Development of Ag/WO3/ITO Thin Film Memristor ...

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Development of Ag/WO3/ITO Thin Film Memristor Using Spray. Pyrolysis Method. T. D. Dongale,. 1. S. V. Mohite,. 2. A. A. Bagade,. 2. P. K. Gaikwad,. 3. P. S. Patil ...
Electron. Mater. Lett., Vol. 0, No. 0 (0000), pp. 1-5 DOI: 10.1007/s13391-015-4180-4

Development of Ag/WO3/ITO Thin Film Memristor Using Spray Pyrolysis Method T. D. Dongale,1 S. V. Mohite,2 A. A. Bagade,2 P. K. Gaikwad,3 P. S. Patil,1,4 R. K. Kamat,3 and K. Y. Rajpure2,* 1

Computational Electronics and Nanoscience Research Laboratory, School of Nanoscience and Biotechnology, Shivaji University, Kolhapur, 416004, India 2 Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, 416004, India 3 Embedded Systems and VLSI Research Laboratory, Department of Electronics, Shivaji University, Kolhapur, 416004, India 4 Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur, 416004, India (received date: 9 June 2014 / accepted date: 4 June 2015 / published date:

)

The unique nonlinear relationship between charge and magnetic flux along with the pinched hysteresis loop in I-V plane provide memory with resistance combinations of attribute to Memristor which lead to their novel applications in non volatile memory, nonlinear dynamics, analog computations and neuromorphic biological systems etc. The present paper reports development of Ag/WO3/ITO thin film memristor device using spray pyrolysis method. The structural, morphological and electrical properties of the thin film memristor device are further characterized using x-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and semiconductor device analyzer. The memristor is simulated using linear dopent drift model to ascertain the theoretical and experimental conformations. For the simulation purpose, the width of doped region (w) limited to the interval [0, D] is considered as a state variable along with the window function characterized by the equation f (x) = w (1 − w). The reported memristor device exhibits the symmetric pinched hysteresis loop in I-V plane within the low operating voltage (±1 V). Keywords: memristor, spray pyrolysis, thin films, fourth element

1. INTRODUCTION

memristor, with its behavior characterized by memory resistance property. The first physical realization was confirmed by HP research team and they attributed movement of oxygen vacancies as the basis of the memory resistance characteristics.[2] In recent years memristor is once again in limelight as it is considered as promising candidate for potential applications Resistive Random Access Memory (RRAM), chaos theory, biomedical and neuromorphic applications.[2-10] Recently Xia et al. reported memristor-CMOS based hybrid integrated circuits for reconfigurable logic.[11] Pershin et al. reported the spin memory effects, making them more apt for RRAM applications. A thorough analysis on timedependent spin transport at a semiconductor/ferromagnetic

Established framework of traditional Electrical Engineering (EE) advocates resistor, inductor and capacitor as the merely three fundamental passive circuit elements. The resistor is interrelated with voltage (v) and current (i), inductor with flux (φ) and current (i) and capacitor with charge (q) and voltage (v) respectively. Amidst the above mentioned fundamental passive elements, a possibly omitted relation was postulated by L. Chua based on charge and magnetic flux in 1971.[1] This first time lead to the conception of *Corresponding author: [email protected] ©KIM and Springer

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junction has strengthened the memristive behavior.[12] Researchers (including our group) are also striving hard to synthesize memristors using different techniques. Recently we have reported the hydrothermally grown TiO2 thin film memristor. The developed memristor device reveals symmetric bipolar resistive switching behavior within the low operating voltage (±0.7 V) range.[13] Yoon et al. reported similar memristor behaviors in TiO2 film sandwiched between top Pt and bottom SrRuO3 electrodes.[14] Amongst others the molecular dynamic study reported by Savel’ev et al. for oxide memristors.[15] Kim et al. reported effective dynamic resistive switching model of oxide based memristor device.[16] Advances in nanotechnology and improved synthesis and characterization of memristors are seen to be going hand-inhand. In this context He et al. reported the memristive characteristics in WO3 nanowires. They also investigated the effect of sweep range, temperature, and Ohmic contacts on WO3 memristor device.[17] Zhang et al. reported the memristive characteristics of ZnO/NiO stacked heterostructure by ultrasonic spray pyrolysis technique.[18] Ying-Tao et al. reported the resistive switching behaviours in WO3 based RRAM devices which is similar to memristor.[19,20] The nonvolatile multilevel memory effect was demonstrated by Li et al. in Cu/WO3/Pt device structures. The reported device shows various resistance states and these resistance states can be reproducible over 100 dc switching cycles. The reported stability of such device may withstand over 104 seconds.[21] Researchers are adopting different synthesis routes for realization of memristor. Among various deposition methods, spray pyrolysis seems to be the most convenient method owing to its simple and inexpensive experimental arrangement, high growth rates and wide possibility of varying the film properties by changing preparative parameters mainly the substrate temperature. It has been successfully used to synthesize many transparent conducting oxide films.[22,23] The present paper reports the development of Ag/WO3/ITO thin film memristor device using spray pyrolysis method. The structural, morphological and optical properties of Ag/ WO3/ITO thin film memristor device are characterized using x-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and semiconductor device analyzer respectively. The paper is structured as follows. Section 1 covers the introduction, international scenario and rationale behind the work. This is followed by synthesis and characterization details in section 2, thereafter discussion of results and revelation of confirmatory correlation between the simulation and experimental investigations.

2. SYNTHESIS AND CHARACTERIZATION The tungsten oxide (WO3) thin films were deposited on the ITO substrates using spray pyrolysis technique. The

Fig. 1. Schematic structure of the fabricated Ag/WO3/ITO memristor device (MD).

precursor solution for the spray was prepared by dissolving 12.5 mM tungstic acid in a solution containing ammonia and double distilled water in the ratio of 3:7. The total 100 mL precursor solution was sprayed onto ITO substrates at different substrate temperatures 300°C, 325°C, 350°C and 375°C. Other preparative parameters uniformly preserved during the development process were: spray rate (4 mL/ min.), solution concentration (12.5 mM), total quantity of spraying solution (100 mL) and nozzle to substrate distance (32.5 cm). Films were annealed at 500°C for 5 hr. The typical structure of developed memristor devices (MD) is shown in Fig. 1. The MD is formed with three layers viz. bottom, active and top. The bottom layer comprising of ITO is used to ensure good Ohmic contacts. The top Ag layer was deposited by using vacuum deposition technique using Ag foil as a target. The middle layer which is the active layer of the device comprises of WO3 prepared by spray pyrolysis technique. The identification of phase and crystal structures were carried out by Bruker x-ray diffractometer Model D2: phaser with CuKα (λ = 1.5406 Å) radiation in the span of 20 - 600. Surface morphology of WO3 films were studied by using JEOL JSM-6360 scanning electron microscopy (SEM), Japan. Current-voltage (I-V) characteristic of memristor was recorded by applying the bias voltage from −1 to 1 V across the device. The memristor device performance was characterized using a semiconductor device analyzer (B1500A, Agilent, USA).

3. RESULTS AND DISCUSSION 3.1. X-ray diffraction studies The phase formation of WO3 films were analyzed by x-ray diffractogram in the 2θ range of 20 - 600 as shown in Fig. 2. The polycrystalline nature of WO3 films were observed in x-ray pattern with monoclinic crystal structure (JCPDS card No. 01-072-1465). The most intense peaks (002), (020) and (200) planes (2θ = 23.14°, 23.61° and 24.37°) were observed corresponding to 325°C substrate temperature film. The average grain size of polycrystalline WO3 films was calculated using the Scherrer relation for the most intense peak.[24]

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films exhibit porous interconnected mesh-like microstructures. The size of the interconnected mesh component increases with substrate temperature due to sintering at higher temperatures. Figure 3(b) portrays interconnected porous microstructure composed with quasi nanoflakes, due to increase of metal nucleation centres which confirms the enhancement in crystallinity. The irregularity in morphology increases after 325°C substrate temperature.

Fig. 2. X-ray diffraction patterns of deposited WO3 thin film prepared at various substrate temperature using spray pyrolysis technique.

0.9λ D = -------------βcosθ

(1)

where D is crystallite size, λ is the of x-ray wavelength of Cu Kα line, β is the Full width at half maximum of the reflection peaks, θ is the Bragg angle. The average crystallite size of WO3 thin film was observed as about 51 nm. 3.2. Morphological studies The surface morphology of WO3 thin films with different substrate temperature are shown in Fig. 3. The WO3 thin

3.3. Simulation of Memristor In the present investigation we have used linear dopent drift model of memristor proposed by HP team.[2] The width of doped region (w) was considered as a state variable which is limited to the interval [0, D] and window function was considered to be f (x) = w(1−w).[2] For the simulation Mefficiency factor (ROFF/RON) is considered as 50. The other parameter used for simulation are: thickness of memristor (D) = 550 nm, width (w) = 250 nm, amplitude of input signal = 1 V at 5 Hz frequency. Figure 4(a) depicts simulation plots of flux Vs. charge. The magnetic flux is taken as a singlevalued function of charge. The monotonically increasing functions of magnetic flux (φ) and charge (q) relation is presented in equation (2). This equation implies that memristor does not store charge itself and therefore exhibits a characteristics of passive element.[1,2] lim M( q) = RON and lim M( q ) = ROFF

q → –∞

q→∞

(2)

where, 0 < RON < ROFF. Figure 4(b) represents typical pinched hysteresis loop of memristor. The characteristic of memristor

Fig. 3. SEM images of WO3 thin films prepared at different substrate temperatures using spray pyrolysis technique.

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Fig. 4. Simulation of memristor device (a) Charge Vs Flux, (b) Current Vs Voltage, (c) Relationship between Current and Voltage Vs Time, (d) Simulative Plot of Time (s) Vs Memristance (Ω).

has governed by equation (3) and (4).[2] It is observed that current in the memistor becomes nonlinear with the applied voltage, resulting in hysteresis loops rather than straight lines. The equation (3) and (4) confirms that with decrease in the thickness of memristor, its memristance property becomes more prominent.[2] RON w( t )⎞ w(t) V( t ) = ⎛ ------------------+ R ⎛ 1 – ----------⎞ i( t) ⎝ D ⎠ OFF⎝ D ⎠

(3)

where, state variable ‘w’ can be represented as, μv RON dw( t -) -i(t) -----------= η -------------D dt

(4)

Figure 4(c) reveals simulation plot of current and voltage Vs time. Current appears to be lagging with respect to voltage only by the factor of amplitude and not by phase, which confirms the memristor formation. Figure 4(d) represents simulation plot of time (s) Vs memristance (Ω). It is observed that memristance having periodic response with respect to time. 3.4. Memory device performance The resistive switching property of the Ag/WO3/ITO thin film memristor device prepared by spray pyrolysis technique is shown in Fig. 5. The 325°C substrate temperature WO3 thin film is analyzed for memristor property. I-V plot portrays the voltage dependent bipolar resistive switching characteristics of developed device which is typical memristor property.[2] The voltage is applied between the top (Ag) and

Fig. 5. I-V curve of typical Ag/WO3/ITO memristor device (MD). The pinched hysteresis loop is observed in the developed device.

bottom (ITO) electrodes with the latter being grounded for IV measurement. The external voltage sweeps in the cycle of 0 V → +1 V → 0 V → −1 V → 0 V.[13] As we decrease the positive voltage steadily, the current drops smoothly and at negative −1 V the device exhibits RESET i.e. high resistance state (switching from Low Resistance State to High Resistance State). During positive voltage bias, the current jumps suddenly at a voltage of about +1 V showing the SET or low resistance state.[2,25-28] The pinched hysteresis loop (I-V curve) is linear and symmetric which is due to fact that contact between active

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layer (WO3) and top and bottom metal layer becomes Ohmic.[17,27] The HP memristor model considered drift of oxygen vacancies as a state variable.[2] Our memristor device too shows same dependency on drifting of oxygen vacancies. The large concentration of oxygen vacancies in WO3 makes it a promising material for memristor device.

4. CONCLUSIONS The Ag/WO3/ITO memristor device has been successfully realized by using spray pyrolysis technique. The top metal electrode (Ag) was patterned at ambient conditions using vacuum evaporation technique while the active WO3 layer was put in place by employing spray pyrolysis technique. The morphological studies of the films reveal the interconnected porous microstructure composed with quasi nanoflakes. Simulation of memristor has also been carried out using linear dopent drift model. The simulation results have been validated with experimental ones to confirm the characteristics. The developed Ag/WO3/ITO device exhibits bipolar resistive switching behavior within ±1 V which is a typical memristor property. The simulation and experimental results have confirmed that prepared device shows memristor like property. The research group is further working on exploring the applications of the developed device.

ACKNOWLEDGEMENTS The authors are very much thankful to the University Grants Commission (UGC), New Delhi, for the financial support through its project No. ‘‘41-869/2012 (SR)’’.

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