The role of oxygen vacancies on resistive switching properties of oxide materials

: Tuning the level of oxygen vacancies in metal oxide materials is a promising approach to enhance resistive switching properties towards memory applications. To comprehensively understand the microstructure and oxygen vacancy migration mechanism of oxide materials, recent research in controlling the concentration of oxygen vacancies and the relationship between oxygen vacancy and resistive switching behavior as well as computational study have been reviewed in this work. In particular, the role of oxygen vacancies on the resistive switching properties of various metal oxides, including transition oxides, perovskite oxides and complex oxides are discussed in this review. Moreover, different types of processing methodologies of oxygen vacant oxide materials are reviewed and compared in detail. In the end, the future trends in fine tuning the level of oxygen vacancies are reviewed and discussed.


Introduction
Recently, the technological advances in integrated circuits (ICs) have enabled the rapid development of modern electronic devices, such as memory devices and transistors, which has dramatically changed our daily life. However, since the IC complexity evolves exponentially and the integrated device density will face physical limit soon according to Moore's Law, significant studied has been conducted to develop next generation electronic devices as an alternative to replace filament composed of oxygen vacancies are finally generated, and set process is realized as expected. Upon application of a negative voltage, then oxygen vacancies migrate in the opposite direction and recombine with the oxygen ions, leading to dissolved filament and reset process [2,15]. Therefore, in many cases, oxygen vacancies determine the resistive switching process and the performance of the memory devices.
In this review, the electronic structure of the oxygen vacancies, recent efforts on controlling the concentration of oxygen vacancies and their roles on resistive switching properties are analyzed and discussed.

The role of oxygen vacancies-theoretical calculation
Recently, first principles calculations based on density functional theory (DFT) have been used to study the implications of the formation energy and charge states of oxygen vacancy on the switching mechanisms observed in memory devices [16,17]. Oxygen vacancies and other ionic defects can be introduced in the systems and the resulting structural and energy changes can be monitored. For memory devices, it is important to determine the formation energy of oxygen vacancies near the oxide-electrode interface, and to evaluate the feasibility of the migration of oxygen vacancies under electric fields. A new method, namely climbing image nudged elastic band can be used for analyzing the migration of oxygen vacancies. This method allows for the determination of the most probable ionic diffusion paths and estimation of the accompanying kinetic energy barriers. Thus the first-principles calculations can provide atomistic guidance for tuning the resistive switching properties, as those are dominated by the concentration and mobility of ionic defects near the interfaces [18,19].
For metal oxides, it is generally believed that the electronic structure of the transition metal atoms is affected by the on-site Coulomb interactions among the electrons in d orbitals, and sometimes in p orbitals of O for some systems such as TiO 2 [20,21]. There have been some studies reported that all valence states including oxygen p orbitals must apply to self-interaction correction to get the best fit value of band gap energy compared with the experimental one. Specifically, Figure 1 shows schematics of band structure of the rutile TiO 2 using Linear Discriminant Analysis (LDA) + U method. As seen, valence states including oxygen can produce the closest value of bandgap energy of rutile TiO 2 to the experimental value of 3.0 eV. In fact, DFT calculations can address bandgap problems of most materials, but for systems containing d electrons and f electrons, especially transition metal oxides, the traditional DFT calculations are not accurate enough when calculating the strongly correlated systems. Therefore, Hubbard (U) is applied to electrons on d or f orbitals to describe the interaction between electrons, which can correct the failure of DFT to predict correct bandgaps for strongly correlated materials. Reproduced with permission from Ref. [16].
Sun et al. studied the electronic structure and properties of SrZrO 3 (SZO) with ordered and disordered oxygen vacancies (V O s) [22]. They found that the formation of oxygen vacancy row (V Orow) leads to defect assisted conduction filament, which is the "ON"-state of SZO memory device, while the disruption of the ordered V O -row breaks the conduction filament and therefore such structure means the "OFF"-state of SZO memory, and the resistance is quite high compared with "ON" state. The formation and disruption of V O -row in the oxide can be triggered by external electric fields through electron injection and removal. It should be noted that the electrodes also greatly affects these processes, because active metals can be oxidized and reduced and they may have lower activation energy compared with the formation and disruption of V O . Therefore for simulation the role of the electrodes should be considered.

Controlling the concentration of oxygen vacancies (V O s) in oxide materials
Oxygen vacancy (Vo) is one of the most common defects in semiconductor materials, especially in metal oxide semiconductor, which has an important impact on the properties of semiconductor materials. V O s refer to the oxygen atoms (oxygen ions) separation in the lattice of metal oxide or other oxygen-containing compounds, resulting in oxygen loss and formation of vacancy. Consequently, plenty of the studies have been performed on its role on resistive switching in oxide materials during the past decade.  [23,24]. Up to now, a couple of methods have been employed to adjust the lattice structure properties of oxides to generate additional V O s, such as temperature [25], pressure [26] and reducing gas atmosphere sintering [27]. However, it remains a challenge to control oxygen vacancy defect via engineering the structure of oxide materials under ambient conditions. Therefore, choosing a material with a corresponding synthetic method is important for the formation and accurate control of oxygen vacancy. Furthermore, to a specific application or device, a tiny difference occurred in the lattices would lead to severe influences on the materials performance promotion or demotion.

Element doping
In the previous studies, it has been indicated that inserting dopants could alter V O s in transition metal oxide based materials. Additional studies demonstrated that p-type dopants (such as Al, Y, La) imported holes into the structure may greatly reduce the formation energy of oxygen vacancy [28,29]. Hence, V O s can be effectively obtained by element doping. At the same time, the structural expansion of material lattice due to the tensile strain which caused by the dopants are also found. From the research of Fell et al. detailed analysis of the lattice structure through X-ray diffraction may give rise to the revelation of feasible alteration of the lattice structure owing to foreign atoms introduction [30]. Figure 2 shows the introduction of a Ca 2+ ion into the ZrO 2 lattice produces a vacancy on the O 2sub-lattice. This substitution helps to stabilise the cubic fluorite structure of ZrO 2 . In terms of the impact of doping on the performance of resistance switching devices, He et al. reported on fabricating the heterostructure of Pt/Ti-Ta 2 O 5 /Pt with a polycrystalline Ta 2 O 5 film for investigation of resistance switching properties in nonvolatile memories. The results show that in aspect of unipolar RS properties, the resistance switching of Pt/Ti-TaO 5 /Pt is 5000% higher than the Ta 2 O 5 , and it has significant retention behavior under continuous voltage (more than 12 h). Compared to T 2 O 5 , excellent properties are observed such as higher LRS or HRS values and higher resistance switching ratio, since Ti addition in Ti-doped Ta 2 O 5 films can suppress the migration of oxygen vacancies via forming the inactive Ti/O-vacancy complex. Therefore, this research highlights advantages of element doping in excellent RS devices [29].
Several techniques have been successfully employed to dope oxide-based materials to generate V O s, such as, sol-gel [31], hydrothermal [32], (plasma-assisted) chemical vapor deposition (CVD) [33][34][35][36], pulsed laser deposition (PLD) [37], chemical spray pyrolysis (CSP) [38], and atomic layer deposition (ALD) [39], etc. For high temperature methods, such as CVD and PLD, the doping efficiency is relatively high because high temperature means the dopant ions can diffuse into the oxide crystal lattice easily. On the other hand, the solid solubility of the dopants also limits the doping concentration and beyond that, second phase appears and the concentration of oxygen vacancies cannot be further tuned.
The major challenge encountered in solution processed methods, such as sol-gel, hydrothermal and chemical bath deposition lies in doping efficiency. A lot of previous studies have shown that the tendency for dopant ions to be excluded from the oxides during crystal growth in solution, which means the formation of second phase or impurities. This is particularly true for oxides grown from aqueous solution, because it is difficult to control the kinetics in each individual reaction step. That means, simple addition of dopants to the solution hardly results in incorporation of dopants in oxide crystal lattices. Hosono et al. reported the fabrication of Al doped ZnO films through the pyrolytic transformation (500 o C) of AlCl 3 -added layered hydroxide zinc acetate self-templates, in the absence of seed layer [40]. They used non-aqueous system that allows water act only as a reactant in the chemical reactions. An alternative way, electrodeposition has been proved to be successfully doping Co and Ni into ZnO nanowires, which can be conducted on a conductive substrate. For electrodeposition, the nucleation and growth of oxide crystals are determined by applied currents, therefore, the dopant ions can be adsorbed on the nuclei of oxide and incorporated into the crystal lattice. Another thing needs to be considered for electrodeposition is that the doping concentration is still low due to the low temperature (<100 o C) of electrodeposition process. Another effective method is solvo-thermal, where relatively high temperature (~200 o C) is applied and the dopant ions have higher diffusion coefficient compared with electrodeposition process. Different form hydrothermal process, the hydrolysis of metal ions in organic solvents is much slower and the dopant ions can be incorporated into crystal lattice instead of forming second phase.

Irradiation treatment
In a recent study, the lattice structure of oxide materials might be tuned by the exposure time to a source of irradiations such as ultraviolet lamps or microwave reactors. It is worth noting that the lattice system changes may result from the variation of the V O concentrations directly (Figure 3), since the doping process as aforementioned is a practical method for the confirmation. A better understanding of the band theory of oxides will ideally service for describing further related discoveries, because of the tight relations between materials lattice structure and electronic band structure [41][42][43]. Up to date, irradiation treatment, especially the microwave irradiation, has brought in an enormous amount of research interests because it is one of the most effective approaches to control the V O s creation [41,44,45]. A very recent review paper published by Kabongo et al. thorough summarized that the generation of V O s in transition metal oxide based materials assisted by microwave irradiation [46]. It should be noted that though irradiation treatment has shown their benefits like shorten reaction period and enhancing heating speed, this method needs to perform with another process to reach higher efficiency collaboratively. In addition, a series of studies have shown that high-energy electrons and ions can dissociate oxygen ions and neutral atoms on the TiO 2 surface, thereby generating oxygen vacancies. Because high-energy electrons and ions can open the Ti-O bond on the surface, and then remove oxygen atoms or ions oxygen vacancies are generated on the surface. Wang et al. [47] used electron beam irradiation and Ar + bombardment to introduce oxygen vacancies on the surface of TiO 2 . Yim et al. [48] introduced and regulated oxygen vacancies and their concentrations on rutile TiO 2 surface by electron bombardment.

Thermal treatment
Apart from the modification during the synthesis processing, extra V O s also can be generated by thermal treatment under different atmosphere. For instance, V O -rich titanium dioxide (TiO 2-x ) obtained after annealing it at a particular temperature (generally higher than 400 °C) under the oxygen-depleted condition, such as pure helium, argon, nitrogen, gas atmosphere or vacuum condition [49]. It was also reported that reductant-assisted thermal treatment enables the removing oxygen and generating oxygen vacancies simultaneously. These reductants involve from reactive metal (such as Li, Mg, Al) to reductive agent (Boron hydride). Wang et al. successfully modified the aluminothermic reaction to massively produce nonstoichiometric TiO 2-x at a relatively low temperature (i.e. 300-500 °C) [50]. The product shows extreme visible-light absorption, so-called black titania, has been proved through Raman spectroscopy and X-ray photoelectron spectroscopy that this physical property transformation of the samples caused by the V O s generation led to an amorphous surface.
In the earlier of this year (2018), similar results in a series of metal oxides (CeO 2 , ZnO, SnO 2 ) further indicated by Ou's Group used a simple lithium reduction strategy under room-temperature [51]. The Figure 4 shows that the three peaks labelled as O1, O2 and O3 can be presented to the lattice oxygen, oxygen defects and surface adsorbed oxygen species, respectively. It is obviously to find the content of oxygen defects in all the oxides significantly increased after lithium reduction treatment. Additionally, some studies reported cerium oxide nanoparticles were exposed to a high-temperature Carbon monoxide (CO) atmosphere for thermal treatment. The X-ray Absorption Near Edge Spectroscopy (XANES) region of the spectra changes with the procession of reaction, because of the reduction of Ce 4+ to Ce 3+ [52,53]. The number of oxygen vacancies is controlled by this method, which is very meaningful for the design of smart nanoparticles in future. In particular, creating V O s in perovskite oxides aroused the interest of Tan et al.; they mixed and ground the SrTiO 3 (STO) and NaBH 4 in a glove-box before annealing samples at the temperature range from 300 to 375 °C within 60 min under the argon gas protection [54]. The Figure 5 shows the reflectance of the pristine STO starting around 390 nm, which matches the absorption of STO band gap (3.2 eV). For comparison, an additional absorption band over 400 nm extending to infrared regions is observed for the samples after reduction. It has believed that the diverse light absorption of STO is due to the increasing concentration of V O s on the surface of STO [55]. This means the resistive switching properties can be tuned through the band gap of the oxide materials at relatively low temperature.

Mechanochemical synthesis
By an approach of mechanochemical force, which is a common ball milling (BM) method, mechanical shearing force and pressure are applied to the ball milled powder to introduce a large amount of lattice distortion in the powder, which leads to that oxygen atoms are separated from the original lattice position and oxygen vacancies are introduced. Yang et al. used a planetary ball mill to implement mechanochemical forces to introduce oxygen vacancies on the surface of MnO 2 for synthesized MnO x [56]. And the same study also confirmed that mechanochemical forces can introduce oxygen vacancies in oxides.

The effects of oxygen vacancies on resistive switching properties
Resistive switching (RS) was discovered in 1960s in oxides sandwiched between metal electrodes [57,58]. It had been realized that this effect was promising for non-volatile memory applications [59]. Since then resistive switching phenomena has puzzled a lot of scientists. In the past few decades, researchers had an improved comprehension of the mechanism of the RS. Meanwhile, unipolar and bipolar RS performances have been discovered in several classes of materials. Among them, oxides are the largest family of candidates. Herein, we focus on the switching model based on the metal-oxide-metal (MOM) structure is typically resulted by the driftdiffusion of V O s or oxygen ions. The regular viewpoint of MOM structured RS device is that the migration of V O s plays an essential role for the RS behaviour.
Fundamentally, two categories were built for the explanation of the RS mechanism in the VCMtype devices (Scheme illustrates in the Figure 6). On the one hand, the formation/rupture of the conductive filament is caused by the interconnected/disconnected V O s by applying different voltages. On the other hand, the RS cell is dominated by the interfaces, where the switching happens at the interface near the electrode and the oxide [60,61]. Interestingly, in some practical experiments, the above switching modes can be realized in one system, which indicate that the in-depth study remain necessary to clarify the differences [62]. For simplicity, we divided oxides into three types: transition metal oxide (TMO), perovskite oxide (PO), and complex oxide (CO) to discuss the RS behaviour linked with V O s. Nowadays, the resistive switching effect is one of the best-known phenomena in complex oxide-based heterostructures proposed for a novel memory cell. A lot of scientists and engineers are engaged in studies and applications of the resistive switching. Presently, some plausible models for resistive switching in oxides have been reported [6][7][8]. Among them is the voltage-enhanced oxygenvacancy migration model (VEOVM) for bipolar resistive switching [9].

Transition metal oxides
Although the first VCM-type RS phenomenon had been observed at the 1960s in a Nb/NbO xbased cell, the research about the improvement of the NVM in the TMO family has not prevailed until the past twenty years [63]. It should be noted that the performances of the VCM RRAMs have been dramatically developed to reaching the manufacture levels recently. In TMOs, the mobility of V O s is usually higher than metal cations. The distribution and concentration of V O s would affect the valence states of the metal cations effectively. Therefore, it is vital to make the role of V O s movement clear in TMO based RS devices. In this case, several characterization approaches have been conducted to monitor the migration of V O s in the RS process. Yoshida et al. firstly reported the direct observation of V O s drifted in a NiO film during RS property measurement by C-AFM [64]. The same outcome further proved by Lee et al. from Samsung Electronics Co., Ltd. using Current Sensing-AFM [65]. Moreover, in situ TEM technique was used in the Au/CeO 2 /Nb-STO structure to trace the V O s migration under electrical manipulation [66].
In 2017, Rafael et al. systematically studied a series of ceria-based RS devices including their microstructures and RS performances [67]. After that, they fabricated gadolinia-doped ceria with different doping levels to obtain RS devices with the controllable concentration of V O s and designed a model to explain the relationship between V O s and RS behaviour. There is an overview of ceriabased memristive devices and their characteristics in Table 1 [67]. Additionally, they defined V O s as "free" (mobile) or "frozen" (immobile) oxygen-deficiencies and indicated the RS enhancing would result from a high concentration of free V O s, shown in Figure 7. Their outcomes suggested that simply increasing the concentration of V O s in TMO based structure may not improve the RS performance. A link between the ionic conductivity in the oxides layer and their RS behaviour could be analogized the relations between ion diffusion and the ratio of internal V O s type ("free" vs "frozen").

Perovskite oxides
Generally, perovskite oxides could be simply regarded as ternary oxides with formula ABO 3 , where A and B represent metal cations. V O s also play a critical role in RS devices composed of PO. In order to explicate the vital physical mechanism of PO based RS, a number of models, for instance, modify the interface property induced by the motion of V O s has been proposed. By recent results, it is promising to control the PO based RS behaviour by manipulating the concentration of V O s at the interface area during the fabrication process. Jin et al. investigated the RS mechanisms in the oxygen-deficient LaMnO 3 (LMO) and BiFeO 3 (BFO) [68]. The migration of the V O s at the interface of these materials led to the resistance change of the devices, obtained an interface-type RS characteristic. A year later, Panda et al. selected two typical POs, SrZrO 3 (SZO) and SrTiO 3 (STO) as models, to thoroughly summarize the RS properties [69]. The SZO based RRAM devices have been widely accepted as show VCM filamentary type RS performance (Zr 3+ /Zr 4+ ), which formation/rupture filaments made by V O s as a dominant factor for RS behaviours.
For the STO based memory cells, especially the samples doped with iron, two types of VCM RS behaviour (filamentary and interface) can be observed in one and at the same junction by C-AFM [70]. The RS behaviour of a framework Pt/Fe:STO(500 nm)/Nb:STO is shown in Figure 8 as an I-V curve. Later, Lim et al. and Bagdzevicius et al. separately reviewed complex physical phenomena and active mechanisms which were linked to the appearance of RS in PO S , together with several relevant examples for each material system [61]. They also presented some examples of conduction mechanisms explanations and summarizes related resistive switching devices based on perovskite oxides [71]. It noteworthy that several issues are not clear yet, especially for the understanding of the physicochemical effects induced RS behaviour might remain debatable in various actual cases.

Complex oxides
Complex oxides contain at least three elements except for oxygen, such as Pr x Ca 1-x MnO 3 (PCMO), La x Ca 1-x MnO 3 (LCMO) and YBa 2 Cu 3 O 7 (YBCO). Most of the CO was a perovskiterelated V O s-rich material that proved promising for the modern electric device. Tarasova investigated the introduction of low concentrations of fluoride ions increases the ionic component of the conductivity [72], shown in Figure 9. Nowadays, increasingly research on the RS performance in CO-based heterostructures for proposing novel memories. A consensus has been broadly believed that the migration of V O s at the interface area of system junction was considered to be a core factor of the RS in PCMO with the Schottky-like barrier [73]. Wang et al. reported an investigation of the relationship among the configuration or concentration of V O s and RS effects in an epitaxial grown LCMO/STO structure [74]. Rearrangements of V O s diffusing to/away from CO interfaces correlate with the resistance stage changing of the LCMO based memristive device. Plecenik et al. experimentally verified the crystal growth orientation of the YBCO compound has a significant effect on their RS performance. The consequence implies a deep reason that was the fast diffusion of V O s on c-axis, which explained the observation of an asymmetric I-V curve and bipolar RS property of YBCO [75].  [76,77] were applied in their experiment. The +/− gating voltage drives the −/+ charge carriers to gather to the surface region of PBCO film, leading to a decrease/increase of the resistance, i.e. the PBCO based device can be reversibly set/reset from the HRS to LRS, or the other direction.

Summary and prospective
In summary, oxygen vacancy (Vo) plays a key role in determining the resistive switching behaviour of oxide materials. Computer simulation results combined with experimental studies indicate that resistive switching states between high resistance state (OFF) and low resistance state (ON) in oxide materials can be led by the density change of Vo, where Vo assisted conduction channel can result in the "ON" state of the memory device, and the disruption of the ordered Vo breaks the conduction channels and therefore turns off the memory device. Furthermore, the presence of this leading effect of Vo density can be explained by electrons capture and emission under electric field through oxygen vacancies migration. Following this idea, there have been a lot of studies focused on tuning the concentration of Vo in oxides in order to achieve desired resistive switching performance. It should be noted that there is a suitable concentration window of Vo, because too high Vo means the oxide is highly conductive, which results in low ON/OFF ratio. On the other hand, low Vo means limited mobile ions in the oxides and it is more difficult to switch ON/Off the device. In addition, it is essential to control the distribution of Vo in oxides, as this will affect the device uniformity. In summary, recent research progress has demonstrated the significance of Vo in theory and gives physical insights into the switching mechanism and also for improving the device performance.
There are still some challenges in this area, for example, accurate and quantitative theory as well as experimental approach to identify the exact role of Vo in resistive switching are limited. Therefore, (1) new methods should be developed to monitor the generation of Vo during resistive switching process and quantify its concentration. One promising tool is 3D SIMS-SPM prototype, namely secondary ion mass spectrometry (SIMS), which the data is linked with topographical images from scanning probe microscopy (SPM) module to get accurate chemical 3D map of the oxides with and without external electric field. This is particularly important for thin film devices as it is difficult to identify the oxygen vacancy distribution in thin film by conventional characterization technologies.
(2) It would be interesting to introduce Vo at low temperature, as oxide crystals will grow and cracks will form at high temperature, and also for some specific applications, such as flexible electronics, high temperature is harmful for flexible substrates. One of the possible approaches is plasma treatment. Plasma processing can bypass the grain growth and cracks formation, and introduce Vo owing to oxygen deficient atmosphere. Different plasma treating processes, such as Ar plasma, N 2 plasma need to be finely tuned to create a suitable level of oxygen vacancy without introducing other harmful structural defects which will degrade the device performance. (3) Controlling the distribution of Vo in oxide materials can further improve the device uniformity. To achieve this goal, one can tune the morphology of oxide crystals, such as nanocube, nanospheres as Vo is usually distributed on the surface of the oxide crystals and surface of Vo can easily migrate under external electric field. Other direction is to design the oxide/electrode interface with controllable oxygen diffusion barriers, which may further enhance the performance of the devices, and create new functionalities such as memristive switching, and memcapactive switching.