Different current compliance

The Ti/CuO/Pt devices are measured by changing current compliance in order to investigate the relation between current compliance and resistive switching property.

3.3.1 Measurement of different current compliance

Fig. 3.39-46 show I-V curve of switching cycles at each current compliance;

1mA, 3mA, 5mA, 10mA, 20mA, 30mA, 40mA, 50mA by an Agilent 4155C Semiconductor Parameter Analyzer. The detail methods of the measure are illustrated as follow. After forming process, the current compliance is set at 1mA. The turn-on process make device to ON-state. Continuously, the turn-off process are increase negative bias step by step until the device turn off to OFF-state. This method can avoid unnecessary damage in turn-off process. Repeat turn-on and turn-off process until more than ten times. The current compliances are increased to 3mA, 5mA, 10mA, 20mA, 30mA, 40mA, 50mA. The turn-on and turn-off process are repeat at each current compliance value.

There are some resistive switching parameters be defined in order to convenient follow discuss. Fig. 1.2 shows those parameters in I-V curve. The turn-off voltage ( Voff ) is the voltage of the ON-state to OFF-state, the turn-off current ( Ioff ) is the current of the ON-state to OFF-state, and the stop voltage ( Vstop ) is the maximum voltage value of the sweep voltage range when turn-off process.

3.3.2 Discussion of current compliance

Fig. 3.47 shows the relation between current compliance and current at –0.2 V.

The upper error bars are ON-state current variation under switching cycles, while lower error bars are OFF-state current variation under switching cycles. The ON-state current increased with current compliance, and saturate at current compliance value 20mA. The ON-state current increased with current compliance, imply that when current compliance increased, the filaments are stronger than low current compliance value, and conduction cross area more large lead to ON-state resistance decrease.

Fig. 3.48 shows that the turn-off voltage increased with current compliance, and the variation of turn-off voltage is seldom. The insert shows the relationship of current compliance and turn-off voltage is not observed any power law, and the turn-off voltage is weak dependent current compliance.

Fig. 3.49 shows that the turn-off current increased with current compliance, and the variation of turn-off current is seldom. The dotted line mark current compliance equal to turn-off current. The insert shows the relationship of power law between current compliance and turn-off voltage, and the turn-off current is strong dependent current compliance. The current compliance influence turn-off current is more sensitive than turn-off voltage can be explained as follow. The increasing power

contribute from two term

2

P IR I I R Δ = Δ + Δ

Because ON-state is low resistance, first term can be drop. The current mainly contribute to power rather than voltage. This is suggested that power or current dominate turn-off process.

We define that turn-off power is the product of turn-off voltage and turn-off current ( Poff = Ioff*Voff). The turn-off power is the electric power to make device from ON-state to OFF-state. Fig. 3.50 shows that the turn-off power has linear relationship with current compliance, and the variation of turn-off current is seldom. The linear relationship between turn-off power and current compliance can be explained as follow. We make some assumption to explain the linear relationship. First, current density ( J ) is constant current density in filament, and weak dependent current compliance. Second, the effect switching thickness ( ds ) is almost constant and independent current compliance, because the filament has small voltage drop. Third, the defect concentration per volume is independent current compliance.

The effect switching region is modeling a cylinder, is composed with defect, with effect cross section area ( As ). At turn-on process, the current compliance ( Icomp ) is

comp s

I = ⋅ J A

On the other hand, the defect maybe is Cu atoms, Cu ions, oxygen vacancies or CuOx (x<1). The total defect number in cylinder filament is

Therefore, the number of defect proportion to current compliance. In another point of view, the current pass filament in turn-off process. This locally enhances electric field and current density, hence the Joule dissipation and the temperature, which in the further accelerates the dissolution process. This positive feedback is at the basis of the sharp current drop at turn-off process [42]. When the device switched turn-off, the defect will combine with oxygen to form high resistive CuO at the effect switching region. The turn-off power should proportion to number of defect as follow.

The turn-off power would proportion to current compliance.

comp off

I ∝ P

To further confirm Joule heating effect caused the filament rupture model, the various temperature measure to observe turn-off process. Fig. 3.51 shows that turn-off process with different temperature. There is a tendency towards turn-off power decay, as a result of temperature raise.

3.3.3 Relation between temperature and current

At ON-State, as shown in Fig. 3.52, the current is decreasing with raising measurement temperature. This trend indicates that the property of ON-state is metal-like behavior. It is owing to the formation of metallic filaments in thin film. At OFF-state, as shown in Fig. 3.53, an obvious trend is investigated. The current is

can indicate that the property of OFF-state is semiconductor-like or insulator behavior.

Fig. 3.54 shows that double log I-V curve of turn-on process at RT and 150^oC, and Fig.

3.55 shows that double log I-V curve of turn-off process at RT and 150^oC. The ohmic region of OFF-state is larger at 150^oC. The mechanism of ohmic region of OFF-state is different from the ohmic region of ON-state. The ohmic region of OFF-state is due to thermally-generated carrier, rather than injection carrier of metal-like.