Electrical and Physical Properties of the Al/V-doped SrZrO 3 /LaNiO 3 /Pt

4.2.1 Forming Process

At beginning, the sample is at an Original-state lower than L-state. As shown in Fig. 4-12, when the voltage sweeps to a voltage about 5 or -5V, the leakage current suddenly increases and switches to the H-state. Then, the nonpolar resistance switching properties can exist without any delay time between every voltage sweep cycles. The first resistive switching process is called the forming process.

4.2.2 SrZrO3 Sputtering Parameters

The influences of sputtering power on the SZO film have components, deposition rate, crystallization, etc. Fig. 4-13 shows forming voltage uniformity versus deposition time and sputtering power. According to the results, it shows that sputtering power influences the variation of the forming voltage. The forming voltage of SZO film sputtered at 100W is more uniform as compared with that sputtered at 150W. The reason may be the SZO film sputtered at 100W is more uniform result in good uniformity of the forming voltage.

4.2.3 Electrical Properties of the SrZrO3 Resistive Films

Fig. 4-14 depicts the I-V curve of the Al/0.3% V-doped SZO/LNO/Pt device.

While the negative voltage is applied on the top electrode from 0 to -5V, the current rapidly increases at -3.5V, and then the device is switched from L-state to H-state.

During the measurement, the current is restricted to 1mA to prevent the degradation of the device. While the device is switched from L-state to H-state and limited at 1mA, it does not influence the H-state current of the device. The device altered from L-state to H-state is called as on process. Subsequently, the bias voltage sweeps from 0V to -2V and the device is switched from H-state back to L-state at -1.8V. The device altered from H-state to L-state is called as off process. When the positive voltage is applied on the top electrode from 0 to 5V, the device is altered from L-state to H-state at 3V. Then, the bias voltage sweeps from 0V to 2V and the device is changed from H-state to L-state at 1.8V. As shown in Fig. 4-15, the resistance ratio between two current states is over 10⁶ measured at -1V. The resistive switching phenomenon switched by either positive or negative bias voltage can repeat over 10 times. The resistive switching properties of the device altered by either positive or negative bias voltage are called nonpolar resistive switching

characteristic.

Fig. 4-16 shows that I-V curve of the Al/0.3 % V:SZO/LNO device. The tri-layer device is switched from L-state to H-state at -13V, and from H-state to L-state at 10V. The resistance ratio of the Al/SZO/LNO device is 10⁴ at measured -1V as shown in Fig. 4-17. The on and off processes are applied negative and positive bias voltage, respectively, which called bipolar resistive switching characteristic.

Compared Fig. 4-14 with Fig. 4-16, the differences between two devices are H-state current and switching voltage. Because the H-state current of the four-layer device is higher than that of tri-layer device over 1000 times, the resistance ratio between two states of the four-layer device is higher than 10⁶. The switching voltage of the four-layer device is lower than that of the tri-layer device. The results are discussed in the next section.

4.2.4 Schematic Conducting Loop

Because the conductivity of the LNO buffer layer is lower than that of metal electrode, Pt electrode has replaced LNO electrode to be the bottom electrode. This is proved at this section. However, it results in different resistive switching characteristic which is discussed in the previous section.

As shown in Fig. 4-18 (a), the serial resistance of the carrier passing through the path1 is,

Rpath1=RW/Al+RAl+RAl/SZO+RSZO+RSZO/LNO+RLNO+RLNO/Pt+RPt+RPt/LNO+RLNO+

RLNO/Al+RAl+RAl/W………...….(1)

where the RW/Al and RAl/W are the contact resistance of probe and top electrode.

Because the fabricating processes of the SZO and LNO films in the four-layer structure are identical to those of the tri-layer structure, it is considered that the

conductivity of the carrier passed through path2 in four-layer structure is the same in tri-layer structure. As shown in Fig. 4-18 (a) and (b), the serial resistance of the carrier passed through path2 is,

Rpath2=RW/Al+RAl+RAl/SZO+RSZO+RSZO/LNO+RLNO+RLNO/Al+RAl+RAl/W………(2) The Rpath1 estimated by H-state current in four-layer structure shown in Fig.

4-14 is about 20Ω. The Rpath2 estimated by H-state current in tri-layer structure shown in Fig. 4-16 is about 15kΩ. Compared Eqns. (1) with (2), it is proposed that the RLNO is about 15kΩ, and the RPt is lower than 20Ω. Therefore, the conductivity of the LNO electrode worse than that of Pt electrode is proved and the LNO bottom electrode in four-layer becomes a buffer layer to deposit particular orientations of the upper layer.

The resistance ratio of the tri-layer device is three orders of magnitude lower than that of the four-layer device owing to the difference between the H-state current of two devices. According to Eqns. (1) and (2), the crabwise resistance of the LNO bottom electrode of the tri-layer device mainly influences on the H-state current, which is similar to the compliance effect although the resistance of the V:SZO film and parasitic resistance of two devices maybe have some differences under distinct on processes.

Because the conductivity of the LNO film is not very high, when the device is applied positive voltage on the top electrode, the speed of electrons passing through the SZO film is too slow to switch the device from L-state to H-state. Therefore, the switching characteristic is difference between two device structures.

4.2.5 Conducting Mechanisms

Fig. 4-19 depicts the plots of Ln(|I|) versus Ln(|V|) of both H-state and L-state currents for the Al/0.3% V-doped SZO/LNO/Pt device. The slopes of H-state curves

close to unity, indicating that the H-state current is dominated by Ohmic conduction, which is related to thermally excited electrons hopping from one isolated state to the next one [27]. On the other hand, the L-state curves are not straight lines, implying that the L-state current is dominated by other conduction mechanisms. Fig.

4-20 shows the plots of Ln(|I/V|) as a function of of H-state and L-state currents for this device. The linear fittings of the device indicate that the L-state current follows the F-P emission, which is corresponding to field-enhanced thermal excitation of trapped electrons into the conduction band [27].

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4.2.6 Possible Resistive Switching Mechanisms

In the on process, the biased electrons found one or few conduction paths consisting of possible point defects, such as oxygen vacancies and ionic and electronic defects associated with Zr replaced by V. Simultaneously, the electrons hopped passing through the V-doped SZO film in these paths and causing the current to dramatically increase. Consequently, the resistive switching mechanism of the on process is considered to form the formation of current paths [17] as shown in Fig. 4-21 (a). In the off process, while the defects in the V-doped SZO film trap electrons to some degree, and hence, the paths could be considered ruptured as shown in Fig. 4-21 (b).