Effect of Annealing Temperature for Ni/AlOx/Pt RRAM Devices Fabricated with Solution-Based Dielectric

Resistive random access memory (RRAM) devices with Ni/AlOx/Pt-structure were manufactured by deposition of a solution-based aluminum oxide (AlOx) dielectric layer which was subsequently annealed at temperatures from 200 °C to 300 °C, in increments of 25 °C. The devices displayed typical bipolar resistive switching characteristics. Investigations were carried out on the effect of different annealing temperatures for associated RRAM devices to show that performance was correlated with changes of hydroxyl group concentration in the AlOx thin films. The annealing temperature of 250 °C was found to be optimal for the dielectric layer, exhibiting superior performance of the RRAM devices with the lowest operation voltage (104), the narrowest resistance distribution, the longest retention time (>104 s) and the most endurance cycles (>150)

Bismuth halide thin films for resistive random access memory device

Resistive random-access memory (RRAM) is a kind of highly promising non-volatile memory technology. Recently, halide perovskites have aroused attention worldwide because of their outstanding resistive switching performance and ease of fabrication. The advantages of the halide perovskite devices include high ON/OFF ratio and low operation voltage, enabling excellent device performance with low power consumption. Currently, the most widely studied halide perovskites contain lead, which is a toxic element that may incur serious environmental problems and significant harm to human health. In order to address these issues, there is a pressing need to develop lead-free halide perovskites and their derivatives possessing comparable functional properties to their lead-based counterparts. Bismuth-based halide perovskites have emerged as a promising lead-free alternative for applications in RRAM. A great advantage of bismuth-based halide perovskites lies in their high solubility for various elements, thus offering the possibility of the formation of modified compositions to tailor the resistive switching behaviours including ON/OFF ratio, endurance and retention. Cs3Bi2I9 and MA3Bi2I9 (MA = methylammonium) are two common lead-free perovskite halides that have been widely studied for RRAM. However, doping in Cs3Bi2I9 and MA3Bi2I9 is normally conducted on a single chemical site (either A-site or X site) and the impact of co-doping on their resistive switching properties remains less explored. In this project, thin films of several co-doped compositions namely MA2CsBi2BrxI9-x (x=2, 3, 4, 5, 6, 7, 8) were prepared to investigate the double doping (Cs on A-site, Br on X-site) effects on their structural, morphological and electrical properties. In addition, the effect of different top electrodes (Ag and Au) on the electrical performance of the MA2CsBi2BrxI9-x thin films was also studied. It was found that more uniform and denser thin films could be obtained with an increase in Br content. Among the several compositions under investigation, MA2CsBi2Br8I-based thin film with Au top electrodes exhibited typical resistive switching behaviour and an interface-type conduction mechanism. When the perovskites layer was covered by Ag top electrodes, the distinct resistive switching behaviour could be observed with the increase of I content, which could be attributed to the redox reaction of Ag electrodes and iodide ions at the interface between electrodes and the active layer. Compared to other compositions, MA2CsBi2Br2I7-based thin film with Ag electrodes exhibited an outstanding ON/OFF ratio of around 105. Since the MA2CsBi2Br8I perovskite had good endurance and full-coverage surface, the MA2CsBi2Br8I perovskite was employed for further study. Au/MA2CsBi2Br8I/ITO devices with different thicknesses (290 nm, 307 nm, 341 nm and 435 nm) showed stable bipolar resistive switching behaviours. With the increasing thickness, the SET electric field remains around 6.5 V/μm, which is nearly independent of film thickness. When the thickness of the MA2CsBi2Br8I perovskite layer increased from 136 nm to 307 nm, the device demonstrated better stability over 100 cycles and a higher ON/OFF ratio (~10) at a low reading voltage of 0.27 V

Status and Prospects of ZnO-Based Resistive Switching Memory Devices

In the advancement of the semiconductor device technology, ZnO could be a prospective alternative than the other metal oxides for its versatility and huge applications in different aspects. In this review, a thorough overview on ZnO for the application of resistive switching memory (RRAM) devices has been conducted. Various efforts that have been made to investigate and modulate the switching characteristics of ZnO-based switching memory devices are discussed. The use of ZnO layer in different structure, the different types of filament formation, and the different types of switching including complementary switching are reported. By considering the huge interest of transparent devices, this review gives the concrete overview of the present status and prospects of transparent RRAM devices based on ZnO. ZnO-based RRAM can be used for flexible memory devices, which is also covered here. Another challenge in ZnO-based RRAM is that the realization of ultra-thin and low power devices. Nevertheless, ZnO not only offers decent memory properties but also has a unique potential to be used as multifunctional nonvolatile memory devices. The impact of electrode materials, metal doping, stack structures, transparency, and flexibility on resistive switching properties and switching parameters of ZnO-based resistive switching memory devices are briefly compared. This review also covers the different nanostructured-based emerging resistive switching memory devices for low power scalable devices. It may give a valuable insight on developing ZnO-based RRAM and also should encourage researchers to overcome the challenges

Memristive Non-Volatile Memory Based on Graphene Materials

Resistive random access memory (RRAM), which is considered as one of the most promising next-generation non-volatile memory (NVM) devices and a representative of memristor technologies, demonstrated great potential in acting as an artificial synapse in the industry of neuromorphic systems and artificial intelligence (AI), due its advantages such as fast operation speed, low power consumption, and high device density. Graphene and related materials (GRMs), especially graphene oxide (GO), acting as active materials for RRAM devices, are considered as a promising alternative to other materials including metal oxides and perovskite materials. Herein, an overview of GRM-based RRAM devices is provided, with discussion about the properties of GRMs, main operation mechanisms for resistive switching (RS) behavior, figure of merit (FoM) summary, and prospect extension of GRM-based RRAM devices. With excellent physical and chemical advantages like intrinsic Young’s modulus (1.0 TPa), good tensile strength (130 GPa), excellent carrier mobility (2.0 × 105 cm2∙V−1∙s−1), and high thermal (5000 Wm−1∙K−1) and superior electrical conductivity (1.0 × 106 S∙m−1), GRMs can act as electrodes and resistive switching media in RRAM devices. In addition, the GRM-based interface between electrode and dielectric can have an effect on atomic diffusion limitation in dielectric and surface effect suppression. Immense amounts of concrete research indicate that GRMs might play a significant role in promoting the large-scale commercialization possibility of RRAM devices

Tailored electrical characteristics in multilayer metal-oxide-based-memristive devices

Auf Mehrlagen-Metalloxiden basierende memristive Bauelemente sind einer der vielversprechendsten Kandidaten für neuromorphes Computing. Allerdings stellen spezifische Anwendungen des neuromorphen Computings unterschiedliche Anforderungen an die memristiven Bauelemente. Eine ungelöste Herausforderung in der technologischen Entwicklung ist daher das maßgeschneiderte Design von memristiven Bauelementen für spezifische Anwendungen. Insbesondere die unterschiedlichen Materialien des Schichtstapels erschweren die Herstellungsprozesse aufgrund einer großen Anzahl von Parametern, wie z. B. der Stapelsequenzen und -dicken und der Qualität sowie der Eigenschaften der einzelnen Schichten. Daher sind systematische Untersuchungen der einzelnen Bauelementparameter besonders entscheidend. Darüber hinaus müssen sie mit einem tiefgreifenden Verständnis der zugrundeliegenden physikalischen Prozesse kombiniert werden, um die Lücke zwischen Materialdesign und elektrischen Eigenschaften der resultierenden memristiven Bauelemente zuschließen. Um memristive Bauelemente mit unterschiedlichen resistiven Schalteigenschaften zu erhalten, werden verschiedene Abfolgen und Kombinationen von drei Metalloxidschichten (TiOx, HfOx, und AlOx) hergestellt und untersucht. Zunächst werden einschichtige Oxidbauelemente untersucht, um Kandidaten für mehrschichtige Stapel zu identifizieren. Zweitens werden zweischichtige TiOx/HfOx Oxidbauelemente hergestellt. Anhand von systematischen Experimenten und statistischen Analysen wird gezeigt, dass die Stöchiometrie, die Dicke, und die Fläche des Bauelements die Betriebsspannungen, die Nichtlinearität beim resistiven Schalten und die Variabilität beeinflussen. Drittens werden TiOx/AlOx/HfOx-basierte Bauelemente hergestellt. Durch das Hinzufügen von AlOx in die zweischichtigen Oxidstapel weisen diese dreischichtigen Bauelemente optimale elektrische Eigenschaften für den Einsatz in neuromorpher Hardware auf, wie z. B. elektroformierungsfreies und strombegrenzungsloses Schalten sowie eine lange Lebensdauer. Die entwickelten memristiven Bauelemente werden in Systeme, wie Kreuzpunkt-Strukturen und Ein-Transistor-ein-Memristor-Konfigurationen integriert. Hier wird die Eignung für effizientes neuromorphes Computing bewertet. Außerdem werden Methoden zur stufenlosen analogen Einstellung des Widerstands der Bauelemente demonstriert. Diese Eigenschaft ermöglicht effiziente neuromorphe Rechenschemata. Diese umfassende Studie beleuchtet die Beziehung zwischen den Bauelementparametern und den elektrischen Eigenschaften von mehrschichtigen memristiven Bauelementen auf Metalloxidbasis. Auf dieser Grundlage werden maßgeschneiderte Methoden für spezifische neuromorphe Anwendungen entwickelt.Multilayer metal-oxide-based-memristive devices are one of the most promising candidates for neuromorphic computing. However, specific applications of neuromorphic computing call for different requirements for memristive devices. Therefore, an open challenge in technological development is the tailored design of memristive devices for specific applications. In particular, multilayer stacks complicate fabrication processes due to a large number of device parameters such as staking sequences and thicknesses, quality, and property of each layer. Therefore, systematic investigations of the individual device parameters are particularly decisive. Moreover, they need to be combined with a profound understanding of the underlying physical processes to bridge the gap between material design and electrical characteristics of the resulting memristive devices. To obtain memristive devices with different resistance switching characteristics, various sequences and combinations of three metal oxide layers (TiOx, HfOx, and AlOx) are fabricated and studied. First, single-layer oxide devices are investigated to find desirable multilayer stacks for memristive devices. Second, TiOx/HfOx-based bilayer oxide devices are fabricated. Via systematic experiments and statistical analysis, it is shown that the stoichiometry, thickness, and device area influence operating voltages, non-linearity in resistive switching, and variability. Third, TiOx/AlOx/HfOx-based devices are fabricated. By adding AlOx into the bilayer oxide stacks, these trilayer devices present favorable electrical features for use in neuromorphic hardware, such as electroforming-free and compliance-free switching as well as long retention. The developed memristive devices are integrated into systems such as crossbar structures and one-transistor-one-memristor configurations. Here, suitability for efficient neuromorphic computing is assessed. Also, methods to tune the device resistance gradually in an analog fashion are demonstrated. This feature allows for efficient neuromorphic computation. This comprehensive study highlights the relationship between device parameters and electrical properties of multilayer metal-oxide-based memristive devices. On this basis, tailoring methodologies are established for specific neuromorphic applications

Perspective: Zinc-Tin Oxide Based Memristors for Sustainable and Flexible In-Memory Computing Edge Devices

As the Internet of things (IOT) industry continues to grow with an ever-increasing number of connected devices, the need for processing large amounts of data in a fast and energy-efficient way becomes an even more pressing issue. Alternative computation devices such as resistive random access memories (RRAM), or memristors, started taking centre stage as prime candidates to tackle this issue due to their in-memory computation capabilities. Amorphous oxide semiconductors (AOSs), more specifically eco-friendly zinc-tin oxide (ZTO), show great promise as a memristive active material for flexible and sustainable applications due to its low required fabrication temperature, amorphous structure, low-cost, and critical-raw-material-free composition. In this perspective article, the research progress on ZTO-based memristors is reviewed in terms of device structure and material compositions. The effects on the electrical performance of the devices are studied. Additionally, neuromorphic and optoelectronic capabilities are analyzed with the objective of finding the best approaches toward implementing these devices in novel computing paradigms

Low-temperature amorphous oxide semiconductors for thin-film transistors and memristors: physical insights and applications

While amorphous oxides semiconductors (AOS), namely InGaZnO (IGZO), have found market application in the display industry, their disruptive properties permit to envisage for more advanced concepts such as System-on-Panel (SoP) in which AOS devices could be used for addressing (and readout) of sensors and displays, for communication, and even for memory as oxide memristors are candidates for the next-generation memories. This work concerns the application of AOS for these applications considering the low thermal budgets (< 180 °C) required for flexible, low cost and alternative substrates. For maintaining low driving voltages, a sputtered multicomponent/multi-layered high-κ dielectric (Ta2O5+SiO2) was developed for low temperature IGZO TFTs which permitted high performance without sacrificing reliability and stability. Devices’ performance under temperature was investigated and the bias and temperature dependent mobility was modelled and included in TCAD simulation. Even for IGZO compositions yielding very high thermal activation, circuit topologies for counteracting both this and the bias stress effect were suggested. Channel length scaling of the devices was investigated, showing that operation for radio frequency identification (RFID) can be achieved without significant performance deterioration from short channel effects, which are attenuated by the high-κ dielectric, as is shown in TCAD simulation. The applicability of these devices in SoP is then exemplified by suggesting a large area flexible radiation sensing system with on-chip clock-generation, sensor matrix addressing and signal read-out, performed by the IGZO TFTs. Application for paper electronics was also shown, in which TCAD simulation was used to investigate on the unconventional floating gate structure. AOS memristors are also presented, with two distinct operation modes that could be envisaged for data storage or for synaptic applications. Employing typical TFT methodologies and materials, these are ease to integrate in oxide SoP architectures

Size-Dependent Metal-insulator Transition in Pt-Dispersed Sio2 Thin Film: A Candidate for Future Non-Volatile Memory

Non-volatile random access memories (NVRAM) are promising data storage and processing devices. Various NVRAM, such as FeRAM and MRAM, have been studied in the past. But resistance switching random access memory (RRAM) has demonstrated the most potential for replacing flash memory in use today. In this dissertation, a novel RRAM material design that relies upon an electronic transition, rather than a phase change (as in chalcogenide Ovonic RRAM) or a structural change (such in oxide and halide filamentary RRAM), is investigated. Since the design is not limited to a single material but applicable to general combinations of metals and insulators, the goal of this study is to use a model material to delineate the intrinsic features of the electronic metal/insulator transition in random systems and to demonstrate their relevance to reliable memory storage and retrieval. We fabricated amorphous SiO2 thin films embedded with randomly dispersed Pt atoms. Macroscopically, this random material exhibits a percolation transition in electric conductivity similar to the one found in various insulator/metal granular materials. However, at Pt concentrations well below the bulk percolation limit, a distinct insulator to metal transition occurs in the thickness direction as the film thickness falls below electron’s “diffusion” distance, which is the tunneling distance at 0K. The thickness-triggered metal-to-insulator transition (MIT) can be similarly triggered by other conditions: (a) a changing Pt concentration (a concentration-triggered MIT), (b) a changing voltage/polarity (voltage-triggered MIT), and (c) an UV irradiation (photon-triggered MIT). The resistance switching characteristics of this random material were further investigated in several device configurations under various test conditions. These include: materials for the top and bottom electrodes, fast pulsing, impedance spectroscopy, static stressing, retention, fatigue and temperature from 10K to 448K. The SiO2-Pt RRAM exhibits fast switching speed (~25 ns), high resistance ratio (\u3e100), long retention time/write time ratio (\u3e1012), multi-bit storage and extraordinary performance reproducibility. The device switches by a purely electronic mechanism: electron trapping makes it an insulator; charge detrapping returns it to a metal. The switching voltages are low, ~ 1 V, and are independent of size, thickness, composition, temperature and write/erase time. The insulator state has a conductance that exponentially decays with the thickness