Cationic Interstitials: An Overlooked Ionic Defect in Memristors

Introduction

As the memories and CPUs are separated in the current von Neumann computer system, the data have to be transferred between them through the limited bandwidth buses, which limits the time and energy efficiencies in the data processing. Such issue could be addressed by achieving an in-memory computing paradigm, for which the memristor is a suitable device because of its higher data storage density (Cheng et al., 2017; Chen et al., 2019) and excellent physical characteristics of conditional switching and physical MAC operation (Yang et al., 2017; Zhou et al., 2019; Xu et al., 2021; Li et al., 2022). Meanwhile, they can also bridge various electrical devices and be applied in energy storage, remote sensing, low-power applications, etc. (Chen, 2017; Han et al., 2020; Wang et al., 2020; Zhang and Sun, 2021) Thus, memristors are crucial for non-volatile memory, logic operations, Internet of Things, and neuromorphic computing in the big data era (Qin et al., 2020; Xu et al., 2020; Yao et al., 2020; Huang et al., 2021).

The memristor is a two-terminal electrical device that regulates the flow of electrical current in a circuit and remembers the amount of charge that has previously flowed through it even after removing the bias voltage (Fu et al., 2020). The original concept for memristors was proposed by Leon Chua in 1971, which was described as a nonlinear, passive two-terminal electrical component that linked electric charge and magnetic flux (Chua, 1971). This conceptual device has been firstly linked to a kind of physical resistive switching device (ReRAM) by HP labs in 2008 (Strukov et al., 2008). Nowadays, the definition of memristor has been broadened to the arbitrary form of non-volatile memory with the foundation principle of resistance switching.

So far, the state transition phenomenon in different material systems is employed to trigger the resistive switching (RS) behavior and further construct the different types of memristors. Except for the well-studied RS behavior in metal-oxide materials (Illarionov et al., 2020; Liu et al., 2021; Wang et al., 2021), phase change materials (Hazra et al., 2021), organic materials (Chen, 2017; Cheng et al., 2017), ferroelectric materials (Guan et al., 2017), and magnetic materials (Park et al., 2018) are also reported to exhibit the macroscopic RS behavior because of the transition of crystalline phase, ferroelectric polarization, and spin polarization respectively. Among these numerous material systems, the metal-oxide-based memristors are promising owing to their low cost, simple process, and high compatibility with complementary metal-oxide-semiconductor (CMOS) technology (Mohammad et al., 2016; Illarionov et al., 2020; Nili et al., 2020).

However, compared to the basic metal-oxide-semiconductor (MOS) transistor which is the foundation device of constructing the current computing system, the memristor is still suffering from the relatively low reliability caused by device fluctuation, limited stability, and durability to maintain the resistance value or to improve repeated erasable times. Therefore, rational design and optimization of the memristor active layer through material engineering for enhancing the performance of memristors are expected. The works on metal-oxide-based RS have been mainly focused on the migration or ionization of oxygen vacancies (V_O) or cations from active electrodes (Waser et al., 2009; Chen et al., 2013; Kamiya et al., 2014; Tang et al., 2015). However, due to the high mobility of V_O and relative low endurance of cation-based resistive random-access memories (RRAM), it is a challenge to maintain stable intermediate states, which greatly limited the applications of memristors in multi-level RS and artificial synapses.

Compared to Vo and cations from electrodes, another common ionic defect in metal oxides, cationic interstitial (C_int) can also contribute to the successful RS, which has been less focused on previously. In the limited reports, C_ints have shown great potential in enhancing the performance of memristors with better stability and endurance, higher ON/OFF ratio, lower operation voltage, etc. Multi-level RS and synaptic RS have also been realized via modulating C_ints in memristors. Hence, in this mini-review, we focused on studying the C_int-induced RS behaviors from the previous reports. Both theoretical and experimental works have been investigated, which may provide reference and inspiration for the rational design of multifunctional memristors from a new perspective and may shed some light on the increments in memristors.

Review of the Works Related to the First-Principle Studies

First-principles calculations have been always employed to investigate the mechanism of the RS behavior through some calculations in terms of formation energy, the density of states, partial charge densities, etc. For C_ints-induced RS, there are two main contributions from C_ints to realize or enhance the RS: forming a conductive path and promoting charge transfer in the metal oxides. Related works have been summarized in Table 1.

TABLE 1

TABLE 1. Theoretical works on the C_int-induced RS behavior.

Gu et al. have compared the formation possibility of conductive paths with Cu interstitials (Cu_int) and Vo in the Ta₂O₅ atomic switch through first-principles studies (Gu et al., 2010): Figure 1A shows that Cu_int can form an effective conduction channel in the Ta₂O₅, as the formation of Cu_int, may connect the two adjacent Ta-O planes through the simulation process, while Vo failed to form such a conductive channel. But it should be noted that the conductive path formed by C_ints is sensitive to the concentrations of interstitials, which need to be tuned carefully in the experiments.

FIGURE 1

FIGURE 1. (A) Isosurface plot of the partial charge density corresponding to the defect state induced by the interstitial Cu_int and Vo in Ta₂O₅ (reproduced with permission (Gu et al., 2010). Copyright 2010, American Chemical Society); (B) deformation electron density in [110] for the defected TiO₂ with the Cu_int, Ti_int, and Zr_int (reproduced with permission (Li et al., 2015). Copyright 2015, Lei Li et al.); (C) isosurface plots of Ti_int and Ti_sub (reproduced with permission (Hussain et al., 2018). Copyright 2018, Springer-Verlag GmbH Germany).

The formation of the conductive path with C_ints is sensitive to the valences of the doped cations as well. Li et al. have systematically calculated the TiO₂ and ZrO₂ with different C_ints and investigated how C_ints with different valence states may affect the transport coefficients (Li et al., 2015). Figure 1B illustrates the deformation electron densities for the TiO₂ with Cu_int, Ti_int, and Zr_int, respectively. The blue region around Cu_int indicates that the loss of e⁻ from Cu could form ionic bonds with nearby O atoms, while such a phenomenon has not been observed in the situation of Ti_int and Zr_int. The calculation results indicate that the transport coefficients of the materials with Ti_int and Zr_int are higher than that with Cu_int. To optimize the RS behavior, the doping of metals with +4 or higher valences could be employed as it may enhance the transport properties.

Similarly, Zr_int and Ti_int in CeO₂ and Ta_int in Ta₂O₅ have also been confirmed to contribute to the formation of conductive paths in memristor. As it is shown in Figure 1C, Ti_int can form a more obvious conductive path in CeO₂ compared to the Ti substitution (Ti_sub). And then, Ti_int and Zr_int have been introduced in experiments and successfully improved the RS performance of CeO₂ RRAMs (Hussain et al., 2018). Zhu et al. compared the V_O and Ta_int in the Ta₂O₅-based RRAM and confirmed the contribution of Ta_int in realizing RS under oxygen-poor conditions (Zhu et al., 2016). Thus, it is concluded that the C_ints can introduce more defect states to above metal oxides than that of Vo, and the C_int-induced RS can be enhanced under an electric field.

The synergistic effects of Vo and C_ints for achieving RS have also been identified in memristors. In the Au-doped HfO₂, it has been confirmed that both Vo and Au_int are involved in the formation of conductive filaments (Tan et al., 2018). Similarly, Abdelouahed et al. compared the TiO₂ with Vo and Ti_int and revealed the co-formation of both defects, which induced a net dipole moment, and enhanced RS behavior under an electric field (Abdelouahed and Mckenna, 2015).

Review of Experimental Works

C_int-induced or enhanced RS behavior in memristors has also been confirmed in experimental works (as summarized in Table 2).

·The formation of C_ints and C_int-induced RS behavior

TABLE 2

TABLE 2. Experimental works on the C_int-induced RS behavior.

The C_ints can be introduced to metal oxides by modifying the synthesis parameters, such as the annealing conditions, oxygen partial pressure, doping concentration, etc. For example, by changing the annealing temperature and improving the oxygen concentration during the annealing process, Cu_ints have been successfully formed in the Cu_xO, and the RS can be enhanced by tuning the Cu_ints in the memristors (Rehman et al., 2018). In the sputtering process, by adjusting the oxygen partial pressure, the formation of O_int or Zn_int could be controlled, and interestingly, it is found that the bipolar and unipolar RS behavior can be tuned by forming O_int and Zn_int in the Al/ZnO_x/Al memory device, respectively (Wu et al., 2014). In the pulsed laser deposition (PLD), rapid thermal annealing may also change the defects in ZnO/Al₂O₃ memristor: the trap-controlled-space-charge (TCSC) limited conduction mechanism has been observed when Vo dominates, while, diode-like RS behavior has been identified in the case of Zn_int dominating. In the latter, the stability, endurance, and ON/OFF ratio of the memristor have been significantly improved as it is shown in Figure 2A (Sekhar et al., 2015). Such transition is ascribed to the formation of ZnAL₂O₄ as an interlayer, which acts as the e⁻ trapping/detrapping area and achieved successful RS.

FIGURE 2

FIGURE 2. Enhanced RS performance and the cross-section images of the device based on (A) Al₂O₃/ZnO/Al₂O₃ memristors (reproduced with permission (Sekhar et al., 2015). Copyright 2015, Elsevier B.V.) and (B) CeO₂/Ti/CeO₂ (reproduced with permission (Rana et al., 2017). Copyright 2017, Anwar Manzoor Rana et al.); the multilevel RS and the cross-section images of the device based on (C) Mn-doped SnO₂ (reproduced with permission (Xu et al., 2018). Copyright 2018, Elsevier Ltd.) and (D) CH₃NH₃PbI₃ thin films (reproduced with permission (Choi et al., 2016). Copyright 2016, John Wiley & Sons, Inc.).

In the memristors fabricated from solution-processed, C_ints are usually introduced via doping. In the NiO:SnO₂ memristor, Ru_int, and Al_int can be achieved through Ru and Al co-doping in the sol-gel process (Li et al., 2014). Compared to the Vo induced RS, the RS behavior conducted by Ru_int and Al_int could be greatly improved and show a larger ON/OFF ratio. The enhanced RS is ascribed to the increased trapped states between the equilibrium Fermi level and conduction band by Ru_int and Al_int (Li et al., 2014). Mn_ints have been achieved in SnO₂ by increasing the Mn-doping level to 12.5 mol% in the liquid-liquid interface hydrothermal process and compared to the pure SnO₂ with Vo, Mn-doped SnO₂ memristor shows more effective and stable RS with significantly larger ON/OFF ratio and better intermediate state retention (Xu et al., 2018).

Adding a buffer layer is another method to introduce the C_ints. As it is illustrated in Figure 2B, Ti_ints have been introduced by a Ti buffer layer in the TaN/CeO₂/Ti/CeO₂/Pt memory device, in which Ti_ints assisted the formation of conductive filaments in CeO₂. Compared to the device without the Ti buffer layer, the device’s stability and endurance could be significantly improved alongside the lower SET voltage and larger memory window (Rana et al., 2017). Similarly, by alternately depositing the SnO₂ layer with Vo and Mn-doped SnO₂ layer with Mn_int, Mn_ints have been introduced to the SnO₂-based RRAMs, which significantly enhance the RS behavior with a higher ON/OFF ratio and better stability and endurance (Xu et al., 2017).

·C_int-induced Multi-level RS

Multilevel RS has also been investigated in C_int-induced memristors. Intrinsic multi-state RS behavior with good endurance and stability has been observed in Mn-doped SnO₂-based memristor by increasing the Mn-doping concentration, as it is illustrated in Figure 2C (Xu et al., 2018). By comparing the RS behavior of Mn-doping, Al-doping, and In-doping in SnO₂ together with the XPS results and the calculated defect formation energies, the multi-level RS has been ascribed to Mn_int instead of Vo. Iodine interstitials induced multi-level RS has also been achieved in the Ag/CH₃NH₃PbI₃/Pt cells as shown in Figure 2D (Choi et al., 2016). Owing to the relatively low activation energies, the migration of I_int enables filament formation and annihilation at a relative operation voltage.

·Synergistic RS induced by C_int and other defects

Furthermore, the synergistic effect of C_ints with other ionic defects in memristor has been confirmed in experiments more than the theoretical results above (Lee et al., 2021). The synergistic effect of Ti_int with Vo has been confirmed in the Au/TiO₂ nanotube/Ti memory (Hazra et al., 2021). In the Ti/MoO₃/FTO memory cell, it is identified that Mo_int, surface defects, and Vo have contributed together to the multilevel RS behavior (Patil et al., 2021). Zn_int together with Vo enables the formation and rupture of conducting filaments in the Cu-doped ZnO, and both electric controlled and white light modulated RS has been achieved (Saini et al., 2021). Similarly, Vo and I_int assisted RS via a Schottky barrier tuning has also been verified in the Au/CH₃NH₃PbI₃/TiO₂/FTO memory device (Lee et al., 2021).

In addition, C_int can act as assistance or a game-changer in metal oxides. Figure 3A illustrates the defect-abundant memory device of Ag/TiO₂-LPE (known as lime peel extract)/FTO, in which Ti_int from TiO₂, Ag⁺ oxidized from the Ag electrode, and K⁺ from the LPE synergistically contribute to the RS behavior. Ti_ints provide active paths for cation migrations, which enhanced the stability and endurance of bipolar RS of the memory cell with low operation voltage and high ON/OFF ratio (Abbasi et al., 2020). In the B-doped LaAlO₃, B_ints realized charge injection into the neighboring cations, which enables remarkable electrical RS and transformed the oxide into a ferromagnetic ionic-electronic conductor at the same time, as it is shown in Figure 3B. This extends applications of C_ints to contribute to energy-efficient and spin-based devices (Park et al., 2018).

FIGURE 3

FIGURE 3. (A) Enhanced retention and the schematic of Ti_int assisted the conductive filament with Ag⁺ and K⁺ (reproduced with permission (Abbasi et al., 2020). Copyright 2020, American Chemical Society); (B) the remarkably enhanced RS and ferromagnetic behavior by B_int in LaAlO₃(1-x):LaBO₃(x) (reproduced with permission (Park et al., 2018). Copyright 2018, John Wiley & Sons, Inc.).

Summary and Outlook

In summary, the cationic interstitials induced RS behavior in metal-oxide-based memories has been summarized. For a defect that has been less focused, there are very few reports on the formation, contribution, and mechanism of C_int-induced RS behavior compared to those on Vo or active electrodes. However, from both theoretical and experimental aspects, the C_int-induced or enhanced RS behavior has been confirmed in recent years. As discussed above, diversified C_ints provide more opportunities to tailor the metal oxides for different electronic devices. The rational fabrication of memristors with C_ints may give rise to remarkable enhancement in RS performance with better stability and endurance, lower operation voltage, higher ON/OFF ratio, faster device speed, etc. However, C_int-based memristors are sensitive to the concentration and valence state of C_int, which makes the formation of C_int in metal oxide synthesis need to be carefully modulated. By adjusting the C_ints, suitable electric structures would be established in the metal oxides, which helps improve the performance of the electronic devices.

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.

Funding

ZX acknowledges financial support from the Beijing Technology and Business University (Grant No. 19005902140). YH, WW, and NX acknowledge financial support from the National Natural Science Foundation of China (Grant No. 61832007) and the Research Foundation from the National University of Defense Technology (Grant No. ZK20-02).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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