Perovskites in Next Generation Memory Devices

Abstract

Although perovskites are widely employed in other industries such as photovoltaics and light-emitting diodes (LEDs), digital technology is rapidly gaining pace in today’s market and shows no signs of abating. As a result, the progress of system memory and memory storage has accelerated into new inventions. The invention of dynamic Random-Access Memory (RAM) in the 1960s laid the groundwork for today’s multibillion-dollar memory technology sector. Resistive switching (RS) capabilities of perovskite-based materials such as perovskite oxides and metal halides have been extensively studied. Chemical stability, high endurance, quick writing speed, and strong electronic interaction correlation are some of the benefits of employing perovskites in RS devices. This chapter will investigate the progress of system memory and memory storage employing perovskites, the advantageous properties of perovskites utilized in memory devices, the various types of RS employing perovskites, as well as the research challenges that perovskite-based memory systems face in future commercial development.

1. Introduction

Perovskites, which have the same crystal structure as calcium titanium oxide (CaTiO₃), are semiconductor materials that have gained massive interest in various technology. When exposed to light, the structure and characteristics of these materials allow them to transfer electric change. These materials are very beneficial for system memory and memory storage in computer memory. Perovskites’ resistive switching (RS) properties enable fast writing speed and long durability. Thus, RS is employed in the most recent computer memory technology, Resistive Random Access Memory (ReRAM), which is expected to replace flash memory [1, 2].

In memory devices, there are many forms of RS. Bipolar and unipolar switching, write-once-read-many (WORM) [3], and multilevel RS [4] are examples of these. This chapter will discuss the advancements in the use of perovskites for RS memory. Oxide perovskites, halide perovskites, and layered perovskites are some of the perovskite materials employed in this application. Lastly, some design challenges are discussed, and future work is proposed.

2. System memory and memory storage technology

Digital technology is being employed extensively in today’s economy, and it shows no signs of abating. As a result, the progress of system memory and memory storage has accelerated, resulting in new improvements. The system memory is where the computer stores currently running applications and data. On the other hand, memory storage is generally for the goal of orderly retrieval and documentation. Figure 1 depicts the chronology of the evolution of system memory and memory stage.

Figure 1.

A timeline depicting the progress of system memory and memory storage over time.

In 1932, Gustav Tauchek invented the drum memory technology which marked the beginning of system memory [5, 6]. The drum memory was cylindrical in form, with an outside covering comprised of recordable ferromagnetic elements, and it could store up to 500,000 bits, or 62.5 kilobytes of memory [7]. Eventually, in the mid-1940s, the delay line memory was found, which was a refreshable memory that used sequential access and was constructed of mercury. This unique mercury delay line was capable of transmitting data at a rate of around 5,000,000 binary digits per second. It was not until World War II that the United States Navy adopted the initial drum memory idea and refined it into the magnetic drum memory system. The magnetic core memory was then constructed using tiny toroidal ferrimagnetic ceramic ferrites. The memory was stored via an induced magnetic field, which could store one bit depending on the magnetization direction [8]. Twister memory, which used magnetic tape instead of rings to replace core memory, was introduced in 1968 at Bells Lab but received little attention [9]. The magnetic tape was intentionally chosen to enable magnetization only down the length of the tape. As a result, only one point of the twistor would have the proper field direction to ever get magnetized. On the other hand, bubble memory which is a sort of non-volatile memory employs a small layer of magnetic material in its fabrication due to the influence of an external magnetic field. This contains little magnetized patches known as bubbles or domains, each of which may retain one bit of data [10, 11]. Similarly, bubble memory also suffered the same fate as twister memory since both were eclipsed by the development of dynamic RAM.

The invention of dynamic Random-Access Memory (RAM) in the 1960s laid the groundwork for today’s multibillion-dollar memory technology sector. Every sort of memory technology described above is rendered obsolete by the discovery of RAM. The earliest architecture of dynamic RAM was a square array with a capacitor and a transistor for each data bit [12]. In today’s technology, a broad range of RAM technologies have been researched for their commercialization potential. The advancement of memory storage technology began in 1976 with the usage of punched cards, with certain holes on them as a set of instructions for digital programs [13]. It was then refined further into punch tapes or paper tapes. Similarly, paper tapes were developed to replace punched cards, which were considerably more convenient since they provided a continuous set of data or instructions without the need to insert punched cards one at a time. The substance used to create the paper tapes was then altered and replaced with magnetic materials.

Magnetic tapes were significantly easier to use and could contain far more data than paper tapes. This has transformed the broadcasting industry by allowing live broadcasts to be recorded and replayed at any time [5]. It was not until 1969 that memory storage technology was substantially influenced by the invention of magnetic discs, which can store databases and vast volumes of data. As a result, floppy discs were inspired by magnetic discs, which were portable and generally available to the public. Flash drives, which are based on Erasable Programmable Read Only Memory (EPROM) and Electrically Erasable Programmable Read Only Memory (EEPROM) technologies, are no longer rare in today’s technology [14, 15]. Future predictions for system memory and memory storage technologies have focused on a few advancements, including the ReRAM technology, which was projected to replace flash memory.

3. Resistive switching in memory devices

3.1 Bipolar and unipolar switching

The rapid switching speed, lower power consumption, and excellent scalability, ReRAM has emerged as the most encouraging choice for the future use of non-volatile devices [1, 2]. A dual terminal ReRAM device has an insulating layer wedged between two conducting electrodes. An external electric field can cause the memory cell to flip between two resistance states, known as the low resistance state (LRS, ON state) and high resistance (HRS, OFF state) state [16]. ReRAM is divided into two switching modes: unipolar and bipolar. The polarity of the switching voltage is inconsequential in unipolar switching, whereas in bipolar switching, the electrical polarity required to change from an HRS to an LRS is the inverse of that required to switch from an LRS to an HRS. Switching materials such as organics, binary oxides, and perovskite oxides, have been studied to achieve ReRAM with reliable switching, high ON/OFF current ratio, and long retention period [17, 18, 19].

Non-volatile memory (NVM) is a classification of computer memory that cannot be deleted or removed even when the power is turned off. In today’s technology, NVMs are typically utilized for long-term data storage or secondary storage. Flash drives, magnetic storage devices, ferroelectric RAM, magnetoresistive RAM, ReRAM, and optical discs are examples of NVM devices. In today’s storage technology, NAND (Boolean operator and logic gate) flash memory are widely used in most goods. However, the demand for faster writing speeds, higher density, and lower cost drives new research into developing technologies such as ReRAM. In the late 1990s and early 2000s, researchers investigated ReRAM technology, which allows RS between two resistance states to be exchanged via a thin film layer [20]. It works on the principle of applying a high voltage across a dielectric to transition from insulating to conductive qualities via a conduction filament pathway. Classification of ReRAM mechanism as memristors is controversial [21]. Theoretically, their RS curves differ from each other. Figure 2 shows a typical current-voltage (I-V) characteristic obtained for a ReRAM device [2]. Such classifications of ReRAM and memristors are still a source of contention today.

Figure 2.

(a) The predicted and typical types of (a) unipolar switching and (b) bipolar switching ReRAM curves. Adapted from Figure 4 [2].

The memristor is the fourth fundamental circuit element identified, after resistors, capacitors, and inductors. Chua et al. suggested the notion of memristor in 1971 [22], and it was validated in 2008. There are several materials having memristive characteristics which have been discovered since 1962. Memristors, like resistance in Ohms, are defined as:

$$M=\frac{d\phi }{dq}$$E1

where φ is magnetic flux and q is electric charge. As observed in its theoretical memristor curves, memristor curves are typically non-linear devices. Real-world memristors, on the other hand, have a comparable I-V characteristic to ReRAM. Some researchers hypothesized that such behavior was caused by conducting filament [23], active memristor [24], and non-zero crossing [25]. ReRAM curves, on the other hand, can be classified as bipolar or unipolar switching. To generate the RS curve, unipolar switching uses the same polarity of the swept bias with variable magnitudes, but bipolar switching requires various polarities [26]. ReRAM devices, in general, consist of an insulator layer sandwiched between two electrodes. Conduction pathways were formed by the flow of charge carriers alternating between the cathode and anode, which was induced by several physical mechanisms that are still commonly utilized today. Furthermore, the conduction methods differ depending on the materials utilized and the device’s overall architecture. Szot et al. were the first to detect conductive filament using an electron microscope, which they attributed to filament build-up and rupture [27].

3.2 Write-once-read-many (WORM)

WORM devices, in general, are devices in which data that has been written cannot be manipulated or removed. WORM memory is used in fields such as healthcare, security, taxation, and accounting where the data cannot be tampered with or updated to secure information. The most prevalent WORM devices in use today are the Compact Disc Recordable (CD-R) and Digital Versatile Disc Recordable (DVD-R). Furthermore, the “read-many” element implies that the device’s data can be read an infinite number of times, with the only limitation being the device lifetime. The WORM memory pixels are read according to the rows, with an unwritten pixel labeled as logical “0” and a written pixel labeled as logical “1” [3].

Today’s WORM memory technology is based on the electrically or laser programmed fuse WORM type. However, in current WORM research, the emphasis was shifted to organic materials or solution process techniques in order to achieve rapid switching rate, lower power consumption,, large storage density, simplicity, and being cost-effective, which has been dubbed WORM RS [28, 29]. Unlike ReRAM, the intrinsic features of WORM RS were sufficient to oppose the applied electric field, resulting in an irreversible shift. A typical WORM RS I-V curve obtained when a voltage is supplied is shown in Figure 3 [30]. The change between resistance states (OFF to ON) is an irreversible process, indicating WORM features.

Figure 3.

A typical WORM curve transiting from OFF to ON process. Adapted from Figure 3 [30].

3.3 Multilevel resistive switching

The potential of multilayer RS effect has been documented in several inorganic materials due to its superior memory performance, but the difficult fabrication method and stiffness restrict the development for ReRAM [31, 32, 33]. Organometal halide perovskites (OHPs) have recently sparked a lot of interest in the ReRAM community because to the high flexibility, variable band gaps, huge absorption coefficients, and long electron-hole diffusion length [34, 35]. Furthermore, OHPs highly feature defect-tolerant, simple, and cost-effective solution-processed procedures for fabricating the OHPs layers. As a result, they envision it being used in multilevel RS, which is advantageous for multilevel data storage. Typically, these devices feature numerous resistance states that can be changed within the device. Figure 4 shows a typical multilevel perovskite memory consisting of a all-inorganic CsPb_1–xBi_xI₃ perovskite film [36]. This reveal that multilevel RS was accomplished by altering the reset stop voltages.

Figure 4.

I-V curves of Au/KCl-MAPIC/ITO/glass indicating two V_SETs of 0.8 V and 1.0 V. Adapted from figure 3 [36].

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 ps 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.

Figure 5.

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

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 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 La_0.1Bi_0.9FeO₃(LBFO) layer has been introduced on a 0.7 wt% Nb-doped SrTiO₃ (001) single-crystal substrate to form a Pt/La_0.1Bi_0.9FeO₃/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 SrFeO_x as a model, the system demonstrated that in the ON state, SrFeO₃ nanofilaments are produced and stretch essentially through the BM SrFeO_2.5 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/SrFeO_x/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.

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, TiO_x/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 La_0.7Sr_0.3MnO₃ (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.

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].

An active layer for resistive memory has been developed using CsPbBr₃ single-crystal 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 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 CH₃NH₃PbI_3−xCl_x/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.

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 CH₃NH₃PbI_3−xCl_x (MAPbI_3−xCl_x), 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 MAPbI_3−xCl_x 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/CsPbCl_xBr_x (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 V_SET and V_RESET 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 transistor-based 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 V_SET, 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.

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].

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 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 plate-like 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 Bi₆Ti_2.8Fe_1.52Mn_0.68O₁₈ (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 all-inorganic 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.

Acknowledgments

This work was financially supported by the AUA-UAEU Joint Research Grant Project (IF016-2021 and G00003485), UNITEN BOLD grant J5150050002/20021170, Fundamental Research Grant Scheme (Project No.: FP113-2019A), Geran Putra-Inisiatif Putra Muda (GP-IPM/2018/9667000) and SATU Joint Research Scheme (ST002-2021).