N.C Kim, and J Lee, J Korean Phys Soc 54, 873 (2009) B.T Phan, T Choi, A Romanenko, and J Lee, Solid-State Electron 75, 43 (2012) Y.C Chen, C.F Chen, C.T Chen, J.Y Yu, S Wu, S.L Lung, R Liu, and C.Y Lu, IEDM Tech Dig (2003), pp 905–908 10 B.J Choi, D.S Jeong, S.K Kim, S Choi, J.H Oh, C Rohde, H.J Kim, C.S Hwang, K Szot, R Waser, B Reichenberg, and S Tiedke, J Appl Phys 98, 033715 (2005) 11 K Jung, H Seo, Y Kim, H Im, J.P Hong, J.W Park, and J.K Lee, Appl Phys Lett 90, 052104 (2007) 12 A Chen, S Haddad, Y.C Wu, Z Lan, T.N Fang, and S Kaza, Appl Phys Lett 91, 123517 (2007) 13 C.Y Lin, C.Y Wu, C Hu, and T.Y Tseng, J Electrochem Soc 154, G189 (2007) 14 T Le, H.C.S Tran, V.H Le, T Tran, C.V Tran, T.T Vo, M.C Dang, S.S Kim, J Lee, and B.T Phan, J Korean Phys Soc 60, 1087 (2012) 15 N.K Pham, D.T Nguyen, B.T.T Dao, K.H.T Ta, V.C Tran, V.H Nguyen, S.S Kim, S Maenosono, and B.T Phan, J Electron Mater 43, 2747 (2014) 16 T.B.T Dao, K.N Pham, Y.L Cheng, S.S Kim, and B.T Phan, Curr Appl Phys 14, 1707 (2014) 17 K.N Pham, M.S Choi, C.V Tran, T.D Nguyen, V.H Le, T Choi, J Lee, and B.T Phan, J Electron Mater 44, 3395 (2015) 18 J.B Park, K.P Biju, S.J Jung, W.T Lee, J.M Lee, S.H Kim, S.S Park, J.H Shin, and H.S Hwang, IEEE Electron Device Lett 32, 476 (2011) 19 J.W Seo, Doctoral Thesis, Korean Advanced Institute of Science and Technology, 2011 20 D.S Jeong, R Thomas, R Katiyar, J.F Scott, H Kohlstedt, A Petraru, and C.S Hwang, Rep Prog Phys 75, 076502 (2012) 21 Y.E Syu, T.C Chang, T.M Tsa, G.W Chang, K.C Chang, Y.H Tai, M.J Tsai, Y.L Wang, and S.M Sze, Appl Phys Lett 100, 022904 (2012) 22 B.U Jang, A.I Inamdar, J.M Kim, W Jung, H.S Im, H.S Kim, and J.P Hong, Thin Solid Films 520, 5451 (2012) 23 D.S Hong, Y.S Chen, Y Li, H.W Yang, L.L Wei, B.G Shen, and J.R Sun, Sci Rep 4, 4058 (2014) 24 J Wua and J Wang, J Appl Phys 108, 034102 (2010) 25 W.Y Yang and S.W Rhee, Appl Phys Lett 91, 232907 (2007) 26 N.F Mott and R.W Gurney, Electronic Processes in Ionic Crystals (Oxford: Clarendon, 1940) 27 S.M Sze, Physics of Semiconductor Devices, 2nd ed., Vol (New York: Wiley, 1981), pp 72–75 28 C Charles, N Martin, M Devel, J Ollitrault, and A Billard, Thin Solid Films 534, 275 (2013)  ... Table I Dominant electrical conduction mechanisms in Ag /WO3/ Pt, Ag /WO3/ FTO, and Ti /WO3/ FTO devices +Vmax fi Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive RandomAccess... electrode Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive RandomAccess Memory Fig Switching model for clockwise I–V hysteresis devices: Ag filament paths mediated... conducted Understanding Electrical Conduction States in WO3 Thin Films Applied for Resistive RandomAccess Memory Fig I–V curve of LRS of Ag /WO3/ Pt device under fi +2 V process plotted in terms