1 Introduction

The increasing demand for data storage and computing applications has attracted researchers toward resistive random-access memory (ReRAM) devices due to its low power consumption, high operating speed, reduced potential, and compatibility with complementary metal-oxide-semiconductor (CMOS) technology [1, 2]. The ReRAM is a simple capacitor device that consists of an insulating/semiconducting layer sandwiched between two metal electrodes. The ReRAM device is capable of switching between two or more resistance states upon applying a bias voltage [3, 4]. ReRAM devices have been fabricated using different metal oxides such as tungsten oxide [5], tantalum oxide [6], nickel oxide [7], titanium oxide [8], hafnium oxide [9], zinc oxide [10], and cobalt ferrite [11]. Oxide based ReRAM devices have been mostly observed to exhibit digital switching. However, there are few reports of the observation of analog switching also [12,13,14,15,16,17]. The electroforming free resistive switching can also be realized using nanostructured material [18].

ReRAM device can be used as synaptic device for neuromorphic computing but for these application ReRAM devices must exhibit analog switching [14, 15]. Recently there are reports on observing analog switching in inorganic oxide materials like titanium oxide [15], zinc oxide [16], and tantalum oxide [17]. Among all the oxide materials, ZrO2 can also be interesting material to explore resistive switching as it has high dielectric constant, large bandgap, stable thermal properties and can exhibit resistive switching at low bias field [19]. ZrO2 films are prepared via sol–gel deposition technique, sputtering technique, and atomic layer deposition technique [4, 19, 20]. Lee et al. studied the effect of UV irradiation on the switching properties of ZrO2 films which showed stable digital resistive switching property after UV irradiation [21]. Lee et al. reported the presence of synaptic characteristics after insertion of SiO2 thin films in Cu/SiO2/ZrO2/Pt devices [22].

In the present work, we report the synthesis of ZrO2 nanorods by hydrothermal method and the fabrication of FTO/ZrO2/Ag resistive switching device using ZrO2 nanostructured film exhibiting analog switching. The mechanism of resistive switching is also presented by analyzing I–V characteristics of the device in high resistive and low resistive state.

2 Experimental details

2.1 Material synthesis

The ZrO2 nanostructured powder was prepared using the hydrothermal method [23]. Initially, 0.5 M zirconyl nitrate hydrate (ZrO(NO3)2.xH2O) solution was prepared in deionized (DI) water. 5 M NaOH solution prepared in DI water was slowly added to zirconyl nitrate solution under continuous stirring followed by sonication for 30 min to obtain a homogeneous solution. A 70 ml of the prepared solution was loaded into a 100 ml Teflon-lined autoclave.

The autoclave was sealed and maintained at temperature 180 °C for 24 h (hydrothermal treatment time). It was then allowed to cool down to room temperature naturally. The resultant precipitate was filtered, washed with distilled water to remove the soluble nitrates, and with ethanol to reduce agglomeration, and later dried at 100 °C for 12 h. This ZrO2 powder was used as the precursor for the fabrication of ZrO2 thin film on the FTO substrate using spray deposition technique (Fig. 1).

Fig. 1
figure 1

Schematic for synthesis of ZrO2 powder

Full size image

2.2 Device fabrication

The FTO/ZrO2/Ag trilayer structure (Fig. 2) was fabricated using ZrO2 film which was deposited by spray coating the ZrO2 nanopowder on the FTO substrate. Before deposition of the film, the FTO substrate was cleaned via the conventional ultrasonication method (ultrasonication in deionized water, acetone, and isopropyl alcohol for 10 min each). A part of the FTO substrate was covered with Kapton tape for getting the bottom contact of the device. ZrO2 powder was mixed in ethanol and was stirred for 30 min. Afterward, a diluted citric acid (in DI water) was slowly added to ZrO2. The resultant solution was loaded in a spray gun and was spray coated precisely on the FTO substrate. The thickness of the ZrO2 film was 350 nm. After the deposition, the film was dried at 80 °C for an hour. The contact pads of Silver (Ag) on the device were deposited by e-beam evaporator using metal shadow mask with holes of diameter 0.5 mm. The thickness of the silver contact pad on ZrO2 film was 100 nm.

Fig. 2
figure 2

Schematic of FTO/ZrO2/Ag resistive switching device

Full size image

2.3 Characterizations

In order to characterize the ZrO2 film grown on FTO substrate, powder X-ray diffraction (XRD) was performed on a Rigaku Ultima IV diffractometer equipped with Ni-filtered Cu Kα radiation (l.54 Å) [24]. Diffraction patterns were recorded for this film in the range of 2\(\theta \) from 20° to 80° at a scanning rate of 2°/min. A scanning electron microscope (SEM, TM3000, Hitachi) equipped with an EDAX (SwiftED3000 Oxford) was used to collect SEM images and to analyze composition [25]. FEI Tecnai transmission electron microscope (accelerating voltage = 200 kV) was used to conduct a transmission electron micrograph (TEM) [26]. Electrical transport measurement was performed by Keithley 2400 source meter. Two probe point contact was made on the device with a sharp conducting probe tip made of copper. The data of current (I) versus voltage (V) were recorded via Labview interfaced computer program.

3 Result and discussion

X-ray diffractometer (XRD) pattern of ZrO2 nanorods is shown in Fig. 3. The XRD pattern of ZrO2 nanorods has the characteristic peaks at 24.20°, 28.40°, 30.46°, 31.71°, 34.12°, 35.52°, 38.57°, 40.87°, 44.67°, 49.38° and 50.15° corresponding to (110), (− 111), (011), (111), (020), (002), (120), (− 112), (− 202), (220) and (022) planes, respectively (JCPDS card no. 37–1484). No unidentified peaks were observed in the XRD pattern, indicating the synthesis of single-phase ZrO2. The average crystallite size of ZrO2 nanorods was obtained as ∼ 50 nm. Scherer formula is used to determine the average crystallite size for ZrO2 nanorods, which is given as [27];

$$ t = \frac{0.9 \lambda }{{\beta {\text{cos}}\theta }} $$
(1)

where β represents the full width at half the maximum of the XRD peak, θ is Bragg’s angle, (1.542 Å) is the wavelength of X-ray, and t is the average crystallite size. The average crystallite sizes for ZrO2 nanorods are estimated as ~ 50 nm, respectively.

Fig. 3
figure 3

XRD pattern of ZrO2 nanostructured film spray deposited on FTO substrate

Full size image

The morphology and chemical composition of the ZrO2 nanostructure were studied by SEM and EDAX analysis. The SEM image of the ZrO2 nanostructure is shown in Fig. 4a. The ZrO2 nanostructure shows the irregular-shaped overlapped morphology due to the aggregation of nanorods. The chemical composition of the ZrO2 nanorods was examined by EDAX (Fig. 4b) which shows that the Zr and O elements are present in the stoichiometric ratio.

Fig. 4
figure 4

a SEM image and, b EDAX patterns of ZrO2 nanorods, c TEM images of ZrO2 nanorods and d Size distribution histograms of ZrO2 nanorods

Full size image

The TEM micrograph of ZrO2 nanostructure was recorded to determine the shape, size, and uniformity of the nanorods. The TEM micrograph of ZrO2 nanostructure shows the nanorods-like structure (Fig. 4c) with the average length of the nanorods as 45 nm, as observed by the size distribution histograms (Fig. 4 d).

4 Electrical transport properties

For recording of I–V characteristics of the device a direct current (DC) voltage sweep from 0 → 1 → 0 → − 1 → 0 with a step of 0.01Volt and a maximum applied voltage of 1 V with compliance current (ICC) of 10 mA was applied and the result is shown in Fig. 5a. In the first voltage sweep 1, the device was in the high-resistance state (HRS). In the resistive cycle, on returning from 1 → 0 there is a gradual change in the resistance, and the device switches from HRS (high-resistance state) to LRS (low-resistance state) at 0.8 Volt. Similarly, in cycle 3(0 → − 1) device stays in the LRS state, and the cycle from − 1 → 0 around—0.7 V the device switches to HRS again. I–V characteristics of the FTO/ZrO2/Ag device were recorded for several cycles to check the reproducibility of switching device. Figure 5b shows the I–V characteristics for 1 to 100th cycle at each 10th interval. Exact overlapping of all I–V characteristics clearly shows that the performance of resistive switching is very stable. A small switching window can be seen in all the cycles. As in all 100 cycles, there is a small change in the resistance between the HRS and LRS, and there is no sharp transition between the HRS and LRS, the switching of device can be described as analog switching. Figure 5c shows the resistance values in both LRS and HRS with Roff/Ron ratio as 1.86 at 0.35Volt. The I–V measurement was performed on 6 devices and we got similar kind of switching pattern as show in Fig. 5c. We have also performed the experiment for − 2 to + 2 Volt separately and it has been observed for 2 Volt of applied bias, the device shows breakdown behavior and no switching window is seen Similar type of behavior of occurring breakdown for higher applied bias is reported in earlier also [28].

Fig. 5
figure 5

a I–V measurement of FTO/ZrO2/Ag device with Vmax =  ± 1Volt, (b). Repeated cycles of I–V characteristics from 1to 100th cycles, (c). Magnified view of resistance values in on and off state of device

Full size image

5 Mechanism of switching

In the present device, the HRS to LRS switching is not abrupt and there is a continuous variation for changing for HRS to LRS. In order to understand the mechanism of the present analog switching, we carried out a detailed analysis of I–V characteristics both in HRS and LRS. In Fig. 6a, b, I–V characteristics have been plotted in logarithmic scale for both HRS and LRS. It is observed that the initial bias voltage the variation of current (I) with voltage (V) is linear however for larger bias the characteristics become nonlinear. For the HRS, the slope of the ln-I-ln-V curve is 2 for the bias voltage from V1 = 0.23 Volts to V2 = 0.53 Volts, and for V > 0.53 Volts the slope becomes 3. For LRS, the slope of I–V is equal to 2 from 1 to 0.25 Volts, and it is equal to 1 from 0.25 to 0 Volts of applied bias.

Fig. 6
figure 6

Experimental I–V curve fitted to I = \(K{V}^{m}\) for the resistive switching devices, a High Resistance State (HRS), and b Low Resistance State (LRS)

Full size image

The ohmic conduction equation is given by [29]

$$J_{{{\text{ohmic}}}} = \sigma E = q\mu N_{c} E\,{\text{exp}}\left[ {\frac{{ - \left( {E_{{\text{c}}} - E_{{\text{F}}} } \right)}}{KT}} \right]$$
(2)

where σ is electrical conductivity, µ is the electron mobility, Nc is the effective density of states of the conduction band, EC is the conduction band and EF is the Fermi energy level. The current (I)–Voltage (V) characteristics of the present resistive switching device follows I α Vm [30, 31] which is a clear signature that the trap assisted space charge limited conduction (SCLC) is the dominant mechanism of charge traps.

Conduction process in our device is influenced by trapping and de-trapping of charge carriers in an externally applied electric field. In the trapping/de-trapping mechanism of conduction, the trapping of charges provides a conduction path and thus increases the current [31, 32]. For the cycle 0 → 1, when biasing voltage is less than 0.23 Volt thermally generated charge carriers dominate and the ohmic conduction takes place for which the slope (m) is equal to 1. When the biasing voltage is raised from 0.23 Volt to 053 Volt the traps (voids/defect) states got occupied by the charge carriers. As the traps getting filled, it becomes easier for electrons to flows and there is a rise in current which leads the slope (m) equals to 2 [31, 32]. Further increasing the applied bias voltage (0.53 < V < 1), all the traps present in the film got filled with the charge carriers and the current is increased further which changes the slope (m) from 2 to 3. In the phenomenon of trapping, more number of charges present in the active layer which form a conduction path between top and bottom electrode. The process of conduction happens due to trap assisted space charge limited conduction (TSCLC) in the active layer this leads the exponential rise in the current [31]. For the cycle 1 → 0, initially, trap states were filled with the charges. As the applied voltage is decreased from 1 to 0 Volt, the charges start de-trapping from its trapping centers due to presence of opposite lower applied bias. Although the de-trapping of charges got started, there are still trapped charges till 0.25 Volt which causes the slope (m) equals to 2. Due to few trapped charges in the film till 0.25 volts the conduction process is again governed by the TSCLC process. Further when the charges got fully de-trapped below 0.25 Volt, the ohmic conduction process is dominated which causes the slope (m) to change from 2 to 1 [31,32,33].

6 Conclusions

Analog resistive switching device with FTO/ZrO2/Ag structure was fabricated using spray deposited ZrO2 nanostructured film. ZrO2 nanostructure was prepared using the hydrothermal technique. XRD studies confirmed the formation of single-phase ZrO2 film and the TEM micrograph of ZrO2 nanorods showed the nano-rod-like structure. The fabricated device exhibits analog switching behavior with gradual switching from HRS to LRS with the resistance ratio Ron/Roff as 1.86 at 0.35 Volts. Analysis of I–V characteristics of HRS and LRS revealed that in the higher applied bias transport is dominated by SCLC mechanism and the switching for HRS to LRS is due to filling and unfilling of trap states.