Figure 1a shows a schematic of the Pt/TiO2/HfAlOx/TiN device. XPS analysis was performed as the X-ray was incident from the TiO2 surface, and analysis was conducted on the inner thin film through Ar+ etching. Figures 1b and c show the Ti 2p spectra for etch level 2 and etch level 35, which correspond to the TiO2 layer and TiN bottom electrode, respectively. The Ti 2p XPS spectra include Ti 2p3/2 and Ti2p1/2, as shown in Figs. 1b and c. The peak binding energies of Ti 2p3/2 and Ti 2p1/2 are located at approximately 458.94 eV and 464.5 eV for Ti-O bonds . However, the peak binding energies of Ti 2p3/2 and Ti 2p1/2 are found at approximately 454.92 eV and 460.93 eV for Ti-N bonds . Figure 1d shows the Hf 4f spectra at an etch level of 9. Hf 4f7/2 and Hf 4f5/2 doublet peaks are found at 18.63 eV and 20.03 eV, respectively, for Hf-O bonds in the Hf-dominated HfAlOx film. Figure 1e shows the Al 2p spectra at an etch level of 9. Because the Al content is low in the HfAlOx film, the Al intensity is low and some noise is observed, but a distinct peak is observed for the Al-O bond at approximately 75.4 eV .
Figure 2a shows the I-V current characteristics, including 200 cycles of the Pt/TiO2/HfAlOx/TiN device. The set process occurs by sweeping from 0 V to -0.7 V. An abrupt set transition is observed from the HRS to LRS in which the decrease in resistance is due to soft breakdown in the two insulator layers. Here, a compliance current (CC) of 1 mA was used to tightly control the conducting filament. Moreover, the oxygen vacancies in the double insulating layers were created under the applied bias. It should be noted that the negative bias on the Pt/TiO2/HfAlOx/TiN device is relatively more favorable for resistive switching because the TiON interfacial layer acts as an oxygen reservoir. The oxygen can easily move to the TiN layer to form TiON when a negative bias is applied on Pt. The detailed mechanisms are discussed in previous studies . The reset process is conducted to return the device to the HRS by sweeping from 0 V to 1 V. The reset process occurs because of the rupture of the conducting filament based on oxygen vacancies. The oxygen ions move from the TiON layer under a positive bias, and the recombination occurs with oxygen vacancies. Subsequently, the conducting filament can be ruptured, which indicates a decrease in conductance. On the other hand, resistive switching is not fluent under the opposite polarity bias (not shown here) because Pt, which is an inert metal, is less reactive toward oxygen than TiN. Figure 2b shows up to 200 cycles in which LRS and HRS possess a read voltage of 0.3 V. Uniform LRS and HRS are achieved even though a small on/off ratio is observed. To increase the on/off ratio, the CC and reset stop voltages can be increased. The LRS current increases with an increase in CC, and the HRS current decreases with an increase in the reset stop voltage. An on/off ratio of 10 times or more was achieved by a CC of 10 mA and reset sweep voltage of 2 V for 200 endurance cycles, as shown in Fig. 2c. Good retention properties in the LRS and HRS were observed for 1 h (Fig. 2d).
Next, we demonstrate the fine modulation of the LRS and HRS by varying the reset stop voltage. Figures 3a–c show the I-V characteristics of the Pt/TiO2/HfAlOx/TiN device with reset stop voltages of 1.1 V, 1.2 V, and 1.3 V, respectively. Stable and uniform switching is observed for 21 cycles in each case. It is noted that the LRS and HRS currents decrease with the reset stop voltage. The higher the voltage, the more the conducting filament ruptures, and thus, the HRS current decreases. It is also observed that the current gradually decreases as the reset stop voltage increases in the DC sweep mode, as shown in Fig. 3e. However, MLC can be achieved by CC for the set process, which is not desirable for device control.
To overcome the limitation of the abrupt set process, we used different current regions in self-compliance for the gradual set process. Figure 4a shows the self-compliance curve after the CC controlled I-V curves of the Pt/TiO2/HfAlOx/TiN device. The self-compliance region is clearly observed on the linear scale in Fig. 4b. The current gradually increases with the voltage in the self-compliance region after an abrupt transition from 432.7 µA to 2 mA at -0.55 V. This gradual transition region is suitable for achieving MLC. The self-compliance behavior during set transition due to the TiON layer acts as a series resistance . Furthermore, the gradual reset switching is maintained after the self-compliance set process.
Next, the potentiation and depression characteristics were mimicked to apply the Pt/TiO2/HfAlOx/TiN device to a neuromorphic system. For potentiation, repetitive pulses with a voltage of -0.8 V and a width of 1 ms were used, as shown in Fig. 5. A low read pulse voltage (0.3 V) was used to not affect the conductance change during the reading. It should be noted that for gradual conductance modulation in the self-compliance region. We demonstrated 10 cycles of long-term potentiation and long-term depression and found no significant degradation of the MLC.