electronics Article
Solution-Processable ZnO Thin Film Memristive Device for Resistive Random Access Memory Application Swapnil R. Patil 1,† , Mahesh Y. Chougale 1,† , Tushar D. Rane 1,† , Sagar S. Khot 1 , Akshay A. Patil 1 , Ojus S. Bagal 1 , Sagar D. Jadhav 1 , Arif D. Sheikh 1 , Sungjun Kim 2, * and Tukaram D. Dongale 1, * 1
2
* †
Computational Electronics and Nanoscience Research Laboratory, School of Nanoscience and Biotechnology, Shivaji University, Kolhapur 416004, India;
[email protected] (S.R.P.);
[email protected] (M.Y.C.);
[email protected] (T.D.R.);
[email protected] (S.S.K.);
[email protected] (A.A.P.);
[email protected] (O.S.B.);
[email protected] (S.D.J.);
[email protected] (A.D.S.) School of Electronics Engineering, Chungbuk National University, Cheongju 28644, Korea Correspondence:
[email protected] (S.K.);
[email protected] (T.D.D.); Tel.: +043-261-3327 (S.K.); +0231-260-9490 (T.D.D.) Authors contributed equally to this manuscript.
Received: 1 November 2018; Accepted: 13 December 2018; Published: 17 December 2018
Abstract: The memristive device is a fourth fundamental circuit element with inherent memory, nonlinearity, and passivity properties. Herein, we report on a cost-effective and rapidly produced ZnO thin film memristive device using the doctor blade method. The active layer of the developed device (ZnO) was composed of compact microrods. Furthermore, ZnO microrods were well spread horizontally and covered the entire surface of the fluorine-doped tin oxide substrate. X-ray diffraction (XRD) results confirmed that the synthesized ZnO was oriented along the c-axis and possessed a hexagonal crystal structure. The device showed bipolar resistive switching characteristics and required a very low resistive switching voltage (±0.8 V) for its operation. Two distinct and well-resolved resistance states with a remarkable 103 memory window were achieved at 0.2-V read voltage. The developed device switched successfully in consecutive 102 switching cycles and was stable over 102 seconds without any observable degradation in the resistive switching states. In addition to this, the charge–magnetic flux curve was observed to be a single-valued function at a higher magnitude of the flux and became double valued at a lower magnitude of the flux. The conduction mechanism of the ZnO thin film memristive device followed the space charge limited current, and resistive switching was due to the filamentary resistive switching effect. Keywords: memristive device; ZnO; resistive switching; doctor blade method
1. Introduction Memristor/memristive devices are popular in academia as well as in industry due to their simple structure, zero power requirement for sustaining resistive states, and high speed of operation [1]. These devices can be used as a basic building block for neuromorphic computing [2,3], nonvolatile memory [4,5], and signal processing applications [6,7]. They were theoretically predicted by Leon Chua as a fourth basic circuit element in 1971 [8] and were experimentally realized by a team of Hewlett Packard researchers in 2008 [9]. Pinched hysteresis loops in the current–voltage (I–V) plane and single-valued charge–magnetic flux (q–ϕ) relations are some of the defining characteristics of the memristor device and can be experimentally realized by properly engineering the active material.
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A literature survey suggested that different kinds of materials could be used for the development of memristive devices. The transition metal oxides [10], perovskite oxides [11], chalcogenides [12], and organic compounds [13] are some of the materials that show resistive switching behavior. Among them, metal-oxides including ZnO [14,15], NiO [16], TiO2 [17,18], and WO3 [19] are promising materials because of their low power consumption, multistate resistive switching characteristics, and simple chemical composition. Out of these materials, ZnO is a versatile material for technological applications. It is a wide band gap semiconducting material and has attracted much attention due to its excellent optical, electrical, and piezoelectric properties [20]. At room temperature, the electron hall mobility of ZnO single crystals is in the order of 200 cm2 /Vs and they also show large exciton binding energy (around 60 meV) [21]. In recent years, bipolar resistive-switching-based memristive devices have been developed using different physical and chemical techniques. Recently, Gul et al. developed a sputter-deposited ZnO memristive device and demonstrated its bipolar resistive switching characteristics in a Al/ZnO/Al-based memristive device [22]. A bipolar resistive-switching-based memristive device with a low operating voltage (±0.88 V) was developed by Dongale et al. They studied the effect of temperature on the developed device using a thermal reaction model [15]. Choi et al. fabricated a reliable and cost-effective ZnO memristive device using an electrohydrodynamic printing technique. The device required 1.6 V/−2 V operating voltage and provided an on/off ratio in the order of ~10:1 [23]. Many reports suggest that a ZnO-based memristive device can be useful for memory and neuromorphic computing applications [24,25]. However, very few reports are available that consider a low-cost fabrication methodology for ZnO memristive devices [23,26–28]. In addition to this, the ZnO memristive devices reported in the existing literature require a higher resistive switching voltage (VSET and VRESET > 1 V), and very few reports demonstrate the charge–magnetic flux characteristics. In the present work, we investigated the simplest way to fabricate a bipolar resistive-switchingbased Ag/ZnO/Fluorine-doped tin oxide (FTO) thin film memristive device using the doctor blade method. Morphological, structural, and electrical characterizations of the ZnO thin film memristive device were carried out using scanning electron microscopy (SEM), X-ray diffraction (XRD), and a memristor characterization platform, respectively. The developed device showed the fingerprint pinched hysteresis loop in the I–V plane with a low resistive switching voltage (±0.80 V). An excellent 103 memory window with stable nonvolatile memory (endurance and retention) properties was achieved at a 0.2-V read voltage. The time domain flux, time domain charge, charge–magnetic flux, and charge–voltage characteristics of the ZnO thin film memristive device were also determined. The excellent electrical results and fabrication-friendly procedure of the present work could help to develop cost-effective and rapidly produced devices for nonvolatile memory applications. 2. Experimental Details 2.1. Materials and Method All the reagents used for synthesis were of analytical grade and were used without further purification. Zinc acetate (SD-fine, Mumbai, India) and ammonia (SD-fine, Mumbai, India) were used for the synthesis of ZnO powder, whereas ethyl cellulose (SD-fine, Mumbai, India), lauric acid (Himedia, Mumbai, India), terpineol (Loba chemie, Mumbai, India), and ethanol (SD-fine, Mumbai, India) were used for the thin film development (doctor blade method). The FTO coated on a glass substrate (10 Ω/sq.) was used as a bottom electrode. The FTO substrates were cleaned with laboline and distilled water and were finally rinsed with acetone. The doctor blade technique was used for the development of the ZnO active layer on the FTO substrate. 2.2. Synthesis of ZnO Powder and Development of ZnO Thin Film Using the Doctor Blade Method Figure 1 depicts the schematic representation of the ZnO powder synthesis procedure. In the typical process, 0.1 M zinc acetate (C4 H6 O4 Zn·2H2 O) solution was prepared in 50 mL
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of double-distilled water (DDW). This solution was kept on a magnetic stirrer until it became Electronics x PEER REVIEW of Electronics 2018, 2018, 7, 7,and x FOR FOR PEERafter REVIEW of 12 12 homogeneous clear, which ammonia (NH3 ) was added dropwise with continuous33 stirring. After the addition of a few milliliters of NH3 , the solution initially became precipitated and then addition aa few milliliters of 33,, the solution became and colorless. The addition of of few milliliters of NH NHwas thetransferred solution initially initially became precipitated precipitated and then then The colorless. The resultant system to a stainless-steel autoclave and colorless. hydrothermally resultant was transferred to a stainless-steel autoclave and hydrothermally treated at 80 °C resultant system system was transferred to a stainless-steel autoclave and hydrothermally treated at 80 °C treated at 80 ◦ C for 1 h. After completion of the reaction, the resultant system was allowed to cool for for 11 h. h. After After completion completion of of the the reaction, reaction, the the resultant resultant system system was was allowed allowed to to cool cool down down to to room room down to room temperature. The synthesized powder was washed two times with ethanol and distilled temperature. temperature. The The synthesized synthesized powder powder was was washed washed two two times times with with ethanol ethanol and and distilled distilled water water and and water and thenatair-dried at room temperature for the 2 h.as-synthesized Finally, the as-synthesized ZnO powder was then air-dried room temperature for 2 h. Finally, ZnO powder was annealed then air-dried at◦ room temperature for 2 h. Finally, the as-synthesized ZnO powder was annealed at at annealed at 350 Figure C for 1 h. Figure 2 shows the schematic representation of the deposition doctor blade deposition 350 350 °C °C for for 11 h. h. Figure 22 shows shows the the schematic schematic representation representation of of the the doctor doctor blade blade deposition technique technique technique for the development of the ZnO thinthis film. In this technique, ZnO powder (1 g) was blended for for the the development development of of the the ZnO ZnO thin thin film. film. In In this technique, technique, ZnO ZnO powder powder (1 (1 g) g) was was blended blended with with aa with a mixture of ethyl cellulose (0.3 g) and lauric acid (0.1 g) in a mortar under vigorous grinding mixture with mixture of of ethyl ethyl cellulose cellulose (0.3 (0.3 g) g) and and lauric lauric acid acid (0.1 (0.1 g) g) in in aa mortar mortar under under vigorous vigorous grinding grinding with aawith a pestle, pestle,to towhich whichfive fiveto sixdrops dropsof terpineolwas wasadded. added. During grinding, a few drops of alcohol During grinding, aa few drops of pestle, to which five totosix six drops ofofterpineol terpineol was added. During grinding, few drops of alcohol alcohol were added in order to reduce viscosity and mix the precursors properly. This mixture was blended were added in order to reduce viscosity and mix the precursors properly. This mixture was blended were added in order to reduce viscosity and mix the precursors properly. This mixture was blended for 1 h to obtain a uniform and lump-free paste. The prepared paste was coated on a conducting for for 11 h h to to obtain obtain aa uniform uniform and and lump-free lump-free paste. paste. The The prepared prepared paste paste was was coated coated on on aa conducting conducting side sideside ◦ ◦ precleaned FTO substrate. at 120 °C 10 min, 200 °C 10 ofof Thissubstrate substratewas wassintered sintered 120 C for min, C for 10 min, ofa aaprecleaned precleanedFTO FTO substrate. substrate. This This substrate was sintered at at 120 °C for for 10 10 min, 200200 °C for for 10 min, min, and then calcined at 450 and C for 30 min minto toremove removethe thebinder. binder. andthen thencalcined calcinedat at 450 450 ◦°C °C for 30 30 min to remove the binder.
Figure 1. synthesis procedure. Figure 1. 1. Schematic Schematic representation ofofZnO ZnO powder synthesis procedure. Figure Schematicrepresentation representationof ZnOpowder powder synthesis procedure.
Figure Schematic representation of the doctor blade deposition technique for the development Figure of the Figure2.2. 2.Schematic Schematicrepresentation representation of ofthe thedoctor doctor blade blade deposition deposition technique technique for for the the development development of of the ZnO thin film. ZnO thin film. the ZnO thin film.
2.3. 2.3. Development Development and and Characterizations Characterizations of of ZnO ZnO Memristive Memristive Device Device
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2.3. Development and Characterizations of ZnO Memristive Device In the present investigation, Ag acted as a top electrode, ZnO as an active layer, and FTO as a bottom electrode. The active memristive layer (ZnO) was coated on the FTO substrate using the doctor blade method. The Ag was patterned on the ZnO layer so as to work as a top electrode for the memristive device. The Ag layer (~500 nm) was patterned using a thermal evaporation system (Vacuum Techniques, Model – VT-ACG-03, Bengaluru, India). In this experiment, Ag evaporation slugs (Sigma-Aldrich, Mumbai, India) were kept in the evaporation boat and a 10−5 -Torr vacuum environment was created. This resulted in a good-quality top Ag contact for electrical measurements. The synthesized ZnO thin films were characterized by morphological, structural, and electrical characterization techniques. The surface morphology of the ZnO thin film was investigated using SEM (JEOL-JSM 6360 A, Japan). The phase and crystal structures of the ZnO thin film were examined using XRD with CuKα λ = 1.5406 Å (Bruker Model D2 phaser, United States). The electrical measurements of the Ag/ZnO/FTO thin film were recorded using an electrochemical workstation (Autolab N-Series) and memristor characterization platform (ArC ONE). During all electrical measurements, we biased the top Ag electrode with respect to bottom FTO electrode. The endurance and retention measurements were obtained with the help of a pulsed measurement protocol. The time domain flux, charge, charge–magnetic flux, and charge–voltage characteristics were calculated using experimental I–V data by employing Equations (2)–(6), which are shown in the next section. 3. Results and Discussions The scanning electron micrograph of the ZnO thin film is shown in Figure 3a. The surface micrographs suggested that the ZnO thin film was composed of compact microrods. Furthermore, ZnO microrods were well spread horizontally and covered the entire surface FTO substrate. The cross-sectional SEM image of the ZnO thin film is shown in the inset of Figure 3a. The thickness of the thin film was found to be 34 µm. The cross-sectional image suggested that the uniform deposition of ZnO was obtained by the doctor blade method. ZnO is an II–VI binary compound semiconductor that has a cubic zinc-blende or hexagonal wurtzite crystal structure where each anion is wrapped by four cations at the corners of the tetrahedron. This tetrahedral coordination has substantial ionic behavior with sp3 covalent bonding. The ionicity of ZnO resides at the borderline between a covalent and ionic semiconductor. Figure 3b shows the XRD pattern of the ZnO powder sample. XRD results suggested that the prepared ZnO sample was nanocrystalline in nature and matched well with the hexagonal (wurtzite) crystal structure (JCPDS No.–36-1451). The large broadening of the ZnO peaks was due to the very small crystallite size. The major peaks (i.e., (100), (002), and (101)) confirm the hexagonal (wurtzite) crystal structure. Some other Bragg’s peaks, such as (102), (110), (103), (200), (112), (201), (004), and (202), were also observed with relatively lower intensities. The average crystallite size (D) was calculated from the XRD pattern by using Scherer’s relation, as given in Equation (1): D=
0.9λ βcosθ
(1)
where D is the crystallite size, λ is the wavelength of X-ray (1.5406 Å), β is the fullwidth at half-maxima, and θ is the angle of diffraction. The average crystallite size of the prepared ZnO sample was found to be 62 nm, which confirmed the nanocrystalline nature of ZnO. The lattice parameters of the prepared ZnO sample were a = b = 3.2548 Å and c = 5.2052 Å. In a nutshell, the XRD results confirmed that the synthesized ZnO sample was oriented along the c-axis and possessed a hexagonal crystal structure.
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Figure Figure 3. 3. Morphological Morphological and and structural structural characterization characterization of of the the ZnO ZnO thin thin film. film. (a) (a) Surface Surface morphology morphology of of the the ZnO ZnO thin thin film. film. The The inset inset represents represents the the cross-sectional cross-sectional SEM SEM image image of of the the ZnO ZnO thin thin film; film; (b) (b) XRD XRD patterns patterns of of ZnO ZnO powder powder sample. sample.
The The ideal ideal memristor memristor device device is is aa passive passive circuit circuit element element with with inherent inherent memory memory and and nonlinearity nonlinearity properties. The extended class of memristor device, popularly known as the memristive device, is more properties. The extended class of memristor device, popularly known as the memristive device, is practical and suitable for a wide of applications. The memristive device is generally more practical and suitable forrange a wide range of applications. The memristive device isrecognized generally by a pinchedbyhysteresis loop in the I–V plane andI–V oneplane such I–V the Ag/ZnO/FTO recognized a pinched hysteresis loop in the andcharacteristic one such I–Vofcharacteristic of the thin film device is film shown in Figure 4a.inThe inset in Figure the zero crossing Ag/ZnO/FTO thin device is shown Figure 4a.shown The inset shown4a in represents Figure 4a represents the zero property the memristive device. Thedevice. fingerprint hysteresis was clearly for crossing of property of the memristive The pinched fingerprint pinchedloop hysteresis loopobserved was clearly the Ag/ZnO/FTO thin film device, which suggested that the developed device acted as a memristive observed for the Ag/ZnO/FTO thin film device, which suggested that the developed device acted as device. In order to obtain thetopinched hysteresis loop in theloop I–Vinplane, theplane, voltage 0 a memristive device. In order obtain the pinched hysteresis the I–V the swept voltagefrom swept to +0.8 V, +0.8 to 0 V, 0 to − 0.8 V, and − 0.8 to 0 V. Initially, the device was in the high-resistance from 0 to +0.8 V, +0.8 to 0 V, 0 to −0.8 V, and −0.8 to 0 V. Initially, the device was in the high-resistance state (HRS) at at00V.V.The Thecurrent current(I)(I) device increased as sweep the sweep voltage increased 0 state (HRS) of of thethe device increased as the voltage increased from from 0 to 0.8 to V. At the device to change its resistive switching state. ThisON is the ON V. 0.8 At 0.8 V, 0.8 the V, device startedstarted to change its resistive switching state. This is the state, orstate, lowor low-resistance state (LRS), of the device and the corresponding voltage is known as the SET voltage. resistance state (LRS), of the device and the corresponding voltage is known as the SET voltage. This This made uninterrupted progress up−0.8 to −V. 0.8After V. After 0.8the V, the device again started to change statestate made uninterrupted progress up to −0.8−V, device again started to change its its resistive switching state, and thecorresponding correspondingvoltage voltageisisknown knownas asthe the RESET RESET voltage. voltage. For For aa clear resistive switching state, and the clear understanding, understanding, the the continuous continuous arrows arrows represent represent the the LRS LRS of of the the device, device, whereas whereas the the dotted dotted arrows arrows represent represent the the HRS HRS of of the the device. device. In In order order to to test test the the repeatability repeatability of of the the measurements, measurements, we we measured measured the asas shown in in Figure 4b.4b. TheThe results suggested thatthat the the I–V I–Vcharacteristics characteristicsfor for100 100consecutive consecutivecycles, cycles, shown Figure results suggested Ag/ZnO/FTO thin film memristive device possessed reliability and repeatability in the measurements. the Ag/ZnO/FTO thin film memristive device possessed reliability and repeatability in the The memory property of the memristive device is described by memristance (M). It is divided measurements. into two states, namely, and HRS. Furthermore, the by transition between two resistance The resistance memory property of the LRS memristive device is described memristance (M). It is divided states dictates the application domain the memristive device. abrupt transition from HRS to into two resistance states, namely, LRS of and HRS. Furthermore, theAn transition between two resistance LRS and vice versa is useful fordomain resistiveof switching memorydevice. applications, whereas a smooth transition states dictates the application the memristive An abrupt transition from HRS to of resistive switching states is useful for neuromorphic computing applications. The endurance and LRS and vice versa is useful for resistive switching memory applications, whereas a smooth transition retention characteristics of the Ag/ZnO/FTO thin film memristive device are shown in Figure 4c,d, of resistive switching states is useful for neuromorphic computing applications. The endurance and respectively. For the nonvolatile memory measurements, the pulsed and nondisruptive memory retention characteristics of the Ag/ZnO/FTO thin film memristive device are shown in Figure 4c,d, measurement protocol was used. In the typical measurement, write pulses with ofmemory ±0.8-V respectively. For the nonvolatile memory measurements, thea series pulsedofand nondisruptive magnitude were applied and theIn resistance of measurement, the device was measured withpulses a 0.2-V read pulse. measurement protocol was used. the typical a series of write with of ±0.8-V Throughout the measurement, a 300-µs pulse duration was maintained for write and read pulses. Two magnitude were applied and the resistance of the device was measured with a 0.2-V read pulse. distinct and well-resolved resistance states withduration a remarkably higher memory window (HRS/LRS) Throughout the measurement, a 300-µs pulse was maintained for write and read pulses. were for the developed device. In the present we achieved a 103 memory memory window Two observed distinct and well-resolved resistance states withwork, a remarkably higher window at a 0.2-V read a higher memory window is required for random access (HRS/LRS) werevoltage. observedSuch for the developed device. In the present work, weresistive achieved a 103 memory memory [29–31]. The endurance be used to probe cycle-to-cycle window applications at a 0.2-V read voltage. Such a higherproperty memorycould window is required forthe resistive random resistive switching behavior of the memory device. In the present case, the developed device switched access memory applications [29–31]. The endurance property could be used to probe the cycle-tosuccessfully in switching consecutive 102 switching cycles. The stability ofpresent LRS and HRSthe states were studied cycle resistive behavior of the memory device. In the case, developed device 2 s without any using a retention test. inIt consecutive was observed the LRS and HRS were stable over 10HRS switched successfully 102that switching cycles. The stability of LRS and states were observable degradation intest. the resistance states. that the LRS and HRS were stable over 102 s without studied using a retention It was observed any observable degradation in the resistance states.
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Figure 4. (a) Representative current–voltage (I–V) characteristics of the Ag/ZnO/FTO thin film Figure 4. (a) Representative current–voltage (I–V) characteristics of the Ag/ZnO/FTO thin film memristive device and (b) repetitive I–V characteristics for 100 consecutive cycles. (c) Endurance and memristive device and (b) repetitive I–V characteristics for 100 consecutive cycles. (c) Endurance and (d) retention characteristics of the developed memristive device. The inset shown in (a) represents (d) retention characteristics of the developed memristive device. The inset shown in (a) represents the the zero crossing property of the memristive device. The direction of resistive switching is denoted zero crossing property of the memristive device. The direction of resistive switching is denoted by by arrows. arrows.
The ideal memristor can be recognized by the charge–magnetic flux (q–ϕ) relation and pinched The ideal memristor can be recognized by the charge–magnetic flux (q–φ) relation and pinched hysteresis loop in the I–V plane. The mathematical formulation of the memristor suggested that the hysteresis loop in the I–V plane. The mathematical formulation of the memristor suggested that the q–ϕ characteristics must be a nonlinear, continuously differentiable, and monotonically increasing q–φ characteristics must be a nonlinear, continuously differentiable, and monotonically increasing single-valued function [8]. In view of this, memristor devices can be defined as the basis of a charge single-valued function [8]. In view of this, memristor devices can be defined as the basis of a charge and magnetic flux relation, such that [32] and magnetic flux relation, such that [32] fMf(φ, q) q=)0= 0 M ( ϕ,
or
or ϕ=φ f (=q)f (q)or or q =qg=( ϕg)(φ)oror
dϕ 𝒅𝝋 = 𝑴. = M. dq 𝒅𝒒
(2) (2)
The voltage (v) across or current (i) through the memristor can be obtained by differentiating The voltage (v) across or current (i) through the memristor can be obtained by differentiating Equation (1) w.r.t. ‘t’, [32]: Equation (1) w.r.t. ‘t’, [32]: vv == M (q) i (3) M(q)i (3) M (q) i iv==W(ϕ)v 𝒅𝝋
(4) (4)
𝒅𝒒
where, v𝒗 = dϕ and 𝒊 = dq. Equation (3) is a current-controlled memristor, whereas Equation (4) is where, dt and i =𝒅𝒕dt . Equation (3) is a current-controlled memristor, whereas Equation (4) 𝒅𝒕 is known a voltage-controlled device. In this q and as state variables, whereas known as as a voltage-controlled device. In this case,case, the qthe and φ actϕasactstate variables, whereas M(q) M(q) and W(ϕ) are known as memristance and memductance, respectively. In the present case, and W(φ) are known as memristance and memductance, respectively. In the present case, we applied we applied external voltage and measured the current the device. In classical terms, wecontrolled controlled the external voltage and measured the current of theofdevice. In classical terms, we memristance of the device by the instantaneous time integral of the current. Therefore, the developed memristance Ag/ZnO/FTO thinfilm filmdevice deviceacted actedas asaacurrent-controlled current-controlled memristive memristive device. In order to investigate Ag/ZnO/FTO thin the basic flux and charge characteristics, characteristics, we used Equations (5) and (6) and and experimental experimental I–V data (time-dependent) [33]: (time-dependent) [33]: Z t
q(t) = 𝒒(𝒕) =
𝒕
i (t)dt
−∞ 𝒊(𝒕)𝒅𝒕
(5) (5)
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𝛗(𝒕) =
(6)
𝒗(𝒕)𝒅𝒕 Z t
ϕ(t) = v(t)dt (6) The time domain flux and charge characteristics −∞of the Ag/ZnO/FTO thin film memristive device are shown in Figure voltage stimulus and current response are shown in thedevice inset The time domain5a,b. flux The and external charge characteristics of the Ag/ZnO/FTO thin film memristive 𝐍 of Figure 5a,b, respectively. The initial (A 1), half-period (BCW), final-period (A2), and turning (𝐁𝐂𝐖 ) are shown in Figure 5a,b. The external voltage stimulus and current response are shown in the pointsofrepresent the respectively. resistive switching states(Aof),the memristive device. The device was in the HRS inset Figure 5a,b, The initial 1 half-period (BCW ), final-period (A2 ), and turning Nthe initial and final-period points and went into the LRS at half-period points. The obvious at (BCW ) points represent the resistive switching states of the memristive device. The device was in the symmetric domain flux characteristics were observed forLRS theat developed device. This was due to HRS at the time initial and final-period points and went into the half-period points. The obvious fact that the voltage stimulus was symmetric in nature and its integration ( 𝝋(𝒕) ) had to be toa symmetric time domain flux characteristics were observed for the developed device. This was due symmetric function. However, the time domain charge characteristics were found to be asymmetric fact that the voltage stimulus was symmetric in nature and its integration (ϕ(t)) had to be a symmetric in nature (final charge value). This kindcharge of asymmetric behavior suggested pinched hysteresis function. However, the time domain characteristics were found tothat be the asymmetric in nature loop of the developed device was asymmetric in nature. In addition to this, the shape of theoftime (final charge value). This kind of asymmetric behavior suggested that the pinched hysteresis loop the domain charge characteristics was asymmetric in nature. This kind of asymmetric behavior (final developed device was asymmetric in nature. In addition to this, the shape of the time domain charge charge value and led toindouble-valued q–φ of characteristics which were the opposite ofand the characteristics wasshape) asymmetric nature. This kind asymmetric behavior (final charge value definition of the ideal memristor device [8]. One such characteristic is shown in Figure 5c. It is worth shape) led to double-valued q–ϕ characteristics which were the opposite of the definition of the ideal 𝐍 mentioningdevice that the (AFigure 2) points dictate the nature of the q–φ 𝐂𝐖 ) and final-period memristor [8]. turning One such( 𝐁 characteristic is shown in 5c. It is worth mentioning that the N characteristics. In the present case, the single-valued q–φ curve was observed at the LRS turning (BCW ) and final-period (A2 ) points dictate the nature of the q–ϕ characteristics. Inand the became present double valued at the HRS. The double-valued q–φ curve at the HRS was due to the incomplete case, the single-valued q–ϕ curve was observed at the LRS and became double valued at the HRS. breaking of the conductive andwas some of parasitic capacitance inductance present in The double-valued q–ϕ curvefilament at the HRS duesort to the incomplete breaking oforthe conductive filament the device [34]. In addition to this, the asymmetric nature of the device can be represented with the and some sort of parasitic capacitance or inductance present in the device [34]. In addition to this, helpasymmetric of charge–voltage in Figure of the HRS to LRS and the nature ofcharacteristics, the device can as beshown represented with5d. the The helptransition of charge–voltage characteristics, vice versainand the 5d. asymmetric natureofare fromversa the charge–voltage characteristics. as shown Figure The transition theclearly HRS toobserved LRS and vice and the asymmetric nature are This kind of novel representation is useful for the identification of the ideal memristor device from clearly observed from the charge–voltage characteristics. This kind of novel representation is useful nonideal memristor devices. for the identification of the ideal memristor device from nonideal memristor devices. BCW
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Figure Figure 5. 5. (a) (a) Time Timedomain domainflux fluxand and(b) (b)time timedomain domaincharge chargecharacteristics characteristicsofofthe theAg/ZnO/FTO Ag/ZnO/FTO thin thin film Insets shown shown in in (a) (a) and and (b) (b) represent film memristive memristive device. device. Insets represent the the applied applied voltage voltage signal signal and and the the corresponding output current signal, respectively. (c) Nonlinear and double-valued charge magnetic corresponding output current signal, respectively. (c) Nonlinear and double-valued charge magnetic flux thin film film memristive memristive device. The CW flux and and (d) (d) charge–voltage charge–voltagecharacteristics characteristicsof ofthe theAg/ZnO/FTO Ag/ZnO/FTO thin device. The CW represents feature of the signal. The transition of the device the from high-resistance representsthe theclockwise clockwise feature of input the input signal. The transition of the from device the highstate (HRS)state to the(HRS) low-resistance state (LRS) and vice versaand are vice represented by represented dotted arrows. resistance to the low-resistance state (LRS) versa are by dotted
arrows.
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The conduction mechanism of the Ag/ZnO/FTO memristive device was obtained by plotting the conduction mechanism of thescale. Ag/ZnO/FTO memristive device wasdouble obtained by plotting I–V the I–V The characteristics on a log–log Figure 6a,b represents the logarithmic I–V characteristics on ZnO a log–log scale. Figure 6a,b thenegative double logarithmic I–V characteristics characteristics of the memristive device in represents positive and bias, respectively. The slopes of memristive device positive negative bias, respectively. The of the low-voltage of the the ZnO low-voltage range (0 to in ±0.1 V) andand high-voltage range (±0.1 to ±0.8 V)slopes were calculated and are range (0 to 0.1 Figure V) and6a,b. high-voltage range (±0.1 range, to ±0.8the V) slope were calculated depicted the depicted in±the For the low-voltage was ~1 in and bothare bias regions.inThis Figure 6a,b. For the low-voltage range, the slope was ~1 in both bias regions. This suggested that the suggested that the current of the device was directly proportional to the applied voltage, which current of the device was directly proportional to the applied voltage, which thatrange. the Ohmic confirmed that the Ohmic conduction mechanism was dominated in theconfirmed low-voltage The conduction mechanism was dominated inin thethe low-voltage range. The current of thethe device increased current of the device increased suddenly high-voltage range and, therefore, magnitude of suddenly theincreased high-voltage range and, therefore, the magnitude of thethe slopes also increased in both the slopesinalso in both bias regions. In order to investigate conduction mechanism of bias regions. In order to the investigate the conduction mechanism of theashigh-voltage range,6c,d. the Child’s the high-voltage range, Child's law characteristics were plotted, shown in Figure Child's 2 equal law were as shown in Figure 6c,d. lawRwas well to fitted to the experimental law characteristics was well fitted to plotted, the experimental data, with theChild’s adjusted 0.9915 and 0.9850 for 2 data, with thenegative adjustedbias R equal 0.9915 and 0.9850 for positive and negative This indicated positive and data. to This indicated that Child's law dominated inbias the data. high-voltage range. that law dominated in the high-voltage range. It is a(SCLC) well-known fact that the spaceappears charge It is aChild’s well-known fact that the space charge limited current conduction mechanism limited conduction mechanism appears with theand Ohmic conduction at with thecurrent Ohmic (SCLC) conduction mechanism at the low-voltage range Child's law at themechanism high-voltage the low-voltage range Child’s law at the high-voltage In a nutshell, the SCLC conduction range. In a nutshell, theand SCLC conduction mechanism was range. responsible for the charge transport of the mechanism was responsible for the charge transport of the Ag/ZnO/FTO memristive device. Ag/ZnO/FTO memristive device. (a)
(b)
-3 -3
10
Slope = 1.19
-4
10
Current (|A|)
Current (A)
10
Slope = 1.01
-5
10
1E-3
0.01
Slope = 1.27
-4
10
Slope = 0.98
-5
10
0.1
1
1E-3
0.01
Voltage (V)
0.1
1
Voltage (|V|)
-3
1.8x10
(c)
Expt. Data Linear Fit
-3
1.5x10
-3
2.1x10 1.8x10
Adj. R2 = 0.9915
-3
-3
1.2x10
Current (|A|)
Current (A)
(d)
Expt. Data Linear Fit
-3
-4
9.0x10
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1.5x10
Adj. R2 = 0.9850
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0.6
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Voltage (|V |)
Figure (a) Log–log Log–log I–V I–V characteristics characteristicsofofthe theAg/ZnO/FTO Ag/ZnO/FTO thin film memristive device during Figure 6. 6. (a) thin film memristive device during (a) 2 of2the (a) positive negative bias. Child’s plot: current voltage of the high slope of the positive andand (b) (b) negative bias. Child's lawlaw plot: current vs. vs. voltage high slope partpart of the (c) (c) positive and (d) negative bias data, respectively. positive and (d) negative bias data, respectively.
The The electrical electrical results results showed showed the the abrupt abrupt increase increase in in the the current current at at the the SET SET and and RESET RESET voltage voltage point. distinct and well-resolved resistance states were observed nonvolatile point. Furthermore, Furthermore,two two distinct and well-resolved resistance states were during observed during memory measurements. In general, this kind of result was observed only when the filamentary nonvolatile memory measurements. In general, this kind of result was observed only when the type of resistive dominated in the memristive device. Considering the electrical filamentary type switching of resistiveeffect switching effect dominated in the memristive device. Considering the characteristics of the Ag/ZnO/FTO thin film memristive device, the possible filamentary type resistive electrical characteristics of the Ag/ZnO/FTO thin film memristive device, the possible filamentary switching mechanism is mechanism shown in Figure 7. Ininthe present case, acted case, as a top is type resistive switching is shown Figure 7. In theAg present Ag electrode. acted as a Ittop aelectrode. well-known fact that a Ag electrode work as an electrochemically active component in filament It is a well-known fact that a Ag electrode work as an electrochemically active component formation the rupture [35].process When a[35]. positive voltage was applied theapplied top Agto electrode in filamentand formation and process the rupture When a positive voltage to was the top + cations were generated with respect to the bottom FTO electrode, oxidation of Ag occurred and Ag Ag electrode with respect to the bottom FTO electrode, oxidation of Ag occurred and Ag+ cations were generated (Ag ⟶ Ag+ + e−). These cations traveled through the active ZnO layer and reduced at
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(Ag → Ag+ + e− ). These cations traveled through the active ZnO layer and reduced at the bottom + + e− ⟶ Ag). The precipitations of Ag metal atoms at the bottom FTO + + e− → the bottom FTO electrode (Ag FTO electrode (Ag Ag). The precipitations of Ag metal atoms at the bottom FTO electrode electrode resulted in the growth of Ag in filament in the ZnO layer. This metal filament finallyatended at resulted in the growth of Ag filament the ZnO layer. This metal filament finally ended the top the top electrode, as in shown in7a. Figure The fully conductive grown conductive to the switch the electrode, as shown Figure The 7a. fully grown filamentfilament helped helped to switch device device to the ON or LRSInstate. In the nextancase, an electrochemical dissolution of Agplace tookdue place to the ON or LRS state. the next case, electrochemical dissolution of Ag took to due the to the change in the polarity of thevoltage. appliedThis voltage. This in the polaritythe ruptured the change in the polarity of the applied change in change the polarity ruptured conductive conductive anddevice droveinto the the device the OFF or In HRS state. In the a nutshell, theand formation filament andfilament drove the OFFinto or HRS state. a nutshell, formation ruptureand of rupture of the conductive filament gave rise to the bipolar resistive switching effect in the the conductive filament gave rise to the bipolar resistive switching effect in the Ag/ZnO/FTO thin Ag/ZnO/FTO thin film memristive device. film memristive device.
Figure 7.7.Possible Possible filamentary resistive switching mechanism of the Ag/ZnO/FTO thin film Figure filamentary resistive switching mechanism of the Ag/ZnO/FTO thin film memristive memristive device during ON state or LRS device during (a) ON state (a) or LRS and (b) OFF and state(b) or OFF HRS.state or HRS.
The solutionThe performance performance comparison comparison of of ZnO ZnO memristive memristive device device with with existing existing ZnO-related ZnO-related solutionprocessable devices is summarized in Table 1. These our solution-processable processablememory memory devices is summarized in Table 1. results These indicate results that indicate that our solutionZnO memristive is a gooddevice candidate nonvolatile resistive applications. processable ZnOdevice memristive is aforgood candidate for memory nonvolatile resistive memory applications. Table 1. Performance comparison of ZnO memristive devices. Device Structure
Device Structure
Table 1. Performance comparison of ZnO memristive devices. Resistive Memory Endurance Retention Reference Switching Voltage Window (Cycles) Resistive Memory Endurance (Seconds) Retention
ITO/GaZnO/ITO +5/−7.5 V Switching Voltage Au/ZnO/Au ±4 V ITO/GaZnO/ITO +5/−7.5 V Pt/a-IGZO/Pt +1.7/−1 V Au/ZnO/Au Ag/ZnMn2 O4 /p+−Si ±4 V+8/−10 V Pt/a-IGZO/Pt +1.7/−1+3/ V −1.5 V Ag/ZnO/ITO Ag/ZnO/FTO Ag/ZnMn2O4/p+−Si +8/−10 V±0.8 V
15 Window 102 15 2 10 102 102 102 10 102 103 10
300 (Cycles) 300 100120100 100 120
Reference
- (Seconds) [36] [37] [36] [38] 104 [37] 5 [39] 10 [38] [40] 4 × 103 104 2 5 Present Work 10 10 [39]
Ag/ZnO/ITO +3/−1.5 V 4 × 103 [40] Present 4. Conclusions Ag/ZnO/FTO ±0.8 V 103 100 102 Work In conclusion, we have developed a filamentary resistive-switching-based ZnO thin film memristive device using the doctor blade method. The surface micrographs suggested that the 4. Conclusions ZnO thin film was composed of compact microrods. The microrods spread horizontally and covered In conclusion, developedXRD a filamentary resistive-switching-based ZnO film the entire surface of we the have FTO substrate. results confirmed that the synthesized ZnOthin sample memristive the doctor blade method. The surface micrographs suggested that pinched the ZnO was orienteddevice alongusing the c-axis and possessed a hexagonal crystal structure. The fingerprint thin film was composed of compact microrods. The microrods spread horizontally and covered the hysteresis loop was clearly observed for the Ag/ZnO/FTO thin film device, which suggested that entire surface of the FTO substrate. XRD results confirmed that the synthesized ZnO sample was the developed device acted as a memristive device. A remarkably low resistive switching voltage 3 memory along the c-axis and possessed a hexagonal structure. pinched (oriented ±0.8 V) with a 10 window was achieved for thecrystal developed device.The The fingerprint nonvolatile memory hysteresis loop was clearly observed for the Ag/ZnO/FTO thin film device, which suggested that the properties, such as endurance and retention, suggested that the device switched successfully in developed device acted as a memristive device. A remarkably low resistive switching voltage (±0.8 V) with a 103 memory window was achieved for the developed device. The nonvolatile memory properties, such as endurance and retention, suggested that the device switched successfully in
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consecutive 102 switching cycles and was stable over 102 seconds without any observable degradation in the resistive switching states. The ideal memristor can be recognized by the single-valued q–ϕ curve. In the present case, the q–ϕ curve was observed to be a single-valued function at a higher magnitude of the flux (or at LRS) and became double valued at a lower magnitude of the flux (or at HRS). The double-valued q–ϕ curve at the HRS was due to the incomplete breaking of the conductive filament and some sort of parasitic capacitance or inductance present in the device. These results suggest that the developed device is an extended class of memristor device or, more specifically, it is a memristive device. The conduction mechanism investigations suggest that the SCLC conduction mechanism dominated and the bipolar resistive switching effect was due to the formation and rupture of the metallic conductive filament. Author Contributions: Authors T.D.D, S.K. and A.D.S. conceptualized the idea. S.R.P., M.Y.P. and T.D.R. performed the synthesis. S.S.K., A.A.P., O.S.B. and S.D.J. did the characterizations. S.R.P., M.Y.P. and T.D.R. performed the electrical measurements. T.D.D., S.K. and A.D.S. analyzed the data. S.R.P., M.Y.P. and T.D.R. wrote the first draft and T.D.D., S.K. and A.D.S. finalized the manuscript. All authors reviewed the manuscript. All authors read and approved the final manuscript. Funding: The author T. D. D. thanks Shivaji University, Kolhapur for the financial assistance under the “Research Initiation Scheme”. A. D. S. thanks the Department of Science and Technology (DST), Ministry of Science and Technology, Government of India, New Delhi for the INSPIRE faculty research grant (Award No. DST/INSPIRE/04/2015/002601). This work was supported by a grant from the National Research Foundation of Korea (NRF), funded by the Korea government (MSIP) (2018R1C1B5046454). Conflicts of Interest: The authors declare no conflict of interest.
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