Nonvolatile memory devices prepared from sol-gel derived niobium pentoxide films

Effect of electrode materials on resistive switching behaviour of NbOx-based memristive devices

Memristive devices that rely on redox-based resistive switching mechanism have attracted great attention for the development of next-generation memory and computing architectures. However, a detailed understanding of the relationship between involved materials, interfaces, and device functionalities still represents a challenge. In this work, we analyse the effect of electrode metals on resistive switching functionalities of NbOx-based memristive cells. For this purpose, the effect of Au, Pt, Ir, TiN, and Nb top electrodes was investigated in devices based on amorphous NbOx grown by anodic oxidation on a Nb substrate exploited also as counter electrode. It is shown that the choice of the metal electrode regulates electronic transport properties of metal-insulator interfaces, strongly influences the electroforming process, and the following resistive switching characteristics. Results show that the electronic blocking character of Schottky interfaces provided by Au and Pt metal electrodes results in better resistive switching performances. It is shown that Pt represents the best choice for the realization of memristive cells when the NbOx thickness is reduced, making possible the realization of memristive cells characterised by low variability in operating voltages, resistance states and with low device-to-device variability. These results can provide new insights towards a rational design of redox-based memristive cells.

Resistive switching and role of interfaces in memristive devices based on amorphous NbOx grown by anodic oxidation

Memristive devices based on the resistive switching mechanism are continuously attracting attention in the framework of neuromorphic computing and next-generation memory devices. Here, we report on a comprehensive analysis of the resistive switching properties of amorphous NbO_x grown by anodic oxidation. Besides a detailed chemical, structural and morphological analysis of the involved materials and interfaces, the mechanism of switching in Nb/NbO_x/Au resistive switching cells is discussed by investigating the role of metal-metal oxide interfaces in regulating electronic and ionic transport mechanisms. The resistive switching was found to be related to the formation/rupture of conductive nanofilaments in the NbO_x layer under the action of an applied electric field, facilitated by the presence of an oxygen scavenger layer at the Nb/NbO_x interface. Electrical characterization including device-to-device variability revealed an endurance >10³ full-sweep cycles, retention >10⁴ s, and multilevel capabilities. Furthermore, the observation of quantized conductance supports the physical mechanism of switching based on the formation of atomic-scale conductive filaments. Besides providing new insights into the switching properties of NbO_x, this work also highlights the perspective of anodic oxidation as a promising method for the realization of resistive switching cells.

Resistive random access memory: introduction to device mechanism, materials and application to neuromorphic computing

The modern-day computing technologies are continuously undergoing a rapid changing landscape; thus, the demands of new memory types are growing that will be fast, energy efficient and durable. The limited scaling capabilities of the conventional memory technologies are pushing the limits of data-intense applications beyond the scope of silicon-based complementary metal oxide semiconductors (CMOS). Resistive random access memory (RRAM) is one of the most suitable emerging memory technologies candidates that have demonstrated potential to replace state-of-the-art integrated electronic devices for advanced computing and digital and analog circuit applications including neuromorphic networks. RRAM has grown in prominence in the recent years due to its simple structure, long retention, high operating speed, ultra-low-power operation capabilities, ability to scale to lower dimensions without affecting the device performance and the possibility of three-dimensional integration for high-density applications. Over the past few years, research has shown RRAM as one of the most suitable candidates for designing efficient, intelligent and secure computing system in the post-CMOS era. In this manuscript, the journey and the device engineering of RRAM with a special focus on the resistive switching mechanism are detailed. This review also focuses on the RRAM based on two-dimensional (2D) materials, as 2D materials offer unique electrical, chemical, mechanical and physical properties owing to their ultrathin, flexible and multilayer structure. Finally, the applications of RRAM in the field of neuromorphic computing are presented.

Ternary Flash Memory with a Carbazole-Based Conjugated Copolymer: WS2 Composites as Active Layers

For nonvolatile memory devices, the design and synthesis of their substrate materials are very important. Due to the versatility and large-area fabrication of the low-temperature spin coating process, organic/inorganic nanomaterials as active layers of memory devices have been deeply studied. Inorganic nanoparticles can engage in interactions with polymers via external voltage. WS₂ NPs have a large specific surface area and good conductivity. They can be used as the charge trap center in the active layer, which is conducive to the charge transfer in the active layer. Poly[2,7-9-(9-heptadecanyl)-9H-carbazole-co-benzo[4,5] imidazole[2,1-α] isoindol-11-one] (PIIO) was synthesized via the Suzuki coupling reaction. ITO/PIIO/Al and ITO/PIIO:WS₂ NP/Al devices were prepared by the spin coating method and vacuum evaporation technology. All devices showed tristable switching behavior. The influence of the WS₂ mass fraction on memory performance was studied. The device composite with 6 wt % WS₂ NPs showed the best storage features. The OFF/ON1/ON2 current ratio was 1: 1.11 × 10¹: 2.03 × 10⁴, and the threshold voltage V_th1/V_th2 was -0.60 V/-1.05 V. The device is steady for 12,000 s in three states-high-resistance state (HRS), intermediate state (IRS), and low-resistance state (LRS). After reading 3500 times, the switch-state current displayed no obvious attenuation. This work shows that the polymer and its composites have broad prospects in next-generation nonvolatile storage.

Recent Progress in Solution-Based Metal Oxide Resistive Switching Devices

Metal oxide resistive switching memories have been a crucial component for the requirements of the Internet of Things, which demands ultra-low power and high-density devices with new computing principles, exploiting low cost green products and technologies. Most of the reported resistive switching devices use conventional methods (physical and chemical vapor deposition), which are quite expensive due to their up-scale production. Solution-processing methods have been improved, being now a reliable technology that offers many advantages for resistive random-access memory (RRAM) such as high versatility, large area uniformity, transparency, low-cost and a simple fabrication of two-terminal structures. Solution-based metal oxide RRAM devices are emergent and promising non-volatile memories for future electronics. In this review, a brief history of non-volatile memories is highlighted as well as the present status of solution-based metal oxide resistive random-access memory (S-RRAM). Then, a focus on describing the solution synthesis parameters of S-RRAMs which induce a massive influence in the overall performance of these devices is discussed. Next, a precise analysis is performed on the metal oxide thin film and electrode interface and the recent advances on S-RRAM that will allow their large-area manufacturing. Finally, the figures of merit and the main challenges in S-RRAMs are discussed and future trends are proposed.

Silicon oxide-niobium oxide mixture films and nanolaminates grown by atomic layer deposition from niobium pentaethoxide and hexakis(ethylamino) disilane

Amorphous SiO₂-Nb₂O₅ nanolaminates and mixture films were grown by atomic layer deposition. The films were grown at 300 °C from Nb(OC₂H₅)₅, Si₂(NHC₂H₅)₆, and O₃ to thicknesses ranging from 13 to 130 nm. The niobium to silicon atomic ratio was varied in the range of 0.11-7.20. After optimizing the composition, resistive switching properties could be observed in the form of characteristic current-voltage behavior. Switching parameters in the conventional regime were well defined only in a SiO₂:Nb₂O₅ mixture at certain, optimized, composition with Nb:Si atomic ratio of 0.13, whereas low-reading voltage measurements allowed recording memory effects in a wider composition range.

Characterization of electron beam deposited Nb2O5 coatings for biomedical applications

Niobium oxide coatings deposited on Ti₆Al₄V substrates by electron beam deposition and annealed in air at 600 °C and 800 °C were evaluated for their suitability towards dental, maxillofacial or orthopaedic implant applications. A detailed physico-chemical properties investigation was carried out in order to determine their elemental and phase composition, surface morphology and roughness, mechanical properties, wettability, and corrosion resistance in simulated body fluid solution (pH = 7.4) at room temperature. The biocompatibility of the bare Ti₆Al₄V substrate and coated surfaces was evaluated by testing the cellular adhesion and viability/proliferation of human osteosarcoma cells (MG-63) after 72 h of incubation. The coatings annealed at 800 °C exhibit more phase pure nanocrystalline Nb₂O₅ surfaces with enhanced wettability, reduced porosity and enhanced corrosion resistance properties making them good candidate for dental, maxillofacial or orthopaedic implant applications.

Inorganic nanoparticle multilayers using photo-crosslinking layer-by-layer assembly and their applications in nonvolatile memory devices

We introduce a general and facile method for the preparation of organic/inorganic nanoparticle (NP) nanocomposite multilayer films that allows vertical growth of various NP layers (i.e., metal or transition metal oxide NPs) in a densely packed structure. Our approach is based on the successive photo-crosslinking layer-by-layer (LbL) assembly between hydrophobic ligands onto a NP surface and photoinitiator (PI) molecules. Therefore, our approach requires neither the additional surface modification needed for well-defined NPs synthesized in organic media nor the deposition step that inserts a polymer layer bridge between adjacent inorganic NP layers in the preparation of traditional LbL-assembled NP films. We also demonstrate that photo-crosslinking LbL-assembled (metal oxide NP)n films could be used as a nonvolatile memory layer without a high-temperature thermal treatment, unlike conventional vacuum-deposition- or sol-gel-derived memory devices, which require thermal treatments at temperatures greater than 200 °C. This robust method could open a facile route for the design of functional NP-based electronic devices.