Excellent selector performance in engineered Ag/ZrO₂:Ag/Pt structure for high-density bipolar RRAM applications

Resistance random access memory (RRAM) devices have recently attracted significant attention for memory, logic and neuromorphic applications.^1,2 Among them, electrochemical metallization memory (ECM) devices have shown excellent resistive switching properties such as fast programming speed, large memory window, low power consumption, and high storage density.^3–5 The simplicity of the metal–insulator–metal (MIM) structure enables RRAM to be easily integrated in passive crossbar arrays and to achieve the highest storage capacity with the smallest cell size of 4F²/n (F is the minimum feature size; n is the stacking layer number of the crossbar arrays).⁶ However, a fundamental problem for such passive array is ‘sneak paths’, which corresponds to unintended multiple parallel sneak paths in the crossbar network, leading to the failure of write and read operations.⁷ Thus, a two-terminal selector device with high selectivity and low off-current is essential to suppress the sneak current. Several types of selector devices have been extensively investigated because of their promising selection properties, including a metal-insulator transition (MIT) device,⁸ and Ovonic threshold switching (OTS) device.⁹ Nonetheless, these selector devices cannot sufficiently suppress the leakage current, which are not meet the requirement for achieving high density RRAM integration. Subsequently, the field-assisted-superlinear-threshold (FAST) device has been reported with attractive performance but undisclosed materials.¹⁰ 

Recently, a new type of selector which behaves volatile threshold switching (TS) characteristics with high selectivity is causing widespread interests and it shows great potential for a cross-point array implementation.^11–16 However, previous reported selector with TS characteristics was usually unidirectional switching owing to the absence of active metal source and limited for unipolar selector applications. Contrastively, the overall performance of a bipolar-type RRAM is better than that of a unipolar-type RRAM, especially switching uniformity and endurance.^17,18 Hence, in order to alleviate the sneak paths issue and achieve reliable high-density cross-point memory arrays, bidirectional TS property for bipolar selector applications has been recently proposed.^19–24 

In this work, the transition from unidirectional to bidirectional threshold switching characteristics was achieved in the Ag/ZrO₂/Pt stack structure by the optimized annealing treatment. Forming-free characteristics, reduced threshold voltages, higher selectivity and excellent uniformity were realized as well. The thermally induced diffusion of Ag clusters was responsible for the reliable threshold switching behaviors and the volatile characteristics under a higher programming current of 10 mA were verified by microstructural analysis. Furthermore, we found a proper bipolar RRAM device which was well-matched with the bipolar selector. By the integration of the Ag/ZrO₂/Pt selector with the bipolar Ti/ZrO₂/Pt RRAM device, a bipolar 1S–1R memory device was demonstrated, in which the undesired sneak current was significantly suppressed.

The TS selector with Ag/ZrO₂/Pt structure was prepared using the following processes. A 20 nm Ti adhesion layer and a 100 nm Pt bottom electrode (BE) were successively deposited on a SiO₂/Si substrate through electron beam evaporation. Subsequently, a thick ZrO₂ film (80 nm) was grown at 150 ^oC under a base pressure of 8.5×10^-4 Pa via electron beam evaporation. Then, 100 nm Ag top electrode (TE) and 50 nm Au protective layer were deposited by magnetron sputtering and patterned into circular shape with a diameter of 250 μm by lift-off technology. To obtain a selector with bidirectional TS characteristics, prepared devices were annealed at various temperatures: 200, 400, 600, and 800 ^oC in N₂ ambient, respectively. After that, a 200×200 μm² RRAM device, consisting of three sequential layers of Ti/ZrO₂/Pt (with thicknesses of 70/30/70nm, respectively), was stacked in succession on the Au protective layer by electron-beam evaporation. And the ZrO₂ film (30 nm) was grown at ambient temperature under a base pressure of 9.6×10^-4 Pa. Physical analyses were performed by TEM and EDS spectrum. Electrical measurements were carried out by using Agilent B1500A semiconductor characterization system. During all the measurements, the Pt bottom electrode was grounded, while the bias voltage was applied on the Ag top electrode for the Ag/ZrO₂/Pt selector and Ti electrode for the Ti/ZrO₂/Pt/Ag/ZrO₂/Pt 1S–1R memory device, respectively.

Fig. 1(a) shows a schematic diagram of the fabricated selector device with Ag/ZrO₂/Pt stacked structure. The as-fabricated devices are in the high resistance state (HRS) with a low current level near ∼300 pA at a read voltage of 0.1 V. Fig. 1(b) depicts the typical I-V curve of the device under a compliance current of 100 nA. Initially a forming process is needed to activate the device. In the subsequent I-V cycles after the initial forming process, the device can switch abruptly to the low resistance state (LRS) at a lower threshold voltage (V_th) of about 0.7 V. However, when the voltage reaches a certain value called hold voltage (V_hold∼0.25 V) during the backward voltage sweeping, it cannot remain the ON state and the current decreases to the initial high resistance level, indicating a typical TS behavior. This unidirectional TS behavior was also reported in other ECM-based TS devices and explained by the spontaneous self-rupturing of the unstable filaments only at switching current of <1 mA owing to the electromotive force or Rayleigh instability.^13,16,19 Because of the absence of active metal source near the BE, this selector with TS behavior merely exhibits unipolar characteristics. In order to obtain expected selector with bidirectional TS characteristics, an optimized annealing treatment was performed in the as-fabricated devices to enhance the Ag diffusion into the ZrO₂ film. As shown in Fig. 1(c), when the voltage is swept to a negative polarity, a dramatic current increase is observed at ∼ –0.3 V in the device annealed at 600 ^oC, exhibiting bidirectional TS characteristics. It is noteworthy that no forming process is needed to activate the device, which is promising for practical applications. Furthermore, we investigate the annealing temperature dependence of TS characteristics in the Ag/ZrO₂/Pt device. As shown in Fig. 1(d), when the annealing temperature is high (800 ^oC), substantial Ag atoms are incorporated into the ZrO₂ matrix, leading to a metallic behaviour. On the other hand, when the annealing temperature is lower than 600 ^oC, the device exhibits unidirectional TS, indicating that the temperature is insufficient for the Ag diffusion. Therefore, the proper control of the Ag diffusion is crucial to obtain the bidirectional TS property.

Next, various electrical characteristics of proposed Ag/ZrO₂:Ag/Pt threshold selector device were evaluated in order to access its performance as a selector. Highly reversible and uniform bidirectional TS behavior of the device in 100 consecutive cycles under compliance current of 100 μA is verified as illustrated in Fig. 2(a). The selector device shows high selectivity (∼10⁵) and steep slope (<5 mV/dec). Tight distribution of the HRS resistances, shown in Fig. 2(b), proves good switching uniformity of the device. Moreover, the corresponding narrow distributions of V_hold and V_th are also depicted in Fig. 2(b). It can be seen that the threshold voltage (∼0.2 V) of this device is lower than that of the as-fabricated device (∼0.6 V). To examine the effect of compliance current on the bidirectional TS behaviors, the I-V curves of Ag/ZrO₂/Pt device under different compliance currents varied from 100 nA to 10 mA are conducted and shown in Fig. 2(c). Tremendous difference against the previous report,^14,15 in which a transition from volatile threshold switching to non-volatile memory resistive switching is obtained by applying a high compliance current, volatility is independent of compliance current up to 10 mA for Ag/ZrO₂:Ag/Pt ECM. When the compliance current of 10 mA is applied, a rectifying ratio of about 1×10⁷ can be obtained at ±0.1 V. And the rectifying ratio can be further enhanced prospectively in smaller size cells since the HRS resistance exhibits dependence on the device area while the LRS resistance is insensitive to size based on the filament model in ECM devices.³ Another important property of threshold switching is the volatilizing speed. In Fig. 2(d), a 500 ns/1.3 V switching pulse followed by a 1 μs/0.2 V reading pulse verify that the device has relaxed back to the HRS under zero bias within 5 μs after switching to the LRS. In addition, the threshold selector device shows fast response time of <20 ns, as shown in the inset of Fig. 2(d), which indicates that a switching speed of selector is fast enough for its selection usage in any RRAM devices.

To examine the bidirectional switching mechanism, the sample of Ag/ZrO₂:Ag/Pt device was delivered to analytical TEM. The inset of Fig. 3 shows the cross-sectional TEM image of Ag/ZrO₂:Ag/Pt stack. As expected, the thickness of ZrO₂ film is approximately 80 nm and apparent lattice fringes of small ZrO₂ crystal grains with different orientations can be observed, suggesting the polycrystalline nature induced by the annealing process. However, for the as-deposited ZrO₂ films, there is no obvious diffraction peaks, indicating an amorphous structure (not shown). As shown in Fig. 3, the EDX spectrum was collected from the bottom of the ZrO₂ active layer. Besides the Zr and Pt, Ag signals are detected obviously, indicating that Ag atoms are diffused into the ZrO₂ matrix during the annealing process. Based on the experimental results and previous reports,^25,26 the mechanism of stable bidirectional TS behaviors in Ag/ZrO₂:Ag/Pt devices can be interpreted as follows: During the thermal annealing procedure, Ag atoms diffuse into the matrix along the defects, such as grain boundaries and dislocations, and some injected Ag atoms aggregate into nanoclusters. The dispersed metallic Ag clusters cause significant reduction in effective film thickness. When applying a bias, defects existed in the matrix provide easy segregation paths for metal ions to form filaments. Consequently, the initial forming process is eliminated, and the threshold voltages are reduced, compared with the unannealed counterpart. Meanwhile, the formation of Ag filaments is confined along the pre-existed Ag nanoclusters owing to the enhanced local electric field near themselves, which improves the uniformity of the device. The metallic Ag clusters, either dispersing in ZrO₂ films or accumulating at the ZrO₂/Pt interface, can act as the effective Ag source, which is responsible for the formation of Ag filaments under negative bias. Thus, the device exhibits bidirectional TS behaviour. Recent studies show that interfacial energy can facilitate filament rupture in volatile switching.^22,27 In the annealed devices, conduction channels are formed by the growth and clustering of silver clusters distributed in the dielectric under bias. To minimize the interfacial energy, clusters slowly diffuse to the minimum energy positions and merge into larger clusters. The procedure of minimizing the interfacial energy between the silver clusters and the dielectric acts as the driving force for spontaneous rupture of filament, which results the threshold switching in ECM device, even though the switching current is high (10 mA). In addition, the dissolution of the filament may also be influenced by the Joule heating during the operation of the device and grain boundaries, which provides paths for metal atoms to diffuse. Therefore, the selectivity can be modulated from 10² to 10⁷ by engineering the compliance currents in a range of 100 nA to 10 mA. More importantly, the devices can be functional even under high compliance currents, which is very promising for high-stability integration applications.

Furthermore, to confirm the feasibility that the bidirectional TS device with simple Ag/ZrO₂/Pt structure can be utilized as a bipolar selector in passive crossbar arrays, a forming-free bipolar RRAM device was integrated on top of the selector device and the 1S-1R memory device with Ti/ZrO₂/Pt/Ag/ZrO₂/Pt stacked structure was fabricated. The bipolar resistive switching behavior of a Ti/ZrO₂/Pt RRAM device is depicted in Fig. 4(a). Initial resistance of the device is low and a reset process is needed to convert the state to HRS. According to our previous study, the resistive switching behavior of the RRAM device can be explained by the formation and rupture of the conductive filament through the migration of oxygen vacancies under the applied bias conditions.²⁸ Fig. 4(b) shows the I–V characteristics of the integrated 1S-1R memory device with the structure of Ti/ZrO₂/Pt/Ag/ZrO₂/Pt under compliance current of 400 μA. Initially a reset process is needed to convert the RRAM device to HRS. Since the Ag/ZrO₂/Pt selector is in the OFF state from -V_th to V_th, current flowing through the unselected cell is greatly prevented when 1/2V_read (0 < 1/2V_read < V_th) is applied to the unselected cell. Compared with the RRAM device, the leakage current of the 1S-1R memory device is enormously reduced by 4 orders of magnitude at 1/2V_read. The corresponding cumulative probability of the resistances at V_read (0.7 V) and 1/2 V_read for the 1S-1R memory device are shown in Fig. 4(c). Tight distribution of the HRS and LRS resistances is verified and the on/off ratio of 9 has no visible degradation after 100 successive switching cycles. Due to the ability of the selector to effectively suppress current at low bias, a high selectivity (>10⁴) is maintained during cycling. Fig. 4(d) represents the calculated readout margin using the parameters extracted from I-V data of the 1S-1R device. The leakage current of the 1S-1R device at V_read/2 is significantly reduced, therefore, the readout margin dramatically improves compared to the RRAM. According to the above results, we can suggest that the Ag/ZrO₂/Pt device has a great potential for use in the selector for suppressing the sneak current in a cross-point array.

In summary, we have proposed a bidirectional TS device with Ag/ZrO₂/Pt structure by optimized annealing treatment for high-density resistive memory applications. The bidirectional TS device exhibits excellent switching uniformity and the selectivity can be modulated from 10² to 10⁷ by engineering the compliance currents in a range of 100 nA to 10 mA. Our research can be applicable to other ECM devices and provides an approach for achieving bidirectional TS characteristics for bipolar selector applications. Furthermore, the 1S-1R memory device is successfully fabricated and confirmed the significant suppression of the leakage current compared to the RRAM device. These results indicate that the bidirectional TS device with Ag/ZrO₂/Pt structure can be used to realize ultra-high-density RRAM arrays.