Evolution of conductive filament and its impact on reliability issues in oxide-electrolyte based resistive random access memory.

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.

Ferromagnetic conductive filaments (CFs) were formed in a conductive-bridge random access memory (CBRAM) with a Ni electrode using high current compliances during a set process. We investigated CFs in a Ni/HfO2/Pt CBRAM using the current compliance dependences of the set process, low-temperature characteristics, and anisotropic magnetoresistance (AMR). Set processes occurred when a positive bias was applied to the Ni electrode only; therefore, the switching phenomena showed polarity. The resistance of the ON state (low resistance state) was dependent on the current compliance between 2 and 5 mA. The ON state of the device showed a metallic conduction property, suggested by the temperature dependence of resistance. When a high current compliance (5 mA) was used for programming, the ON state showed AMR, which was direct evidence of ferromagnetic CF formation. This suggests that the formation of a ferromagnetic CF is associated with the accumulation of Ni ions that diffused from the Ni electrode. The OFF-state (high resistance state) resistance slightly increased with decreasing temperature and AMR was not observed.

Conductive-bridging random access memory (CBRAM) using a cobalt (Co) electrode has recently featured a CMOS-compatible process, excellent data retention, and a sub-μA operating current level, which are difficult to achieve by conventional CBRAM. However, the resistive switching (RS) mechanism of Co CBRAM has not been extensively explored compared to that of the conventional CBRAM cells using Ag, Cu, or Ni as active metal electrodes. Because only implicit inferences based on electrical measurements have been made, the formation of Co filaments is not yet clearly understood. This study presents evidence of Co filament formation in Co/10 nm SiOx/TiN resistive random access memory (RRAM) using direct transmission electron microscopy (TEM) observations. Co protrusions larger than 5 nm are observed, and their exact atomic composition is investigated by spectroscopy. We explain the RS operation of Co/SiOx/TiN cells based on the Co electromigration-mediated filament development process through Co electrode deformation, protrusion, and subsequent Co conductive bridge formation. An asymmetric RS operation and negative temperature correlation of the resistance are found in low resistance state (LRS) cells, which further supports the Co-involved RS operation in Co/SiOx/TiN devices.

Here, we present the synaptic characteristics of AlN-based conductive bridge random access memory (CBRAM) as a synaptic device for neuromorphic systems. Both non-volatile and volatile memory are observed by simply controlling the strength of the Cu filament inside the AlN film. For non-volatile switching induced by high compliance current (CC), good retention with a strong Cu metallic filament is verified. Low-resistance state (LRS) and high-resistance state (HRS) conduction follow metallic Ohmic and trap-assisted tunneling (TAT), respectively, which are supported by I–V fitting and temperature dependence. The transition from long-term plasticity (LTP) to short-term plasticity (STP) is demonstrated by increasing the pulse interval time for synaptic device application. Also, paired-pulse facilitation (PPF) in the nervous system is mimicked by sending two identical pulses to the CBRAM device to induce STP. Finally, potentiation and depression are achieved by gradually increasing the set and reset voltage in pulse transient mode.

In resistive random access memory devices (RRAM), materials selection plays a critical role in the tuning of properties like switching voltages, retention, and endurance. Aside from choosing between transition metal-oxide matrices like HfOx, TiOx, TaOx, NiOx, and AlOx, doping in the oxide can also be a powerful tool for device optimization. However, only a handful of dopants have been studied in detail [1-3], and it is currently not known which will offer the greatest benefit for device performance. The filamentary switching mechanism in RRAM devices, in which charged oxygen vacancies (VO’s) coalesce into conductive filaments during switching, has been widely documented [4-9]. Dopants have been shown to affect the switching characteristics of RRAM devices by decreasing the formation energy of VO’s and potentially stabilizing conductive filaments [1-3]. In this investigation, first principles modeling is used to characterize 42 possible dopants in HfOx. In addition to quantifying their effects on HfOx devices, trends are extracted which can serve as general guidelines for doping in any transition metal-oxide, and which are applicable to conductive bridge random access memories (CBRAM) as well. This work identifies a critical new metric for predicting dopant behavior in RRAM and CBRAM: the relative favorability of the dopant forming on substitutional or interstitial sites in the oxide lattice. Dopants that form on interstitial sites, such as Ag, Cu, and Ni, also tend to form cation filaments and thus CBRAM devices. Conversely, dopants that strongly prefer to substitute onto the Hf lattice site are unlikely to produce competing cation filaments, leaving the VO-based switching mechanism intact. The relative favorability is also independent of dopant chemical potential, allowing a direct one-to-one comparison of different dopant species in HfOx. In this work, the quantity of relative stability is formulated and quantified over all 42 cation dopants. The results are shown in Fig. 1. The impact of these dopants on VO formation energy is also analyzed, as shown in Fig. 2. The effect of dopants on device properties is strongly dependent upon dopant species, and Fig.’s 2 and 3 clearly show that these effects follow predictable trends. Finally, the trends underlying dopant behavior are explored, and a model is developed to predict dopant behavior in the oxide system. The behavior of the dopant was found to vary predictably as a function of six key quantities: (1) the valence difference between the dopant and Hf, (2) the dopant atomic radius, (3) the dopant oxide enthalpy of formation, (4) the change in dopant coordination from native oxide to HfOx, (5) the magnetization of the doped system, and (6) the perturbation of neighboring ion charge states by the dopant. The derived composite function predicts the relative dopant site favorability with 95% accuracy. Since these quantities can all be calculated for different oxides, it is expected that these trends can be used as guidelines for doping in any transition metal-oxide for both RRAM and CBRAM applications. [1] D. Duncan, B. Magyari-Köpe, and Y. Nishi, Appl. Phys. Lett. 108, 043501, 2016. [2] L. Zhao, S. W. Ryu, A. Hazeghi, D. Duncan, B. Magyari-Köpe, and Y. Nishi, Tech. Digest VLSI Tech. Symp., T106-T107, 2013. [3] D. Ning, P. Hua, and W. Wei, Chin. Phys. B 23(10), 107306, 2014. [4] D. Duncan, B. Magyari-Köpe, and Y. Nishi, Elec. Dev. Lett. 37 (4), 400-403, 2016. [5] B. Magyari-Köpe, S.G. Park, H.-D. Lee, and Y. Nishi, J. Mater. Sci. 47, pp. 7498-7514, 2012. [6] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B, 82, 115109, 2010. [7] H.-D. Lee, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B 81, 193202, 2010. [8] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Tech. Digest VLSI Tech. Symp., 2011. [9] B. Magyari-Köpe, M. Tendulkar, S.G. Park, H.-D. Lee, and Y. Nishi, Nanotechnology 22, 254029, 2011. Figure 1

The impact of a TaOx nanolayer at the GeSex/W interface on the performance of resistive switching memory in an Al/Cu/GeSex/TaOx/W structure has been examined. All materials and the memory structure have been investigated using high-resolution transmission electron microscopy, energy dispersive x ray spectroscopy, and x ray photo-electron spectroscopy analyses. A conically shaped crystalline Cu (111) nanofilament with a diameter of around 17 nm in the TaOx nanolayer after a current compliance (CC) of 500 μA has been observed, and this has been also characterized by fast Fourier transform. The low resistance state (LRS) decreases as the current compliances (CCs) increased from 1 nA to 1 mA, since the nanofilament diameter increased from 0.04 to 23.4 nm. This is also estimated by bipolar resistive switching characteristics. The resistivity of this crystalline Cu nanofilament is approximately 2300 μΩ.cm. The nanofilament has a cylindrical shape, with CCs ranging from 1 nA to 10 μA and a conical shape with CCs ranging from 50 μA-1 mA. The resistive switching mechanism has been explained successfully under SET and RESET operations. Improved resistive switching parameters, such as SET voltage, LRS, and high resistance state with consecutive switching cycles are obtained and compared to those of pure GeSex and TaOx materials. Extrapolated, long program/erase endurance of > 106 cycles, attributed to the Al/Cu/GeSex/TaOx/W structure design, is observed. This resistive switching memory structure shows extrapolated 10 years data retention with a resistance ratio of > 10 at a low CC of 0.1 μA at 50 °C. A large memory size of ∼ 6 Pbit/sq. in. is obtained, considering the nanofilament diameter at a low CC of 0.1 μA. This study is important not only for improving the performance of low-power resistive switching memory, but also helpful for designing other nonvolatile memory devices.

Memristor effect in ZnO thin films was investigated. It was shown, that increase in the number of laser pulses during the formation of a thin ZnO film from 1000 to 3000 leads to increase resistance of ZnO film in the high resistance state (HRS) from 28.31±8.27 kΩ to 1943.53±123.11 kΩ and decrease resistance of ZnO film in the low resistance state (LRS) from 3.85±2.15 kΩ to 3.22±1.32 kΩ, respectively. Memristor structure fabrication technique was developed. Al2O3/TiN/ZnO/Ti memristor structure was fabricated and investigated. Resistive switching from HRS to LRS occurred at 0.4±0.1 V, and from LRS to HRS at -0.72±0.2 V. Endurance test shown that HRS is 72.41±6.22 kΩ, LRS is 1.05±0.32 kΩ. It was shown, that HRS/LRS ratio was about 69.7 at read voltage 0.3 V. As a result, Al2O3/TiN/ZnO/Ti memristor structure fabrication allowed to decrease switching voltage from 3.2±0.6 V to 0.4±0.1 V for SET, and from -3.5±1.1 V to -0.72±0.2 V for RESET, decrease current from 0.9±0.4 mA to 5.2±2.2 μA, and get less resistance dispersion, than Al2O3/TiN/ZnO structure.

Two-dimensional (2D) materials have received significant attention for their potential use in next-generation electronics, particularly in nonvolatile memory and neuromorphic computing. This is due to their simple metal-insulator-metal (MIM) sandwiched structure, excellent switching performance, high-density capability, and low power consumption. In this work, using comprehensive material simulations and device modeling, the thinnest monolayer hexagonal boron nitride (h-BN) atomristor is studied by using a MIM configuration with Ta electrodes. Our first-principles calculations predicted both a high resistance state (HRS) and a low resistance state (LRS) in this device. We observed that the presence of van der Waals (vdW) gaps between the Ta electrodes and monolayer h-BN with a boron vacancy (VB) contributes to the HRS. The combination of metal electrode contact and the adsorption of Ta atoms onto a single VB defect (TaB) can alter the interface barrier between the electrode and dielectric layer, as well as create band gap states within the band gap of monolayer h-BN. These band gap states can shorten the effective tunneling path for electron transport from the left electrode to the right electrode, resulting in an increase in the current transmission coefficient of the LRS. This resistive switching mechanism in monolayer h-BN atomristors can serve as a theoretical reference for device design and optimization, making them promising for the development of atomristor technology with ultra-high integration density and ultra-low power consumption.

The repeatability of set/reset errors has been investigated in 40nm TaOx based ReRAM cells. Errors of the Low Resistance State (LRS) in specific cells are observed repeatedly, and such cells are recovered by DC read operation. When error cells are recovered, the LRS cell current of the recovered cells shows a sudden jump up to large cell current in certain set cycles. Then, the High Resistance State (HRS) current is also recovered from almost no current to normal HRS current. The phenomena of repeating error can be explained as lack of oxygen vacancy in specific cells which have a wide conductive filament. Moreover, the error repeated cells have larger LRS cell current than the normal LRS cell current after recovery. Thus, the potential error repeated cells can be detected by cell current check sequences, which detect to repeat the larger LRS cell current. The failure rate can be reduced by replacing the error repeated cells with redundant cells.

The Mo/Si3N4:Pt/Pt nanometallic resistive switching devices with ultra-fast write/erase speed (<50 ns) were fabricated. Other than conventional electrical switching, a mechanical stress-induced switching was demonstrated. Such mechanical stress was provided by momentum transfer of 30 keV Ga+ ions in a focus ion beam system, enabling a one-way high resistance state (HRS) to low resistance state (LRS) transition. The capability of mechanical stress switching provides evidence that electron trapping/detrapping mechanism is responsible for nanometallic resistive switching. It was further demonstrated that HRS (trapping state) is a meta-stable state, while LRS (detrapping state) is a stable state. Strong mechanical stress facilitates local bond distortion in dielectric films and thus lowers the energy barrier between HRS and LRS, eventually leading to a barrier-less state transition. A quantitative model based on stress-mediated parallel conduction paths were established to provide a more accurate description of the resistive switching devices.

The effect of Gaussian noise on the switching of a ZrO2(Y) based memristor from the low resistance state (LRS) into the high resistance state (HRS) including transitions from the LRS into intermediate metastable states has been studied. The series of positive (with addition of the noise signal or without the one) and negative rectangular voltage pulses were used as the switching signals. The adding of noise to the switching signal initiated the switching of the memristor from the LRS into the HRS at smaller pulse magnitudes than in the case of switching by the rectangular pulses without adding the noise. A necessary (preset) HRS can be achieved passing the intermediate states by adding the noise with certain parameters to the rectangular switching pulses. The resistive switching is performed without application of adaptive switching protocols. The results of the present study can be applied in the development of innovative memristor switching protocols.

The low-frequency noise in a nanometer-sized virtual memristor consisting of a contact of a conductive atomic force microscope (CAFM) probe to an yttria stabilized zirconia (YSZ) thin film deposited on a conductive substrate is investigated. YSZ is a promising material for the memristor application since it is featured by high oxygen ion mobility, and the oxygen vacancy concentration in YSZ can be controlled by varying the molar fraction of the stabilizing yttrium oxide. Due to the low diameter of the CAFM probe contact to the YSZ film (∼10 nm), we are able to measure the electric current flowing through an individual filament both in the low resistive state (LRS) and in the high resistive state (HRS) of the memristor. Probability density functions (Pdfs) and spectra of the CAFM probe current in both LRS and HRS are measured. The noise in the HRS is found to be featured by nearly the same Pdf and spectrum as the inner noise of the experimental setup. In the LRS, a flicker noise 1/fγ with γ ≈ 1.3 is observed in the low-frequency band (up to 8 kHz), which is attributed to the motion (drift/diffusion) of oxygen ions via oxygen vacancies in the filament. Activation energies of oxygen ion motion determined from the flicker noise spectra are distributed in the range of [0.52; 0.68] eV at 300 K. Knowing these values is of key importance for understanding the mechanisms of the resistive switching in YSZ based memristors as well as for the numerical simulations of memristor devices.

Data retention characteristics and a failure mechanism of TaN∕CuxO∕Cu resistive memory device are investigated by a temperature-accelerated test method. Data retention capability at 85°C is sufficiently longer than 10years by using two different methods: simple extrapolation and Arrhenius equation. The high resistance state fails to low resistance state and low resistance state fails to high resistance state at the elevated temperature. It is attributed that different retention-failure mechanisms are responsible for high resistance state and low resistance state, respectively. A filament/charge trapped combined model is presented to clarify the retention-failure mechanism.

A highly transparent resistive random access memory with a configuration of multilayer graphene (MLG)/Dy2O3/indium tin oxide (ITO) structure is demonstrated in this paper. The fabricated device is transparent, with 80% transmittance at 550 nm. The MLG/Dy2O3/ITO device shows unipolar resistance switching with a low operation current ( 104), fast switching speed ( 200 cycles), which makes a step toward the realization of low-power transparent electronics for next-generation nonvolatile memory application. The MLG/Dy2O3/ITO device exhibited typical filamentary-conduction-type resistive random access memory behavior and no forming process is required. High resistance state (HRS) increases with a reduced device area while low resistance state (LRS) is insensitive to the device sizes. Moreover, Raman spectra obtained in pristine state, HRS, and LRS indicate that the lower power consumption of MLG/Dy2O3/ITO device is attributable to the formation of oxygen graphene layers.

In order to improve the performance of HfO x -based resistive random access memory (RRAM), a VO x buffer layer was introduced in the Cu/HfO x interface of Cu/HfO x /TiN RRAM cell in this paper. Their resistive switching characteristics (such as I–V characteristics, endurance and retention) and the switching mechanism were investigated. Results show that the VO x buffer layer acts as a barrier which avoids excessive Cu ion reaching to HfO x layer as result to improve the device performances. The current conduction mechanism of low resistive state (LRS) is Ohmic conduction while the high resistive state (HRS) is Schottky emission. Based on the negative temperature coefficient of LRS resistance and conduction mechanism, we believe that the resistive switching between HRS and LRS is attributed to the Cu-CF's formation and rupture.