NiO x based RRAM

For NiOx-based RRAM, there are three parts, including the material analysis, resistive switching character, and conduction mechanism analysis, which will be discussed in this section.

First, material analyses include the microstructure. Figure 4-1 shows the HRTEM image of our NiOx film in the metal-insulator-metal (MIM) structure. According to this image, we can observe clearly (especially in the enlarge image of figure 4-2) that the polycrystalline NiOx is everywhere in this film. Moreover, the SAED image (inset of figure 4-2) also shows the polycrystalline structure in NiOx film. Several studies^[79-81]

also reported the polycrystalline structure in their NiOx RRAM research.

Figure 4-1. HRTEM image of polycrystalline NiOx film

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Figure 4-2. The enlarged part (dash mark in fig.4-1) of polycrystalline NiOx film. Inset shows the SAED image.

Second, for resistive switching character, both unipolar and bipolar switching characters have been demonstrated in other studies^[82-84]. These studies indicated the bistable resistive switching character in NiOx film. For the bipolar resistive switching NiOx RRAM, the resistance state is dependent on the direction of the applied voltage.

The HRS and LRS is formed by opposite bias. For the unipolar resistive switching NiOx

RRAM, the resistance state is dependent on the amount of applied voltage. The LRS and HRS is formed by large and small applied voltage, respectively.

In this study, we only found the bipolar resistive switching character in NiOx film.

Figure 4-3 shows the thickness-dependent bipolar switching phenomenon of NiOx films.

In our experiment, the thickness above 30 nm was necessary because the bistable resistive switching phenomenon disappeared when the thickness was below 30 nm.

With the increase of thickness, the more obvious resistive switching phenomenon was observed. Moreover, these thicker samples exhibited larger on/off ratio in the cycle endurance test. Accordingly, we defined the bistable states as high resistance state (HRS and low resistance state (LRS). In the programming operation, the resistance state is changed from HRS to LRS with an applied positive bias voltage. Similarly, the resistance state is changed from LRS to HRS by means of an applied negative bias voltage.

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-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 1E-12

1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4

Current (A)

Voltage (V)

30nm 75nm 150nm

Figure 4-3. The resistive switching character of NiOx films.

Figure 4-4 shows the relationship between sample thickness and the on/off ratio.

The ratio shows the thickness effect in this figure. The on/off ratio is about 0.5 when the thickness is about 30 nm. As the thickness increases, the on/off ratio also increased. The on/off ratio is about 100 when the thickness increases to 150 nm. This result indicates the resistance switching performance dependent on the thickness of NiOx.

Figure 4-4. The relationship between sample thickness and on/off ratio.

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Figure 4-5 shows more than 200 times of resistive switching processes of cycle endurance test of thick (75 nm and 150 nm) NiOx films. In the endurance test, it was obvious that the resistive state is rising in this cycling process. It is attributed to the damage of the NiOx film and this damage induced the rising of resistance state. This phenomenon was also reported by Rossel et al.^[52] and it is attributed to the localized

“burned” pits with create-like geometry. Therefore, the on/off ratio also showed a dropping in the endurance measurement. Figure 4-6 shows the on/off ratio dropping phenomenon and it indicates the on/off ratio dependence on the thickness of NiOx.

Figure 4-5. The endurance test of different thickness NiOx films.

Figure 4-6. The on/off ratio of different thickness NiOx films in the endurance measurement.

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The oxygen content of NiOx film is known to play an important role in resistive switching character. Figure 4-7 shows the resistive switching character (I-V curve) of 150 nm NiOx film with various oxygen flow ratio. In this experiment, we defined O2/(Ar+O2) as oxygen flow ratio and this ratio shows a strong relationship with on/off ratio. The relationship between oxygen flow ratio and the on/off ratio is shown in figure 4-8. It is clear to see that the on/off ratio dramatically increases from X3 to X100 as we increase oxygen flow ratio from 33% to 43%. In this figure, it indicates that the high oxygen flow ratio sample exhibits better performance.

-20 -15 -10 -5 0 5 10 15 20

Figure 4-7. The resistive switching character of NiOx film with different oxygen flow ratio.

Figure 4-8. The relationship between on/off ratio and oxygen flow ratio.

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Electrical analysis found the conduction mechanism was the Schottky emission in NiOx. Figure 4-9 shows the Schottky emission fitting curve in our sample. The linear dependency indicates that the transport mechanism is strongly dependent on the barrier height and the dielectric constant, as is shown in Eq. (3-2). The calculation of dielectric constants and the (B), which was defined by the barrier high difference between HRS and LRS by fitting the Schottky emission, were summarized in figure 4-10. The trend of

(B) is similar to the on/off ratio. It indicates the strong correlation between barrier high and on/off ratio. Several studies^[85,86] also reported the relationship between barrier high and the resistive switching character. Moreover, a very high dielectric constant was obtained in the samples with oxygen flow ratio less than 33%, and the opposite case was observed in the sample with oxygen flow ratio higher than 43%. A sample with higher dielectric constant means it owns higher charge storage capability, which also influences the electric character in the resistive switching process.

2.0 2.5 3.0 3.5

10 12 14

ln ( J ) ( A/cm2 )

V^1/2

Figure 4-9. Schottky emission approximation of NiOx film.

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20 30 40 50 60

10 100 1000

Set Reset

 ()

O2 / Ar+O

2 (%)

 i  ()

Figure 4-10. Dielectric constant and the barrier high change with various oxygen flow ratio.

Figure 4-11 illustrates how the internal electric field affects the resistive switching process for samples with various oxygen flow ratio. If we apply the same voltage in two different permittivity samples, the real voltage in those samples will be different. For the high permittivity sample, it can storage more charges in the interface and those charges form an opposite voltage in the insulator at the same time. Therefore, the real voltage in the sample is lower than the applied voltage. Similarly, there are small opposite voltage formed in the insulator of the low permittivity sample. This might be attributed to the fact that the samples with few oxygen ratios needed higher voltage to switch the resistance state.

Figure 4-11. The influence of dielectric constant in the operation process.

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Figure 4-12. The temperature dependent electric character of NiOx film.

Figure 4-12 shows the temperature-dependent electric character of NiOx film. It is clear to see the resistance decreases when the temperature decreases from room-temperature to 4K. The resistance of NiOx closes to linear decreasing trend when the temperature was above 60K. The minimum resistance appeared when the temperature was below 60K and it remained the same one. This result indicated a metal conduction character of our NiOx film.

In summary, NiOx shows the polycrystalline microstructure. The I-V curve shows clear bipolar resistive switching character. Both sample thickness and oxygen content influenced the bistable resistive switching phenomenon. The conduction mechanism follows the Schottky emission and the resistive switching character shows barrier high dependence relationship. Due to the temperature-dependent electric character, the NiOx

film showed a metal conduction character.

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