HfO2-based Resistive Random Access Memory Devices Research Articles

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.

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.

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

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.

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.

Influence of the bottom metal electrode and gamma irradiation effects on the performance of HfO2-based RRAM devices

ABSTRACTHafnium oxide-based Resistive Random Access Memory (RRAM) devices have been fabricated with Ag as the top electrode. Au and Pt have been used as bottom electrodes in two different sets to study the effects of oxidation on the performance of RRAM devices. Further, the influence of gamma irradiation on the performance of these devices has also been studied. It is shown that the diffusion of O vacancies and oxidation at electrodes are more effective for the migration of metal ions from the top electrode in determining the switching behavior of these devices. Pt-based devices are found to be more susceptible to gamma irradiation when compared to Au-based devices. The performance of the device is improved for lower doses (12 kGy), may be due to possible irradiation-induced annealing effects. Significant deterioration is observed at 24 kGy and the devices have totally failed at a dose of 48 kGy. These studies provide useful information about the radiation damage and reliability of HfO2-based RRAM devices.

Transmission electron microscopy assessment of conductive-filament formation in Ni–HfO2–Si resistive-switching operational devices

Resistive random-access memory (ReRAM) devices are currently the object of extensive research to replace flash non-volatile memory. However, elucidation of the conductive-filament formation mechanisms in ReRAM devices at nanoscale is mandatory. In this study, the different states created under real operation conditions of HfO2-based ReRAM devices are characterized through transmission electron microscopy and electron energy-loss spectroscopy. The physical mechanism behind the conductive-filament formation in Ni/HfO2/Si ReRAM devices based on the diffusion of Ni from the electrode to the Si substrate and of Si from the substrate to the electrode through the HfO2 layer is demonstrated.

Electrical study of radiation hard designed HfO2-based 1T-1R RRAM devices

ABSTRACTIn this work the electrical performance of a radiation hard designed 1T-1R resistive random access memory (RRAM) device is investigated in DC (voltage sweep) and AC (pulsed voltage) modes. This new device is based on the combination of an Enclosed Layout Transistor (ELT) used as selector device and a TiN/ HfO2/ Ti/TiN RRAM stack used as resistive device. The high cell to cell variability in the DC mode makes it difficult to define an electrical gap between the High Resistive State (HRS) and the Low Resistive State (LRS). The strong reduction of the variability by the use of Incremental Step Pulse with Verify Algorithm (ISPVA) makes the later a mandatory programming approach. The Quantum Point Contact (QPC) model defines an energy barrier located in the rupture point of the filament in HRS. The compensation between the width and height variations of this barrier during cycling could explain the stability of HRS and LRS. The good performance of the proposed device using the ISPVA programming approach makes it a good candidate for Rad-Hard Non Volatile Memories integration.

Statistical analysis of random telegraph noise in HfO2-based RRAM devices in LRS

In this work, we present a thorough statistical characterization of Random Telegraph Noise (RTN) in HfO2-based Resistive Random Access Memory (RRAM) cells in Low Resistive State (LRS). Devices are tested under a variety of operational conditions. A Factorial Hidden Markov Model (FHMM) analysis is exploited to extrapolate the properties of the traps causing multi-level RTN in LRS. The trapping and de-trapping of charge carriers into/out of defects located in the proximity of the conductive filament results in a shielding effect on a portion of the conductive filament, leading to the observed RTN current fluctuations. It is found that both oxygen vacancies and oxygen ions defects may be responsible for the observed RTN. The variations of the current observed at subsequent set/reset cycles are instead attributed to the stochastic variations in the filament due to oxidation/reduction processes during reset and set operations, respectively.

An overview of the switching parameter variation of RRAM

Resistive random access memory (RRAM) has been considered as one of the most promising candidates for next-generation nonvolatile memory, due to its advantages of simple device structure, excellent scalability, fast operation speed and low power consumption. Deeply understanding the physical mechanism and effectively controlling the statistical variation of switching parameters are the basis of fostering RRAM into commercial application. In this paper, based on the deep understanding on the mechanism of the formation and rupture of conductive filament, we summarize the methods of analyzing and modeling the statistics of switching parameters such as SET/RESET voltage, current, speed or time. Then, we analyze the distributions of switching parameters and the influencing factors. Additionally, we also sum up the analytical model of resistive switching statistics composed of the cell-based percolation model and SET/RESET switching dynamics. The results of the model can successfully explain the experimental distributions of switching parameters of the NiO- and HfO2-based RRAM devices. The model also provides theoretical guide on how to improve the uniformity and reliability such as disturb immunity. Finally, some experimental approaches to improve the uniformity of switching parameters are discussed.

Impacts of Au-doping on the performance of Cu/HfO2/Pt RRAM devices

The impact of Au-doping on the resistive switching properties of a Cu/HfO2/Pt resistive random access memory (RRAM) device is investigated. The significantly enhanced performances are achieved in the Cu/HfO2: Au/Pt device, including low set voltages, improved uniformity of switching parameters, enough ON/OFF ratio. This performance improvement in Cu/HfO2: Au/Pt is clarified to the suppressed stochastic formation of oxygen vacancy conducting filaments in the HfO2 film by AuO bond effect. The validity of the trivalent dopants in future application of HfO2-based RRAM devices is also identified.

Improved Resistive Switching Characteristics by Al2O3 Layers Inclusion in HfO2-Based RRAM Devices

A series of complex HfO2/Al2O3 layer by layer resistive random access memory(RRAM) structure grown by atomic layer deposition are investigated. The modulation of forming voltage can be achieved by controlling the number of Al2O3 layers in HfO2 devices. In addition, the crystallization temperature of HfO2 based RRAM devices can also be improved by insetting Al2O3 layers in HfO2 film. Compared with pure HfO2 device, a significant improvement in resistive switching properties such as forming voltage variation and the distribution of HRS/LRS during resistance switching is demonstrated in the HfO2/Al2O3 layer by layer devices. Moreover, good endurance characteristic and highly reliable multibit operation are also achieved in this device structure.

Highly Stable Radiation-Hardened Resistive-Switching Memory

In this letter, the resistive random access memory (RRAM) with metal-insulator-metal structure is investigated for the first time under radiation conditions. The fabricated Cu-doped HfO2-based RRAM devices are found to have immunity from 60Co γ ray of various dose ranges. The basic RRAM parameters such as high-resistance state, low-resistance state, SET/RESET voltages, operation speed, and endurance have nearly no degradation after 60Co γ ray treatment with a total dose as high as 3.6 × 105 rad (Si). Furthermore, a retention characteristic of 105 s is also achieved during radiation. The highly stable characteristics of Cu-doped HfO2 -based RRAM devices under radiation provide RRAM a great potential for aerospace and nuclear applications.