HfO2-based Resistive Random Access Memory Research Articles

Investigation on self-rectifying properties of Pt/HfO2/Ti with rivet-like structure based on ALD conformal technology

This work explores an architecture for nonvolatile resistive random-access memory (RRAM) systems. The study proposes a self-rectifying RRAM device utilizing a two-terminal 1R selector that harmonizes the gate control efficacy of transistors with the inherent simplicity of diode structures. A rivet-like HfO2-based RRAM array is meticulously constructed through atomic layer deposition (ALD), aiming to enhance device performance and retention stability. The conformal fabrication technique of ALD method is critical in achieving uniform coverage of isolation layers and precise electrode placement, which is instrumental in the fabrication of high-performance memory cells. Empirical analyses indicate significant improvements in rectification and ON/OFF ratios compared to existing RRAM models, bolstered by compatibility with established CMOS processes. It reveals that these advances are conducive to scalable, high-density memory integration, positioning the RRAM as a viable contender for future computational applications that require high efficiency and neuromorphic computing capabilities.

Ab initio study of oxygen vacancy filament formation at Ta/HfO2 interface

The resistive switching (RS) behavior of resistive random access memory (RRAM) based on oxygen vacancy (VO) conduction is significantly affected by the interface properties between metal electrode and oxide layer, yet the modulation between the RS behavior and the physico-chemical properties of the interface is still not very clear. In this study, the correlative role of Ta/HfO2 interface with the RS behavior in HfO2-based RRAM is explored at atomic level. First-principles thermodynamic calculations show that the strong interaction between three-fold oxygen vacancies (VO3) leads to a formation of VO3-based conductive filament (CF) along direction perpendicular to the interface. Four-fold oxygen vacancies (VO4) make a major contribution to the re-formation and growth of CFs during the set process by diffusing into the residual filaments. The results of electronic properties further indicate that as the number of VOs perpendicular to the interface increases, the charge redistribution between O and Ta atoms at the interface is significantly increased, and more electron clouds are gathered around VOs. This is the underlying mechanism of forming a conductive channel. This study reveals the important regulation mechanism of the interface characteristics between metal electrode and oxide layer in RRAM on the formation and growth of VO-based CFs.

Performance enhancement of HfO2-based resistive random-access memory devices using ZnO nanoparticles

In this study, resistive random-access memory (ReRAM) devices with ZnO nanoparticles (NPs) are suggested to enhance performance and reduce variation in device switching parameters. The ZnO NPs are formed by annealing ZnO prepared via atomic layer deposition on HfO2, which is verified using transmission electron microscopy, x-ray diffraction pattern, and atomic force microscopy. The depth profile analysis of x-ray photoelectron spectroscopy shows that oxygen diffuses from HfO2 to ZnO NPs during annealing. This can be explained by the calculation results using density functional theory (DFT) where the formation energy of oxygen vacancies is reduced at the interface of ZnO NPs and HfO2 compared to single HfO2. The fabricated ZnO NPs ReRAM demonstrates reduced forming voltage, stable resistive switching behavior, and improved cycle-to-cycle uniformity in a high-resistance state.

Ti/HfO2-Based RRAM with Superior Thermal Stability Based on Self-Limited TiOx

HfO2-based resistive random-access memory (RRAM) with a Ti buffer layer has been extensively studied as an emerging nonvolatile memory (eNVM) candidate because of its excellent resistive switching (RS) properties and CMOS process compatibility. However, a detailed understanding of the nature of Ti thickness-dependent RS and systematic thermal degradation research about the effect of post-metallization annealing (PMA) time on oxygen vacancy distribution and RS performance still needs to be included. Herein, the impact of Ti buffer layer thickness on the RS performance of the Al/Ti/HfO2/TiN devices is first addressed. Consequently, we have proposed a simple strategy to regulate the leakage current, forming voltage, memory window, and uniformity by varying the thickness of the Ti layer. Moreover, it is found that the device with 15 nm Ti shows the minimum cycle-to-cycle variability (CCV) and device-to-device variability (DDV), good retention (105 s at 85 °C), and superior endurance (104). In addition, thermal degradation of the Al/Ti(15 nm)/HfO2/TiN devices under different PMA times at 400 °C is carried out. It is found that the leakage current increases and the forming voltage and memory window decrease with the increase in PMA time due to the thermally activated oxidation of the Ti. However, when the PMA time increases to 30 min, the Ti can no longer capture oxygen from HfO2 due to the formation of self-limited TiOx. Therefore, the device shows superior thermal stability with a PMA time of 90 min at 400 °C and no degradation of the memory window, uniformity, endurance, or retention. This work demonstrates that the Ti/HfO2-based RRAM shows superior back-end-of-line compatibility with high thermal stability up to 400 °C for over an hour.

Oxide thickness-dependent resistive switching characteristics of Cu/HfO2/Pt ECM devices

HfO2-based resistive random-access memory devices are promising candidates for new memory and computing applications. Hereby, scaling of the devices is a key issue, where overall fundamental switching and conduction mechanisms can be easily influenced by changes in the oxide layer thickness. This work addresses the oxide thickness-dependent resistive switching characteristics in Cu/HfO2/Pt memory devices through bipolar DC switching characterization. Forming, reset, and set characteristics are investigated depending on the oxide layer thickness, revealing a significant difference for thicker compared to thinner films. Thicker samples tend to show a more abrupt reset behavior and a larger set voltage variance, while for thinner samples, a more gradual reset behavior and a low set variance is found. These phenomena can be explained by a model based on thermally assisted electrochemical metallization. Furthermore, to understand the conduction mechanism of the devices, current–voltage curves of the set process were investigated. The devices are found to have an Ohmic conduction mechanism in the lower voltage region generally, while thinner samples tend to show an additional space-charge-limited current conduction mechanism in a higher voltage region.

Modulating the resistive switching stability of HfO2-based RRAM through Gd doping engineering: DFT+U.

Oxide-based resistive random access memory (RRAM) is standing out in both non-volatile memory and the emerging field of neuromorphic computing, with the consequence of increasing performance demands. Rare-earth doping is often used as an effective means for performance modulation. In this work, the modulation mechanism of the resistive switching (RS) behaviors in trivalent rare-earth Gd-doped HfO2-based RRAM has been carefully investigated using first-principles calculations. The results of electronic structure analysis show that Gd doping would lead to a change in the local geometry of the m-HfO2 defect system and would enhance the Coulomb interaction between the atoms around Gd and oxygen vacancy (VO), which may be one of the reasons for the enhanced conductivity of the HfO2-based RRAM after Gd doping. Thermodynamic and kinetic study results indicate that there is a strong interaction between Gd and its surrounding VO defects, and this strong interaction would not only attract more oxygen vacancies (VOs) to be generated near the dopant Gd, but also increase the migration energy barrier of the +2 charged VOs around the Gd doping site, thus suppressing the random generation of VO filaments, which leads to a better uniformity of the switching parameters during the RS process and improves the performance stability of the devices. The results of this work will provide new insights into modulating the RS behaviors and improving the device performance of HfO2-based RRAM through doping engineering.

Exploring the direction-dependency of conductive filament formation and oxygen vacancy migration behaviors in HfO2-based RRAM.

Oxygen vacancy (VO) defects play an essential role in governing the conductivity of semiconductor materials. The direction-dependency of oxygen vacancy conductive filament (CF) formation and VO migration behaviors in HfO2-based resistive random access memory (RRAM) were systematically investigated through first-principles calculations. The energetic and electronic structural analyses indicate that the continuous distribution of 3-fold oxygen vacancy (VO3) or 4-fold oxygen vacancy (VO4) is more favorable for the CF formation along [010] and [001] directions, and a continuous distribution between VO3 and VO4 in the m-HfO2 system can also combine to promote the formation of CFs along a particular direction. Furthermore, the high annealing temperature and low oxygen partial pressure (PO2) could effectively reduce the VO formation energy and promote the formation of CFs, resulting in a lower applied voltage of the devices. Our results indicate that q = 0 and q = +2 are the most probable charge states for VO3 and VO4 in m-HfO2. Subsequently, it is found that the low activation energy of VO originates from the +2q charged VO3 or VO4 migrating in the CFs along a particular crystallographic [001] direction. The diffusion coefficient (D) of the oxygen atom along the [001] direction is much higher than that of all the other possible pathways considered, due to the lower energy barrier. This demonstrates that the growth of CFs is potentially direction-dependent, and that a lower forming voltage and lower SET voltage are required when the CFs are grown along a particular direction in RRAM devices. The present work would help to provide a fundamental guide and new understanding for the development and application of HfO2-based RRAM.

Process Optimization to Reduce Power in HfO2-Based Rram Devices for in-Memory Computing

Recently, HfO2-based resistive random-access memory (RRAM) devices have shown promise as candidates for in-memory computing applications. By engineering the distribution of defects or oxygen vacancies, the switching dielectric can potentially enable low power switching and multi-conductance levels. A higher concentration of oxygen vacancies closer to the top electrode a two-terminal RRAM device with a HfO2/Al2O3 bilayer structure reduces switching energy (1). Introducing excess oxygen vacancies near the top electrode through a hydrogen plasma treatment with a Ru as top electrode reduces the switching power of the device (2).In this work we study the pulsed SET operation of different HfO2-based RRAM devices for their possible uses as multi-level cells. We have compared two different RRAM devices with HfO2-based dielectrics. Device-A is prepared with hydrogen plasma treatment at mid-point of HfO2 deposition (10nm Ti/50nmTiN/3nm HfO2(plasma treatment) 3nm HfO2/5nm Ru/5nm ALD TiN/ 50nm PVD TiN). Whereas Device-B constitutes a HfO2/Al2O3 bilayer structure (10nm Ti/50nmTiN/7nm HfO2/1nm Al2O3/5nm Ru/5nm ALD TiN/50nm PVD TiN). Both the devices have Ru as the top metal.Fig. 1 and Fig. 2 show the conductance modulation obtained by applying successive pulse sequences (80 pulses) with increasing the pulse width tp (from 4 \U0001d707s to 10 ms) every 20 pulses where the pulse height, Vp, remained fixed at 2V during the experiment. A read pulse with voltage Vr=0.1V and pulse width tr=1ms is applied immediately after each pulse. The forming compliance current in Device-A is seven time lower than Device-B. Both devices clearly show promising behavior of multi-level conductance. Figure 1

Effects of Ambient and Annealing Temperature in HfO2 Based RRAM Device Modeling and Circuit-Level Implementation

This article focuses on the relevance of the effect of ambient temperature and annealing in the context of compact modeling of metal oxide resistive random access memory (RRAM) devices. The ambient temperature affects the conduction characteristic of resistive switching memories, so it becomes an essential factor to include when adjusting the experimental data. Reported the fabricated results and memory switching parameters with the defined set (Vset) and reset (Vreset) transition voltages for the fabricated annealed HfO2-based RRAM. Additionally, to illustrate the importance of this characteristic in the form of the I-V curve, the Stanford model (SFM) for RRAM devices is enhanced by incorporating the annealing temperature as an additional parameter in the script of the Verilog-A model. Stanford and modified Stanford model (MSFM) are analyzed at the device level using cadence circuit simulator and implemented in the nonvolatile memory circuit (3 *3 memory arrays). Results confirmed that the experimental switching voltages, Vset, Vreset are 1.7 V, −0.8 V. These values are well suited along the simulated MSFM switching voltages of, Vset, Vreset (1.8 V, −0.7 V). The mean error percentage of the MSF is 18.42%.

A Large-Size HfO2 Based RRAM Structure Suitable for Integration of One RRAM with One InGaZnO Thin Film Transistor for Large-Area Applications

This work aims at finding a HfO2-based resistive random-access memory (RRAM) structure suitable for the integration of one RRAM with one InGaZnO thin film transistor (TFT) for large-area applications such as flexible electronic circuits. One of the major concerns is that the compliance current (CC) required for the formation of stable and strong conductive filaments in the forming and set processes as well as the maximum current required in the reset process in a large-size RRAM should be lower than that of the maximum current a TFT can deliver. In this work, an ultrathin Al2O3 layer of 2 nm was inserted between the HfO2 switching layer and the reactive Ti layer of the top electrode in the RRAM with the structure of Pt (bottom electrode)/HfO2/Al2O3/Ti/TiN (top electrode). With the ultrathin Al2O3 layer, the forming voltage was greatly reduced, and the CC for stable forming and set operations and maximum reset current can reach a low current level that an InGaZnO TFT is able to provide, while the device-to-device variation of the forming operation and cycle-to-cycle resistance variations of the set and reset operations are improved significantly.

Effects of Oxygen Precursor on Resistive Switching Properties of CMOS Compatible HfO2-Based RRAM

In this work, we investigate the resistive switching behaviors of HfO2-based resistive random-access memory (RRAM) in two different oxidants (H2O and O3) in an atomic layer deposition system. Firstly, the surface characteristics of the Ni/HfO2/Si stack are conducted by atomic force microscopy (AFM). A similar thickness is confirmed by scanning electron microscope (SEM) imaging. The surface roughness of the HfO2 film by O3 (O3 sample) is smoother than in the sample by H2O (H2O sample). Next, we conduct electrical characteristics by current–voltage (I–V) and capacitor–voltage (C–V) curves in an initial process. The forming voltage of the H2O sample is smaller than that of the O3 sample because the H2O sample incorporates a lot of H+ in the film. Additionally, the smaller capacitor value of the H2O sample is obtained due to the higher interface trap in H2O sample. Finally, we compare the resistive switching behaviors of both samples by DC sweep. The H2O sample has more increased endurance, with a smaller on/off ratio than the O3 sample. Both have good non-volatile properties, which is verified by the retention test.

Atomistic Insights on the Full Operation Cycle of a HfO2-Based Resistive Random Access Memory Cell from Molecular Dynamics.

We characterize the atomic processes that underlie forming, reset, and set in HfO2-based resistive random access memory (RRAM) cells through molecular dynamics (MD) simulations, using an extended charge equilibration method to describe external electric fields. By tracking the migration of oxygen ions and the change in coordination of Hf atoms in the dielectric, we characterize the formation and dissolution of conductive filaments (CFs) during the operation of the device with atomic detail. Simulations of the forming process show that the CFs form through an oxygen exchange mechanism, induced by a cascade of oxygen displacements from the oxide to the active electrode, as opposed to aggregation of pre-existing oxygen vacancies. However, the filament breakup is dominated by lateral, rather than vertical (along the filament), motion of vacancies. In addition, depending on the temperature of the system, the reset can be achieved through a redox effect (bipolar switch), where oxygen diffusion is governed by the applied bias, or by a thermochemical process (unipolar switch), where the diffusion is driven by temperature. Unlike forming and similar to reset, the set process involves lateral oxygen atoms as well. This is driven by field localization associated with conductive paths.

Realizing forming-free characteristic by doping Ag into HfO2-based RRAM

In this work, Ag-doped HfO2-based resistive random access memory (RRAM) with high on-off ratio, low-power consumption and forming-free properties was investigated. We propose the fabrication flow of the RRAM with via-hole structure. After doping Ag into HfO2 as the switching layer, the devices could execute resistive switching without a high-voltage forming process. The conduction mechanism was subsequently validated by a current fitting analysis. Electric field simulation was also utilized to observe the electric field distribution and finally a physical model was proposed to provide an explanation for the formation and dissolution of the filament.

Improving Performance by Inserting an Indium Oxide Layer as an Oxygen Ion Storage Layer in HfO₂-Based Resistive Random Access Memory

This study investigates an improvement in memory characteristics through depositing an In2O3 layer as an oxygen ion reservoir in the switching layer (SL) of resistive random access memory (RRAM). The deposition of In2O3, denoted as Pt/HfO2/In2O3/TiN device, provides better memory characteristics including lower forming voltage ( ${V}_{F}$ ), set/reset voltage ( ${V}_{\mathrm {SET}}/{V}_{\mathrm {RESET}}$ ), larger memory window, and higher resistances with higher uniformity at low resistance state (LRS) and high resistance state (HRS), compared to the devices without the In2O3 deposition, denoted as Pt/HfO2/TiN device. The conduction mechanisms, verified through fitting current–voltage ( ${I} - {V}$ ) curves, are consistent with the results of a varied temperature ${I} - {V}$ experiment. Finally, a physical model is proposed to explain our observations.

Effects of swift heavy ion irradiation on the performance of HfO2-based resistive random access memory devices

In this work, the effects of 120 MeV Ag ion irradiation on the switching properties of Au/HfO2/Au-based Resistive Random Access Memory (RRAM) devices are reported. The ion fluence is varied between 5 × 1010 and 5 × 1012 ions/cm2 while two device sizes, with active areas 10 µm × 10 µm and 20 µm × 20 µm, are tested. In each case, 16 devices are subjected to ion irradiation and it is shown that the set voltages are generally lower and the spread in the switching voltages is reduced for the irradiated samples in comparison to the pristine devices. The existence of a critical dose of 5 × 1011 ions/cm2 up to which an improvement in the device performance is observed. Photoluminescence studies indicate the presence of oxygen-related vacancies in both pristine and irradiated samples, which may be the reason for the observed forming free switching behavior. Swift heavy ion irradiation is, thus, a simple but effective technique to tune the performance of HfO2-based resistive switching devices. The study also indicates the significance of radiation damage and reliability of these devices beyond a critical fluence.

Oxygen Vacancy Density Dependence with a Hopping Conduction Mechanism in Multilevel Switching Behavior of HfO2-Based Resistive Random Access Memory Devices

We report a switching model that directly explains the change in activation energy (EAC) at different RESET stop voltages (Vstop) in HfO2-based resistive random access memory devices. The dependenc...

Programming Pulse Width Assessment for Reliable and Low-Energy Endurance Performance in Al:HfO2-Based RRAM Arrays

A crucial step in order to achieve fast and low-energy switching operations in resistive random access memory (RRAM) memories is the reduction of the programming pulse width. In this study, the incremental step pulse with verify algorithm (ISPVA) was implemented by using different pulse widths between 10 μ s and 50 ns and assessed on Al-doped HfO 2 4 kbit RRAM memory arrays. The switching stability was assessed by means of an endurance test of 1k cycles. Both conductive levels and voltages needed for switching showed a remarkable good behavior along 1k reset/set cycles regardless the programming pulse width implemented. Nevertheless, the distributions of voltages as well as the amount of energy required to carry out the switching operations were definitely affected by the value of the pulse width. In addition, the data retention was evaluated after the endurance analysis by annealing the RRAM devices at 150 °C along 100 h. Just an almost negligible increase on the rate of degradation of about 1 μ A at the end of the 100 h of annealing was reported between those samples programmed by employing a pulse width of 10 μ s and those employing 50 ns. Finally, an endurance performance of 200k cycles without any degradation was achieved on 128 RRAM devices by using programming pulses of 100 ns width.

Toward a Reliable Synaptic Simulation Using Al-Doped HfO2 RRAM

The potential in a synaptic simulation for neuromorphic computation has revived the research interest of resistive random access memory (RRAM). However, novel applications require reliable multilevel resistive switching (RS), which still represents a challenge. We demonstrate in this work the achievement of reliable HfO2-based RRAM devices for synaptic simulation by performing the Al doping and the postdeposition annealing (PDA). Transmission electron microscopy and operando hard X-ray photoelectron spectroscopy results reveal the positive impact of Al doping on the formation of oxygen vacancies. Detailed I-V characterizations demonstrate that the 16.5% Al doping concentration leads to better RS properties of the device. In comparison with the other reported results based on HfO2 RRAM, our devices with 16.5% Al-doping and PDA at 450 °C show better reliable multilevel RS (∼20 levels) performance and an increased on/off ratio. The 16.5% Al:HfO2 sample with PDA at 450 °C shows good potentiation/depression characteristics with low pulse width (10 μs) along with a good On/Off ratio (>1000), good data retention at room temperature, and high temperature and good program/erase endurance characteristics with a pulse width of 50 ns. The synapse features including potentiation, depression, and spike time-dependent plasticity were successfully achieved using optimized Al-HfO2 RRAM devices. Our results demonstrate the beneficial effects of Al doping and PDA on the enhancement of the performances of RRAM devices for the synaptic simulation in neuromorphic computing applications.

Variable-temperature activation energy extraction to clarify the physical and chemical mechanisms of the resistive switching process.

This study investigates the physical and chemical mechanisms during the resistive switching process by means of obtaining the activation energy in the reaction procedure. From the electrochemical and electrical measurement analysis results of HfO2-based resistive random access memory (RRAM), it can be observed that the chemical reaction during the reset process is consistent with the first-order reaction law. The activation energy, Ea, is determined from the reaction rate constant k under a varying-temperature environment in the reset process. The whole reset chemical reaction process can be divided into five phases involving N-O bond breaking, O-O bond breaking and triple-step oxygen ion migration. The methodology of the activation energy determination carried out in this study showcases a distinct approach to elucidate the resistive switching mechanism of RRAM and offers insight into RRAM design for future potential application.

Interface modification of HfO2-based ReRAM via low temperature anneal

Resistive Random Access Memory (ReRAM) is a novel form of non-volatile memory with a variety of applications ranging from neuromorphic circuits used for artificial intelligence applications, to a potential replacement for flash memory. Each application has specific requirements including Roff/Ron ratio, maximum variability tolerance, retention, endurance, and operating voltages. Tuning of individual performance metrics for a particular application often results in performance trade-offs. In this work, we demonstrate the adjustment of the above-mentioned parameters via manipulation of the pre-forming oxygen vacancy gradient using a controlled annealing step with nanoscale HfO2-based ReRAM devices. ReRAM devices of 100 × 100 nm2 were implemented between the metal 1 and 2 interconnect in a custom designed split via process, using 65 nm CMOS processing technology. A temperature study on these devices was performed to optimize the oxygen vacancy gradient from the switching layer to the oxygen scavenger layer. Devices in a 1 transistor/1 ReRAM (1T1R) configuration were subjected to 300 °C–400 °C, for 1–120 min and then electrically characterized using DC switching and endurance testing. Based on the results of this study, we outline the temperature constraints for post-fabrication drive-in of oxygen vacancies via annealing-based treatment of HfO2 based ReRAM devices.