**4. Recent advancements of perovskites in memory devices**

### **4.1 Oxide perovskites in memory devices**

In recent oxide perovskites based works, a Ag/BaTiO3/Nb:SrTiO3 ferroelectric tunnel junction (FTJ) with the quickest switching speed of 600 *p*s has been constructed [37]. When the sub-nanosecond switching action is maintained at 112.85°C, the device exhibits great temperature resilience. In addition, the gadget established 32 states or 5 bits of states for each cell, which is regarded to be the highest in the class. The combination of a high carrier concentration in the Nb:SrTiO3 electrode and the low work function of silver metal has resulted in significant increases in operation speed with a low current density of 4 × 103 A cm−2. The device performance can be seen through the rapid resistance switching at normal temperature depicted in **Figure 5.**

The advancement of fabrication for BaTiO3 has been established by growing this material on pre-deposited SrTiO3 substrate using the epitaxy technique to form a free-standing film. Then, they are transferred onto a silicon substrate for integration into complementary metal-oxide-semiconductor (CMOS) devices [38]. Not only

*Perovskites in Next Generation Memory Devices DOI: http://dx.doi.org/10.5772/intechopen.105360*

#### **Figure 5.**

*(a) Graph of resistance vs. driven voltage (Vd), and (b) common resistance switching between ON state and various high-resistance states at pulse duration, td = 600 ps. Adapted from Figure 2 [37] .*

the performance of the device is comparable with previously reported work [39], but it also exhibits a large ON state current with good bias voltage measurement of 0.2 V. This enables a non-destructive readout during the operation process. With an optimal depiction of SrRuO3/BaTiO3 dual buffer layers, highly structured BiFeO3 films were prepared oriented on flexible mica substrates [40]. The BiFeO3 films possess a stable ferroelectric polarization with over 100 bending cycles at a 5 mm radius. Therefore, it results in the continuously controllable resistance memristor characteristics which suggests the feasibility for solid synaptic devices. Moreover, using interactive supervised learning, the handwritten digits reveals excellent recognition accuracy valued at 90% in artificial neural network simulations which displays the potential for flexible ferroelectric memristors in wearable devices (data storage and computation). Ferroionic tunnel junctions have been suggested to make a huge electroresistance in ReRAM based BaTiO3 [41]. In low-resistance states, it works as a ferroelectric tunnel junction and as a Schottky junction which is due to changes within the interface caused by a field. This device significantly employs the ferroelectric barrier (BaTiO3) and Nb-doped SrTiO3 as the bottom electrode. The giant electroresistance result in ON/OFF ratios of 5.1 × 107 and 2.1 × 109 at room temperature and 10 K, respectively. The movement of oxygen vacancies from polarization reversal caused by the bias voltage could significantly alter the dimension of the interface barriers [42].

The ferroelectric manipulation of spin-filtering BaTiO3/CoFe2O4 composite barriers has been demonstrated in multiferroic FTJ synapses [43]. By manipulating the polarization switching of BaTiO3, it is possible to establish long-term memory and a constant conductance change achieving a 544,400% ON/OFF current ratio. On a crossbar neural network, supervised learning simulations applies the Spike-Timing Dependent Plasticity (STDP) outcomes as a database for weight training which achieved recognition accuracy rates above 97%. As a result, there is an approximately 10-fold shift in tunneling magnetoresistance ratio including a turnaround relying on the resistance state of the electrodes when the polarization is switched. A novel approach to multiferroic neuromorphic devices with energy-saving electrical exploitation is provided by these studies, notably the switchable spin polarization.

Additional FTJ device options include ferroelectric oxide-grown spinel ferrite barriers, which open a wider range of potential applications.

On the other hand, SrTiO3 has been used to develop the Pt/CeO2/Nb: SrTiO3 heterostructure which demonstrates outstanding memory behavior with a highest RS ratio of 3 × 104 [44]. Under the irradiation of an ultraviolet 405 nm laser beam, an obvious photoresponse was detected, which also corresponds to substantial switching characteristics in a high resistance state. This device demonstrates light-manipulated RS and voltage dependent photoresponse, which are two different types of RS. The RS and photoresponse features were contributed from the Schottky barrier at the Pt/CeO2 interface, as well as the electron trapping, and de-trapping caused by oxygen vacancies at the interface. An ultrathin (6.2 nm) ferroelectric La0.1Bi0.9FeO3(LBFO) layer has been introduced on a 0.7 wt% Nb-doped SrTiO3 (001) single-crystal substrate to form a Pt/La0.1Bi0.9FeO3/Nb-doped SrTiO3 heterostructure [45]. The ferroelectricity of the LBFO film was extremely high, but the coercive field was extremely low. By adjusting the thickness of the LBFO film, it was possible to produce a high resistance OFF/ON ratio of up to 2.8 × 105 for the Pt/LBFO (6.2 nm)/NSTO heterostructure. Moreover, the heterostructure exhibited multi-level storage and outstanding retention properties, as well as steady bipolar resistance switching behavior, which is suitable for application in ferroelectric memristors. On the LaBiFeO3/Nb-doped SrTiO3 interface, the resistance switching behavior has been demonstrated to be caused by a modulating impact of ferroelectric polarization turnaround on both the breadth of the depletion area and the potential barrier's heigh.

Using pulsed-laser deposition (PLD) technology, epitaxial BiFeO3 (BFO) thin films were fabricated to produce the robust in-plane domain dynamic process created when applied under the influence of external electric fields [46]. It has also been noticed that the retention and repeatability are good, particularly at high temperatures. Besides, by forming a heterostructure consisting of indium tin oxide (ITO), BFO, and strontium ruthenium oxide (SRO), an optically triggered non-volatile memory has been demonstrated [47]. In comparison to traditional devices, in which optical excitations often increase conductivity, the constructed structure demonstrates a significant drop in conductivity (1 × 10−4) following laser illumination at wavelengths 405, 532, and 1064 nm, indicating that the device is poorly conductive. Additionally, optical stimuli may be used to reset the negative optoelectronic memory, and an electrical pulse could be used to establish the memory. It was discovered that this property could be inhibited by annealing in an oxygen-rich environment, but that it could be restored by annealing in an oxygen-depleted atmosphere. Based on investigations of the transport and dielectric characteristics, it has been determined that the optical/electrical RS behavior observed at the ITO/BFO interface is caused by the potential profile's modulation at the ITO/BFO interface caused by optical and electrical excitations.

Fundamental research into the reversible topotactic phase change between the insulating brownmillerite (BM) phase and the conducting perovskite structure is critical for the creation of RS memories. Using SrFeOx as a model, the system demonstrated that in the ON state, SrFeO3 nanofilaments are produced and stretch essentially through the BM SrFeO2.5 matrix, and that in the OFF state, they are ruptured, indicating indisputably the presence of a filamentary RS process [48]. The nanofilaments are roughly 10 nm in diameter, permitting for the first time the downscaling of Au/SrFeOx/SrRuO3 RS devices to the 100 nm range. They have exceptional performance, with an ON/OFF ratio of up to 104 , a retention time of more than 105 s, and an endurance of up to 107 cycles, among other things.

#### *Perovskites in Next Generation Memory Devices DOI: http://dx.doi.org/10.5772/intechopen.105360*

On the other hand, an investigation into the process of irreversible RS conversion from bipolar to unipolar process is conducted in a capacitor model composed of SrZrO3, TiOx/Pt, which has been produced on a substrate composed of Pt, Ti, SiO2, and SiO2 [49]. The *I-V* properties of bipolar RS memory were seen in the RS operating voltage range spanning from +2.5 V to −1.9 V, and the memory exhibits outstanding durability and retention characteristics. An additional forming step happens when the voltage bias is raised above +4 V, irrevocably changing the RS mode from bipolar to unipolar. This process occurs as the voltage bias is raised above +4 V. In this study, two materials are combined with two different switching processes to create an RS memory with acceptable properties in various current regions. Furthermore, a bipolar RS characteristic was investigated in a Pt/LaNiO3/Nb: SrZrO3/Cu structure, where the Cu and LNO layers function as capping and buffering layers, respectively [50]. It was feasible to achieve a high endurance performance for the bistable bipolar switching characteristic at room temperature, which was measured up to 1.2103 times. The Cu layer was utilized as a reservoir layer to change the distribution of oxygen vacancies and traps inside the films, resulting in a steady RS response, low operation voltage, and extended retention duration.

A nanocomposite consisting of La0.7Sr0.3MnO3 (LSMO) and reduced graphene oxide (rGO) has been identified as a viable option for non-volatile memory applications in oxide electronic devices [51]. Individual component phases were identified because of the structural characterization process. A minimal switching speed of 1.1 μs and a relatively steady switch mechanism over 1000 switching cycles were obtained from the device. The switching behavior is also shown to be resilient against variable voltage sweeping rates. It is highlighted that the transportation of the oxygen ions at the SET and RESET voltages result in alterations of the resistance states due to the conduction filaments forming or rupturing. LSMO is also combined with BTO to form a heterostructure of non-volatile and reversible RS [52]. It was revealed that altering the electric field orientation caused variations in the LSMO layer's resistivity and metal-insulator transition temperature (TMI). When the BTO layer is subjected to a negative electric field, the resistivity for the accumulation state of hole carriers drops while the TMI for the accumulation state of hole carriers rises. When a positive electric field is applied to the BTO layer, the resistivity rises while the TMI falls for the hole carrier depletion condition.

### **4.2 Halide perovskites in memory devices**

An organo-metal halide source has been shown to produce perovskite layers through one-step spin-coating technique for the construction of unipolar RS devices in a cross-bar array design utilizing a simple one-step spin-coating procedure [53]. With gold as the electrodes, these unipolar perovskite RS devices attain a high ON/ OFF ratio of up to 108 while operating at a small operating voltage, with high stability, over 1000 writing cycles, and retention over 104 s. The memory devices were successfully incorporated into an 8 8 crossbar array design with up to 94 percent yield. Furthermore, as shown in **Figure 6**, the 1D-1R system of selective activation of memory cells was shown by eliminating crosstalk interference between nearby cells linked by external diodes.

An active layer for resistive memory has been developed using CsPbBr3 singlecrystal film (SCF) [54]. With an extremely high switching ratio of above 109 and a rapid switching speed of 1.8 s, the Ag/CsPbBr3/Ag memory cells demonstrate repeatable RS. A large interface contact is produced when the metal/CsPbBr3 SCF contact

**Figure 6.**

*A diagram of the identical reading procedure in a 2 × 2 array in a 1D-1R architecture with memory cells connected by external diodes. Adapted from Figure 4 [53].*

has an interface S value of 0.50, indicating that a large interface contact has been established. Because of the high RS ratio at the interface, the high interface contact leads to the stable high resistance state (HRS), and the steady HRS leads to an ultrahigh RS ratio. Besides, it has been demonstrated that the use of vacancy defects in lead halide perovskite structures may create excellent performance nano floating gate memory (NFGMs) [55]. A CdS nanoribbon (NR) surface was evenly covered with CH3NH3PbBr3 nanocrystals (NCs) using a simple dip-coating procedure, resulting in a core-shell structure composed of CdS NR/CH3NH3PbBr3 NC cores and shells. It is noteworthy that the device exhibited a very large memory window of up to 77.4 V and a long retention time of 12,000 s, as well as a high current ON/OFF ratio of 7 × 107 and long-term air stability for 50 days, all of which were attributed to the presence of sufficient carrier trapping states in CH3NH3PbBr3 NCs.

On the other hand, a CH3NH3PbI3−xClx/FTO RS device structure has been proven to retain information in dual levels of resistance states generated by electrical probe stimulation [56]. The device with the silver probe demonstrates bipolar RS behavior after formation, with a 106 ON/OFF resistance ratio, showing that it is bipolar RS. The constructed probe-based memory cell has a minimum endurance of 104 cycles and a minimal retention length of 2 × 103 s, which are both good performance features. Thus, organic-inorganic lead halide perovskite (OILHP) materials are highly proposed as a feasible candidate for usage as a storage layer for probe-based storage memories. Thin polyethyleneimine (PEI) interfacial layers have been introduced between the layers to avoid direct contact between the perovskite layer and the top and bottom electrodes, resulting in a device structure consisting of ITO/PEI/ CH3NH3PbI3/PEI/metal [57]. This device can reach over 4000 durability cycles while maintaining a low operating voltage of around 0.25 V. Aside from that, the repeatability of memory switching behavior was proven across 180 devices manufactured using eight different device batching settings.

#### *Perovskites in Next Generation Memory Devices DOI: http://dx.doi.org/10.5772/intechopen.105360*

Recently, 2D/3D perovskite heterostructure films consisting of 2D perovskite (phenethylammonium lead iodide, PEA2PbI4)/3D perovskite (MAPbI3) have been produced using a low-temperature all-solution technique [58]. The integration of 2D and 3D perovskite RS memories displayed remarkable efficiency, with an overall durability of 2700 cycles, an ON/OFF ratio of more than 106 , and a reaction time of 640 μs. Both the expected activation energy for thermally aided ion hopping and the time-of-flight secondary ion mass spectrometry data suggested that the 2D perovskite layer effectively blocked Ag ion migration through into the 3D perovskite film. By putting n-butylammonium iodide above CH3NH3PbI3−xClx (MAPbI3−xClx), another 2D/3D memory device was demonstrated [59]. The perovskite film is made in a single step by heating molten salt methylammonium acetate to room temperature and spinning it in the air. When compared to their 3D counterparts, RS memory devices with a 2D/3D perovskite heterostructure provide a significantly better switching window with an ON/OFF ratio of more than 103 while needing a lower operating voltage. The 2D/3D perovskite heterostructure is advantageous for manufacturing uniform-crystalline-grain, highly compact structures, and it can passivate defect states for the MAPbI3−xClx film and interface, resulting in improved memory properties for both the film and the interface.

In contrast, the cube of CsPbX3 was used in an Al/CsPbClxBrx (x = 3, 1.5, 0)/ITO/ PET memory device that demonstrated a bipolar RS pattern at a low working voltage [60]. When compared to all other memory devices developed, the CsPbBr3-based system has the most obvious RS qualities, such as the lack of an initial forming procedure, reproducibility, uniform switching, and a long retention period with a high ON/ OFF ratio. The multilayer data storage potential of flexible memory devices may also be evaluated by making minor changes to current compliance and stopping voltage.

Recently, flexible wearable electronic materials and fiber-shaped resistive random access memory based on MAPbI3, an organic-inorganic halide perovskite semiconductor, have been created utilizing a simple and cost-effective cheap deep coating process [61]. After refining the manufacturing settings, a well-arranged pinhole-free layer was coated on the aluminum fiber. The device has a bipolar RS feature with a roughly 106 ON/OFF ratio, a low working voltage, and a retention duration of more than 104 s. More crucially, the switching mechanism has remained nearly unchanged with up to a 45° bending angle.

In addition, our previous study investigated increasing the molar ratio of Pb to Ti by 5% in the MAPbI3-TiO2 layer, which increased VSET and VRESET values to 3.6 V and 1.1 V, respectively [59]. This study is regarded as a forerunner in understanding the use of a single layer MAPbI3-TiO2 in several applications. In the meanwhile, Cs3Sb2I9 inorganic halide perovskite has been developed as a lead-free source of high ReRAM and artificial synaptic devices [62]. A vapor-assisted solution technique was used to create this 2D perovskite (VASP). The memristive devices not only feature reproducible bipolar RS with a massive ON/OFF ratio of 104 at a low working voltage of 0.4 V, but they also have superb retention over 104 s and extraordinary resistance to environmental degradation. The ReRAM devices show promise to produce phototunable memories and artificial synaptic devices with the capacity to perform concurrent processing and learning due to high light-matter interaction in the perovskite and an intrinsic electronic–ionic connection.

#### **4.3 Layered perovskites in memory devices**

Dion-Jacobson organic-inorganic halide perovskite (OIHP) has been proven as a resistive switching memory (RSM), with grain size varying to improve grain boundaries [63]. By adjusting the ratio of *N, N*-dimethylformamide to dimethyl sulfoxide in the reaction mixture, the grain structure of the OIHP may be easily controlled. The controlled grain sizes in RSM can alter the paths for halide ion migration, enabling for a shift in the ON/OFF ratio by modifying the grain size. Large memory applications also need the use of the cross-point array structure. However, because sneak-current paths may generate undesirable current flow across unselected memory cells in a cross-point array structure, it is critical to minimize leakage current from surrounding cells by integrating selector devices in the design. We demonstrate the usage of selector devices in combination with the devices to avoid sneak current paths in OIHP-based RSMs. These findings suggest that OIHP might be utilized in high-density memory applications.

An Al2O3/2D Ruddlesden-Popper perovskite (2D PVK) heterostructure dielectric architecture, on the other hand, has been utilized to create ambipolar SnO transistorbased non-volatile memories with multibit memory behavior and ultralong retention time of >105 s [64]. The unique storage features are attributed to the decreased gate leakage generated by the Al2O3 layer, as well as the hopping-like ionic transport in 2D PVK with changeable activation energy under various light intensities. Because of the photoinduced field-effect process, it is feasible to operate a top-gated transistor in the presence of light, which would not be possible in the absence of light. As a result, it has exceptional photoresponsive properties, such as an extremely high specific detector detectivity of 2.7 × 1015 Jones and a large bandwidth spectrum differentiating capacity (375–1064 nm). The shape of metal halide perovskite layers, rather than grain size, is critical for high-performance memory systems. By including the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) into the perovskite formulation, the microstructure of solution-processed layered Ruddlesden–Popper-phase perovskite films based on phenethylammonium lead bromide ((PEA)2PbBr4) can be manipulated [65]. The hole is transported in the CB-BTBT, while the charge is stored in the (PEA)2PbBr4. With the combination of the (PEA)2PbBr4/C8-BTBT channels, the transistor-based memory device exhibited a huge record memory window over 180 V, large erase/write channel current ratio 104 , excellent data retention, and hood durability over 104 cycles. Moreover, a Ruddlesden-Popper-phase strontium titanate, SrO(SrTiO3)n (n = 1) and conventional perovskite SrTiO3 have been combined to form a heterojunction thin-film on an FTO substrate using the sol-gel method [66]. The Au/Sr2TiO4/SrTiO3/FTO/glass memory device performed a stable switching ratio over 102 under a high operating voltage of 8 V.

Generally, ion migration in the *I-V* hysteresis is known as the drawback of halide perovskite optoelectronic. This unwanted issue has been solved by employing layered Ruddlesden-Popper perovskites (RPPs) [67]. As a result, the memory devices of RPP with indices n = 5 show the largest ON/OFF ratio of 104 , operated in low VSET, in comparison to n = 1 and the 3D indices composition of perovskite, as presented in **Figure 7** In addition, the device can last for 500 cycles in an inert environment with data retention of 250 h. The data retention can be extended when the device is operated under 60% relative humidity. These results are due to the chemical interaction between moving ions and the external contacts, which results in a modification of the charge transfer barrier at the interface, which subsequently modifies the device's resistive states.

On the other hand, neuromorphic computing requires extremely minimal operating energy to provide huge parallel data processing that mimics the human brain. It is necessary to attain this aim by using resistive memory that is based on materials that have good ionic transport and operate at very low currents. Extremely low operating

*Perovskites in Next Generation Memory Devices DOI: http://dx.doi.org/10.5772/intechopen.105360*

**Figure 7.**

*Electrical characteristics of various ReRAM devices: (a) I-V curves for devices incorporating perovskites with varying n indices (n = 1, 5/3D) in an ITO/PEDOT:PSS/perovskite/PCBM/Ag setup (b) overview of SET voltages and ON/OFF ratios for various devices. Adapted from Figure 3 [67].*

current facilitates low-power operation by minimizing program, erasing, and read currents. The mixed electronic and ionic transport, as well as the ease with which they may be produced, make the 2D Ruddlesden-Popper phase hybrid lead bromide perovskite single crystals appealing materials for low operating current nanodevice applications [68]. The migration of bromide ions across the exfoliated 2D perovskite layer demonstrates ionic transport in the layer. Resistance memories with the lowest program currents down to 10 pA with a 100 ON/OFF ratio. Furthermore, the resistive memory demonstrated 400 fJ/spike synaptic functioning, which is comparable to the energy consumption required to convey information in the normal nervous system.

The use of lead-free perovskite through Aurivillius phase thin films have been suggested to improve their ferroelectric characteristics by modifying the growth, texture, and orientation [69]. In particular, liquid injection chemical vapor deposition (LI-CVD) was used to grow c-plane oriented Bi6Ti3Fe2O18 (B6TFO) functional oxide Aurivillius phase thin films on *c-*plane sapphire substrates, which were then annealed at 850°C to generate highly crystalline, well-textured single-phase Aurivillius platelike shapes with 110 nm average film thickness and 24 nm roughness. Piezoresponse force microscopy (PFM) shows in-plane polarization enhanced by adjusting the deposition of *a-*axis oriented grains along the plane of the B6TFO films. Interestingly, the device shows a large and stable ferroelectric polarization switching under high operating temperatures of up to 200°C even after 20 h of PFM scanning. These investigations show the promise of B6TFO thin films for high-temperature piezoelectric applications and non-volatile ferroelectric memory applications.

It is also important to highlight that multiferroic materials with associated ferroelectric and ferromagnetic order characteristics might be used to store data by writing bits electrically and reading them magnetically. For example, Aurivillius phase Bi6Ti2.8Fe1.52Mn0.68O18 (B6TFMO) produced by chemical solution deposition (CSD) shows magnetoelectric coupling at ambient temperature [70]. The in-plane ferromagnetic signature can be enhanced by manipulating the deposition method using the liquid injection chemical vapor deposition technique, which is related to the formation of Aurivillius phase [71]. Under magnetoelectric coupling, the ferroelectric switching volume increased by up to 14 percent as compared to CDS-grown films, and irreversible and reversible magnetoelectric domain switching was observed. This

demonstrates that B6TFMO thin films are a viable choice for in-plane RAM applications as well as future high data storage multistate memory devices.

### **5. Design challenges of perovskites based memory devices**

Understanding the RS properties is critical when building a RS device. According to published research, RS is often associated with oxygen or halide vacancies, metallic defects, and dislocations in perovskite thin films [72]. The kind of perovskites, film thickness, inclusion of dopants, and selection of bottom/top electrodes are all basic elements to consider in memory design construction. To fabricate a stable and efficient perovskite film device, preparation processes of perovskite thin films and understanding of the interaction between each layer are critical.

Wearable gadgets are gaining popularity among researchers in the age of wearable technology. Conventional electrodes, such as metals or transparent conducting oxides, are inflexible and readily shattered under stress. On the other hand, perovskites may be prepared via low temperature solution processing methods, Flexible devices could be considered while memory performance is maintained as published for study involving polyethylene naphthalate (PEN) or polyethylene terephthalate (PET) substrates [73, 74].

For further development of halide perovskites with high performance RS memory, the stability issue must be addressed due to the sensitivity to heat and moisture. This obstacle must be overcome to compete with metal oxide-based memory, which are more stable and easier to manufacture. To slow down the deterioration process while boosting stability, thick encapsulation might be applied to the devices. This layer may consist of a thin metal oxide or polymer-based layer [75]. Similar, to perovskite solar cells, the usage of lead compound is a key drawback in halide perovskite-based RS devices. Since lead is a poisonous chemical, the environmental impact of lead leaking may be disastrous. As a result, researchers are investigating lead alternatives such as lead-free perovskites. One example that researchers are looking at is tin-based perovskites, which have similar RS capability. Ji et al. discovered a lead-free allinorganic cesium tin iodide perovskite (CsSnI3) [76]. The (Ag or Au)/PMMA/CsSnI3/ Pt/SiO2/Si bipolar RS could represent an ecologically acceptable option.

### **6. Conclusions**

RS memory is one of the most sophisticated techniques for next-generation storage class memory, with lower power consumption, high density, and better performance. RS devices are regarded as one of the viable technologies for next-generation non-volatile memory. In addition to the well-studied usage of perovskite in perovskite solar cells, the use of perovskites in memory devices might be a fascinating subject to examine. Perovskites of various classes, such as perovskite oxides, perovskite halides, and layered perovskites, can be used in various RS devices. There are still numerous options to investigate for RS devices. The interactions of these perovskites with other elements are currently understudied and need to be investigated further. However, they still face significant challenges in entering the commercial sector. Although memory performance is advancing at a rapid pace, fundamental challenges in stability, reproducibility, and real-world applications are being addressed. As a result, perovskite may continue to play a significant role in dominating the memory storage sector soon.
