TiO x based RRAM

For TiOx–based RRAM study in this section, we discuss three parts, including resistive switching character, conduction mechanism, and the interface contribution.

Figure 4-13 shows the schematic core-section of TiOx-based RRAM. The titanium oxide film is placed on the top of the 0.18 um diameter W-plug, and this W-plug is also called contact (CT) in the semiconductor fabrication. This CT is the connection between the memory cell and the source site, and the memory state is dependent on the resistance of titanium oxide.

Figure 4-13. Cross-sectional view of TiOx-based RRAM.

Figure 4-14. The bipolar resistive switching characterics of TiOx film.

First, the electric character exhibits the bipolar resistive switching character, which is shown in figure 4-14. The positive switching voltage is about +5 V and the negatvie switching voltage is about -2 V. This asymmetrical resistive switching behavior is different with NiOx film. Park et al.^[87] also reported such an asymmetrical resistivite switching character in TiOx film.

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Figure 4-15. The resistive switching phenomenon from high resistance state to low resistance state.

Figure 4-15 shows the resistive switching phenomenon from HRS to LRS with linear scale of current. We can find a sudden rise at +5.5V in the rising curve and the CF is formed at the same time.

Figure 4-16. Voltage dependent on/off ratio.

Figure 4-16 shows the relationship between the applied voltage and on/off ratio. In this figure, we found the maximum on/off ratio is 20 at the applied voltage about 4 V.

Also, we define this applied voltage as the transition voltage (VT) and we can use this

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parameter to observe the electric character. The reading voltage here must be below the VT to avoid influencing the resistance stste.

Figure 4-17 shows resistive switching characters of TiOx at various thicknesses.

The electric characterstics indicate that these resistance switching behaviors are in thickness-indepentent relationship. All three I-V curves show similar resistive switching behavior and the resistance of these samples also shows thickness independent.

Figure 4-17. The resistive switching character of TiOx film at various thicknesses.

Moreover, both on/off ratio and VT are independent with sample thicknessas shown in figure 4-18. These results indicate the interface contribution of this TiOx material.

Figure 4-18. The thickness relationshop with transition voltage and the on/off ratio.

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The curve fitting shown in figure 4-19 shows that all TiOx films obey the Schottky emission mechanism. According to this result, we got one more proof of interface contribution in our TiOx-based RRAM.

Figure 4-19. The curving fitting of I-V data for TiOx atvarious thicknesses.

Figure 4-20. The temperature dependent electric characteristics of TiOx film.

Figure 4-20 shows the temperature-dependent electric characteristics. In addition, according to the calculation of Schottky emission curve, we know the barrier high of HRS and LRS are about 0.6 eV and 0.73 eV, respectively.

In order to check this interface contribution, we prepared a hybrid sample with a 40 Å SiO2 layer between TiOx and bottom electrode. This hybrid sample (TiOx-SiO2) shows great improvement in the data retention test, and it also improves the on/off ratio.

These improvements are shown in figure 4-21. In this figure, the LRS and HRS almost overlap, and the on/off ratio almost disappears after 100-sec retention test for TiOx film.

However, the on/off ratio always keeps at about 20 times for the hybrid sample.

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Figure 4-21. Data retention of TiOx-based RRAM with/without SiO2 hybrid system.

Figure 4-22 shows the cycle endurance test of TiOx-based RRAM. In this figure, the resistance increases in the cycling test. It is attributed to the damage of the TiOx film and this damage induces the rising of resistance. This result is similar to the NiOx film.

After 50 times of cycling operation, the LRS is close to the initial of HRS.

Figure 4-22. Cycle endurance test of TiOx-based RRAM.

In hybrid system, the cycle endurance performance is also improved. Figure 4-23 shows the cycle endurance test of TiOx-based RRAM with SiO2 hybrid system. It is clear to see both LRS and HRS keep their resistance in the cycling operation test. This figure shows above 1000 times of cycle endurance performance of our hybrid system, and the on/off ration keeps at about 100X in this test. This hybrid system experiment also indicates the interface contribution of TiOx RRAM.

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Figure 4-23. Cycle endurance test of TiOx-based RRAM with SiO2 hybrid system.

Figure 4-24 shows the read disturb test of the hybrid system. The reading voltage is only 50 mV to avoid damaging the memory cell. This figure indicates over 500 times read disturb performance of our hybrid system. Both LRS and HRS keep their resistance state, and the on/off ratio almost keeps at about 200X in this test. In this figure, the HRS decreases slowly in the read disturb test. It indicates that small reading voltage influences the HRS.

Figure 4-24. Read disturb test of TiOx-based RRAM with SiO2 hybrid system.

In summary, the TiOx exhibits asymmetrical bipolar resistive switching character, and it shows interface contribution. The conduction mechanism follows the Schottky emission and the resistive switching character shows barrier high dependence relationship. The barrier high of LRA and HRS are about 0.6 and 0.73 eV, respectively.

Moreover, the TiOx-SiO2 hybrid system also indicates the interface contribution, which improves the data retention cycle endurance and read disturb performance.

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