Structure of SiOx based devices
Figure 1a schematically illustrates the cross-bar array architectures of the SiOx and SiOx@TiO2 NPs resistive switching devices. And, to observe the cross-sectional information of both devices, TEM is measured as shown in Fig. 1b. The TEM image of the SiOx device shows that the ITO/SiOx/ITO structure is sequentially stacked, and the interface of TE/SiOx is clearly formed. In contrast, the SiOx@TiO2 NPs device shows the slightly rough interface of TE/SiOx@TiO2 NPs, which is related to the insertion of TiO2 NPs profoundly affecting the roughness of SiOx@TiO2 NPs. To examine the composition of the SiOx and SiOx@TiO2 NPs device structures, ToF–SIMS was measured from top to bottom electrode during O ion sputtering with 2 keV. Figure 1c shows that the spectra of ToF–SIMS can be divided into three regions; the first region is only ITO (top electrode), the second region is the SiOx or SiOx@TiO2 NPs layer, and the last region is ITO (bottom electrode). For the SiOx switching device, Si+ is increased in the second region, while In+ and Sn+ are drastically decreased. O+ is continuously detected in all regions, because oxygen is included in all layers. Ti+ is not detected in the first and second regions. In the third region, Ti+ is found due to the glass substrate, thus it can be negligible32. In the case of the SiOx@TiO2 NPs device, the behaviors of Si+, In+, Sn+, and O+ are almost similar to those of the SiOx device. However, a considerable quantity of Ti+ is detected in the second region, and we can recognize that the TiO2 NPs is well inserted into the SiOx matrix.
Analysis of the chemical bonding states
Figure 2 shows the compositions and chemical bonding states of the SiOx and SiOx@TiO2 NPs films by using XPS measurement. Both films are composed with O, Si, and a small amount of C, and Ti is included to ~ 1.5% in the SiOx@TiO2 NPs layer, as shown in Fig. 2a. To elucidate the chemical bonding states, the core-level spectra of Si 2p and O 1 s were normalized, and deconvoluted into Gaussian peaks. In the case of O 1 s, it is composed with three Gaussian peaks according to Si–O bonds (O1s), oxygen deficient states (O2), and hydroxyl groups (O3), as shown in Fig. 2b and c26,33. The prepared SiOx and SiOx@TiO2 NPs films have a lot more O2 and O3 states than does the conventional SiO2 film. Generally, this is related to the solution processed SiOx obtaining a large amount of defect states, such as oxygen vacancies or OH groups, which affects the stoichiometry of the SiOx (x < 2)26,28. In the Si 2p spectra, the regular SiO2 (Si4+) and oxygen deficient SiO2-x (Si3+) are indicated, as shown in Fig. 2b and c34,35. The prepared SiOx films synthesized by solution process show higher composition of oxygen vacancies than the SiO2 synthesized by vacuum process (thermal oxidation or chemical vapor deposition). In general, the amount of oxygen vacancies is expected to change due to the difference in bond dissociation energy in the TiO2 NPs inserted SiOx system27,36. However, in our system, the chemical bonding states are almost similar, due to the small amount of TiO2 NPs in SiOx matrix. Therefore, the change of the chemical bonding states of SiOx is imperceptible.
Resistive switching characteristics
Figure 3 shows the resistive switching performance of the SiOx and SiOx@TiO2 NPs devices. First, the voltage is swept to transit from the pristine state to LRS with the compliance current of 0.1 mA. Both devices show bipolar resistive switching behavior with SET process, which is obtained by sweeping the negative voltage, while the RESET process is obtained by sweeping the positive voltage. These processes can be reversibly changed by controlling the voltage. In detail, the SiOx device switches from HRS to LRS (SET process) at − 1.7 V, while the device switches continuously from LRS to HRS (RESET process) by applying voltage to 2.0 V. For the SiOx@TiO2 NPs device, the SET process occurs at − 1.1 V, which is smaller than the SET voltage of the SiOx device. In addition, different from the SiOx device, the RESET process is obtained twice over; the first RESET process appears at 0.7 V, then the second RESET process with slight resistance change is obtained while increasing the voltage to 2.0 V. The switching speed is obtained that the RSs change under the pulse width of 3 us in SiOx device. In case of SiOx@TiO2 NPs device, the RSs transit with the pulse width of 200 ns, as shown in Fig. 3a and b. To evaluate the reliability of devices, the retention and endurance were conducted for SiOx and SiOx@TiO2 NPs. The retention test was performed by probing each LRS and HRS for 103 s at room temperature, and reading the current at 0.1 V. The SiOx device maintains the LRS/HRS ratio of approximately 20 for 103 s. For the SiOx@TiO2 NPs device, three well-defined RSs (LRS, HRS1, and HRS2) are maintained for 103. Also, the dotted lines indicate the expectation of the lifetime of two devices. The SiOx device shows the expectation of the lifetime almost 103 s, otherwise the SiOx@TiO2 NPs device is expected to the lifetime above 104 s. To examine the endurance performance, the SET/RESET cycling test was conducted for 102 times, and the current level was recorded at 0.1 V. Both devices show a stable LRS/HRS ratio for 102 cycles. In particular, the SiOx@TiO2 NPs exhibits the multiple RSs for 102 cycles. Moreover, the endurance of SiOx@TiO2 NPs is evaluated for 104 cycles under the pulse width of 200 ns. The device shows the stable operation during 104 cycles. Long retention time and stable endurance indicate the high reliability of the solution processed SiOx-based resistive switching devices. The distribution of SET and RESET voltages are measured for 102 times, and the current level was recorded at 0.1 V for examining the cycle-to-cycle variability of devices. Both devices show a stable LRS/HRS ratio for 102 cycles. Especially, the SiOx@TiO2 NPs exhibits the multiple RSs for 102 cycles, and the stable resistive switching occurs in the SiOx@TiO2 NPs device, compared to SiOx device. The stable endurance, retention, cycle-to-cycle variability performance indicate the high reliability of the solution processed SiOx-based resistive switching devices. From the resistive switching characteristics, it is concluded that the SiOx@TiO2 NPs device can be expected to high performance and low-power non-volatile memory due to a lower operation voltage, higher on/off ratio, and fast switching speed23. Moreover, due to multi-level switching, the SiOx@TiO2 NPs device is also applicable to multi-level memory.
Resistive switching mechanism
To discover the origin of the enhancement of performance in the SiOx@TiO2 NPs device, we clarified the resistive switching mechanism of the SiOx and SiOx@TiO2 NPs devices. The I–V curves are re-plotted as log I–log V, as shown in Fig. 4a and b. In the SET process of the SiOx resistive switching device, the I–V curve of the HRS exhibits trap-controlled space charge limited current (SCLC) conduction, which is composed of three parts: the Ohmic region (I ∝ V), the Child’s law region (I ∝ V2), and the steeply increasing region (I ∝ Vn, n > 2)37. The oxygen vacancies in the SiOx matrix serve as an electron trap, and form the conductive filament. Thus, the migration of oxygen vacancies is an important role in the deviation of slopes. In the high-voltage region, all traps are filled with electrons, and excessive electrons flow through the conduction band of SiOx (achievement of the SET process). The I–V curve of the LRS shows a linear Ohmic behavior with a slope of 1.07. Likewise, the RESET process is also in good agreement with the trap-controlled SCLC mechanism in HRS. In the SiOx@TiO2 NPs, the resistive switching mechanism is similar to that of the SiOx device, as shown in Fig. 4b. The electrons are transported according to traps, such as oxygen vacancies, into the SiOx matrix, as well as TiO2 NPs, and with the application of high voltage, then flow into the conduction band of SiOx and TiO2. This behavior is associated with the bulk-controlled mechanism, such as the conductive filament model based on oxygen vacancy. As a result, the resistive switching mechanism of the SiOx and SiOx@TiO2 NPs devices based on the conductive path can be dominated by valence change memory. The traps are a key factor to form the conductive paths into oxide matrix.
Furthermore, to predict the difference in conduction mechanism based on the oxygen vacancies (VO) defects of the SiOx and SiOx@TiO2 NPs devices, DFT calculations for the defects in each oxide were conducted, as shown in Fig. 5. By considering stable crystal structure, α phase of SiO2 and the anatase phase of TiO2 were chosen for the calculations. Figure 5a and b show the Fermi level-dependent formation energies of the VO defects (VOn, where n = (− 2, − 1, 0, + 1, and + 2)) and the band structures containing the energy levels for each charging state of oxygen vacancies in SiO2 and TiO2, respectively. In addition, in the case of the VO in SiO2, the + 2 is the stable charging state for the energy range (0.0–3.2) eV, 0 is stable for the range (3.2–6.6) eV, and − 2 becomes stable above 6.6 eV. For the TiO2, the + 2 charging state is found to be the most stable state over the entire range of bandgap. Considering the Fermi level of SiO2 and TiO2, which is measured in the valance band spectrum of XPS, the most stable charging states of VO are 0 and + 2 for SiO2 and TiO2, respectively. Since the formation energy of VO in TiO2 is smaller than that of the VO in SiO2, VO is more easily generated in TiO2 than in SiO2. Moreover, since the e-field induced migration of VO can occur for charged states, VO in TiO2 that has 2 + charging states can migrate with smaller e-field.
Figure 5c and d show the schematic energy band diagrams of the SiOx and SiOx@TiO2 NPs devices from the results of energy levels for stable charging states. Band alignments are estimated based on the valance band spectrum in XPS. For both the SiO2 and TiO2 systems, VO generates defect states in the bandgap, thus the resistance change of the RERAM device can occur through the generation (SET) and curing (RESET) of VO in oxide. However, considering the band alignment, the activation energy from the Fermi level to defect states is smaller in TiO2 than in SiO2. In addition, the barrier for carrier injection from ITO electrode is lower in TiO2 with VO than in SiO2 with VO. Therefore, the resistance of LRS in TiO2 is lower than the resistance of LRS in SiO2.
To understand the formation/rupture of the conductive path based on the oxygen vacancies for the SiOx and SiOx@TiO2 NPs devices, Fig. 6 shows the stochastic circuit breaker (CB) simulation that was conducted. The simulation method was benchmarked to previous research, as reported by Brivio et al.38 In the SiOx resistive switching device, both the experimental and simulated results show good agreement, which is also exhibited by the bipolar resistive switching behavior. The relative relations between parameters for SiO2 and TiO2 in CBs were determined based on DFT calculation. Table 1 tabulates the parameters. Figure 6b shows that the resistances of CBs are initialized with two values of Rhigh,S and Rlow,S to simulate the insulating oxide and conducting oxide layer, such as SiO2 and oxygen-deficient SiO2-x, respectively. To emulate the oxygen-deficient SiOx matrix, most of the CBs are initialized with Rhigh,S, while the rest of the CBs are initialized with Rlow,S in the initial state. In this case, the ratio of Rhigh,S:Rlow,S is set to approximately 6:4, and the switching probabilities of Rhigh,S and Rlow,S depend on the electric field and temperature by Joule heating. On applying the negative volage, a few of the CBs are changed from Rhigh,S to Rlow,S in sequence from #1 to #2 in Fig. 6b. Then, the voltage is applied above the SET voltage (> − 1.7 V), almost all CBs abruptly transform to Rlow,S, and the device achieves the SET process, as shown in #3 of Fig. 6b. In contrast, when the positive voltage is swept to the RESET voltage, Rhigh,S is continuously increased, then the CB network finally reaches the RESET process (according to the blue arrows in sequence from #3 to #5 in Fig. 6b). This cycle of SET/RESET is reversibly obtained on sweeping the voltage. Likewise, the experimental I–V curve of the SiOx@TiO2 NPs device agrees well with the simulated result, as shown in Fig. 6c. In Fig. 6 d, the maps of CB network are composed with four values of CBs of Rlow,S, Rhigh,S, Rlow,T, and Rhigh,T, which indicate SiO2-x, SiO2, TiO2-x, and TiO2, respectively. Initially, almost all CBs with Rhigh,S and Rlow,S are randomly allocated in places. Based on the XPS analysis, the ratio of oxygen deficient Rlow,S is equalized to that of the SiOx device. The Rhigh,T and Rlow,T of TiO2 NPs are also randomly distributed with the proportion of about 5%, to mimic the TiO2 NPs inserted SiOx matrix. Similarly, the initial ratio of Rhigh,S:Rlow,S and Rhigh,T:Rlow,T is established to be about 6:4, respectively, as shown in Fig. 6d. On increasing the negative voltage, the CBs related to TiO2 NPs are more rapidly transited from Rhigh,T to Rlow,T than those of SiO2 under the applied voltage. And, when further voltage is applied to the SiOx@TiO2 NPs device, the CBs related to SiOx are also changed from Rhigh,S to Rlow,S, and the SET process is achieved according to the red arrows (in sequence from #1 to #3) in Fig. 6d. This is related to the TiO2 NPs assisting the construction of the conductive path in SiOx@TiO2 NPs, and causes lower SET voltages than that of the pristine SiOx device. Under the positive voltage sweeps, the CBs of TiO2 NPs are rapidly changed from Rlow,T to Rhigh,T, while the CBs of SiOx are slightly transited. Also, the first-RESET process can be achieved in sequence from #3 to #4, as shown in Fig. 6d. On further increasing the positive voltage, the Rhigh,S is increased, then the RS gradually reaches second-HRS (in sequence from #4 to #6 in Fig. 6d). The 2-step RESET processes can be obtained by controlling the RESET voltages.
In this study, the simulation method, which is simply expressed by the formation/rupture of conductive path with the stochastic CB model in the case of oxygen-deficient oxide matrix and nanoparticle-inserted oxide resistive memory devices, enables lower computational load for each CB network simulation than the conventional simulation methods. From the CB simulation, the conductive path based on the oxygen vacancies is stochastically examined under sweeping the external voltage. The difference in the switching of the SiOx and SiOx@TiO2 NPs devices, such as the multiple-RESET, lowering switching voltage, and increase of on/off ratio, can be obtained by inserting the TiO2 NPs.
In our case, the SET process is abrupt by applying the negative voltages, which is related to the electric field inducing the defect migration, and then causing an increase of the current. In contrast, the gradual RESET process is due to the conductive filament being gradually ruptured when the positive voltage is applied to the devices11,39. Also, the improvement of the LRS/HRS ratio is noteworthy, as shown in Fig. 3a and b. This can be correlated to the environment for conductive filament growth inside the RS layer being changed due to the insertion of TiO2 NPs40. By applying the positive voltage, the conductive filament can easily rupture due to the existence of TiO2 NPs inside the SiOx matrix, and carriers have difficulty in flowing inside the RS layer. Therefore, the current level of HRS for the SiOx@TiO2 NPs is lower than that of the SiOx device.