Forming‐Free ZnO‐Based Resistive Random Access Memory with Supercritical Fluid Hydrogenation Technique

In this study, an advanced supercritical fluid (SCF) technique that is suitable for materials and electronic devices with low temperature (80 °C) and high pressure (4000 psi) is proposed for a ZnO-based resistive random access memory (RRAM). After an SCF treatment, X-ray photoelectron spectroscopy (XPS) analysis confirms that the concentration of oxygen ions increases in the zinc oxide thin film. Moreover, electrical measurements confirm that the treated ZnO-based random access memory exhibits a lower forming voltage, lower set voltage, and higher ratio of high and low resistance states compared with a device without treatment. Next, the carrier transport mechanism is investigated by fitting the current–voltage curves. Finally, physical models are introduced to illustrate the extra oxygen ions that create better memory characteristics in the random access memory device after the SCF treatment. This significant SCF technique increases the content of oxygen ions without altering the original element ratio and maintains a simple device structure. Thus, the technique shows potential for feasible material improvement capable of effective real applications in the near future.

Zinc oxide (ZnO) is an amorphous oxide semiconductor (AOS) material valued for its high carrier mobility, transparency to visible light, and low-temperature manufacturing process compared to silicon-based semiconductors. These desired features make ZnO-based AOSs widely popular as the channel layer in TFTs and as key dielectric layers in electronic devices like light sensors, gas detectors, resistive random-access memory (RRAM), and more. Among them, Conductive Bridge Random Access Memory (CBRAM) with a dielectric switching layer sandwiched between the electrochemically active metal (Ag, Cu) top electrode and the inert bottom electrode, is of a class of RRAM that offers low-voltage operation, high-density storage device, non-volatile, and compatibility with standard CMOS processes [1]. The CBRAM operates based on the principle of conductive filament formation and rupture. During operation, conductive filaments will be formed in switching layer by connecting metal ions which diffuse from the electrode after applying a positive bias. The device will switch to a low resistance state (LRS) after the conductive filaments were formed. After this, applying a negative bias will break the conductive filaments and switch the device back to a high resistance state (HRS). The HRS and LRS are the 0 or1 state of the memory operation. Studies and works have shown that controlling the distribution and diffusion of electrode metal ions in the switching layer is challenging because the conductive filaments formed can vary significantly in strength, ranging from robust to fragile [2]. This leads to instability in the operation of memory devices and encountering difficulties in the integration with TFT pixel array for memory-in-pixel (MiP) power-saving displays [3] or CMOS-integrated compute-in-memory applications [4].In this work, we proposed a novel CBRAM device featuring ZnO nanostructures as the resistive switching layer. The combination of a sputter-deposited seed layer with the water bath method enables the production of high-quality ZnO nanostructures at temperatures below 90°C. These nanostructures play a crucial role in constraining metal ion diffusion and filament formation, leading to the formation of uniform conductive filaments. This improvement significantly enhances the operational stability of the CBRAM device. The experimental results have demonstrated superior memory characteristics with a highly uniform distribution achieved successfully in the ZnO nanostructure-embedded CBRAM, compared to its counterpart lacking ZnO nanostructures. Furthermore, post-treatments are implemented in this study to further promote electrical performance of the nanostructure-embedded CBRAM, like operation voltage, memory window, retention, and endurance. Reference: [1] C. Gopalan, Y. Ma, T. Gallo, J. Wang, E. Runnion, J. Saenz, F. Koushan, P. Blanchard, S. Hollmer, “Demonstration of Conductive Bridging Random Access Memory (CBRAM) in logic CMOS process”, Solid-State Electronics, vol. 58 (1), p. 54-61, 2011.[2] C. C. Hsu, P. T. Liu, K. J. Gan, D. B. Ruan, Simon M. Sze, “Investigation of deposition technique and thickness effect of HfO2 film in bilayer InWZnO-based conductive bridge random access memory”, Vacuum, vol. 201, 111123, 2022.[3] S. H. Lee, B. C. Yu, H. J. Chung, and S. W. Lee, “Memory-in-pixel circuit for low-power liquid crystal displays comprising oxide thin-filmtransistors,” IEEE Electron Device Lett., vol. 11 (11), pp. 1551–1554, 2017.[4] W. Wan, R. Kubendran, C. Schaefer, et al., “A compute-in-memory chip based on resistive random-access memory”, Nature, vol. 608, p. 504–512, 2022.

Resistance random access memory (RRAM) based on resistive switching in metal oxides has attracted considerable attention as a promising candidate for next-generation nonvolatile memory due to its high operating speed, superior scalability, and low power consumption. However, some operating parameters of RRAM cannot meet the practical requirement, which impedes its commercialization. A lot of experimental results show that doping is an effective method of improving the performance of RRAM, while the study on the physical mechanism of doping is rare. It is generally believed that the formation and rupture of conducting filaments, caused by the migration of oxygen vacancies under electric field play a major role in resistive switching of metal oxide materials. In this work, the first principle calculation based on density functional theory is performed to study the effects of transition metal element X (X=Mn, Fe, Co, and Ni) doping on the migration barriers and formation energy of oxygen vacancy in ZnO. The calculation results show that the migration barriers of both the monovalent and divalent oxygen vacancy are reduced significantly by Ni doping. This result indicates that the movement of oxygen vacancies in Ni doped ZnO is easier than in undoped ZnO RRAM device, thus Ni doping is beneficial to the formation and rupture of oxygen vacancy conducting filaments. Furthermore, the calculation results show that the formation energy of the oxygen vacancy in ZnO system can be reduced by X doping, especially by Ni doping. The formation energy of the oxygen vacancy decreases from 0.854 for undoped ZnO to 0.307 eV for Ni doped ZnO. Based on the above calculated results, Ni doped and undoped ZnO RRAM device are prepared by using pulsed laser deposition method under an oxygen pressure of 2 Pa. The Ni doped ZnO RRAM device shows the optimized forming process, low operating voltage (0.24 V and 0.34 V for Set and Reset voltage), and long retention time (>104 s). Set and Reset voltage in Ni doped ZnO device decrease by 80% and 38% respectively compared with those in undoped ZnO device. It is known that the density of oxygen vacancies in the device is dependent on the oxygen pressure during preparation. The Ni doped ZnO RRAM device under a higher oxygen pressure (5 Pa) is also prepared. The Ni doped ZnO RRAM device prepared under 5 Pa oxygen pressure shows a little higher Set and Reset voltage than the device prepared under 2 Pa oxygen pressure, while the operating voltages are still lower than those of undoped ZnO RRAM. Thus, the doping effect in the ZnO system is affected by the density of oxygen vacancies in the device. Our work provides a guidance for optimizing the performance of the metal oxide based RRAM device through element doping.

Effects of antimony doping and film preferred crystal orientation on the performance and stability of electro-plated cuprous oxide (Cu2O) resistive random access memory (RRAM) devices were studied. Previous study indicated that electro-plated Cu2O with a small amount of antimony doping improves film uniformity, surface roughness1. The Sb doped film (Cu2O:Sb) also has a smaller grain size as compared to that of an un-doped cuprous oxide (u-Cu2O) film at same PH plating solution. SEM image of the grain boundaries of the Cu2O:Sb film also indicated that the film is more vertically oriented as compare to that of the u-Cu2O film. As some literatures reported, the conductive path (or charge hopping distance) of the RRAM is likely shorter if electrons hopping take place along the vertical-like grain boundaries. It will also reduce the voltage to set the conductive path formation (or set up of the charge hopping path). Moreover, uniform film and smoother surface are likely to have a stable set/reset performance because more uniform charge hopping distance and/or hopping site distribution.Our studies show that the forming voltages of a 700nm electroplated Cu2O:Sb (~2 at. %) and u-Cu2O (strongly (111) oriented) films are about 4V and 7V, respectively. Some of Cu2O:Sb RRAMs are forming free. The set voltages (Vset) of Cu2O:Sb and u-Cu2O RRAMs are ranging from 1.5V to 2.6V and from 1.2V to 9V, respectively. The reset voltages (Vreset) are 1.2V ~ 1.5V and 1V ~ 2V, respectively. The ratio of high/low resistance state (HRS/LRS) for both of the RRAMs is about two orders of magnitude, and no decay is observed for retention test of 5000 seconds at 0.1V. These results indicate that Sb doping does reduce forming (likely forming free) and significantly stabilize its set voltage.Surface morphology and texture of a (200)-preferred Cu2O film is very different from (111)-preferred film2 − 4. For a (200)-preferred film, its grains’ boundaries are more perpendicular to the plating surface (or electrode). This kind of film texture is also likely can help forming shorter conductive paths and/or stabilize the charge hopping resistance. It is similar to our Cu2O:Sb film texture as previous described. For Cu2O:Sb film, the XRD intensity ratio (R) of (111) to (200) phase is ~1. While the u-Cu2O is strongly (111)-preferred film with R= 12. Preferred crystal orientation change will give rise to varying in vacancy, defects, and trap charge distribution along the hopping paths. These variations can affect charge hopping resistance and its characteristics. Charge hopping in film with vertical-like grain boundaries is believed to be a shorter and easier path.Performance of preferred crystal orientation dependence of the u-Cu2O RRAM is investigated. A (200)-preferred u-Cu2O film with R= 0.185 was used. Its RRAM performance improved in forming voltage of ~5V and in HRS/LRS ratio with three orders of magnitude.In short, Cu2O:Sb film can be a good candidate for RRAM application because of its low cost, potential forming free, low set/reset voltages and stability. Sb doping may have helped forming shorter conductive paths due to its vertical like gains boundaries, less charge hopping resistance due to more uniformly distributed Sb induced hopping sites. Improved performance of (200)-preferred u-Cu2O RRAM may result from vacancy, defects and trap charge distribution change along the hopping paths.Reference 1. Baek, S. K., Kwon, Y. H., Shin, J. H., Lee, H. S., & Cho, H. K. “Low‐Temperature Processable High‐Performance Electrochemically Deposited p‐Type Cuprous Oxides Achieved by Incorporating a Small Amount of Antimony” Advanced Functional Materials, 25, (2015) 52142. J. Siegfried and K.S. Choi, “Electrochemical Crystallization of Cuprous Oxide with Systemic Shape Evolution” Advanced Materials, 16 (2004) 17433. E. Rakhshani and J. Varghese “Surface texture in electrodeposited films of cuprous oxide” J. Materials Science 23 (1988) 38474. H. Lee, I.C. Leu, C.L. Liao, K.Z. Fung “The structural evolution and electrochemical properties of the textured Cu2O thin films” J. Alloy Compounds 436 (2007) 241

Resistive random access memory (ReRAM) devices are considered to be one of the most promising candidates for the next generation of nonvolatile memory devices because of their superior properties such as low power consumption, simple structure, high integration density, and fast operation speed. In this study, we used zinc oxide (ZnO) thin films to fabricate ReRAM devices comprising Au/ZnO/Al and Al/Au/ZnO/Al structures. We observed that the operating stability of the device containing the Al/Au/ZnO/Al structure improved. The devices displayed effective and stable operational characteristics. The operational voltage of the ZnO ReRAM devices was less than 3 V, and the reset operational voltage was less than −1.5 V.

Zinc oxide (ZnO) thin films have been reported to suffer from degradation in electrical properties, when exposed to elevated heat and humidity, often leading to failures of electronic devices containing ZnO films. This degradation appears to be linked to water and oxygen penetration into the ZnO film. However, a direct observation in the ZnO film morphological evolution detailing structural and chemical changes has been lacking. Here, we systematically investigated the chemical and morphological heterogeneities of ZnO thin films caused by elevated heat and humidity, simulating an environmental aging. X-ray fluorescence microscopy, X-ray absorption spectroscopy, grazing incidence small angle and wide angle X-ray scattering, scanning electron microscopy (SEM), ultra-high-resolution SEM, and optical microscopy were carried out to examine ZnO and Al-doped ZnO thin films on two different substrates—silicon wafers and flexible polyethylene terephthalate (PET) films. In the un-doped ZnO thin film, the simulated environmental aging is resulting in pin-holes. In the Al-doped ZnO thin films, significant morphological changes occurred after the treatment, with an appearance of platelet-shaped structures that are 100–200 nm wide by 1 μm long. Synchrotron x-ray characterization further confirmed the heterogeneity in the aged Al-doped ZnO, showing the formation of anisotropic structures and disordering. X-ray diffraction and X-ray absorption spectroscopy indicated the formation of a zinc hydroxide in the aged Al-doped films. Utilizing advanced characterization methods, our studies provided information with an unprecedented level of details and revealed the chemical and morphologically heterogeneous nature of the degradation in ZnO thin films.

The resistive random access memory (RRAM) device has been widely studied due to its excellent memory characteristics and great application potential in different fields. In this paper, resistive switching materials, switching mechanism, and memory characteristics of RRAM are discussed. Recent research progress of RRAM in high-density storage and nonvolatile logic application are addressed. Technological trends are also discussed.

ZnO based RRAM performance enhancement by 100 MeV Ag9+ irradiation

Widely utilized polyester fibers generally have excellent color fastness, making it quite challenging to controllably modify their color strength ( K / S value), especially in regulated decolorization. In this study, the relation between the color strength and the relative dye concentration of dyed polyester fabrics was investigated based on the principle that dyes may migrate and reach redistribution equilibrium through supercritical carbon dioxide fluid treatment. The results demonstrated that the relation agreed well with an exponential decay fitting, allowing for the controllable modification of the color strength of dyed polyester fabrics using the derived regression formulas. Furthermore, the correlation between the residual dye in the supercritical fluid treatment vessel and the color strength of the previously treated fabric was clarified. To verify the universality of the above strategies, three dyes with representative anthraquinone, heterocyclic, and azo structures were employed in this study. Furthermore, it was observed that the dyes were evenly diffused throughout the polyester fibers following the supercritical fluid treatment, promising the fibers’ good color fastness. These findings might be significant for exploring the dyeing behavior of fibers in supercritical fluids, stimulating the reutilization of colored polyester and carbon neutrality of textiles, and expanding the potential applications of supercritical fluid technology. • Exponential decay describes K/S vs. dye concentration for dyed polyester fabrics. • The K/S value of dyed polyester fabrics was controllably modified. • The residual dye in the scCO 2 treatment vessel was clarified. • Three dyes with representative structures were employed.

Resistive random access memory (ReRAM) is a novel form of non-volatile memory expected to replace FLASH memory in the near future. As a further step toward use of ReRAM in computer architectures, we have developed a CMOS/ReRAM hybrid process, integrating ReRAM into a CMOS process flow. In this study, we investigate yield, reliability, endurance, threshold voltages (forming, set and reset voltages) of ReRAM devices implemented in the back end of the line (BEOL). The focus of this study is on single ReRAM devices operated with an external transistor (1T1R configuration). Yield and endurance of these 1T1R configured devices show a strong dependence on device position on the wafer, the device size, and the current compliance used during the initial forming and subsequent set sweeps. Elevated temperatures resulted in a shift of the optimum operation conditions and could be used to improve device performance. Variance in performance of the ReRAM devices was directly correlated with manufacturing issues and material properties to gain maximum information for future improvements

Transition metal doped Zinc oxide (ZnO) thin films with wide band gap semiconducting nature have diverse range of applications including, gas sensors, optical and optoelectronic devices, electronics and spintronics spintronic devices etc. In the present study, Manganese (Mn)‐doped ZnO thin films deposited on glass substrates using RF‐magnetron sputtering have been investigated for their optical and electrical behavior aiming at resistive random access memory applications. To study the influence of Mn doping and correlation between the structural and the physical properties of the films, the samples were characterized by X‐ray Diffraction (XRD), Raman spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive X‐ray Spectroscopy (EDS), UV‐VIS spectrophotometry and X‐ray photoelectron spectroscopy (XPS). The films are found to be crystalline and a decrease in lattice parameter from 2.6049 Å to 2.5845 Å, an increase in optical band gap from 3.27 eV to 3.35 eV and a decrease in Urbach energy from 0.222 eV to 0.171 eV, is observed with increase in Mn‐concentration. The electrical performance of the film with highest Mn‐content is found to be more suitable for resistive memory applications. Tailoring the electrical behavior of the film by incorporating Mn as dopant is an important approach to find suitable material combination for novel memory devices.

Effect of Al doping on structural and optical properties of atomic layer deposited ZnO thin films

In the rapidly evolving landscape of modern electronics, memory devices and circuits play a pivotal role in determining system performance, energy efficiency, and scalability. Traditional memory technologies, such as Dynamic Random Access Memory (DRAM) and NAND flash, face challenges related to scaling limits, latency, power consumption, and endurance, particularly as demands for faster and more efficient data storage grow. Emerging non-volatile memory (NVM) technologies like Resistive Random Access Memory (ReRAM), Phase-Change Memory (PCM), and Spin-Transfer Torque Magnetic Random Access Memory (STT-MRAM) offer potential solutions to these challenges, but their combination into existing systems poses design and manufacturing hurdles. The primary problem lies in balancing storage performance with power efficiency while ensuring scalability for advanced applications such as artificial intelligence (AI), Internet of Things (IoT), and high-performance computing (HPC). Innovations in circuit design, including advanced error-correction mechanisms, adaptive voltage scaling, and low-power architectures, are critical in enhancing the overall performance of memory systems. This study employs a hybrid approach, combining novel circuit design techniques with the combination of emerging NVM technologies. Simulations were performed to evaluate the performance metrics, including write latency, energy consumption, and endurance. Results show that ReRAM-based memory designs achieved a 35% reduction in write latency and a 50% improvement in energy efficiency compared to traditional DRAM, while PCM demonstrated superior endurance with up to 10^8 write cycles. Furthermore, STT-MRAM circuits showed promise with a 40% reduction in standby power consumption. These advancements in memory circuits and devices highlight the potential for significant improvements in storage performance and efficiency, addressing the growing needs of modern electronic systems and paving the way for future innovations in computing.

Random telegraph noise (RTN) is an important intrinsic phenomenon of any logic or memory device that is indicative of the reliability and stochastic variability in its performance. In the context of the resistive random access memory (RRAM), RTN becomes a key criterion that determines the read disturb immunity and memory window between the low (LRS) and high resistance states (HRS). With the drive towards ultra-low power memory (low reset current) and aggressive scaling to 10 × 10 nm2 area, contribution of RTN is significantly enhanced by every trap (vacancy) in the dielectric. The underlying mechanisms governing RTN in RRAM are yet to be fully understood. In this study, we aim to decode the role of conductance fluctuations caused by oxygen vacancy transport and inelastic electron trapping and detrapping processes. The influence of resistance state (LRS, shallow and deep HRS), reset depth and reset stop voltage (VRESET-STOP) on the conductance variability is also investigated.

The present investigation deals with the NO2 gas detection via nanostructured zinc oxide (ZnO) thin films harvested over a glass substrate via a simple and facile successive ion layer adsorption and reaction (SILAR) approach. ZnO nanostructured thin films are developed by different SILAR cycles (ZnO:15, ZnO:30, ZnO:45, ZnO:60). The physicochemical, optical, and morphological properties of prepared thin films are explored by various characterization techniques. The phase purity and grains size are confirmed by X‐ray diffraction (XRD) and Scherrer formula, which reveal the hexagonal phase structure with variant grain size (≈16–21 nm). The surface morphology of deposited films shows the well‐dispersion of ZnO nanoparticles, which is picturized by field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), attenuated total reflectance (ATR). UV–Vis and FTIR spectroscopy elaborates on the optical properties and functional groups of ZnO films, respectively. Further, the sensing ability of ZnO thin films is studied with different concentrations (50–250 ppm) of NO2 gas in the span of 50 ppm by recording the resistance transient. The ZnO:30 thin‐film displays an enhanced gas sensing response of 1.42 GΩ at 100 °C compared to other synthesized films and at different temperature ranges. The ZnO:30 film shows the highly selective response toward the NO2 gas compared to other interference gas species. Thus, the present study highly recommends the investigated method for the ZnO thin film formation to be employed toward NO2 gas sensing.