Bias Polarity Dependent Threshold Switching and Bipolar Resistive Switching of TiN/TaO_x/ITO Device

Abstract

In this work, we demonstrate the threshold switching and bipolar resistive switching with non-volatile property of TiN/TaO_x/indium tin oxide (ITO) memristor device. The intrinsic switching of TaO_x is preferred when a positive bias is applied to the TiN electrode in which the threshold switching with volatile property is observed. On the other hand, indium diffusion could cause resistive switching by formation and rupture of metallic conducting filament when a positive bias and a negative bias are applied to the ITO electrode for set and reset processes. The bipolar resistive switching occurs both with the compliance current and without the compliance current. The conduction mechanism of low-resistance state (LRS) and high-resistance state (HRS) are dominated by Ohmic conduction and Schottky emission, respectively. Finally, threshold switching and bipolar resistive switching are verified by pulse operation.

1. Introduction

Resistive switching random access memory (RRAM) is attracting attention due to its versatile memory device characteristics. The unipolar resistive switching occurs in the same polarity bias [1]. The set and reset processes occur by the soft breakdown under the electric field and Joule heating. The unipolar resistive switching observed in NiO and TiO₂-based RRAMs has the advantage of integration with selector [2]. A unidirectional selector, such as a diode, can be connected with the unipolar RRAM to suppress the low-resistance state (LRS) current in a low-voltage regime. However, the short endurance cycle and the large variability of the low-resistance state (LRS) and high-resistance state (HRS) are not suitable for mass production. On the other hand, the set and reset processes of bipolar resistive switching occur in different bias polarities [3]. Generally, the bipolar resistive switching which is observed in many dielectrics, such as metal oxide [4,5,6,7,8,9,10,11,12] and metal nitride [13,14,15] shows good resistive switching memory performances such as long endurance (10¹² cycle) [16] and fast switching speed (10 nm) [16]. Especially metal oxide RRAM with bipolar resistive switching offers reproducible and stable resistive switching for mass production. For the bipolar resistive switching, if a metal with good diffusion properties such as Ag or Cu is used, the generation and rupture of Ag or Cu filaments in the metal oxide leads to the resistive switching of the RRAM [17,18,19]. On the other hand, metal oxide RRAM with non-diffusion electrodes uses the change of oxygen vacancies for resistive switching [20,21,22,23]. This type is very promising for high-density memory applications due to its complementary metal-oxide semiconductor (CMOS) compatibility and high memory performances such as high endurance and fast switching speed [16].

The threshold switching of metal oxide provides other valuable applications such as selector and neuromorphic systems [24]. The selector element is essential to two-terminal memory devices, such as RRAM and phase-change random access memory (PCM) [25], adopting the cross-point array structure to mitigate the sneak current among memory cells. Moreover, the natural current decay behavior of metal oxide RRAM can be used for energy-efficient neuromorphic system applications that mimic biological nerve systems. For example, RRAM device with current decay could minimize the training cost in a recurrent neural network [26]. Threshold switching is observed in several metal oxide RRAM systems. It has been reported that when Ag or Cu metal oxide with electrodes is used, the current is exponentially reduced during the relaxation time after the generation of the conducting filament is temporarily formed through current limitation [27]. Moreover, VO₂ and NbO₂ with metal–insulator transition (MIT) effect offer natural threshold switching [28].

In this work, we fabricated the TiN/TaO_x/ITO RRAM device and demonstrated both the threshold switching and memory switching by selecting the bias polarity. The intrinsic threshold switching of TaO_x is achieved under positive bias. On the other hand, bipolar resistive switching is preferred with a negative bias due to the indium ion migration. Moreover, the conduction mechanisms of TiN/TaO_x/ITO RRAM are discussed to distinguish the two switching modes. Finally, the pulse operation is verified for threshold switching and the set and reset processes of bipolar resistive switching. The fact that two functions coexist in one device has the potential to be used as an advantage in the process.

2. Materials and Methods

TiN/TaO_x/ITO RRAM devices were fabricated as follows. Commercially available ~40 nm–thick ITO (sheet resistance of ~60 Ω/sq.) on SiO₂/glass was used as the bottom electrode. A 15 nm–thick TaO_x was deposited by pulsed DC sputtering at 0.5 kW and 50 kHz. The gases of Ar (8 sccm) and O₂ (12 sccm) were used at sputtering pressure of 1 mTorr for deposition. A 100 nm–thick TiN as the top electrode was deposited by reactive type (Ti target + nitrogen) in DC sputtering via shadow mask, containing circular pattern with a diameter of 100 μm. A Keithley 4200-SCS semiconductor parameter analyzer (SPA, Keithley Instruments, Cleveland, OH, USA) and a 4225-PMU pulse measurement unit in the probe station were used to measure electrical characteristics using DC sweep mode and transient features. A bias was controlled on TiN the top electrode and ITO bottom electrode were grounded for electrical measurement. For the measurement, Ag paste was applied over the ITO. X-ray photoelectron spectroscopy (XPS) analysis was conducted, using a Nexsa (ThermoFisher Scientific, Waltham, MA, USA) with a Microfocus monochromatic X-ray source (Al-Kα (1486.6 eV)), a sputter source (Ar⁺), an ion energy of 2 kV, a sputter area of 1 mm × 1 mm, a sputter rate of 0.5 nm/s for SiO₂ and a beam size of 100 µm.

3. Results and Discussion

Figure 1a,b shows the XPS spectra of Ta 4f and O 1s of the DC sputtering deposited TaO_x film, respectively. The XPS peaks for Ta 4f_7/2 and Ta 4f_5/2 of Ta-O bonds are located centered at around 26.72 eV and 28.65 eV, respectively (Figure 1a). On the other hand, the peak of Ta 4f for metallic Ta is not particularly detectable between 20 and 25 eV (Figure 1a). The main peak for O 1s of TaO_x film is located at 531.38 eV, which is related to the oxygen lattice. On the other hand, a small peak is observed at the binding energy of 532.65 eV. This peak indicates the oxygen-deficient Ta-O bonds related to the oxygen vacancies.

Figure 2a shows the typical I–V curve (linear scale) for threshold switching of TiN/TaO_x/ITO device. A dual I–V sweep is conducted to test the threshold switching. The current suddenly increases at turn-on voltage. The compliance current of 500 μA is essential to prevent the hard breakdown of the device. It is confirmed that the current decreased again at the turn-off voltage through the return voltage sweep. Even if only a forward sweep is performed without a return sweep, the threshold switching has a volatile characteristic, so the current has an off state again. In an actual RRAM memory cell, the set voltage should be greater than the turn-on voltage of the selector, and the reset current of the RRAM cell should be lower than the on-current of the selector. Figure 2b shows I–V curves (log scale) by an iterative sweep. There is a slight variation in turn-on voltage, which may adversely affect device operation when connected to RRAM devices. Therefore, more improvement is required through optimization of process conditions, etc. Next, we investigated the conduction mechanism of TiN/TaO_x/ITO devices in the threshold switching mode. We performed various I–V fittings to check the conduction transport possible in the metal–insulator–metal (MIM) structure and confirmed that the I–V fitting for Schottky emission was the most accurate. Figure 2c shows the ln(I) versus sqrt (V) for Schottky emission. The result shows an exact curve of I–V fitting for Schottky emission in the regime before the turn-on voltage. That is until the device is turned on, electrons from the ITO electrode can jump over TaO_x barrier layer by thermionic emission. A similar fitting result was reported in Pt/TaO_x/Pt [29]. It is noted that the oxygen exchange could be dominated between TiN and TaO_x layers for threshold switching when a positive bias is applied to the TiN electrode.

Next, the bipolar resistive switching with non-volatile property of TiN/TaO_x/ITO devices is implemented. Figure 3a,b shows the repetitive I–V curves with compliance current (CC) and without CC, respectively. The indium ion could diffuse the dielectric and indium-based conduction filament could be formed when a negative bias is applied on TiN top electrode of the device. In the set process caused by indium diffusion from ITO electrode, the set voltage is very low regardless of the dielectric medium [30]. The reset process can be explained by an indium-based conducting filament or an increase in oxygen within TaO_x layer. The current can be lowered at CC of 500 μA than the self-compliance current mode. This indicates the current overshoot can be minimized, and the current can be controllable by the CC. It is noted that negative-reset behavior that can make device failure does not occur at the large reset sweep voltage (~3 V) in Figure 3a. On the other hand, the current is limited by self-compliance during the set process (Figure 3b). The self-compliance could occur by the oxygen accumulation toward ITO electrode, which acts as series resistance. When the internal resistance of TaO_x decreases during the set process, a relatively higher voltage could be applied to the oxygen-rich ITO side. The reset process is completed with a gradual increase in current. It is difficult to read the states at a positive bias because the set voltage is very low. However, it is possible to distinguish the states at the read voltage at a lower than reset voltage. It is noted that threshold switching does not occur well after bipolar resistive switching. This indicates that an indium ion could remain after the reset process. Figure 3c shows the log–log scale of LRS for Ohmic conduction in which the slope has 1. This indicates metallic filament composed of indium could be formed in the LRS. On the other hand, the HRS follows the Schottky emission, which is judged by the linear relationship of ln(I) versus sqrt (V) in Figure 3d. The Schottky barrier could exist after the rupture of indium filament in the HRS. Thus, the conduction occurs when electrons overcome the Schottky barrier formed at the interface between the TiN and TaO_x layer with thermal energy. Figure 3e shows the retention test of LRS and HRS in which resistance values are extracted from reading voltage of −0.1 V. Two states have no degradation for 10,000 s, which means TiN/TaO_x/ITO devices in the bipolar resistive switching mode has good data-storage ability.

Next, we demonstrated the pulse operation for threshold switching and set and reset processes in the bipolar resistive switching. Figure 4a shows the voltage and current as a function of time for the threshold switching. The read pulses with the voltage of 0.1 V before and after the pulse for threshold switching are applied to the device. Sufficient pulse width (1 ms) is applied to observe sufficient transient characteristics. After the pulse is applied, the current increases rapidly at about 300 μs, but the current is not maintained and decreases. Finally, the current is not detected by the read pulse, so we confirm the threshold switching with volatile memory property through this series of pulses. The set and reset processes for bipolar resistive switching are performed in a similar pulse application method. Figure 4a shows the transient characteristics for set process. It is observed that the current does not increase more than 300 μA due to self-compliance after the rapid increase in current. In the second read pulse, the current increase compared to the first, confirming that the set process goes well. It is noted that very low voltage (−0.4 V) can make the set process. Figure 2 exhibits a decrease in current by reset pulse with the amplitude of 0.9 V. The current is abruptly decreased in peak current is less than 500 μA.

4. Conclusions

In summary, we characterized both threshold switching and bipolar resistive switching from TiN/TaO_x/ITO devices by controlling the bias polarity. The repetitive I–V curve for threshold switching was achieved with a positive bias on the TiN electrode. On the other hand, the set and reset processes for bipolar resistive switching were observed when a positive and negative bias was applied on the ITO electrode. The possible bipolar resistive switching mechanism can be explained by the formation and rupture of the indium conducting filament. We also demonstrated the pulse operation for the threshold and memory switching. The above result suggests two important switching possibilities simply by bias polarity for the selector and non-volatile memory applications.