**3. Results and Discussion**

In Figure 3, we can see that Φ<sup>B</sup> was modulated by not only the range of the *V*TE readout voltage, but also by the number of consecutive *V*SS sweeps. Moreover, as shown in Figure 3b,c, Φ<sup>B</sup> depends more strongly on OFR in the slope A case (low *V*TE) rather than in the slope B case (high *V*TE). Therefore, the results in Figure 3 provide a clue toward the controllability of the competition between the gradual and abrupt modulations of ΦB. To understand the mechanism for determining the boundary of an abrupt/gradual RS in IGZO memristor devices, we used Figure 3 with the energy band diagram.

First, when *V*TE < *V*PT, the bimodal distribution of Φ<sup>B</sup> into A and B (Figure 3a) can be explained as follows. As shown in Figure 4a, the doubly ionized V<sup>O</sup> (V<sup>O</sup> <sup>2</sup>+) is the well-known metastable state [21,22] and has been frequently pointed out as having a microscopic origin on the device instability under photo-illumination or bias stress [22–26] and persistent photoconductivity [25,26]. From the viewpoint of the subgap density of states (DOSs) in the a-IGZO (Figure 4b), the neutral V<sup>O</sup> states (V<sup>O</sup> 0 s) are transformed into V<sup>O</sup> (V<sup>O</sup> <sup>2</sup>+s) when the process of V<sup>O</sup> <sup>0</sup> <sup>→</sup> <sup>V</sup><sup>O</sup> <sup>2</sup><sup>+</sup> + 2e<sup>−</sup> becomes energetically favorable. These neutral states are very slowly recovered (nonvolatile) [23–26].

In the readout voltage *V*TE-dependent energy band diagrams, which are illustrated in Figure 4c, as *V*TE increases, the Fermi-energy level (*E*F) in IGZO reduces far from the IGZO conduction band minimum (*E*C), and moves closer to the V<sup>O</sup> 0 states above the IGZO valence band maximum (*E*V). It makes the generation of V<sup>O</sup> <sup>2</sup>+s more energetically favorable. When V<sup>O</sup> <sup>2</sup>+s is generated, the concentration of the carrier electrons in *E*<sup>C</sup> increases; the *E*<sup>F</sup> in IGZO again comes closer to *E*C. This situation occurs in non-equilibrium; therefore, the generation of V<sup>O</sup> <sup>2</sup>+s effectively makes Φ<sup>B</sup> lower.

Thus, if the V<sup>O</sup> ionization is nonvolatile, Φ<sup>B</sup> would gradually decrease as the readout voltage *V*TE increases. In other words, Φ<sup>B</sup> has to be inversely linear to *V*TE. However, Φ<sup>B</sup> was classified into two groups (A and B), as seen in Figure 3. Figure 1e,f shows that a large Φ<sup>B</sup> (in low *V*TE) taken from the slope A corresponded to the voltage range where the maximum *V*TE was applied across a thin SiO<sup>2</sup> layer (Figure 1e), whereas a small Φ<sup>B</sup> (in high *V*TE) taken from the slope B corresponded to the voltage range where the maximum increase in *V*TE was mainly applied across the IGZO film (Figure 1f). Then, there would be a significant generation of V<sup>O</sup> <sup>2</sup>+s only in the latter range (Figure 4c). In Figure 2c, *I*mem gradually increased only when *V*TE was in the latter range, that is, in the range 2 V ≤ *V*TE < *V*PT.

diagram.

generation of metastable VO2+ states.

Our discussion indicates that the bimodal distribution of Φ<sup>B</sup> in IGZO memristors originated from the generation of metastable V<sup>O</sup> 2+ *Electronics*  states. **2019**, *8*, x FOR PEER REVIEW 7 of 11

**Figure 4.** Schematic illustration of oxygen vacancies ionization from the viewpoint of (**a**) the atomic structures and (**b**) the subgap DOS in a-IGZO. (**c**) The read voltage *V*TE-dependent energy band **Figure 4.** Schematic illustration of oxygen vacancies ionization from the viewpoint of (**a**) the atomic structures and (**b**) the subgap DOS in a-IGZO. (**c**) The read voltage *V*TE-dependent energy band diagram.

In the readout voltage *V*TE-dependent energy band diagrams, which are illustrated in Figure 4c, as *V*TE increases, the Fermi-energy level (*E*F) in IGZO reduces far from the IGZO conduction band minimum (*E*C), and moves closer to the VO0 states above the IGZO valence band maximum (*E*V). It makes the generation of VO2+s more energetically favorable. When VO2+s is generated, the concentration of the carrier electrons in *E*C increases; the *E*F in IGZO again comes closer to *E*C. This situation occurs in non-equilibrium; therefore, the generation of VO2+s effectively makes ФB lower. Thus, if the VO ionization is nonvolatile, ФB would gradually decrease as the readout voltage *V*TE increases. In other words, ФB has to be inversely linear to *V*TE. However, ФB was classified into two groups (A and B), as seen in Figure 3. Figure 1e,f shows that a large ФB (in low *V*TE) taken from the slope A corresponded to the voltage range where the maximum *V*TE was applied across a thin SiO2 layer (Figure 1e), whereas a small ФB (in high *V*TE) taken from the slope B corresponded to the voltage Next, we investigated the OFR-dependence of ΦB. Figure 5a–c illustrates the energy band diagram of the device fabricated with a high OFR (O-rich device) under three conditions: at a thermal equilibrium (Figure 5a), at a low *V*TE (Figure 5b), and at a high *V*TE (Figure 5c). Figure 5d–f illustrates the energy band diagram of the device fabricated using a low OFR (O-poor device) in three states: at a thermal equilibrium (Figure 5d), at a low *V*TE (Figure 5e), and at a high *V*TE (Figure 5f). As seen in Figure 5a,d, a larger amount of V<sup>O</sup> 0 s existed in the IGZO when the OFR decreased from 1.3 to 1.0 sccm. Then, as the IGZO was O-poorer, the IGZO work function decreased, and Φ<sup>B</sup> became lower, which is consistent with Figure 3b. In addition, as mentioned in Figure 3b,c, the OFR-dependence of Φ<sup>B</sup> was larger in the slope A case (low *V*TE) rather than in the slope B case (high *V*TE). The Φ<sup>B</sup> before the V<sup>O</sup> 2+ generation (at a low *V*TE) was determined mainly by the OFR condition. After a significant amount of V<sup>O</sup> <sup>2</sup>+s were generated at a high *V*TE, the initial OFR-dependence of Φ<sup>B</sup> was combined with the *V*TE-dependence of ΦB. Thus, the OFR-dependence of Φ<sup>B</sup> was diluted in the slope B case (high *V*TE).

range where the maximum increase in *V*TE was mainly applied across the IGZO film (Figure 1f). Then, there would be a significant generation of VO2+s only in the latter range (Figure 4c). In Figure 2c, *I*mem gradually increased only when *V*TE was in the latter range, that is, in the range 2 V ≤ *V*TE < *V*PT. Our discussion indicates that the bimodal distribution of ФB in IGZO memristors originated from the

Next, we investigated the OFR-dependence of ФB. Figure 5a–c illustrates the energy band diagram of the device fabricated with a high OFR (O-rich device) under three conditions: at a thermal equilibrium (Figure 5a), at a low *V*TE (Figure 5b), and at a high *V*TE (Figure 5c). Figure 5d–f illustrates the energy band diagram of the device fabricated using a low OFR (O-poor device) in three states: at a thermal equilibrium (Figure 5d), at a low *V*TE (Figure 5e), and at a high *V*TE (Figure 5f). As seen in

Figure 5a,d, a larger amount of VO0s existed in the IGZO when the OFR decreased from 1.3 to 1.0 37

sccm. Then, as the IGZO was O-poorer, the IGZO work function decreased, and ФB became lower, which is consistent with Figure 3b. In addition, as mentioned in Figure 3b,c, the OFR-dependence of

ФB was larger in the slope A case (low *V*TE) rather than in the slope B case (high *V*TE). The ФB before the VO2+ generation (at a low *V*TE) was determined mainly by the OFR condition. After a significant

the *V*TE-dependence of ФB. Thus, the OFR-dependence of ФB was diluted in the slope B case (high *V*TE).

**Figure 5.** The OFR-dependent energy band diagram and ФB with (**a**)–(**c**) high OFR and (**d**)–(**f**) low **Figure 5.** The OFR-dependent energy band diagram and Φ<sup>B</sup> with (**a**)–(**c**) high OFR and (**d**)–(**f**) low OFR at (**a**,**d**) thermal equilibrium, (**b**,**e**) low *V*TE, and (**c**,**f**) high *V*TE.

OFR at (a,d) thermal equilibrium, (b,e) low *V*TE, and (c,f) high *V*TE. Finally, the evolution of ФB with the increase in the number of consecutive positive *V*SS sweeps is illustrated in the energy band diagrams in Figure 6. When the *V*SS sweeps were repeated four times, ФB gradually decreased because of the gradual increase in VO2+s. However, the process of VO2+ generation followed by ФB lowering was not abrupt; it was gradual because further lowering of the *V*TE-dependent *E*F followed by the VO2+ generation was self-limited due to the increasing of the electron concentration–dependent *E*F. The results in Figure 3c,d explain this well. If *V*TE ≥ *V*PT, the change of *I*mem becomes abrupt because *E*F is aligned with the level of the VO0s peak in DOS (Figure Finally, the evolution of Φ<sup>B</sup> with the increase in the number of consecutive positive *V*SS sweeps is illustrated in the energy band diagrams in Figure 6. When the *V*SS sweeps were repeated four times, Φ<sup>B</sup> gradually decreased because of the gradual increase in V<sup>O</sup> <sup>2</sup>+s. However, the process of V<sup>O</sup> <sup>2</sup><sup>+</sup> generation followed by Φ<sup>B</sup> lowering was not abrupt; it was gradual because further lowering of the *V*TE-dependent *E*<sup>F</sup> followed by the V<sup>O</sup> <sup>2</sup><sup>+</sup> generation was self-limited due to the increasing of the electron concentration–dependent *E*F. The results in Figure 3c,d explain this well. If *V*TE ≥ *V*PT, the change of *I*mem becomes abrupt because *E*<sup>F</sup> is aligned with the level of the V<sup>O</sup> 0 s peak in DOS (Figure 4b).

4b). Therefore, we can classify the operation regime in the two-terminal Au/Ti/a-IGZO/SiO2/p <sup>+</sup>-Si memristors into three parts: (1) low *V*TE (*V*TE < 2 V), (2) high *V*TE (2 V ≤ *V*TE ≤ *V*PT), and (3) higher *V*TE (*V*TE ≥ *V*PT). The boundary between (1) and (2) was approximately 2 V in our case; it was determined by the process/structure details and was controllable using the SiO<sup>2</sup> thickness and the IGZO work function. The *V*TE in regime (1) was adequate for the readout voltage because Φ<sup>B</sup> and *I*mem were determined mainly by the OFR condition. However, the *V*TE in regime (2) can be used as the amplitude of the potential pulse because Φ<sup>B</sup> and *I*mem gradually change in a nonvolatile manner with the increase in the number of consecutive *V*SS sweeps. When the *V*TE in regime (3) was applied to the devices, they operated as abrupt RS switches rather than as gradual RS memristors.

**Figure 6.** Energy band diagram for the evolution of ФB with the increase in the number of consecutive **Figure 6.** Energy band diagram for the evolution of Φ<sup>B</sup> with the increase in the number of consecutive positive *V*SS sweeps.

#### positive *V*SS sweeps. **4. Conclusions**

Therefore, we can classify the operation regime in the two-terminal Au/Ti/a-IGZO/SiO2/p+-Si memristors into three parts: (1) low *V*TE (*V*TE < 2 V), (2) high *V*TE (2 V ≤ *V*TE ≤ *V*PT), and (3) higher *V*TE (*V*TE ≥ *V*PT). The boundary between (1) and (2) was approximately 2 V in our case; it was determined by the process/structure details and was controllable using the SiO2 thickness and the IGZO work function. The *V*TE in regime (1) was adequate for the readout voltage because ФB and *I*mem were determined mainly by the OFR condition. However, the *V*TE in regime (2) can be used as the amplitude of the potential pulse because ФB and *I*mem gradually change in a nonvolatile manner with the increase in the number of consecutive *V*SS sweeps. When the *V*TE in regime (3) was applied to the devices, they operated as abrupt RS switches rather than as gradual RS memristors. It is crucial to have good control over the mechanism on the boundary between the abrupt and gradual RS operations for a systematic design of memristor devices for neuromorphic computing. We investigated the transport and synaptic characteristics of two-terminal Au/Ti/a-IGZO/thin SiO2/p <sup>+</sup>-Si memristors by varying the oxygen content in the a-IGZO film by emphasizing the mechanism determining the boundary of the abrupt/gradual RS. A bimodal distribution of Φ<sup>B</sup> was produced to further lower the *V*TE-dependent *E*<sup>F</sup> followed by the generation of V<sup>O</sup> <sup>2</sup>+s. Based on the proposed model, we explained the influence of the readout voltage, the oxygen content, and the number of consecutive *V*SS sweeps on Φ<sup>B</sup> and *I*mem. Eventually, we proposed three operation regimes: the readout, the potentiation in gradual RS, and the abrupt RS.

**4. Conclusions**  It is crucial to have good control over the mechanism on the boundary between the abrupt and gradual RS operations for a systematic design of memristor devices for neuromorphic computing. We investigated the transport and synaptic characteristics of two-terminal Au/Ti/a-IGZO/thin Our results prove that the Au/Ti/a-IGZO/SiO2/p <sup>+</sup>-Si memristors are promising for the monolithic integration of neuromorphic computing systems because the boundary between the gradual and the abrupt RS can be controlled by modulating the SiO<sup>2</sup> thickness and the IGZO work function. Furthermore, the memristors are expected to be potentially useful for the co-design and joint optimization of the IGZO memristors and TFTs for neuromorphic energy-efficient wearable healthcare circuits and systems.

SiO2/p+-Si memristors by varying the oxygen content in the a-IGZO film by emphasizing the mechanism determining the boundary of the abrupt/gradual RS. A bimodal distribution of ФB was **Author Contributions:** The manuscript was prepared by J.T.J., G.A., S.-J.C., D.M.K., and D.H.K. Device fabrication was performed by J.T.J. and G.A. Results and discussion were performed by J.T.J., G.A., and D.H.K.

produced to further lower the *V*TE-dependent *E*F followed by the generation of VO2+s. Based on the proposed model, we explained the influence of the readout voltage, the oxygen content, and the number of consecutive *V*SS sweeps on ФB and *I*mem. Eventually, we proposed three operation regimes: **Funding:** This work was supported by the national research foundation (NRF) of Korea funded by the Korean government under Grant 2016R1A5A1012966, 2016M3A7B4909668, 2017R1A2B4006982, and in part by an Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government (18ZB1800).

the readout, the potentiation in gradual RS, and the abrupt RS. **Conflicts of Interest:** The authors declare no conflict of interest.

#### integration of neuromorphic computing systems because the boundary between the gradual and the **References**

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Our results prove that the Au/Ti/a-IGZO/SiO2/p+-Si memristors are promising for the monolithic


government under Grant 2016R1A5A1012966, 2016M3A7B4909668, 2017R1A2B4006982, and in part by an Electronics and Telecommunications Research Institute (ETRI) grant funded by the Korean government


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