4-2.1.2 Constant Current Sampling (CCS) in Set Process
Figure 4-8 is the dispersive sampling time of voltage under constant current sampling (CCS). There are four different sampling current (Is). The initial state is OFF state which is high resistance. The voltage measurement initial shows larger, then set process occur, the voltage become smaller cause lower resistance. The switching is abrupt. The transition time depend on the sampling current as shown in figure 4-9. It shows the exponential
relationship, and the electrical chemical reaction rate is exponential relationship, too .Therefore, the set process could consider to electrical chemical reaction which is the same to CVS . In figure 4-8, the power we applied in set process reduced. It should be shown slowly resistive switching cause negative feedback. It implies there are other power sources. It is the charging of parasitic capacitor. There is a current overshot in set process, as shown in figure 4-10[94] and figure 4-11[95]. Figure 4-10 shows the
threshold switch occur the charge which the capacitor store charging. It make a current overshot and the current make high temperature which shown in figure 4-11, which could enhance the set process speed cause the set process maybe electric chemical reaction. For this reason, it shows abrupt switching in CCV in figure 4-8.
4-2.2 Reset Process Sampling
Following, the result of reset process sampling will be shown. The device were operated to ON state, then give a positive constant voltage (current) on top electrode, the current were be measured in time to know the resistive
switching behavior of the device.
4-2.2.1 Constant Voltage Sampling (CVS) in Reset Process
Figure 4-12 shows the current flow through the device under CVS. The current decrease with time going, it means the resistance value become larger.
And the reset process is slowly. Because the power applied on the device decrease, that make the reset process negative feedback. In figure 4-12, there are seven different sampling voltages from 0.9V to 1.2V. the current decrease fast become saturation below one hundred seconds while sampling voltage 1.2V. And the sampling voltage 0.9V needs more than one thousand seconds to be saturation.
Because the resistance transition is not abrupt, we define the transition time t50, the current become to half of initial current, that is , the resistance become twice. The transition time t50 related to sampling voltage (Vs) as shown in figure 4-13, the transition time and sampling voltage are exponential
relationship. it could be considered the set process is electric chemical reaction dominate. The same to set process.
We used some different method to do the sampling, which is shown in figure 4-14. It has the current compliances, the current should be larger than the current compliance initially. In figure4-14, there are sixteen curve, which are four sampling voltage (Vs), 0.85V (red solid line ), 0.9V (green long dash line), 0.95V (purple medium dash line) and 1V (blue short dash line). And four current compliances (Ic), 14mA, 14.5mA, 15mA and 15.5mA. First, it could be compare with four current compliances. No mater the sampling voltage, the high current compliance shows short transition time. It means the current is
important in reset process. In the research of measurement in different temperature, we know the power play a important role in reset process.
Therefore, the current could be considered the power source.
In figure 4-14, it could be found that the curve of same sampling voltage will be matched for a long time even though the current compliances are different, that mean the final state is the same. Like four red solid lines with different current compliance each line, when sampling reach three hundred second the final resistance are almost matched. The other three lines are, too.
And we could find that the larger sampling voltage make the final resistance larger. The resistance at t=300, Vs=0.95V> Vs=0.9> Vs=0.85. It means the final state is defined by the voltage only in enough time which is determined by reaction rate which could be enhanced by power (temperature). It could be found that the resistance at t=300, Vs=0.95V equal to Vs=1V, it means the critical Vs is 0.95V.
In this result, it is match with the DC sweep, the Vstop played the role is the same to Vs.
4-2.2.2 Constant Current Sampling (CCS) in Reset Process
Figure 4-15 shows the voltage measure at top electrode in constant current sampling. The voltage on the device is small initial cause V=IR, and the resistance is small in ON state. After the time going, the voltage abrupt
become larger, it means the resistance increase in a short time. It is different to the CVS in reset process. In this result, the answer could be find by the power applied, the power P=IV, in figure 4-15, the power increases in reset process, it make the positive feedback. The result is matched to the previous result, the
power applied on the device which could increase temperature and enhance the reaction rate.
It is easy to determine the transition time in this case, the time to the voltage abrupt point. Figure 4-16 shows the relationship of transition time and sampling current. The same to previous result, it shows exponential
relationship, it could be consider the reset process is electric chemical reaction.
Figure 4-17 shows the constant current sampling with different voltage compliances the voltage compliances are 1V, 1.2V, 1.4V, 1.6V and 1.8V. The result is the resistance of final state is different. Figure 4-18 shows the
resistance which read at 0.2V after each constant current sampling with the voltage compliances. The different sampling currents which have the same voltage compliance shows the same final resistance and the larger
voltage compliance make higher final resistance. Therefore, the final
resistance depends on the voltage only, the result is the same to the constant voltage sampling. But the result maybe should be under enough reaction rate or time to complete the resistive switching. In figure 4-18, we do the same Vstop
equal to the voltage compliance in constant current sampling. The final
resistance match to the result but Vstop=1V dose not. The reaction rate maybe slow at voltage=1V,
and reaction rate is enough when voltage>1.2V.
In order to know the final resistance dose depend on the voltage. We keep power on the device after reset process. Figure 4-19(a) is the schematic diagram, the power still on after the voltage reach the voltage compliance.
Figure 4-19(b) shows the resistance after the power keeping time. It shows the
power on is useless. Because the final resistance depend on the voltage only.
Figure 4-20 shows the possible reason for the result. Figure 4-20(a) is the ON state, there are many oxygen vacancies in the film, when we applied a positive voltage on the top electrode, a electric field establish, the oxygen which is gettered by Ti would move to the film and recombined with oxygen vacancies, like figure 4-20(b). The larger Vstop make higher final resistance values, like figure 4-20(c), the larger electric field push the oxygen ion farer.
Therefore, under enough reaction time, the final state controlled by the voltage only.