Author’s Accepted Manuscript Fabrication of Zinc Stannate Based All-Printed Resistive Switching Device Junaid Ali, Ghayas-ud-din Siddiqui, Yang-Hoi Doh, Kyung Hyun Choi www.elsevier.com
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To appear in: Materials Letters Received date: 17 October 2015 Revised date: 3 December 2015 Accepted date: 10 December 2015 Cite this article as: Junaid Ali, Ghayas-ud-din Siddiqui, Yang-Hoi Doh and Kyung Hyun Choi, Fabrication of Zinc Stannate Based All-Printed Resistive Switching Device, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.12.045 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication of Zinc Stannate Based All-Printed Resistive Switching Device Junaid Ali1, Ghayas-ud-din Siddiqui1, Yang-Hoi Doh2, Kyung Hyun Choi1* 1
Department of Mechatronics Engineering, Jeju National University, 690-756, Korea
Department of Electronic Engineering, Jeju National University, 690-756, Korea
*Corresponding author. Phone: +82-64-754-3713, Fax: +82-64-752-3174, E-mail: [email protected]
Abstract This paper describes resistive switching in ZnSnO3 thin film deposited by electrohydrodynamic atomization. The field emission scanning electron microscope analysis showed uniform surface morphology for thin films. The active layer, a thin film comprised of ZnSnO3 nano-cubes was printed between screen printed silver (Ag) electrodes on glass substrate. Resistive switching behaviour of the Ag/active layer/Ag sandwich structure was confirmed by current voltage analyses. The 3x3 array of memristors thus fabricated, showed characteristic OFF to ON (high resistance to low resistance) transition at low voltages, when operated between ±2 V, at 100 nA compliance currents. The memristor array exhibited stable room temperature currentvoltage hysteresis, low power operation, retentivity in excess of 24 h. An ROFF/RON ≈ 10:1 was observed at VRead=100 mV for more than 100 voltage stress cycles. All memory bits showed similar current voltage characteristics with respect to resistive switching parameters. Keywords: electrohydrodynamic atomization; memristor; resistive switching; printed memory;
1. Introduction Properties of nanomaterials differ from atoms, molecules, and bulk counterparts [1– 4]. Although resistive switching has been a fringe of research frontiers for over half a century, active research in the field took off after the successful fabrication of memristor at Hewlett-Packard labs, thus proving mathematical model by Chua . The proposed potential applications include, however are not limited to ultimate high density,
application[10,11], and analogue computing . In last couple of decades, research in nanotechnology has delivered numerous new forms of nanomaterials and several different methods to fabricate them. Research on the zero, one and two dimensional nanomaterials has provided many new properties in order to improve device characteristics. The increasing overlap of research areas for daily life applications, has accelerated the growing demand for devices based on biocompatible materials. Among biocompatible materials, ZnSnO3 has been in highlights for non-hazardous biodegradation of various dyes. ZnSnO3 has also been explored for nanogenerator [14,15], multilayer electrodes , photo catalysis, gas [18,19], chemical , and humidity sensing. Electro-hydrodynamic atomization (EHDA) phenomenon, discussed in detail by Poon  has proved itself as preferred low-cost, non-contact, and efficient material printing technique. It has been used in a variety of thin film applications such as, OLED, and Schottky diodes, thin film resistive switches[25–27]. A memristor switches its resistance on passage of suitable magnitude of electric current. Usually a thin film, sandwiched between two electrodes works as an active material. The two resistance states, a high resistance state (HRS), “OFF” state and a low
resistance state (LRS), or “ON” state  are attributed to thermal, electronic and/or ionic effects . This study is primarily focused on exploring a ZnSnO3 based 3x3 array of memristors. EHDA deposited ZnSnO3 thin films were sandwiched between screen printed Ag electrodes on glass substrates. The ZnSnO3 nanocubes were synthesized as previously reported. UV-Visible spectral analysis was used to analyse the transmission/absorption and evaluate the bandgap of the thin film. The thin films showed up to 88% optical transmittance for visible and a relatively higher absorption in UV region of electromagnetic spectrum. The calculated bandgap of the films was about 4.2 eV. Memristive behaviour in Silver (Ag)/active layer/Ag sandwich array was studied using current-voltage hysteresis. The resistive switching devices thus fabricated, showed characteristic OFF to ON (high to low resistance) transition when operated between ±2 V, characterized at 100 nA compliance currents. The fabricated devices exhibited a stable state retentivity of well over 24 h at room temperature, and a promising low power ROFF/RON ≈10:1 for more than 100 switching cycles
2. Experiment details 2.1. Materials and methods For synthesis of ZnSnO3 nanocubes, the precursor zinc sulfate heptahydrate (ZnSO4:7H2O) (F. Wt 287.56) and sodium stannate trihydrate (Na2SnO3:3H2O) (F. Wt 266.71) were purchased from Duksan Pure Chemicals, South Korea. Triton, acetone, and ethanol solvents were purchased from Daejon Chemical and Metal, South Korea. Highly conductive Ag nanoparticle paste was purchased from MicroPE® PARU (Conc. 80 wt %). Viscosity of ZnSnO3 colloidal ink was measured by VM-10A viscometer.
Spectrophotometer, (Shimadzu UV-3150 Japan). A non-contact (NV-2000 Universal) surface profiler in phase shifting interferometry (PSI) mode, with nanoscale accuracy for surface roughness measurement, was used to image the surface topography and (JEOL, JEM 1200EX II) field emission scanning electron microscope (SEM), were used to analyse the surface morphology of printed nanocomposite films on Ag coated glass substrates. Film thickness was measured by a non-destructive, thin film-thickness measurement instrument (K-MAC ST4000-DLX), based on interference spectrum of white light incident on the film surface. The electrical characterizations of resistive switching devices were carried out using a semiconductor device analyser (B1500A, Agilent, USA). The crystal structure of nanocubes was determined by the X-ray diffraction (XRD), Rigaku D/MAX 2200H diffractometer with Cu Kα radiation (𝛌 = 1.5406 Å , 40 kV, 250 mA, 8º min-1) using a fixed glancing incidence angle (2º).
2.2. Device fabrication Prior to deposition, glass substrates were cleaned by acetone, ethanol and deionized water in sequence by bath sonication, for 5 min each, at room temperature and dried in air. Afterwards, the substrates were cleaned by ultraviolet-ozone exposure for 5 min and oxygen plasma treated for 3 min to get rid of organic residues off the substrate surface. A semi-auto screen printer manufactured by SUNMECHANIX was used for electrode deposition. Figure 1 shows complete device fabrication sequence employed here. Bottom Ag electrodes were screen printed on cleaned glass substrates and sintered at 120°C for 30 min. ZnSnO3 active layer was deposited by an in-house built EHDA system[25–27,30], using a metallic capillary nozzle (inner diameter =210 µm) (Havard 33G) at 15 mm standoff distance. ZnSnO3 ink was sonicated and magnetically stirred for 5 minutes each prior to EHDA. Ink (ZnSnO3 in solvent) was fed to the liquid 4
chamber via Teflon tube to nozzle by a syringe pump (Hamilton Model 1001 GASTIGHT syringe). The flow rate was controlled by using the pressure control system. Figure 2 shows the overall operating envelop flow rate (50-800 µl/h) versus applied voltage (kV) using 210 µm diameter nozzle, at constant nozzle to substrate standoff distance of 15 mm. Initially at flow rate of 200 µl/h, a dripping mode appeared from voltage of 3.5 to 3.7 kV. Further increasing voltage, micro-dripping appeared until 4.6 kV. The cone jet remained unstable from 4.6 to 5.5 kV. The desired atomization of ink in the stable cone-jet mode was achieved from 5.5 to 8 kV. Beyond 8 kV, the jet disintegrated, into the multi-jet mode. The substrate was placed on a computer controlled translation stage, and substrate movement was varied from 1 to 3 mm/s to establish uniform coverage of about 1 x 2 cm2, at fixed standoff distance. The optimum atomization mode called stable cone jet mode, was used for final deposition thin films at 5.8 kV applied voltage and 200 µl/h ink flow rate. The film was deposited at translation speed of 5 mm/s. The deposited thin film was sintered at 500 °C for 90 min. Finally Ag screen printed on top of active layer as top electrode to complete the Ag/active layer/Ag MIM sandwich structure as schematically shown in Figure 1. The electrodes sandwiching the ZnSnO3 active layer in between form 3x3 array of MIM resistive switching device. Double voltage sweeps were applied across the two electrodes, to study current-voltage characteristics of the devices.
3. Results and discussions 3.1. Morphological analyses For each thin film used as an active layer, the thickness was measured by taking average of at least 7 measurements at different locations of film, repeated one after the other, in such a way that the set of measurements represented almost the whole top surface of the
sample. The EHDA fabricated ZnSnO3 active layers showed appreciable uniformity with average thicknesses of ~190 ±37 nm, for a nanocube based thin film. Surface morphology analyses of printed active layer by FESEM at two different resolution have been shown in figure 3(a, b). Thus FESEM images reveal uniformity of films, devoid of cracks, voids or humps at nano-scale. This attribute plays important role for reliable working of thin film, employed in electronic devices.
3.2. Optical characterization The optical characterization of thin films by UV/VIS Spectrophotometer, shows that the thin films were optically transparent in visible, showing promise for use in optoelectronic applications as shown in figure 4(a). The composite film showed negligible transmittance in UV region below 288 nm, and a sharp increase in transmittance with gradual increase in incident wavelength from 288 to 377 nm. The transmittance remained about ~88 % beyond 377 nm in visible and NIR regions. The direct energy band gap of ZnSnO3 nanocubes was calculated using absorbance spectrum 4.2 eV using the following equation, (
where ‘’ is the optical absorbance, Eg is band gap energy, ‘ED’ is a constant, ‘h ’ is the photon energy. To find the direct band gap energy, (ah )2 was calculated and was plotted as a function of photon energy as shown in figure 4(b). The linear portions of absorbance were extrapolated to the X-axis to get the approximate direct energy band gap of ZnSnO3 as 4.2 eV [31,32]. Figure 4(c) shows the crystalline phase ZnSnO3 nanocubes by XRD spectrum indexed by JCPDS card no. 28-1486. Additionally, the nanocubes’ studies reveal they are devoid of impure phases, smaller particle size and narrow nanocube size distribution. 6
3.3. Investigation of resistive switching in the memristor device The electrical properties of the fabricated memristors was analysed by resistive switching studies. For current–voltage (I–V) characteristics the Ag/active material/Ag device was biased by connecting the top electrode to driving voltage whereas the bottom electrode was grounded. The 9-bit 3x3 memristor array has been schematically shown in figure 1 with corresponding memristor bit matrix. Pristine ZnSnO3 film showed hysteric loop with figure-eight polarity while regular double voltage sweep was applied on top electrode, showing memristor characteristic of the active layer. The sequence of application of these sweeps, 2 0 -2 0 2, was maintained for all devices used to analyse the resistive switching behaviour. The magnitude of applied voltage is fairly small compared to other printed electronic devices. The applied positive voltage gradually increased the active layer resistance so as to SET the film from the low LRS to the HRS, and with negative bias (-1.1 V) the film is RESET to the LRS. The signature memristor pinched hysteric loop in I-V characteristic curve was observed with bistable resistance switching for multiple read-write cycles. The bipolar behaviour is maintained for multiple voltage sweeps. The device did not require any electroforming as required by Redox-based resistive switching memories [34–36] which is an advantage as device performance does not depend on filament formation. The double voltage sweeps were applied repeatedly to evaluate memristor behaviour at low compliance currents (CC) of 100 nA. The devices exhibited significantly stable resistive switching behaviour, by sustaining the resistive states, for more than 100 repeated double voltage sweeps. The typical pinched hysteresis, semi-log and log-log current voltage (I-V) characteristics, corresponding to 1st, 30th, 50th and 100th voltage sweep have been shown in figure 5 (a-c) respectively. The viability of the device for R-RAM applications depends on the consistency of electrical characteristics of memristor bits.
The resistive switching characteristics for 4th and 6th bit between ±2 V has been shown in figure 5(d). Both the bits showed identical I-V characteristics. The overall variation in resistive switching with 100 consecutive voltage stresses applied on 7th bit have been shown in figure 5(e). This robustness of the MIM structure is both crucial and necessary R-RAM applications. Figure 7(f) shows the retentivity of resistance HRS and LRS for 24 h. Readings were taken at 10 min intervals for 1st h. Afterwards the measurements were taken at 1 h intervals for next 5 h. Finally, the measurement was taken at 24th h to complete retentivity analysis for 1 day.
4. Conclusions A fully printed, a low power resistive switching memristor device has been presented using as-synthesized, solution processed ZnSnO3 nanocubes. The 9 bit memristor fabrication has been carried out using EHDA and screen printing for active layer and electrodes respectively. Commercially available Ag ink is employed for electrodes patterning. The ZnSnO3 ink was prepared by dispersing ZnSnO3 nanocubes in ethanol. The fabricated memristor boasted a steady and consistent resistive switching behavior for over 100 read-write cycles and ROFF/RON~10:1 at very low operating voltage of 100 mV. The memory bits’ current voltage characteristics were identical with respect to resistive switching parameters, hence potential as a printed transparent R-RAM applications. The retentivity of devices was up to 24 h. Transmittance of ~88% in these films show potential in optoelectronic applications.
Acknowledgments This research was supported by the 2015 scientific promotion program funded by Jeju National University.
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List of Figures Figure 1: Schematic illustration of Zinc stannate 3x3 memristor array fabricated by electrohydrodynamic atomization and screen printed electrodes. The inset shows stable cone jet mode employing 210 µm metallic nozzle at 15 mm standoff distance. The digital camera phtograph of final 3x3 memristor array is also shown with memristor bit matrix. Figure 2: Operating envelope illustration for the different EHDA modes at ink flow rate and applied voltage. Fig. 3: (a-b) FESEM images of EHDA deposited ZnSnO3 thin film at different resolutions. The sizes of cubes are also marked ranging from 60 to 200 nm. The high temperature sintering at 500 °C has changed the morphology of nanocubes and they seem to merge within each other. Figure 4: (a) UV/Vis absorbance and transmittance of deposited thin films showing transmittance in excess of 88 %. The thin film absorbance in UV region below 300 nm is tremendously higher than that for visible region of electromagnetic spectrum. (b) Bandgap was evaluated using absorbance data (c) XRD spectrum of ZnSnO3 nanocubes obtained after the calcination at 600 oC. Figure 5: (a) Conventional current voltage characteristics, corresponding (b) semilogarithmic and (c) log-log plots for 1st, 30th, 50th and 100th voltage sweeps. The double voltage sweeps were applied between -2 and +2V at 100 nA CC. (d) The typical bipolar resistive switching characteristics for 4th and 6th bit of Ag/ZnSnO3/Ag device. (e) The Resistive switching for 100 consecutive voltage stress cycles. The linear fit for both HRS and LRS is also plotted to conceive the
overall variation of resistance states. (f) The evolution of HRS and LRS with time for 60 min. The readings were taken at 100 mV at suitable time intervals
Figure 1: Schematic illustration of Zinc stannate 3x3 memristor array fabricated by electrohydrodynamic atomization and screen printed electrodes. The inset shows stable cone jet mode employing 210 µm metallic nozzle at 15 mm standoff distance. The digital camera phtograph of final 3x3 memristor array is also shown with memristor bit matrix.
Figure 2: Operating envelope illustration for the different EHDA modes at ink flow rate and applied voltage.
Fig. 3: (a-b) FESEM images of EHDA deposited ZnSnO3 thin film at different resolutions. The sizes of cubes are also marked ranging from 60 to 200 nm. The high temperature sintering at 500 °C has changed the morphology of nanocubes and they seem to merge within each other.
Figure 4: (a) UV/Vis absorbance and transmittance of deposited thin films showing transmittance in excess of 88 %. The thin film absorbance in UV region below 300 nm is tremendously higher than that for visible region of electromagnetic spectrum. (b) Bandgap was evaluated using absorbance data (c) XRD spectrum of ZnSnO3 nanocubes obtained after the calcination at 600 oC.
Figure 5: (a) Conventional current voltage characteristics, corresponding (b) semi-logarithmic and (c) log-log plots for 1st, 30th, 50th and 100th voltage sweeps. The double voltage sweeps were applied between -2 and +2V at 100 nA CC. (d) The typical bipolar resistive switching characteristics for 4th and 6th bit of Ag/ZnSnO3/Ag device. (e) The Resistive switching for 100 consecutive voltage stress cycles. The linear fit for both HRS and LRS is also plotted to conceive the overall variation of resistance states. (f) The evolution of HRS and LRS with time for 60 min. The readings were taken at 100 mV at suitable time intervals
Deposition of nanocube based thin film as active layer Top and bottom Ag electrodes have been screen printed 3x 3 array of Ag/ZnSnO3/Ag memristors was subjected to multiple voltage sweeps at ROFF/RON ≈ 10:1 operated at low power. All memory bits showed similar current voltage characteristics with respect to resistive switching parameters The films were highly transparent at visible and showed good UV absorption