Oxide-based Resistive Switching Research Articles

Mechanism of resistance distribution properties in oxide-based resistance switching nanodevice

Although oxide-based resistive switching memory (OxRAM) is one of the strong next-generation high-capacity memory candidates, it has an unresolved problem that the higher the resistance levels, the larger the cycle-to-cycle resistance variabilities to be operated in multi-resistance level mode. In this paper, we develop a simple theory from the operating principles of OxRAMs that excellently matches with experiments both qualitatively and quantitatively. From the theory, we can finally understand the fundamental capabilities of multi-resistance level operation of OxRAMs, and how to improve the switching properties within that.

Electrical evidence of multiple-filament formation in tantalum oxide-based resistive switching memory via a novel device structure

Resistive switching memories are the most promising candidate for next-generation nonvolatile memory devices. Resistive switching behavior is believed to be induced by the formation and rupture of conductive filaments between the electrodes, but the characteristics of the filaments remain controversial. Here, a clear evidence of multiple filaments in TaOx-based resistive switching devices is demonstrated using a pillar-type electrode with equivalent circuit analysis.

Classification of Three-Level Random Telegraph Noise and Its Application in Accurate Extraction of Trap Profiles in Oxide-Based Resistive Switching Memory

In oxide-based resistive switching memory (OxRAM), due to the existence of oxygen ions, electron-induced random telegraph noise (e-RTN) and oxygen ion-induced RTN (GR-RTN) coexist and cannot be distinguished directly from the current levels in typical two-level RTN signals. Thus, the accurate extraction of the trap location and energy level (XT, ET) based on the time constants from e-RTN is hindered, which impedes the further investigation of reliability. In this work, three-level RTN in TMOx-based OxRAM was investigated. GR-RTN and e-RTN were both observed and can be distinguished clearly by the comparison of the three discrete current levels. Also, especially for e-RTN, we discussed the bias dependency of time constants of the three-level e-RTN, and the vertical location and energy level of the trap corresponding to this three-level e-RTN were finally extracted. This extraction method after selecting e-RTN from all RTN signals provides a more accurate characterization result of the trap and will be helpful to the investigation of the reliability in OxRAM devices.

Enhancement of resistive switching ratio induced by competing interfacial oxygen diffusion in tantalum oxide based memories with metal nitride electrode

Oxide-based binary resistive switching memories using metal nitride as one of the electrodes usually have a limited ratio of the resistances of the high- and low-resistance states. Here, we propose a competing mechanism to enhance the switching ratio by modifying the high-resistance state with extra inherent interfacial oxygen diffusion against what happens at the oxide/nitride interface. This is implemented in Pt/ZrO2/Ta2O5/TaN bilayer structures, where a resistance ratio above 104, about one to two orders of magnitude greater than that in Pt/Ta2O5/TaN monolayer structures, is achieved. This competing mechanism is further corroborated by the failed enhancement in the switching ratio when using an altered stacking arrangement of the two oxide layers.

Patching of Lattice Defects in Two-Dimensional Diffusion Barriers

Two-dimensional crystals offer promise as diffusion barriers that can also facilitate electronic conduction through the barrier plane via tunneling. We present barriers in which crystal imperfections are patched, leaving the pristine regions of the crystal exposed and able to both prevent diffusion and allow electronic conduction. This is accomplished by atomic layer deposition, where nucleation of patch material is inhibited on the pristine crystal and promoted elsewhere. Demonstrations of the effectiveness of this technique are performed in the contexts of sulfur diffusion control in photovoltaic kesterite devices and oxygen diffusion control in oxide-based resistive switching devices.

Exploring oxygen-affinity-controlled TaN electrodes for thermally advanced TaOx bipolar resistive switching

Recent advances in oxide-based resistive switching devices have made these devices very promising candidates for future nonvolatile memory applications. However, several key issues remain that affect resistive switching. One is the need for generic alternative electrodes with thermally robust resistive switching characteristics in as-grown and high-temperature annealed states. Here, we studied the electrical characteristics of Ta2O5−x oxide-based bipolar resistive frames for various TaNx bottoms. Control of the nitrogen content of the TaNx electrode is a key factor that governs variations in its oxygen affinity and structural phase. We analyzed the composition and chemical bonding states of as-grown and annealed Ta2O5−x and TaNx layers and characterized the TaNx electrode-dependent switching behavior in terms of the electrode’s oxygen affinity. Our experimental findings can aid the development of advanced resistive switching devices with thermal stability up to 400 °C.

Direct Evidence for the Role of Oxygen Vacancy Configuration for Oxide-Based Resistive Switching Devices

The miniaturization of standard silicon-based memory technologies reaches physical limits in terms of size and power dissipation. Novel memory and computing architectures based on resistive switches are promising for future electronics beyond Von Neumann computing architecture. In oxide-based resistive switches, the memristance is associated with the movement of ionic carriers through oxygen vacancies in an oxide film at high electric field strength via structural defect modifications that are still poorly understood. Here, we newly employ an oxide solid solution by systematic extrinsic doping to probe the role of oxygen vacancy configuration either set as free, immobile or clustered to directly answer the fundamental question how the mobility of oxygen vacancies affects the switching characteristics of the devices at high electric fields. Till date strategies to tailor systematically oxygen vacancy concentration and configuration for oxide-based resistive switches are still to be explored. For this, we design resistive switching devices of Ce1-xGdxO2-y as a novel resistive switching oxide constituent. The active oxide layer of the devices, consisting of a 500 nm thick CeO2 film with Gd3+ doping concentrations of 3 to 30 mol% deposited by Pulsed Laser Deposition (PLD), is sandwiched between micro-fabricated Pt-crosspoint electrodes. Cyclovoltammetry measurements show a strong dependence of the ROFF/RON -resistance ratio on the effective gadolinia concentration per bit. At low gadolinia doping concentration < 10 mol%, a purely capacitive behaviour prevails. By increasing up to 20 mol% a significant hystereis in the current-voltage profile is measured with resistance ROFF/RON -ratios up to 100. Turning to high gadolinia concentrations from 20 to 30 mol%, decreases the hysteretic current-voltage profile. To analyse the structural changes related to the oxygen vacancy concentration and configuration, changes opposed by the extrinsic gadolinia dopant in the ceria lattice of the resistive switching devices, we turn to Raman Spectroscopy. From these results we conclude that for moderate gadolinium concentrations up to 20 mol% the concentration of mobile oxygen vacancies steadily increases and is available as "free" carriers for the resistive switches; in this state the oxide is in a fluorite structure type. Further increase in doping, for which we see a reduction of switching characteristics, leads to the formation of oxygen vacancy clusters in the structure, which are simply as a defect configuration "immobile". For this state we confirm a phase change to a more bixbyite structure. Unequivocally the experiments together with the defect chemical model, show that when the vacancies are set as "free", a maximum in memristance is found for 20 mol% Gd3+ doping, which clearly coincides with the maximum in ionic conductivity. In contrast, for higher gadolinia concentration the oxide exhibits only minor memristance, which originates from the decrease in structural symmetry leading to the formation of "immobile" oxygen defect clusters, thereby reducing the density of mobile ionic carriers available for resistive switching. A clear correlation between memristance and oxygen vacancies’ mobility was seen in this defect chemical model experiment on gadolinium doped ceria device structures. Extrinsic doping allows to tune the configuration, concentration and mobility of the oxygen vacancies in a much more systematic and controlled manner than what can be achieved through oxygen-deficient deposition or annealing steps as being the state-of-the-art in resistive switches. The research demonstrates guidelines for engineering of the oxide`s solid solution series to set the configuration of its oxygen vacancy defects and their mobility to tune the memristance for future nonvolatile memory and logic applications.

(Invited) Atomistic Simulations for Understanding Microscopic Mechanism of Amorphous-Tantalum-Oxide-Based Resistive Switching Devices

Among oxide-based resistive switching devices, amorphous-TaOx based ones show good endurance and switching performances. Their switching behaviors are understood based on the formation/rupture of conductive filaments (CFs). However, the atomistic structures of CFs and their microscopic formation/rupture processes have not been sufficiently clarified yet. In this talk, we present the results of our atomistic simulations (most of which are with the density functional theory (DFT)) and discuss several aspects related to the nature and formation processes of CFs as follows. 1) CFs in the Cu/amorphous-Ta2O5/Pt heterostructure: Our simulations reveals that single Cu chains in Ta2O5 cannot work as conductive filaments (CFs), while Cu nanowires with a diameter of three atoms or larger can work as CFs. The stability of the Cu nanowires has been checked by ab initio molecular dynamics [1]. 2) CFs in the Pt/TaOx/Pt heterostructure with x < 2.5: Our results suggest that not O vacancy chain but the Ta-Ta bonding mainly contributes to the CFs [2].3) Difference in the atomistic features between Cu/ Ta2O5 and Pt/Ta2O5 interfaces: In the former, considerable portion of interface Cu atoms tend to migrate to the amorphous Ta2O5 layer, while similar behavior is not seen in the latter [3].4) Ion migration behaviors in TaOx: Our results of ab initio molecular dynamics and nudged elastic band calculations show that the diffusion coefficient in TaOx strongly depends on x [3]. 5) Construction of neural network interatomic potentials: We constructed interatomic potential combining the DFT calculations and neural network for simulations of Cu migration behavior in amorphous-Ta2O5 to achieve computation speed and reliability simultaneously. The pathways and barrier energies for Cu diffusion calculated using the NN potential agree well with those obtained from DFT calculations [5]. The final part of the present works was partly supported by the Support Program for Starting up Innovation hub from Japan Science and Technology Agency (JST), and CREST-JST, Japan.[1] B. Xiao, T. Gu, T. Tada and S. Watanabe, J. Appl. Phys. 115, 034503 (2014).[2] B. Xiao and S. Watanabe, Nanoscale 6, 10169 (2014).[3] B. Xiao and S. Watanabe, ACS Appl. Mater. Interfaces 7, 519 (2015).[4] B. Xiao and S. Watanabe, Extended abstract of SSDM2014 (September 8-11, 2014, Tsukuba, Japan).[5] W. Li, Y. Ando and S. Watanabe, submitted.

Direct Observation of Oxygen Movement in Graphene Oxide-Based Resistive Switching Memory

Direct Observation of Oxygen Movement in Graphene Oxide-Based Resistive Switching Memory

The Role of Ti Buffer Layer Thickness on the Resistive Switching Properties of Hafnium Oxide-Based Resistive Switching Memories.

Ti/HfOx-based resistive random access memory (RRAM) has been extensively investigated as an emerging nonvolatile memory (NVM) candidate due to its excellent memory performance and CMOS process compatibility. Although the importance of the role of the Ti buffer layer is well recognized, detailed understanding about the nature of Ti thickness-dependent asymmetric switching is still missing. To realize this, the present work addresses the effects of Ti buffer layer thickness on the switching properties of TiN/Ti/HfOx/TiN 1T1R RRAM. Consequently, we have demonstrated a simple strategy to regulate the FORMING voltage, leakage current, memory window, and decrease the operation current, etc. by varying the thickness of the Ti layer on the HfOx dielectrics. Accordingly, controllable and reliable bipolar, complementary, and reverse bipolar resistive switching (BRS, CRS, and R-BRS) properties have been demonstrated. This work also provides the direction to avoid unwanted CRS properties during the first RESET operation by decreasing the FORMING voltage. Furthermore, the memory device shows good nonvolatility at ∼1 μA programming current by selecting a proper thickness of Ti buffer layer. To achieve reliable BRS properties for low power application, the operation current has been further optimized, whereas the memory device shows pulse endurance of more than 7 million cycles at a low pulse width of 50 ns and excellent data retention properties of more than 40 h at 150 °C measurement temperature.

In situ transmission electron microscopy of resistive switching in thin silicon oxide layers

Silicon oxide-based resistive switching devices show great potential for applications in nonvolatile random access memories. We expose a device to voltages above hard breakdown and show that hard oxide breakdown results in mixing of the SiOx layer and the TiN lower contact layers. We switch a similar device at sub-breakdown fields in situ in the transmission electron microscope (TEM) using a movable probe and study the diffusion mechanism that leads to resistance switching. By recording bright-field (BF) TEM movies while switching the device, we observe the creation of a filament that is correlated with a change in conductivity of the SiOx layer. We also examine a device prepared on a microfabricated chip and show that variations in electrostatic potential in the SiOx layer can be recorded using off-axis electron holography as the sample is switched in situ in the TEM. Taken together, the visualization of compositional changes in ex situ stressed samples and the simultaneous observation of BF TEM contrast...

Analytical Modeling of Current Overshoot in Oxide-Based Resistive Switching Memory (RRAM)

Current overshoot due to parasitic capacitance during set transition represents a major concern for controlling the resistance and current consumption in resistive switching memory (RRAM) arrays. In this letter, the impact of current overshoot on the low-resistance state (LRS) is evaluated by means of experiments on one-transistor/one-resistor structures of HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> RRAM. We develop a physics-based analytical model, able to calculate the LRS resistance and the corresponding reset current by a closed-form formula. The model allows predicting the current overshoot impact for any value of compliance current, set voltage, and parasitic capacitance.

Design and Application of Oxide-Based Resistive Switching Devices for Novel Computing Architectures

Resistive switching behaviors of oxide-based resistive random access memory (RRAM) and the applications for the data storage and computing systems have been widely studied. In this paper, the critical issues correlated with the applications of oxide-based RRAM are addressed. The physical mechanism and models of oxide-based RRAM are first introduced. Then the optimization design methodology of devices and operation schemes is discussed. Finally, a novel architecture for unifying memory/computing system is presented.

Integration of niobium oxide-based resistive switching cells with different select properties into nanostructured cross-bar arrays

Memristive devices with different underlying physical mechanisms are investigated and compared with respect to their utilization in passive cross-bar arrays for computing and memory applications. Niobium oxide-based metal–insulator–metal structures in various configurations exhibit abrupt filamentary resistive switching, filamentary resistive switching together with a threshold switching effect and analog switching characteristics. It is found that the initial electroforming step, which is mandatory for filamentary cells, causes problems if no individual selector device ensuring internal current compliance is applied. In contrast, cells based on analog switching are forming free and could be operated without difficulty. Thus they might be of value for utilization as passive circuit elements.

Investigation of Single-Bit and Multiple-Bit Upsets in Oxide RRAM-Based 1T1R and Crossbar Memory Arrays

In this paper, the susceptibility of oxide-based resistive switching random memory (RRAM) to heavy ion strikes is investigated. A physics-based SPICE model calibrated with ${\hbox{HfO}}_{\rm x}$ RRAM is employed for circuit and array-level simulations. The RRAM state-flipping is attributed to the transient photocurrents at neighboring transistors. Single-bit-upset (SBU) caused by either single-event upset (SEU) or multiple-event upset (MEU) is modeled and simulated in the one-transistor and one-resistor (1T1R) array, which corroborates with experimental observations. In addition, circuit simulation is performed to investigate the impact of transient-induced soft errors in a $1024 \times 1024$ crossbar array. The sensitive locations in crossbar arrays are the driver circuits at the edge of the array. The simulations show that the crossbar array with ${\hbox{HfO}}_{\rm x}$ RRAM is of high radiation tolerance thanks to the V/2 bias scheme. However, multiple-bit upset (MBU) may occur if using other oxide materials with lower operation voltage. Voltage spikes generated at the edge of the array may propagate along rows or columns as there is no isolation between cells in the crossbar array.

Dynamics of electroforming in binary metal oxide-based resistive switching memory

The onset of localized conduction in TaOx- and TiOx-based resistive switching devices during forming has been characterized. The novel temperature and voltage dependencies of forming times were extracted with pulsed forming experiments that spanned five orders of magnitude in time and showed three different regimes of electroforming. A universal field-induced-nucleation theory which included self-heating effects was used to explain a strong reduction in forming voltage with increasing forming time over all observed regimes of electroforming. It was shown that the effective activation energy for the incubation time changes inversely proportional with the electric field. A diameter of the volatile filament that precedes forming was estimated at ∼1 nm.

Carrier-transport-path-induced switching parameter fluctuation in oxide-based resistive switching memory

A key challenge in resistive switching memory is to reduce the switching parameter fluctuation, which always affects the stability and reliability of an RS device. Numerous methods have been carried out for improving the fluctuation of the switching parameter. However, because the physical nature of the switching parameter fluctuation is, to date, not well understood, a universal identification of the switching parameter fluctuation still has not be achieved. Based on the activation energy of carrier transport from the first-principles calculations, we present a physical model. This proposed model is considering the macroscopic fluctuating I–V curve and material microstructure to analyze the characteristics of carrier transport and the origin of switching parameter fluctuations. The proposed model may specially identify the defect energy level and quantify the distribution of the switching parameter. The model provides possible clues for improving the uniformity of the switching parameter as well.

Role of tantalum nitride as active top electrode in electroforming-free bipolar resistive switching behavior of cerium oxide-based memory cells

Abstract Electroforming-free cerium oxide-based bipolar resistive switching memory devices have been deposited using radio frequency magnetron sputtering technique. These devices demonstrate two types of forming-free cells: some in the low-resistance state and the others in high-resistance state. The transmission electron microscopy and X-ray diffraction analyses illustrate the formation of tantalum oxynitride layer between tantalum nitride (TaN) and cerium oxide (CeOx), which looks to be responsible for the two types of cells as well as their memory performance. Ohmic and Poole–Frenkel conduction mechanisms are found to be responsible for charge transport in the low- and high-resistance states. The current–voltage characteristics and temperature dependence of resistance suggest that resistive switching mechanism in our TaN/CeOx/Pt devices may be explained by the model of connection and disconnection of filamentary paths made of oxygen vacancies. The reliability characteristics of TaN/CeOx/Pt devices indicate better endurance and stable retention performance at relatively lower programming voltages and larger memory window (OFF/ON resistance ratio ~ 103) at room temperature and at 100 °C.

Impact of program/erase operation on the performances of oxide-based resistive switching memory.

Further performance improvement is necessary for resistive random access memory (RRAM) to realize its commercialization. In this work, a novel pulse operation method is proposed to improve the performance of RRAM based on Ti/HfO2/Pt structure. In the DC voltage sweep of the RRAM device, the SET transition is abrupt under positive bias. If current sweep with positive bias is utilized in SET process, the SET switching will become gradual, so SET is current controlled. In the negative voltage sweep for RESET process, the change of current with applied voltage is gradual, so RESET is voltage controlled. Current sweep SET and voltage sweep RESET shows better controllability on the parameter variation. Considering the SET/RESET characteristics in DC sweep, in the corresponding pulse operation, the width and height of the pulse series can be adjusted to control the SET and RESET process, respectively. Our new method is different from the traditional pulse operation in which both the width and height of program/erase pulse are simply kept constant which would lead to unnecessary damage to the device. In our new method, in each program or erase operation, a series of pulses with the width/height gradually increased are made use of to fully finish the SET/RESET switching but no excessive stress is generated at the same time, so width/height-controlled accurate SET/RESET can be achieved. Through the operation, the uniformity and endurance of the RRAM device has been significantly improved.

Thermoelectric Seebeck effect in oxide-based resistive switching memory.

Reversible resistive switching induced by an electric field in oxide-based resistive switching memory shows a promising application in future information storage and processing. It is believed that there are some local conductive filaments formed and ruptured in the resistive switching process. However, as a fundamental question, how electron transports in the formed conductive filament is still under debate due to the difficulty to directly characterize its physical and electrical properties. Here we investigate the intrinsic electronic transport mechanism in such conductive filament by measuring thermoelectric Seebeck effects. We show that the small-polaron hopping model can well describe the electronic transport process for all resistance states, although the corresponding temperature-dependent resistance behaviours are contrary. Moreover, at low resistance states, we observe a clear semiconductor–metal transition around 150 K. These results provide insight in understanding resistive switching process and establish a basic framework for modelling resistive switching behaviour.