Compare different top electrodes

The devices of the oxidation temperature at 400^oC, 60 min, were investigated that provide higher yield and endurance.

3.2.1 Relationship of ratio and voltage

The on/off ratio and voltage bias relationship is showed in Fig. 3.24. The on/off ratio is lager at small bias. Due to this reason and small read bias more damage the memory state, the read bias magnitude is setting 0.2V. The reason of the read bias taken 0.2V is 0.2V is ohmic conduction region by current fitting as show in Fig. 3.18, 3.19, 3.21, 3.22.

3.2.2 Operation mode and polarity

After forming process, the memory device remains at ON-state. Then, we need a voltage to switch the memory device back to the OFF-state and this is what we call turn-off process. If the memory is switched back to the ON-state, this is a turn-on process. In ours research, Pt/CuO/Pt devices belongs to nonpolar switching, and the I-V curve is symmetrical as show in Fig. 3.10-13. On the other hand, even though

Ti/CuO/Pt devices belong to nonpolar switching as show in Fig. 3.6-9, the best mode is located at positive turn-on and negative turn-off mode. From now on, in this thesis, we only sweep for this best mode to investigate the electrical properties. The range of the operation voltage is located at 2V ~ -1.5V, but depicts a larger variation. In order to further confine the resistive switching phenomenon can occur without electrode of active metal, we measure CuO thin film directly by W-probe. The result of measure also show nonploar switching as show in Fig. 3.14-16. Fig. 3.17 shows that compare with Ti top electrode and W-probe, the W-probe can avoid the interface effect and small area of diameter ~ 5um.

3.2.3 Current fitting

Fig. 3.18, 3.19, 3.21 and 3.22 show that the ON-state is ohmic conduction at all

electrodes in both turn-on and turn-off process. It can be noticed that the current has a linear relationship with voltage of ON-state, which indicates that ohmic conduction is dominant in ON-state. It is suggested that conductive filament paths are formed as a bridge between top electrode and bottom electrode after the turn-on process.

Frenkel-poole emission model is extensively used investigate carrier transport behavior in insulator and semiconductor, which describe in section 1.3.4.

Fig. 3.20 and 3.23 show the logarithmic plot of I versus V^1/2 of OFF-state. The experiments data obeys a good linear relationship at high electrical field region. From the ln (I/V)  V^1/2 relation and Frenkel–Poole constant α calculation, carrier trapping and de-trapping of Frenkel–Poole effect is thought of as the main conduction mechanism of OFF-state. Fig. 3.20 shows that I-V curves were well fitted by the formula of Poole–Frenkel emission model with Pt top electrode. Fig. 3.23 shows that I-V curves were well fitted by the formula of Poole–Frenkel emission model with Ti top electrode.

The Pt is deep work function material and Ti is shallow work function. CuO is p-type semiconductor. In generous, the work function is 5.3eV and band gap is about 1.4eV at 400^oC thermal oxidation. A deep work function metal can give an Ohmic contact to a p-type semiconductor, while a Schottky barrier is formed at the interface between a shallow work function metal and a p-type semiconductor. Pt is deep work function which can is formed ohmic contact to a CuO film, but Ti is shallow work function which can give a Schottky barrier contact to a CuO film.

3.2.4 Size effect

Fig. 3.25-28 show I-V curve of Ti/CuO/Pt structure with top electrode area 10um*10um, and diameter 150um, 250um, and 350um. For constant current

compliance value, the ON-state current is independent size, but the OFF-state current is increased with the size of top electrode. The OFF-state is homogenous current conduction, but the ON-state is the filament of inhomogeneous current conduction.

3.2.5 Endurance

The tests of endurance of Ti/CuO/Pt devices are shown in Fig. 3.31-32, and endurance of Pt/CuO/Pt devices are shown in Fig. 3.33, and endurance of W-probe/CuO/Pt devices are shown in Fig. 3.34. The Fig. 3.31 show the Ti/ CuO/Pt devices of turn-on voltage and turn off voltage at different switching cycles, while the Fig. 3.32 show ON-state and OFF-state current at different switching cycles. It is found that the Ti/CuO/Pt devices can be switched over than 500 cycles under the operation of DC sweeps, and turn-on voltage variance are large than turn-off voltage.

The turn-on process is a random process. Because the DC sweeps produce more stress on the semiconductor devices than pulse switching, the Ti/CuO/Pt device expect to have more than 500 cycles with pulse operation.

3.2.5 Retention

Fig. 3.35 depict the retention of Ti/CuO/Pt devices examination. Both results show an excellent retention property of the device, in which the data of ON or OFF-states are retained after 10⁵ at room temperature.

3.2.5 Stress

The stress test of Ti/CuO/Pt devices displays that both ON and OFF state are not modified after stressed under 0.2 V for 6500 s, as shown in Fig. 3.36.

The stress test of Pt/CuO/Pt devices displays that both ON and OFF state are not modified after stressed under 0.2 V for 700 s, as shown in Fig. 3.37.

The stress test of Ni/CuO/Pt devices displays that both ON and OFF state are not modified after stressed under 0.2 V for 12000 s, as shown in Fig. 3.38.