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This paper describes proposal of ReRAM switching mechanism, development of production tool for ReRAM sputtering and improvement in TaOx-ReRAM switching characteristics. We propose that a ReRAM-cell has stack a structure in which an oxygen vacancy supply layer (TaOx) and an oxygen accumulation layer (Ta2O5) sandwiched by the top and bottom electrodes. Resistance change of the ReRAM-cell is caused by the oxygen vacancies migrating between the TaOx and the Ta2O5 layers by applied voltage. This prediction corresponded to the experimental facts. The thickness of Ta2O5 film sputtered by a mass production tool had good uniformity (±1.0%) and excellent stability (±1.0%). Also the sheet resistance uniformity (1σ) of TaOx film had 3.6%. By examining the sputtering conditions, the ReRAM-cell having a Ta2O5/TaOx bi-layer operated in less than 100μA with a forming-free and had excellent endurance property to 1010 cycles at 50nsec.
We have fabricated a ferroelectric resistive switching device of Pt/Bi1-δFeO3 (BFO)/SrRuO3 (SRO) in which the conductivity of BFO layer was controlled by changing the Bi-deficiency concentration. The devices showed a bipolar-type resistive switching effect, i.e., zero-crossing hysteretic current–voltage (I–V) characteristics. In addition, the I–V characteristics in both high and low resistance states are nonlinear, which can avoid a read-error problem in a passive crossbar memory array. Resistive switching characteristics measured in pulse-voltage mode revealed that the resistance values in low resistance states vary with the amplitude and duration time of the pulsed-voltage stresses, indicating possibility of multilevel switching. On the basis of the experimental results, we discuss the potential of the Pt/BFO/SRO device for application in a large-capacity nonvolatile memory.
We present a model for organic bistable devices (OBDs) embedded with metallic nanoparticles. In particular, two device architectures have been studied: a single layer device with metallic nanoparticles dispersed in a organic material matrix and a three layer device where two organic material regions are separated by a layer of heavy packed nanoparticles. The model describes the different behavior, the internal charge and potential distributions in the ON-OFF states. The OFF state is represented by charged nanoparticles forming a space charge layer which limits the current. The ON state occurs with neutral nanoparticles.
Physical properties of filaments in Cu/HfO2/Pt conducting-bridge memory (CB-RAM) were investigated basing on direct observation by conducting atomic force microscopy (C-AFM) and energy dispersive X-ray spectroscopy (EDS), R-T characteristics until liquid nitrogen temperature, and I-V characteristics both in air and in vacuum. As a result, physical picture of filaments in Cu/HfO2/Pt structures was revealed. Filaments consist of Cu containing large residual resistance and the cross-sectional area of the filament, Sfila, was roughly proportional to set voltage, Vset, even when current compliance was kept constant. Interestingly, resistivities of filaments are same among all the filaments in different samples and are invariant even after repetitive switching that changes resistance of the filaments. Cu/HfO2/Pt obeyed the universal relation that reset current, Ireset, is proportional to the inverse of resistance in a low resistance state, 1/RLRS, which is known to be applicable to oxygen-migration-based resistive switching memories such as Pt/NiO/Pt. Considering the invariance of resistivity of the filament, this suggests the fact that Ireset is decided dominantly by Sfila. In addition, it was suggested that moisture is necessary for dissolution and migration of Cu to form filaments.
We proposed and computationally analyzed a nonvolatile power-gating field programmable gate array (NVPG-FPGA) based on pseudo-spin-transistor architecture with spin-transfer-torque magnetic tunnel junctions (STT-MTJs). The circuit employs nonvolatile static random memory (NV-SRAM) cells and nonvolatile flip-flops (NV-FFs) as the storage circuits. The circuit configuration and microarchitecture are compatible with SRAM-based FPGAs, and the additional nonvolatile memory functionality makes it possible to execute efficient power-gating (PG). Break-even time (BET) for the nonvolatile configuration logic block (NV-CLB) of the NVPG-FPGA was also analyzed, and reduction techniques of the BET were proposed, which allows highly efficient PG operations with a fine granularity.
Resistive switches are being explored as a candidate for ultra-dense memory as well as logic circuits. The advantages of the resistive switches include high switching speed and excellent scaling potential. Here, we report for the first time the switching behavior of anti-parallel connected resistive switches (APS), which is a composite device exhibiting bi-directional switching properties. Under the opposite voltage biases, the two anti-parallel cells are alternatively set and reset, rendering the APS switched in both directions. For appropriate ON resistance values and set and reset voltages the two anti-parallel switches can be both set in conductive states. An APS device can be realized in a single switch by two coexisting Cu and oxygen vacancy nanofilaments which are formed and ruptured under opposite voltage polarities. The described APS behavior is of interest to logic applications and in neural networks.
Constant voltage Time-Dependent Forming (TDF) measurements in as-deposited Pt/NiO/Pt stack structures have been conducted. From TDF characteristics, formation of conductive filaments at forming process by applying voltage follows weakest link theory. Furthermore, weakest spots are almost randomly distributed in NiO thin films according to Poisson statistics, each of which can contribute conductive paths locally generated. A “percolating layer” in which the conductive filaments percolate by applying voltage may exist in the NiO thin film. The thickness of the layer is much smaller than that of NiO thin films.
Resistively switching TiO2 thin films show a multitude of resistance states, which are achieved during the programming and erasing of a memory cell. These resistance states depend on the applied voltage and the allowed current. Additionally, the operation time has a relevant influence on the adjusted resistance. This parameterization points out a potential application in future multi-level cell memory systems, but also determines the persistence of the non-volatile nature and provides an additional insight into the physics of the resistance switching. Our devices consist of metal-insulator-metal stacks made of Pt/TiO2/Ti/Pt, which are built up in crosspoint junctions. The maximum programming current and the maximum erase voltage amplitude were used to tune in the low resistance and high resistance state, respectively, in combination with the operation time. The corresponding dependencies were determined by quasi-static voltage sweeps, pulse bursts and single pulses of up to 4 V and down to 10 ns.
A novel resistive device with a floating electrode (RFED) has been manufactured as a stack of layers Cu/TaOx/Pt/TaOx/Cu in a crossbar array comprising two single resistive switches merged antiserially at the common inert Pt floating electrode. The device exhibits four states HRS/HRS, HRS/LRS, LRS/HRS and LRS/LRS, where HRS and LRS are the high and low resistance states, respectively, with only LRS/LRS being fully conductive. When the voltage on one Cu electrode is increased to a value Vth-on(A), a conductive nanofilament (CF) in switch A is formed, while suppressing CF formation in switch B. When the voltage is then extended to negative bias, a sudden jump in the I-V characteristics is observed at Vth-on(B), when the 2nd CF in switch B is formed rendering the RFED device fully conductive. If the current surge just after the formation of CF in 2nd switch exceeds the reset current of the 1st CF, the 1st CF ruptures shortly thereafter, i.e. the conductive state is destroyed as soon as it has been created. This property proves valuable for applications in neural networks where a generation of a current pulse at a critical threshold is required. The height and the time width of the firing pulse is an inherent property of the device and can be controlled by the parameters of the individual switches and their set/reset operations.
The interactions and ordering of oxygen vacancies in rutile TiO2 were thoroughly investigated by density functional calculations to search for atomic configurations of the conductive filament. As random isolated vacancies could not support the low-resistance state conduction in TiO2 ReRAM, vacancy ordering was introduced in  and  directions of the lattice to study the electronic structures. The calculation results revealed that a di-vacancy chain in  direction makes the electrons delocalized in that direction, which is identified as a possible configuration of the conductive filament. This low-resistance state can be effectively disrupted by moving oxygen vacancies out of the filament to reach high-resistance states.
The structures and energies of stoichiometric and oxygen-deficient monoclinic HfO2 were calculated using density functional theory. The electronic interactions in HfO2 were calculated using the LDA+U and GGA+U formalisms, where on-site Coulomb corrections were applied to the 5d electrons of hafnium (Ud) and the 2p electrons of oxygen (Up). Properties calculated using these techniques are compared to results obtained from LDA, GGA, hybrid functionals, and experiment. Ultimately, we show that LDA+Ud+Up and GGA+Ud+Up calculations of HfO2’s electronic and structural properties achieve a level of accuracy on par with much more computationally demanding hybrid functional techniques, such as PBE0 and HSE06.
The resistive switching properties of CMOS compatible TiN/HfO2/TiN resistive-random-access-memory (ReRAM) devices have been investigated after exposure to 1 MeV proton radiation. The HfO2-based ReRAM devices were found to have high total-ionizing-dose (TID) radiation tolerance up to 5 Grad(Si). TiN/HfO2/TiN ReRAM performance parameters include high-resistance state (HRS) resistance, low-resistance state (LRS) resistance, set and reset voltages. HfO2-based ReRAM devices exhibited no degradation in these performance parameters following proton irradiation exposure with TID from 105 to 109 rad(Si). Furthermore, the HfO2-based ReRAM devices exhibited more uniform resistive switching behavior with increased TID. Based on this radiation response it is proposed that the resistive switching mechanism in TiN/HfO2/TiN – trap-assisted tunneling associated with Hf-rich conducting filament formation – may be reinforced through proton exposure which acts to stabilize the formation/rupture of Hf-rich filaments. The high radiation tolerance of HfO2-based ReRAM devices suggests such devices may be potentially attractive for aerospace and nuclear applications.
Research in non-volatile memories (NVM) has intensified in the past few years due to the ever increasing demand for information storage and the near ubiquity of handheld electronics. Resistive memory is a leading contender in this NVM market due to its high endurance, random accessibility, scalability and low programming voltage.
The addition of an external series resistor or imposing current compliance is often used to limit the current through RRAM devices and to prevent “over-programming” and stuck-at-one (SA1) errors. Here, we demonstrate that utilizing an external series resistor is not efficient in preventing over-programming and an on-chip resistor is more desirable.
Poly-silicon bottom electrode based devices (with the poly-silicon electrode acting like an on-chip resistor) and metal bottom electrode devices were fabricated and tested. The presence of the on-chip resistor is shown to enhance the endurance of the RRAM device. This technique of including an on-chip resistor prevents stored current discharge through the device as the device transitions from a high resistance to a low resistance state. A SPICE simulation is also employed to illustrate the benefit of this approach.
Exact locations of conductive filaments formed in NiO-based resistive switching (RS) cells were detected by C-AFM, and their electrical as well as chemical properties were investigated. After a forming process, a part of top electrodes of Pt/NiO/Pt RS cells is deformed. NiO layers are also deformed, and conductive spots, i.e. filaments have been found preferentially along the edges of deformations. Detailed C-AFM investigation has revealed that variation of cell resistances originates from differences in size and shape of filaments, not their resistivity. Furthermore, cross-sectional TEM analysis has demonstrated that filaments determining cell resistance consist of reduced NiO with an inclusion of Pt.
We simulate the thermodynamics and kinetics of the drift/diffusion of oxygen vacancy defects in rutile TiO2, using the density-functional based tight-binding (DFTB) method. Both static and dynamic simulations have been performed. Results indicate that DFTB is well suited to examine the dynamic behavior of oxygen vacancies in TiO2. Detailed analysis shows, that strong model size dependence in relative diffusion barrier heights between different diffusion processes requires great care in defect diffusion simulations in TiO2. Thermodynamic results on the influence of an external electric field show that, due to the large dielectric constant, the coulomb driving force on oxygen vacancy diffusion is very small. Dynamic simulation of the influence of electric fields on the diffusion requires the use of advanced molecular dynamics acceleration schemes.
We investigated the fabrication and the memory characteristics of metal-oxide-semiconductor (MOS) capacitors with GaN quantum-dots (QDs) embedded in the gate insulator. The GaN-QDs, which act as discrete charge storage nodes, were deposited by radio-frequency molecular-beam-deposition (RF-MBD). The influence of the deposition dose on the QDs size and density was investigated by TEM studies. Subsequent electrical characterization measurements on memory capacitors revealed enhanced electron charge trapping leading to significant memory windows. Charge retention measurements at room temperature showed that the sample with the lowest concentration of QDs exhibits a significant programming window after ten-years.
We propose a new nonvolatile resistance device having a metal/Al2O3/3C-SiC/n-Si/metal metal-insulator-semiconductor (MIS) structure. It is explained that the electron trapping states are generated in the Al2O3/3C-SiC interface region of the 3C-SiC layer due to partial oxidation of the 3C-SiC near the interface, and that the on and off states of the device are caused by trapping and detrapping of electrons in the defect states through the Al2O3 layer. The electron capture in the defect states causes high electric-field in the oxide layer which results in high-rate electron tunneling through the oxide layer and lowering the device resistance. We have previously reported the similar memory behavior with a metal/SiO2/SiOx/3C-SiC/n-Si/metal MIS structure, however the new memory exhibits more enhanced endurance characteristics than those of the previous memory, where the trapped electrons are injected and ejected through the 3C-SiC layer.
The effects of temperature and moisture on the resistive switching characteristics of oxide-based atomic switches were investigated to reveal their switching mechanism. The observed temperature variations of the SET voltages can be qualitatively explained by the classical nucleation theory. The moisture absorption in oxides results in the formation of a hydrogen-bond network at grain boundaries, and metal ions are likely to migrate along the grain boundaries. Depending on the strength of hydrogen bonds in oxides, the atomic switches exhibit a different switching behavior to ambient conditions.
In this work, we report on the use ion of implantation to synthesize resistive memory oxides. The surface of copper thin films was converted to copper oxide using oxygen implantation. Devices fabricated from the copper oxide (CuxO) layers exhibited unipolar switching behavior without the need for a forming voltage. Technology scaling was demonstrated by oxygen implanting copper damascene vias. Unipolar switching was observed in via-based devices down to 48 nm. The current-voltage data of devices scaled from 100 μm to 48 nm suggests that the RESET transition is related to localized Joule heating. Tantalum oxide (TaxOy) was also created by oxygen implantation but exhibited bipolar resistive switching. Analysis of the conduction suggests that the difference between the two resistance states in these devices is largely due to a lowering of the Pt-TaxOy Schottky barrier.