Multi-Level Analog Resistive Switching Characteristics in Tri-Layer HfO2/Al2O3/HfO2 Based Memristor on ITO Electrode

Atomic layer deposited (ALD) HfO2/Al2O3/HfO2 tri-layer resistive random access memory (RRAM) structure has been studied with a transparent indium tin oxide (ITO) transparent electrode. Highly stable and reliable multilevel conductance can be controlled by the set current compliance and reset stop voltage in bipolar resistive switching. Improved gradual resistive switching was achieved because of the interdiffusion in the HfO2/Al2O3 interface where tri-valent Al incorporates with HfO2 and produces HfAlO. The uniformity in bipolar resistive switching with Ion/Ioff ratio (>10) and excellent endurance up to >103 cycles was achieved. Multilevel conductance levels in potentiation/depression were realized with constant amplitude pulse train and increasing pulse amplitude. Thus, tri-layer structure-based RRAM can be a potential candidate for the synaptic device in neuromorphic computing.


Introduction
The physical limitation of the conventional flash memory gives rise to the development of resistive random access memory (RRAM) due to its low power consumption, higher density, and simple structure, which consist of mainly transition metal oxides sandwiched between the top and bottom electrodes [1][2][3][4][5][6]. Although, single layer RRAM devices have been found to have uncontrolled filament formation and high switching voltage [7][8][9]. Therefore, research has been focused on gaining low-power RRAM devices for symmetric SET/RESET behavior, gradual conductance change, and high synaptic density. Transition metal oxide, mainly based on Al 2 O 3 , HfO 2 , TaO x , ZrO 2 , TiO 2, and their bilayer and tri-layer structure, was shown to be advantageous for industry-friendly electrical devices and improved multilevel resistive switching properties and its application towards synapses for neuromorphic computing [10][11][12][13][14][15][16].
Bilayer RRAM structures already have shown both abrupt and gradual change in conductance under bipolar resistive switching, which has been proposed effective for neuromorphic computing with tunable potentiation and depression. Among different bilayer structures, recently Al 2 O 3 /Ta 2 O 5 , HfO x /HfO 2 , HfO 2 /TiO x , TaO x /Al 2 O 3 , Al 2 O 3 /TiO 2 have been proposed to modulate the RRAM conductance gradually during resistive switching [5,13,[17][18][19]. In bilayer structures mostly, oxygen vacancies were found to be located at the bilayer interface, thus the push and pull of oxygen vacancies from the interface mainly controls the resistance of the device during switching. Additionally, in other devices, modulation of the conductance can be controlled by the motion of oxygen vacancies between oxygen-deficient and oxygen-rich layers, which further limits the device's operation current and controls low power consumption in RRAM. On the other hand, related to the multi-layer resistive switching, Wang et al., described the tri-layer RRAM structure of Al 2 O 3 /HfO 2 /Al 2 O 3 with abrupt resistive switching performance [20]. In this work, it is considered that the concentration of oxygen vacancy is higher in HfO 2 compared to Al 2 O 3 and interfacial diffusion took place between two dielectrics. Therefore, the interfacial layer helps to exchange oxygen vacancies (V O ), which finally improves resistive switching. A similar phenomenon has been indicated by Lui et al., where multilevel conductance was modulated by V Reset-stop in Al 2 O 3 /HfO 2 /Al 2 O 3 RRAM structure [21]. In the case of unipolar resistive switching, an important finding was explained by Maestro-Izquierdo et al., where 3D simulation suggests that during the RESET process, temperature distributions are different in multilayer structures [22]. Abrupt switching took place due to conductive filament (CF) narrowing in the HfO 2 middle layer due to lower thermal conductivity of HfO 2 (1.0 W m −1 K −1 ) compared to Al 2 O 3 (2.86 W m −1 K −1 ) [23]. Furthermore, the tri-layer formed by inserting an oxygen-deficient ZrO 2-x layer between two ZrO 2 dielectric layers was found to have transitioned from interfacial to filamentary switching characteristics under different SET compliance currents [24]. However, it has previously been reported that the V O from CF has low mobility in the AlO x layer compared to HfO 2 during the reset operation [25]. As compared to other different tri-layer RRAM structures mentioned above, for tri-layer RRAM structure in this study, the Al 2 O 3 layer is placed between two higher Vo contents in HfO 2 layers for better movement of the Vo during SET and RESET operations. In addition, the virtual electrode formed during the electroforming process within Vo rich HfO 2 layers at both ends can help the CF regrowth during SET operation at lower electric field [26].
For the next generation of electronic devices, transparent electronics are recently emerging [27][28][29]. The development of transparent electronics including touch panel, display, energy storage, photodetector, and solar cells have attracted great interest. RRAM on transparent electrodes such as indium tin oxide (ITO), with gradual multilevel resistive switching for logic and memory devices, has gained increasing attention as a reliable synaptic device. Hence, it is highly expected that future RRAM embedded with transparent electrode (ITO) will become a paradigm for future see-through memory devices.
In this work, the proposed atomic layer deposited (ALD) HfO 2 /Al 2 O 3 /HfO 2 tri-layer RRAM structure with a transparent ITO electrode has been studied in detail, which is believed to be more suitable for multilevel resistive switching. In addition, considering HfO 2 has larger oxygen vacancies compared to Al 2 O 3 , this structure has been designed to have two interlayers formed between the top and bottom HfO 2 layers with a middle Al 2 O 3 layer. Excellent multi-resistance states were achieved for tri-layer RRAM to emulate neuromorphic properties. For application in synaptic devices, multilevel conductance was achieved by applying both DC and pulse voltages.

Materials and Methods
Initially, the bottom electrode was taken as commercially available~40-nm-thick ITO (sheet resistance of~60 Ω/sq.) on SiO 2 /glass. Sequential ITO surface cleaning procedure was adopted stepwise with acetone, isopropyl alcohol, and deionized water along with ultrasonication for 5 min. Finally, it was dried using an N 2 blow at room temperature. Cleaned ITO substrates were immediately transferred to the ALD system for Al 2 O 3 and HfO 2 deposition at low substrate temperature. Tri-layer of HfO 2 (5 nm)/Al 2 O 3 (2 nm)/HfO 2 (5 nm) was deposited by using the metal precursors of tetrakis (ethylmethylamino) hafnium (TEMAH) and trimethylaluminum (TMA) for HfO 2 and Al 2 O 3 , respectively. In this ALD technique, H 2 O was used as the oxidant at a substrate temperature of 150 • C. Sputtered TaN was used as a top electrode with Ni capping layer, and electrodes were formed by the liftoff process to achieve an area of 100 × 100 µm. Figure 1 shows the schematics of the fabricated tri-layer RRAM device. Electrical resistive switching (I-V) and pulse measurements to characterize synaptic properties of the fabricated device, a Keithley 4200 SCS semiconductor parameter analyzer (Keithley Instrumnets, Cleveland, OH, USA), and a 4225-PMU ultrafast current-voltage (I-V) Nanomaterials 2020, 10, 2069 3 of 12 pulse module were used. All electrical measurements were obtained by applying a voltage to the top TaN electrode while the ITO bottom electrode (BE) was grounded. ultrafast current-voltage (I-V) pulse module were used. All electrical measurements were obtained by applying a voltage to the top TaN electrode while the ITO bottom electrode (BE) was grounded.

Results and Discussion
The cross-section of the TaN/HfO2/Al2O3/HfO2/ITO tri-layer RRAM structure was investigated by the high-resolution transmission electron microscopy (HRTEM) image and the energy-dispersive X-ray spectroscopy (EDS) compositional mapping as shown in Figure 2. The total thickness of the trilayer was confirmed to be ~12 nm, which was similar to the target thickness, as shown in Figure 2a Figure 2c. To confirm the tri-layer more clearly, X-ray intensities of the line profiles are presented in Figure 2d, which confirmed the presence of HfO2/Al2O3/HfO2 tri-layer without any significant diffusion of In and Sn, which is due to low temperature (150 °C) ALD technique. In addition, from HRTEM and EDS analysis the inter-diffusion at Al2O3/HfO2 can be seen by the presence of hump, which can produce HfAlO, as presented in Figure 2d. According to Lan et al., intrinsic trap sites related to oxygen vacancies can be created due to the inter-diffusion between HfO2 and Al2O3 [30]. This inter-diffusion creates oxygen vacancies due to the presence of HfAlO in thin gate stacks. At both interface regions, the trivalent Al into HfO2 distributes intrinsic oxygen vacancies (VO), which control the gradual resistive switching in the RRAM device discussed in the next section. Figure 3a shows the forming and first RESET characteristics of the TaN/HfO2/Al2O3/HfO2/ITO trilayer RRAM device. The initial forming process shows the similar behavior of multiple devices for the I-V soft breakdown under negative voltage applied to the top TaN electrode where the initial current compliance was set to 10 −5 A. During the electroforming process, the conductive filament forms between two electrodes, which consist of mainly oxygen vacancies (VO). Gradual SET/RESET bipolar switching properties were found after the electroforming process at the SET current compliance of 10 −3 A. Gradual SET/RESET during resistive switching is the requirement for future synaptic device applications for neuromorphic computing [31]. During the SET process, VO regrowth inside both the interfacial layers of Al2O3/HfO2 helps gradual change occur in the current, as shown in Figure 2b. Here, both HfO2 layers act as a virtual electrode (as HfO2 contains more oxygen vacancy compared to Al2O3) and during the RESET process only interface filament is believed to rapture due to positive bias at the top electrode, which controls the resistive switching process stability and reliability [20]. A schematic resistive switching mechanism has been presented in Figure 4.

Results and Discussion
The cross-section of the TaN/HfO 2 /Al 2 O 3 /HfO 2 /ITO tri-layer RRAM structure was investigated by the high-resolution transmission electron microscopy (HRTEM) image and the energy-dispersive X-ray spectroscopy (EDS) compositional mapping as shown in Figure 2. The total thickness of the tri-layer was confirmed to be~12 nm, which was similar to the target thickness, as shown in Figure 2a,b. Different Ta, Hf, Al, O, In, and Sn element mapping confirms the presence of multilayer structures as shown in Figure 2c. To confirm the tri-layer more clearly, X-ray intensities of the line profiles are presented in Figure 2d, which confirmed the presence of HfO 2 /Al 2 O 3 /HfO 2 tri-layer without any significant diffusion of In and Sn, which is due to low temperature (150 • C) ALD technique. In addition, from HRTEM and EDS analysis the inter-diffusion at Al 2 O 3 /HfO 2 can be seen by the presence of hump, which can produce HfAlO, as presented in Figure 2d. According to Lan et al., intrinsic trap sites related to oxygen vacancies can be created due to the inter-diffusion between HfO 2 and Al 2 O 3 [30]. This inter-diffusion creates oxygen vacancies due to the presence of HfAlO in thin gate stacks. At both interface regions, the trivalent Al into HfO 2 distributes intrinsic oxygen vacancies (V O ), which control the gradual resistive switching in the RRAM device discussed in the next section. Figure 3a shows the forming and first RESET characteristics of the TaN/HfO 2 /Al 2 O 3 /HfO 2 /ITO tri-layer RRAM device. The initial forming process shows the similar behavior of multiple devices for the I-V soft breakdown under negative voltage applied to the top TaN electrode where the initial current compliance was set to 10 −5 A. During the electroforming process, the conductive filament forms between two electrodes, which consist of mainly oxygen vacancies (V O ). Gradual SET/RESET bipolar switching properties were found after the electroforming process at the SET current compliance of 10 −3 A. Gradual SET/RESET during resistive switching is the requirement for future synaptic device applications for neuromorphic computing [31]. During the SET process, V O regrowth inside both the interfacial layers of Al 2 O 3 /HfO 2 helps gradual change occur in the current, as shown in Figure 2b. Here, both HfO 2 layers act as a virtual electrode (as HfO 2 contains more oxygen vacancy compared to Al 2 O 3 ) and during the RESET process only interface filament is believed to rapture due to positive bias at the top electrode, which controls the resistive switching process stability and reliability [20]. A schematic resistive switching mechanism has been presented in Figure 4. After applying a negative bias at the electrode, oxygen vacancies are piled up mainly near the interface driven by the external electric field. The virtual electrode (formed during electroforming process) at both ends within HfO2 helps to regrow the CF. During RESET process, the CF is not fully ruptured, and VO recombines with oxygen ions gradually due to the applied positive bias at the top electrode, as shown in Figure 4. According to previous studies, asymmetric CF forms inside two different dielectrics. This further leads to formation of the weakest CF at the interface of two different dielectrics [14,32]. Hence, the redox reaction due to the migration of oxygen ions dominates the RESET process near the interface of two dielectrics, as described, with the switching mechanism in Figure 4 [20,32].The endurance characteristics were obtained for the tri-layer RRAM device with up to 1300 cycles with Ion/Ioff ratio > 10, as shown in Figure 3c, read at 0.1 V. Initial variation of low resistance state (LRS) and high resistance state (HRS) can be due to the large area of the top electrode, where a large number of conductive filaments are created during the forming process. Data retention at a read voltage of 0.1 V, for LRS and HRS, was recorded up to 10 4 s without any significant variation, as shown in Figure 3d. To understand the performance of the proposed tri-layer RRAM device, a detailed comparison of electrical parameters is presented in Table 1.      After applying a negative bias at the electrode, oxygen vacancies are piled up mainly near the interface driven by the external electric field. The virtual electrode (formed during electroforming process) at both ends within HfO 2 helps to regrow the CF. During RESET process, the CF is not fully ruptured, and V O recombines with oxygen ions gradually due to the applied positive bias at the top electrode, as shown in Figure 4. According to previous studies, asymmetric CF forms inside two different dielectrics. This further leads to formation of the weakest CF at the interface of two different dielectrics [14,32]. Hence, the redox reaction due to the migration of oxygen ions dominates the RESET process near the interface of two dielectrics, as described, with the switching mechanism in Figure 4 [20,32].The endurance characteristics were obtained for the tri-layer RRAM device with up to 1300 cycles with I on /I off ratio > 10, as shown in Figure 3c, read at 0.1 V. Initial variation of low resistance state (LRS) and high resistance state (HRS) can be due to the large area of the top electrode, where a large number of conductive filaments are created during the forming process. Data retention at a read voltage of 0.1 V, for LRS and HRS, was recorded up to 10 4 s without any significant variation, as shown in Figure 3d. To understand the performance of the proposed tri-layer RRAM device, a detailed comparison of electrical parameters is presented in Table 1.
Multi-level resistance states depending on the SET current compliance (I cc ) were investigated to find the influence of increasing oxygen vacancy concentration inside conducting filaments (CF) [36][37][38]. Figure 5a shows the resistive switching characteristics with variable I cc during the SET process from 100 µA to 1 mA. From low to high I cc , the resistance of the device continuously decreased as shown in Figure 5a. For the endurance test of each LRS, 30 cycles of each resistance state were monitored. As evident from Figure 5b, HRS remains almost constant and multiple LRS was found to be with 10.54 kΩ, 6.08 kΩ, 3.57 kΩ, 2.13 kΩ, 1.01 kΩ at the read voltage of 0.1 V. The decrease in resistance can be explained by the increased width of CF inside tri-layer dielectric films, concerning the continuous enhancement of V O [36]. The stability of different resistance states was confirmed by the retention test, as shown in Figure 5c, where different LRS maintained up to 10 3 s without any significant variation. Modulation of a multi-level conduction state is a very essential aspect for the synaptic device to realize the high-density memory storage. To achieve multilevel memory states, gradual RESET Nanomaterials 2020, 10, 2069 6 of 12 has been controlled by V reset-stop , as shown in Figure 5d. A similar approach has been shown in recent works on bilayer and tri-layer RRAM structures to obtain multi-state resistance by controlling V reset-stop [21,33,39,40]. Here, the positive RESET voltage was slowly increased to get a gradual RESET process in the HfO 2 /Al 2 O 3 /HfO 2 tri-layer RRAM device, which leads to multiple HRS. Along with gradual conductance change under DC voltage, modulation of resistance was also studied by a sequential paired pulse. Analogous to the bio-synapse, suitable paired pulse application can change the resistance of the RRAM devices with a short interval of time [41][42][43]. So, it is believed that synapse response is higher at the second pulse if a paired pulse is applied to the synapse [44]. Figure 6a,b show the pair-pulse fluctuation (PPF) and paired-pulse depression (PPD) characteristics for TaN/HfO2/Al2O3/HfO2/ITO memristor device. PPF and PPD responses are monitored after implementing paired pulse of −0.8 V/5 ms and +1.2 V/5 ms, respectively, with an interval of 10 ms, which is the short-term change in synaptic weight. In the case of PPF, an increase in current can be noticed at the response of the second pulse compared to the first pulse, which indicates the generation of VO. In the case of PPD, the opposite phenomenon took place, where the pulse current was found  Figure 5f shows the retention properties up to 10 3 s for distinct LRS and six HRS states for testing the reliability of multi-state resistance. From this above experiment, it was confirmed that the HfO 2 /Al 2 O 3 /HfO 2 tri-layer is suitable for high storage and multi-level analog RRAM applications.
Along with gradual conductance change under DC voltage, modulation of resistance was also studied by a sequential paired pulse. Analogous to the bio-synapse, suitable paired pulse application can change the resistance of the RRAM devices with a short interval of time [41][42][43]. So, it is believed that synapse response is higher at the second pulse if a paired pulse is applied to the synapse [44]. Figure 6a,b show the pair-pulse fluctuation (PPF) and paired-pulse depression (PPD) characteristics for TaN/HfO 2 /Al 2 O 3 /HfO 2 /ITO memristor device. PPF and PPD responses are monitored after implementing paired pulse of −0.8 V/5 ms and +1.2 V/5 ms, respectively, with an interval of 10 ms, which is the short-term change in synaptic weight. In the case of PPF, an increase in current can be noticed at the response of the second pulse compared to the first pulse, which indicates the generation of V O . In the case of PPD, the opposite phenomenon took place, where the pulse current was found to be reduced due to recombination of oxygen vacancies and oxygen ions. Therefore, this result indicates that tri-layer RRAM can simulate the bio-synapse in real-time signals. Calculated average PPF and PPD was calculated to be~10.2% and~7.5%, respectively, from the equation, PPF = (I 2 − I 1 )/I 1 × 100% (1) where I 1 and I 2 are the final currents recorded at each paired pulse [45]. Adjustable gradual conductance increase and decrease (potentiation/depression) are very essential for electronic synapse and have been studied in this section. The tri-layer RRAM device exhibits a gradual change in conductance after applying a constant amplitude pulse train of −0.8 V/100 µ s and +1.0 V/100 µ s for potentiation and depression, respectively, as shown in Figure 6c. A consecutive 50 cycles of negative pulses and 50 cycles of positive pulses were applied to achieve gradual conductance change, which is consistent with the DC gradual switching behavior discussed Adjustable gradual conductance increase and decrease (potentiation/depression) are very essential for electronic synapse and have been studied in this section. The tri-layer RRAM device exhibits a gradual change in conductance after applying a constant amplitude pulse train of −0.8 V/100 µs and +1.0 V/100 µs for potentiation and depression, respectively, as shown in Figure 6c. A consecutive 50 cycles of negative pulses and 50 cycles of positive pulses were applied to achieve gradual conductance change, which is consistent with the DC gradual switching behavior discussed before. These properties indicate the synaptic plasticity in response to the pulse train, similar to the long-term potentiation (LTP), and long-term depression (LTD) in biological synapse [46]. This conductance change phenomenon is assumed to be dominated by the separation and recombination of oxygen vacancies (V O ) and oxygen ions near both interfaces of HfO 2 /Al 2 O 3 after applying different polarity pulse at the top electrode. During RESET, oxygen vacancies and oxygen ions recombine slowly and reduce the filament width at the weak filaments formed at both HfO 2 /Al 2 O 3 due to the application of positive pulse train [25]. During the application of the negative pulse train, again gradually the weak filaments formed at both interfaces. To implement more accurately synaptic efficiency, of the tri-layer TaN/HfO 2 /Al 2 O 3 /HfO 2 /ITO memristor, negative pulse, and positive depression pulse with increasing amplitude were applied for potentiation and depression characteristics [47][48][49]. For potentiation and depression, increasing the pulse amplitude from −0.6 to −1.4 V, with a −0.05 V step, and 0.8 to 1.6 V, with a 0.05 V step, respectively, was applied to each tri-layer RRAM device. The applied pulse sequence is presented in Figure 6c, with a read voltage of 0.1 V. A clear gradual increase/decrease in conductance was observed during the long-term potentiation (LTP) and long-term depression (LTD) process, which is similar to the synaptic change in the biological synapse. The change in conductance was measured by the peak current obtained from the read pulse of 0.1 V as shown in Figure 6d. An almost gradual increase in conductance and a gradual decrease in conductance was observed during all 8 cycles. During potentiation at increasing negative pulse voltage to the top electrode oxygen ions were depleted mainly from the HfO 2 /Al 2 O 3 interface and created weak CF. The opposite phenomenon occurs with positive increasing pulse voltage at the top electrode and gradually the CF at the HfO 2 /Al 2 O 3 interface becomes narrow. Successful implementation of eight cycles of potentiation and depression is presented, which proves the reliability of the synaptic property of the tri-layer device. Slightly increasing conductance behavior at LTP can be due to the occurrence of new V O creation at the HfO 2 /Al 2 O 3 interface.
To emulate the Hebbian learning of spiking neural networks (SNN), we focus on mimicking the spike-timing-dependent plasticity (STDP) learning rule in the tri-layer TaN/HfO 2 /Al 2 O 3 /HfO 2 /ITO memristor, having been employed to simulate synapse functionality [50]. This learning rule depends on the relative time difference (∆t) of a set of spikes related to the pre-synaptic and post-synaptic neurons [51]. Design of the pre-spike, post-spike and consequent effective pulse applied to the synapse for time-division multiplexing (TDM) approach are shown in Figure 6e. Using this pulse sequence, the obtained STDP characteristics for synaptic learning rules in the tri-layer RRAM device was employed using the TDM approach, as shown in Figure 6d  Here, it is considered that when the pre-spike precedes the post-spike (∆t > 0), potentiation occurs, and in the opposite case when the post-spike precedes the pre-spike (∆t < 0) the device is depressed. Positive synaptic weight change is observed in the IInd quadrant when ∆t increases from 0 to −100 µs, which indicates synaptic potentiation, and negative synaptic weight change observed in the IVth quadrant when ∆t varies from 0 to 100 µs is described as synaptic depression. The change of synaptic weight can be described as follows: where A + and Aare the synapse maximum weights at ∆t = 0, and τ + and τare the broadening of the STDP window [48]. From Figure 6d, it can be found that the maximum change in synaptic weight [∆W(∆G/G) = (G fin − G min )/G min ] for positive ∆t was 98.3% and for negative ∆t was −94.8%, which is Nanomaterials 2020, 10, 2069 9 of 12 very symmetric for tri-layer RRAM device. These results further confirm the superiority of the stable and reliable synaptic characteristics of the TaN/HfO 2 /Al 2 O 3 /HfO 2 /ITO memristor device.

Conclusions
In summary, bipolar resistive switching behavior and synaptic properties of ALD deposited TaN/HfO 2 /Al 2 O 3 /HfO 2 /ITO memristor were studied in detail. Bipolar resistive switching with a gradual change in conductance was confirmed by DC I-V and pulse application. Uniform gradual resistive switching can be attributed to the formation of HfAlO in the interface of HfO 2 /Al 2 O 3 in the tri-layer dielectric stack, where more than 10 3 cycles of endurance and 10 4 s of retention were achieved. Through applying different pulse sequences, short-term plasticity and symmetrical long-term plasticity were studied by PPF and potentiation/depression. Successful STDP behavior was achieved using the TDM method with conductance change from −94.8% to 98.3%. The above results for tri-layer RRAM predict a promising nonvolatile memory based synaptic device for the next generation. Although optimization of deposition parameters and thickness of the dielectric stacks are needed in the future.
Author Contributions: C.M. designed the experiment concept, conducted the electrical measurements, wrote the manuscript; M.K. writing-review and editing, funding acquisition, investigation; S.K. designed the experiment concept, writing-review and editing, and supervised the study. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:
The authors declare that they have no competing interest.