Bilayer Resistive Random Access Memory Research Articles

Optimization of Bilayer Resistive Random Access Memory Based on Ti/HfO2/ZrO2/Pt.

In this paper, the electrothermal coupling model of metal oxide resistive random access memory (RRAM) is analyzed by using a 2D axisymmetrical structure in COMSOL Multiphysics simulation software. The RRAM structure is a Ti/HfO2/ZrO2/Pt bilayer structure, and the SET and RESET processes of Ti/HfO2/ZrO2/Pt are verified and analyzed. It is found that the width and thickness of CF1 (the conductive filament of the HfO2 layer), CF2 (the conductive filament of the ZrO2 layer), and resistive dielectric layers affect the electrical performance of the device. Under the condition of the width ratio of conductive filament to transition layer (6:14) and the thickness ratio of HfO2 to ZrO2 (7.5:7.5), Ti/HfO2/ZrO2/Pt has stable high and low resistance states. On this basis, the comparison of three commonly used RRAM metal top electrode materials (Ti, Pt, and Al) shows that the resistance switching ratio of the Ti electrode is the highest at about 11.67. Finally, combining the optimal conductive filament size and the optimal top electrode material, the I-V hysteresis loop was obtained, and the switching ratio Roff/Ron = 10.46 was calculated. Therefore, in this paper, a perfect RRAM model is established, the resistance mechanism is explained and analyzed, and the optimal geometrical size and electrode material for the hysteresis characteristics of the Ti/HfO2/ZrO2/Pt structure are found.

Ultrathin HfO2/Al2O3 bilayer based reliable 1T1R RRAM electronic synapses with low power consumption for neuromorphic computing

Neuromorphic computing requires highly reliable and low power consumption electronic synapses. Complementary-metal-oxide-semiconductor (CMOS) compatible HfO2 based memristors are a strong candidate despite of challenges like non-optimized material engineering and device structures. We report here CMOS integrated 1-transistor-1-resistor (1T1R) electronic synapses with ultrathin HfO2/Al2O3 bilayer stacks (<5.5 nm) with high-performances. The layer thicknesses were optimized using statistically extensive electrical studies and the optimized HfO2(3 nm)/ Al2O3(1.5 nm) sample shows the high reliability of 600 DC cycles, the low Set voltage of ∼0.15 V and the low operation current of ∼6 µA. Electron transport mechanisms under cycling operation of single-layer HfO2 and bilayer HfO2/Al2O3 samples were compared, and it turned out that the inserted thin Al2O3 layer results in stable ionic conduction. Compared to the single layer HfO2 stack with almost the same thickness, the superiorities of HfO2/Al2O3 1T1R resistive random access memory (RRAM) devices in electronic synapse were thoroughly clarified, such as better DC analog switching and continuous conductance distribution in a larger regulated range (0–700 µS). Using the proposed bilayer HfO2/Al2O3 devices, a recognition accuracy of 95.6% of MNIST dataset was achieved. These results highlight the promising role of the ultrathin HfO2/Al2O3 bilayer RRAM devices in the application of high-performance neuromorphic computing.

Impact of interfacial engineering on MgO-based resistive switching devices for low-power applications

In this work, the resistive switching characteristics of MgO/Al2O3-based resistive random-access memory (ReRAM) devices have been reported. Analysis shows the change in dominant conduction mechanism from space-charge-limited conduction to Schottky emission owing to the incorporation of an Al2O3 insertion layer. The MgO/Al2O3 bilayer ReRAM devices exhibit lower power operation (50.6% reduction) and better switching uniformity as compared to single-layer devices, depending on the stack configuration. This can be attributed to the lower oxygen vacancy accumulation and filament confinement at the MgO/Al2O3 interface, resulting in a more controllable switching operation. Further X-ray photoelectron spectroscopy (XPS) depth profile analysis of the bilayer device reveals that the switching dynamics are correlated directly with the oxygen vacancy concentrations. These findings indicate the importance of interfacial layer engineering in improving the resistive switching properties of MgO-based memory devices, thus allowing for low-power applications.

Bipolar resistive switching properties of TiO x /graphene oxide doped PVP based bilayer ReRAM

In this paper, firstly, some recently explored promising materials and processes for resistive random access memory (ReRAM) devices with bipolar switching mechanism along with their performance are discussed. Further, resistive switching behaviour of TiO x /graphene oxide (GO):poly(4-vinylphenol) (PVP) based bilayer in ReRAM devices is demonstrated. It was found that bipolar resistive switching behaviour is significantly enhanced by embedding 2D material such as GO in the organic polymer acting as switching layer. ReRAM devices with Ag/PVP:GO/TiO x /fluorine doped tin oxide (FTO) structure exhibited high ON/OFF current ratio (>103), low voltage operation, and high retention time. Bipolar resistive switching from these engineered active layers will have great potential for future large area and sustainable electronics.

Memory state protected from leakage current in Ti/SiN/NiN/Pt bilayer resistive random-access memory devices for array applications

In this work, we present a method to protect the memory state of Pt/NiN/SiN/Ti bilayer memory structures, to integrate resistive switching memory with bipolar resistive switching (BRS) elements into a cross bar array (CBA) structure. In the device, a silicon nitride (SiN) layer acts as area creating BRS, while a nickel nitride (NiN) layer can be used as a selector (S), which results in the selector free resistive random-access memory (RRAM) devices possible to directly integrate into a CBA structure, without the need of additional elements. In addition, the proposed RRAM device shows a stable current ratio of >101 in the repetitive direct current cycling test of 200 times, a long retention time of >105, and a wide read-out margin of 130, which indicates the sneak current path in the crossbar structures was significantly suppressed. We believe that the proposed architecture is a strong candidate for future cross-bar type RRAM applications.

Analog high resistance bilayer RRAM device for hardware acceleration of neuromorphic computation

Analog nonvolatile resistive switching phenomena in metal oxides can potentially be used as a synaptic weight in hardware based neuromorphic computing accelerators. Single layer resistive random-access memory (RRAM) devices have switching currents in the greater than 1 mA range, effectively requiring too much energy for integration in a crossbar array based neural accelerator. This study details the fabrication and characterization of a bilayer RRAM device consisting of a Pt-TaOx-Al2O3-TiN stack which is designed for low current operation. This high resistance bilayer device reduces switching energy to ∼8 pJ during RESET and 15 pJ during SET, at the expense of increased operational noise. Noise increase is expected in this higher resistance device due to electron trapping in levels created by vacancies piling up at the interface between the Al2O3 and TaOx layer. As a result, the simulated performance of these devices used in training a neuromorphic accelerator on the MNIST dataset was 80%, significantly lower than required. Using the difference in current between two devices to represent a digit and using two digits per weight with a technique called periodic carry (for a total of 4 devices), a training accuracy of 93% could be achieved. The device and methods detailed here represent a necessary step toward the realization of energy efficient neuromorphic accelerators.

Revealing conducting filament evolution in low power and high reliability Fe3O4/Ta2O5 bilayer RRAM

In this work, we used the polycrystalline-Fe3O4 to improve the reliability of the Ag/Ta2O5/Pt resistive random access memory (RRAM). In both the Ag/Ta2O5/Fe3O4/Pt and Ag/Fe3O4/Ta2O5/Pt structures, the switching properties for these bilayer RRAMs were measured in atmosphere and vacuum environments. The results demonstrated that the humidity would affect the Ag filament formation in different environments, and the Ta2O5 and Fe3O4 interface in a different sequence would change the performance of the device, particularly the Forming voltage. Furthermore, the switching voltage and reliability of these bilayer RRAMs was better than single-layer RRAM device, which significantly increased endurance, especially in the Ag/Fe3O4/Ta2O5/Pt device. We also observed the conducting filament shape and evolution during Forming via in/ex-situ transmission electron microscopy (TEM) in the Ag/Fe3O4/Ta2O5/Pt system. In low humidity, the conducting filament was composed of many weak filaments in a low-resistance state (LRS), where the grain boundaries in the Fe3O4 layer limited filament size. The results of energy dispersive spectrometry (EDS) analysis demonstrated that the filament was composed of Ag metal. This study provided detailed switching knowledge of the bilayer RRAM for improving the reliability and power consumption of the device and new design viewpoints of the RRAM structure in future applications.

Tri-state resistive switching characteristics of MnO/Ta2O5 resistive random access memory device by a controllable reset process

Tri-state resistive switching characteristics of bilayer resistive random access memory devices based on manganese oxide (MnO)/tantalum oxide (Ta2O5) have been studied. The current–voltage (I–V) characteristics of the Ag/MnO/Ta2O5/Pt device show tri-state resistive switching (RS) behavior with a high resistance state (HRS), intermediate resistance state (IRS), and low resistance state (LRS), which are controlled by the reset process. The MnO/Ta2O5 film shows bipolar RS behavior through the formation and rupture of conducting filaments without the forming process. The device shows reproducible and stable RS both from the HRS to the LRS and from the IRS to the LRS. In order to elucidate the tri-state RS mechanism in the Ag/MnO/Ta2O5/Pt device, transmission electron microscope (TEM) images are measured in the LRS, IRS and HRS. White lines like dendrites are observed in the Ta2O5 film in both the LRS and the IRS. Poole–Frenkel conduction, space charge limited conduction, and Ohmic conduction are proposed as the dominant conduction mechanisms for the Ag/MnO/Ta2O5/Pt device based on the obtained I–V characteristics and TEM images.

Variability Improvement of TiO x /Al2O3 Bilayer Nonvolatile Resistive Switching Devices by Interfacial Band Engineering with an Ultrathin Al2O3 Dielectric Material.

Variability control over the resistive switching process is one of the key requirements to improve the performance stability of the resistive random access memory (RRAM) devices. In this study, we show the improvement of the variability of the resistive switching operation in the TiOx/Al2O3 bilayer RRAM devices. The achievement is based on the thickness engineering of the Al2O3 layer. A thick Al2O3 dielectric actively takes part to control the resistive switching behavior; on the contrary, the ultrathin layer of Al2O3 behaves as the tunnel barrier in the structure. At lower voltage, the low resistance state conductions follow the trap-assisted tunneling and Fowler–Nordheim tunneling for the thick and thin Al2O3 RRAMs, respectively. Finally, the variation control in device forming, SET voltage distribution, high resistance state, low resistance state, and resistance ratio is achieved with the TiOx/Al2O3 bilayer RRAM devices by interfacial band engineering with an ultrathin Al2O3 dielectric material.

Built-in Nonlinear Characteristics of Low Power Operating One-Resistor Selector-Less RRAM By Stacking Engineering

Resistive random access memory (RRAM) using various metal oxides (i.e., SiO2[1], HfO2, NiO[2], Al2O3, NbO) have attracted much attentions since the current nonvolatile memory (NVM) has been approaching the scaling limit. Meanwhile, selector devices are essential to address the sneak path issue which causes reading errors and hinders the implementation of RRAM in high-density cross-bar array architecture [3-4]. However, additional selector devices, e.g. a commonly proposed one selector-one resistor (1S-1R) design, increases the process complexity and cost. In this work, selectorless 1R RRAM devices have been demonstrated by utilizing the nonlinear (NL) resistive switching (RS) characteristics. Pulse operation voltage variation tolerance on SiOx-based stacking structures has also been characterized. The starting substrates are heavily-doped N+ Si wafers, and titanium nitride of 200 nm thickness was deposited on N+ Si substrate as bottom electrode. Then, 9 nm of SiOx and 4 nm of HfOx were deposited as RS dielectric layers for bilayer (HfOx/SiOx) structures by RF sputtering method. Pt (165nm) was deposited as top electrodes for RRAM devices (Fig. 1). Fig. 2 shows bipolar RS I-V characteristics during DC voltage sweeps for single-layer HfOx and bilayer devices. Both SET and RESET voltages are lower in single-layer HfOx devices than in bilayer devices possibly due to higher oxygen concentration. The SET voltage of single-layer HfOx devices is about 0.2 V lower than of single-layer SiOx devices, and RESET voltage is reduced to half (~ 0.5 V) with inserting a HfOx stack layer, which is potentially beneficial for low voltage operating applications. Also, the RESET current is critical for overall switching power in RRAM applications, is ~1 mA and has been found to be independent of the thickness ratio of stacks. Comparing the bilayer structure to the single-layer HfOxstructure, the current at -0.2 V is reduced from ~10-4 to ~10-6 A. It depicts that there was significantly increased resistance at low voltage region, which is proposed as a solution for sneak path issue in crossbar array applications. Nonlinearity (NL) is defined as the ratio of the current at full read voltage (i.e. -0.6 V) to the current at 1/3 read voltage (-0.2 V). The higher nonlinearity, the better ability is to avoid the sneak current interference. The NL characteristics in ILRS of all the devices are showed in Fig. 3. With a thin SiOx layer (2 nm) on the bottom of HfOx (11 nm) layer, the NL is ~3x in comparison to the single-layer HfOx layer devices. Based on our results, the bilayer RRAM device has been found to show the highest NL among the SiOx-based stacking structures. This makes SiOx-based bilayer devices a potential candidate for 1R selectorless RRAMs. To evaluate the voltage variation tolerance in pulse operation, the reset stop voltage effect has been studied. The RS I-V was characterized with increasing RESET stop voltage from -1.4 to -2.6 V for the bilayer and trilayer devices (Fig. 4). For bilayer devices, the high resistance state (HRS) resistance and memory window increase with increasing RESET stop voltage (Fig. 5). The bilayer devices with higher SET/RESET voltage variation tolerance (less variation below stop voltage of 2.2 V) and have been found to exhibit better NL characteristics (~10) than the trilayer devices, which shows larger switching voltage variation with changing reset stop voltage (Fig.6). In this study, build-in nonlinear characteristics have been realized for the SiOx-based one-resistor (1R) device without an additional diode or a selector. The highly nonlinear characteristics observed in SiOx-based bilayer devices are desirable in avoiding the read error and preventing sneak-path currents in crossbar arrays. Our results show that SiOx-based bilayer devices are promising for high-density, low-power, selectorless RRAM applications. Figure 1

Interface-induced two-step RESET for filament-based multi-level resistive memory

In this paper, a two-step RESET switching behavior of Ag/Al2O3/HfO2/Pt bilayer resistive random access memory (RRAM) devices is investigated. The interface between the two oxide layers is responsible for the special two-step RESET switching. When the conducting filaments have ruptured in the lower layer, the interface can protect the Ag ions of the filaments from breaking in the upper layer due to the trapped charges or defects at the interface. Therefore, a stable middle resistance state (MRS) is realized and the device exhibits a terrace-like I–V curve during the RESET operations. A filament-based switching mechanism combined with the electron hopping theory is proposed to explain the physical nature of the two-step RESET behavior. Furthermore, a good multi-level resistive switching performance with excellent endurance and retention reliability is obtained.

Endurance Enhancement by Soft Forming Algorithm on AlO&lt;sub&gt;&lt;italic&gt;x&lt;/italic&gt;&lt;/sub&gt;/WO&lt;sub&gt;&lt;italic&gt;y&lt;/italic&gt;&lt;/sub&gt; Resistive Switching Memory Array

A new soft forming algorithm, proposed in this letter, is demonstrated to improve endurance of an AlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> bilayer resistive random access memory array by two orders of magnitude. Relative to the conventional strong forming algorithm, soft forming creates a thinner conductive filament (CF) by applying voltage pulses that have narrower widths and lower amplitude. The mechanism can be explained by a CF model. soft forming algorithm is beneficial to form a thinner CF, which contains more movable oxygen vacancies (Vo) than that is formed by strong forming method. The rupture of thinner CF requires less movable oxygen ions (O <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sup> ). Consequently, exhaustion of movable Vo/O <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sup> slows down and the number of set/reset cycles is increased, which improves endurance of the array.

Random telegraph noise analysis in AlOx/WOy resistive switching memories

In this Letter, the origins of current fluctuations of Al/AlOx/WOy/W bilayer resistive random access memory (RRAM) devices are investigated through detailed noise analysis. Random telegraph noise (RTN) measurements were performed on RRAMs with three different resistance states. An obvious RTN signal with 40.7% amplitude difference was found at high resistance state, and the trapping/de-trapping process leading to the RTN signal was studied in detail by extracting the trap energy from energy diagram. For median and low resistance states, the resistance fluctuations were 34.0% and 0.3%, respectively. To further study the RTN characteristics, the normalized power spectral density (PSD) was analyzed. It is found that, for one dominant-trap caused RTN phenomena, the normalized noise PSD behaves as 1/f 2 on the high resistance state; while for median and low resistance states, the noise follows 1/f rule, suggesting that the current fluctuations are associated with the envelop of multiple RTNs caused by traps located near/in the conductive filament. Based on the noise analyses in time and frequency domains, a conduction mechanism is proposed to describe the trap effects on the current fluctuations of different resistance states.

Switching mechanism of double forming process phenomenon in ZrOx/HfOy bilayer resistive switching memory structure with large endurance

In this Letter, the mechanism of double forming process phenomenon revealing in ZrO2/HfO2 bilayer resistive random access memory structure is investigated. This phenomenon caused by the formation of TiON interfacial layer can be well explained by using the energy band diagram. The TiON interfacial layer will be a tunneling barrier during the first forming process when a negative voltage applied on the device, while it will breakdown when applying a positive voltage. Besides, due to the double forming process, an asymmetric conductive filament with narrower size at ZrO2/HfO2 interface is formed in the device. The point for formation and rupture of the conductive filament can be confined at the ZrO2/HfO2 interface, and it will suppress the consumption of oxygen ions during endurance test. Therefore, high speed (40 ns) and large endurance (107 cycles) characteristics are achieved in this device structure.

Study of conduction and switching mechanisms in Al/AlOx/WOx/W resistive switching memory for multilevel applications

In this letter, the conduction and switching mechanisms of Al/AlOx/WOx/W bilayer resistive random access memory devices are investigated. Five stable resistance states were achieved through current compliance control. For each resistance state, I-V characteristics at different temperatures were measured. Conduction mechanisms are found to vary with resistance states. At low resistance levels, devices show ohmic conduction with metallic behavior. Conduction at medium resistance levels is due to electron hopping. The carrier transport at high resistance levels is governed by Schottky emission. Based on the resistance-dependent transport characteristics, an oxygen migration model is proposed to explain the switching mechanism between different resistance states.