Electrically tailored resistance switching in silicon oxide

Resistance random access memory (RRAM) is a great potential candidate for next-generation nonvolatile memory due to the outstanding memory characteristic. However, the resistance switching mechanism is still a riddle nowadays. In this letter, the switching mechanism is investigated by current-voltage (I-V) curve fitting in the TiN/WSiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</sub> /Pt RRAM device. The asymmetric phenomenon of the carrier conduction behavior is explained at the high-resistance state in high electric field. The switching behavior is regarded to tip electric field by localizing the filament between the interface of top electrode and insulator.

Among the emerging memories, resistive switching (RS) based resistive random-access memories (RRAMs) are attracting lots of attention due to their simple metal–insulator–metal structures, low power consumption, long endurance and retention characteristics, low fabrication cost, ultrafast switching, and CMOS compatibility. In recent years, several oxides, chalcogenides, polymers and their composites have been explored for RS devices. Many of these studies show a high resistance ratio of the OFF and ON states with good RS characteristics. Some of these studies also show the realization of multifunctional RS devices such as the simultaneous switching of resistance and magnetic states. In order to scale up RS-based RRAMs, a detailed understanding of the occurrence of RS is very much desirable. In this review, we provide an overview of the current understanding, recent advances and future outlook of RS-based RRAM devices along with fundamental concepts of the different types of RS, and conventional as well as novel measurement techniques which are being used to characterize RS devices. Observations of RS in different materials are presented, and RS mechanisms, such as the valence change mechanism and electrochemical metallization memory, are discussed in detail. An overview of multifunctional RS devices and the main challenges faced in scaling up RS devices is also presented.

Here, an optimized method for energy‐efficient and highly reliable multibit operation in a Au/Al2O3/HfO2/TiN resistive switching (RS) device is investigated. A thin Al2O3 layer inserted between the top electrode and the HfO2 RS layer in a capacitor‐like RS device plays the role of an electrical resistor and modulates the external bias‐dependent resistance variation behavior of the RS device. Compared to the case where a single HfO2 layer is used, the device with a Al2O3/HfO2 stacked layer shows slow and nonvolatile resistance state variation with a stepwise increase of the applied voltage, which can significantly reduce the occurrence frequency of the abnormal cases during the incremental‐step‐pulse‐programming (ISPP) and error‐check‐and‐correction (ECC) processes. This phenomenon reduces the number of ISPP/ECC sequences, which is advantageous for achieving energy‐efficient programming in the multibit operation of an RS device. To clarify the role of the Al2O3 thin film layer, a comparative study is performed with an Au/HfO2/TiN stacked device and a serially connected external load resistor. The observed behavior is consistent with that of the device with an Al2O3 layer. This study demonstrates that the simple insertion of an insulating layer in an RS device can facilitate reliable and energy‐efficient multibit operation for future nonvolatile memory applications.

Resistive switching (RS) devices based on Nb:SrTiO3 (NSTO) hold significant potential for next-generation data storage due to their tunable resistance, stability, and low power consumption. The majority of existing research studies have predominantly focused on metal/NSTO structures that utilize single-crystal NSTO. Although this configuration exhibits superior performance, it is costly and cannot be easily integrated. Another important point is that epitaxial structures can enhance device stability because of their excellent interfaces. However, epitaxial growth of metals on NSTO poses significant challenges due to inherently high structural mismatches, which has led to inconsistent results reported by various researchers employing the same metal/NSTO structure. In this study, NSTO films with different Nb doping amounts were prepared on YBa2Cu3O[Formula: see text][Formula: see text] (YBCO) layers to form NSTO/YBCO heterostructures. YBCO serves as both the electrode and the epitaxial heterolayer material for NSTO. The photogenerated voltage and RS properties of epitaxially grown NSTO/YBCO structures were investigated in detail. At a low Nb doping amount, NSTO cannot produce sufficient photogenerated carriers, while excessive Nb doping results in a low interface barrier height, causing the photogenerated open-circuit voltage of the NSTO/YBCO structure to initially increase and then decrease with the increase in Nb doping amount (the peak value is observed at a Nb doping concentration of 1%). On the other hand, the electrical conductivity of NSTO increases with Nb doping, leading to an increase in the photogenerated short-circuit current. Further, the RS characteristics of the NSTO/YBCO structure were investigated, and clockwise and counterclockwise RS behaviors under low and high bias voltages were observed, respectively. At low voltages, the clockwise RS effect is primarily attributed to the increased interfacial barrier resulting from the migration of oxygen vacancies in YBCO. Conversely, at high voltages, the migration of oxygen ions within NSTO leads to a reduction in the interfacial barrier, thereby inducing a counterclockwise RS effect. In addition, at low bias voltages, the resistance ratio between the high-resistance state and the low-resistance state under [Formula: see text] 1/[Formula: see text] 2 V bias voltages first increases and then decreases as the Nb doping amount increases, with the highest resistance ratio (&gt;100) observed at 1% Nb doping. These findings provide novel insights into the RS mechanism, and it is envisaged that they will promote the application of conductive oxides in RS devices and the development of multilevel memory devices.

We report a study of light-activated resistance switching in silicon oxide (SiOx) resistive random access memory (RRAM) devices. Our devices had an indium tin oxide/SiOx/p-Si Metal/Oxide/Semiconductor structure, with resistance switching taking place in a 35 nm thick SiOx layer. The optical activity of the devices was investigated by characterising them in a range of voltage and light conditions. Devices respond to illumination at wavelengths in the range of 410–650 nm but are unresponsive at 1152 nm, suggesting that photons are absorbed by the bottom p-type silicon electrode and that generation of free carriers underpins optical activity. Applied light causes charging of devices in the high resistance state (HRS), photocurrent in the low resistance state (LRS), and lowering of the set voltage (required to go from the HRS to LRS) and can be used in conjunction with a voltage bias to trigger switching from the HRS to the LRS. We demonstrate negative correlation between set voltage and applied laser power using a 632.8 nm laser source. We propose that, under illumination, increased electron injection and hence a higher rate of creation of Frenkel pairs in the oxide—precursors for the formation of conductive oxygen vacancy filaments—reduce switching voltages. Our results open up the possibility of light-triggered RRAM devices.

Oxygen vacancies (VOs) are fundamental for oxide resistance switching (RS) devices, while the introduction of VOs in oxides during the fabrication processes has generally generated unfavorable defects which have limited the improvements in RS performance. Herein, a novel strategy, called ex situ VOs doping, has been proposed to increase the concentration of VOs to fabricate improved ZnO RS devices. The ex situ VOs doping process is similar to modulation doping of heterojunctions, where VOs are intentionally incorporated in a separate oxide material and then migrated toward the target materials. Specifically, HfO2/ZnO architecture with a thin oxygen-deficient HfO2 capping layer is prepared, where VOs in the HfO2 layer facilely migrated toward the underlying ZnO under applying bias. Theoretical calculations and atomic binding energy analysis have verified the effective ex situ VOs doping of ZnO in the HfO2/ZnO architecture. Benefiting from the dominant migration of VOs along grain boundaries, the accumulated VOs around grain boundaries at HfO2/ZnO interface provided preferential nucleation sites for deterministic formation of conductive filaments. Accordingly, improved ZnO RS devices have been demonstrated in terms of increased RS window and enhanced uniformity, stability, and endurance. The ex situ doping of VOs using a heterojunction architecture enables multi-dimensional regulation of the formation of conductive filaments, which will find potential applications in achieving improved oxide RS devices in the future.

The traditional von Neumann architecture limits the increase in computing efficiency and results in massive power consumption in modern computers due to the separation of storage and processing units. The novel neuromorphic computation system, an in-memory computing architecture with low power consumption, is aimed to break the bottleneck and meet the needs of the next generation of artificial intelligence (AI) systems. Thus, it is urgent to find a memory technology to implement the neuromorphic computing nanosystem. Nowadays, the silicon-based flash memory dominates non-volatile memory market, however, it is facing challenging issues to achieve the requirements of future data storage device development due to the drawbacks, such as scaling issue, relatively slow operation speed, and high voltage for program/erase operations. The emerging resistive random-access memory (RRAM) has prompted extensive research as its simple two-terminal structure, including top electrode (TE) layer, bottom electrode (BE) layer, and an intermediate resistive switching (RS) layer. It can utilize a temporary and reversible dielectric breakdown to cause the RS phenomenon between the high resistance state (HRS) and the low resistance state (LRS). RRAM is expected to outperform conventional memory device with the advantages, notably its low-voltage operation, short programming time, great cyclic stability, and good scalability. Among the materials for RS layer, indium gallium zinc oxide (IGZO) has shown attractive prospects in abundance and high atomic diffusion property of oxygen atoms, transparency. Additionally, its electrical properties can be easily modulated by controlling the stoichiometric ratio of indium and gallium as well as oxygen potential in the sputter gas. Moreover, since the IGZO can be applied to both the thin-film transistor (TFT) channel and RS layer, it has a great potential for fully integrated transparent electronics application.In this work, we proposed amorphous transparent IGZO-based RRAMs and investigated switching behaviors of the memory cells prepared with different top electrodes. First, ITO was choosing to serve as both TE and BE to achieve high transmittance. A multi-target magnetron sputtering system was employed to deposit all three layers (TE, RS, BE layers) on glass substrate. I-V characteristics were evaluated by a semiconductor parameter analyzer, and the bipolar RS feature of our RRAM devices was demonstrated by typical butterfly curves. The optical transmission analysis was carried out via a UV-Vis spectrometer and the average transmittance was around 80% out of entire devices in the visible-light wavelength range, implying high transparency. We adjusted the oxygen partial pressure during the sputtering of IGZO to optimize the property because the oxygen vacancy concentration governs the RS performance. Electrode selection is crucial and can impact the performance of the whole device. Thus, Cu TE was chosen for our second type of device because the diffusion of Cu ions can be beneficial for the formation of the conductive filament (CF). A ~5 nm SiO2 barrier layer was employed between TE and RS layers to confine the diffusion of Cu into the RS layer. At the same time, this SiO2 inserting layer can provide an additional interfacial series resistance in the device to lower the off current, consequently, improve the on/off ratio and whole performance. Finally, an oxygen affinity metal Ti was selected as the TE for our third type of device because the concentration of the oxygen atoms can be shifted towards the Ti electrode, which provides an oxygengettering activity near the Ti metal. This process may in turn lead to the formation of a sub-stoichiometric region in the neighboring oxide that is believed to be the origin of better performance.In conclusion, the transparent amorphous IGZO-based RRAMs were established. To tune the property of RS layer, the sputtering conditions of RS were varied. To investigate the influence of TE selections on switching performance of RRAMs, we integrated a set of TE materials, and a barrier layer on IGZO-based RRAM and compared the switch characteristics. Our encouraging results clearly demonstrate that IGZO is a promising material in RRAM applications and breaking the bottleneck of current memory technologies.

Resistive switching mechanism of zirconium oxide-based resistive random access memory (RRAM) devices composed of Cu-doped ZrO2 film sandwiched between an oxidizable electrode and an inert electrode was investigated. The Ag/ZrO2:Cu/Pt RRAM devices with crosspoint structure fabricated by e-beam evaporation and e-beam lithography show reproducible bipolar resistive switching. The linear I–V relationship of low resistance state (LRS) and the dependence of LRS resistance (RON) and reset current (Ireset) on the set current compliance (Icomp) indicate that the observed resistive switching characteristics of the Ag/ZrO2:Cu/Pt device should be ascribed to the formation and annihilation of localized conductive filaments (CFs). The physical origin of CF was further analyzed by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). CFs were directly observed by cross-sectional TEM. According to EDS and elemental mapping analysis, the main chemical composition of CF is determined by Ag atoms, coming from the Ag top electrode. On the basis of these experiments, we propose that the set and reset process of the device stem from the electrochemical reactions in the zirconium oxide under different external electrical stimuli.

Hexagonal boron nitride (h-BN) is an attractive insulating material for nanoelectronic devices due to its high reliability as dielectric and excellent compatibility with other two dimensional (2D) materials (e.g. graphene, MoS2). Multilayer h-BN stacks have been readily grown on Cu and Pt substrates via chemical vapor deposition (CVD) approach, confirming its potential for wafer scale integration. However, the growth of h-BN on other substrates needs to be also achieved in order to expand the use of this material. Recently, the CVD growth of monolayer h-BN on Fe substrates was reported, but it just focused on material structure characterization. Here we present the first fabrication of electronic devices using multilayer h-BN dielectric stacks grown on Fe foils. We fabricate and characterize resistive switching (RS) devices based on Au/Ag/h-BN/Fe nanojunctions, and observe the coexistence of both volatile and non-volatile RS depending on the electrode to which the bias is applied. The characteristics measured agree well with those simulated via SIM2RRAM software and QPC modeling, and the cycle-to-cycle variability is slightly lower than that of transition metal oxide based RS devices.

Nonvolatile resistive switching memories-characteristics, mechanisms and challenges

In this paper, the switching layer thickness and temperature impacts on resistance switching polarity of tantalum oxide (TaO x ) based resistive random access memory (RRAM) have been investigated. Our results show that at room temperature unipolar behavior was activated in thick TaO x but failed in thin TaO x , while in 280 °C condition thin TaO x layer RRAM can also show stable unipolar characteristics. For thick TaO x device, the unipolar behavior is attributed to more heat accumulation in thicker switching layer. In 280 °C condition, stable unipolar characteristic of thin TaO x device is due to extra ambient heat offered. In addition, thin TaO x RRAM with Si0 2 buffer layer is also fabricated to investigate the heat accumulation impact on operation polarity of RRAM devices. These results are helpful in obtaining the insight on understanding the switching mechanism of TaO x RRAM and improve the RRAM device performance and reliability capability.

Resistance random access memory (RRAM) based on resistive switching in metal oxides has attracted considerable attention as a promising candidate for next-generation nonvolatile memory due to its high operating speed, superior scalability, and low power consumption. However, some operating parameters of RRAM cannot meet the practical requirement, which impedes its commercialization. A lot of experimental results show that doping is an effective method of improving the performance of RRAM, while the study on the physical mechanism of doping is rare. It is generally believed that the formation and rupture of conducting filaments, caused by the migration of oxygen vacancies under electric field play a major role in resistive switching of metal oxide materials. In this work, the first principle calculation based on density functional theory is performed to study the effects of transition metal element X (X=Mn, Fe, Co, and Ni) doping on the migration barriers and formation energy of oxygen vacancy in ZnO. The calculation results show that the migration barriers of both the monovalent and divalent oxygen vacancy are reduced significantly by Ni doping. This result indicates that the movement of oxygen vacancies in Ni doped ZnO is easier than in undoped ZnO RRAM device, thus Ni doping is beneficial to the formation and rupture of oxygen vacancy conducting filaments. Furthermore, the calculation results show that the formation energy of the oxygen vacancy in ZnO system can be reduced by X doping, especially by Ni doping. The formation energy of the oxygen vacancy decreases from 0.854 for undoped ZnO to 0.307 eV for Ni doped ZnO. Based on the above calculated results, Ni doped and undoped ZnO RRAM device are prepared by using pulsed laser deposition method under an oxygen pressure of 2 Pa. The Ni doped ZnO RRAM device shows the optimized forming process, low operating voltage (0.24 V and 0.34 V for Set and Reset voltage), and long retention time (&gt;104 s). Set and Reset voltage in Ni doped ZnO device decrease by 80% and 38% respectively compared with those in undoped ZnO device. It is known that the density of oxygen vacancies in the device is dependent on the oxygen pressure during preparation. The Ni doped ZnO RRAM device under a higher oxygen pressure (5 Pa) is also prepared. The Ni doped ZnO RRAM device prepared under 5 Pa oxygen pressure shows a little higher Set and Reset voltage than the device prepared under 2 Pa oxygen pressure, while the operating voltages are still lower than those of undoped ZnO RRAM. Thus, the doping effect in the ZnO system is affected by the density of oxygen vacancies in the device. Our work provides a guidance for optimizing the performance of the metal oxide based RRAM device through element doping.

Mechanism of ferroelectric resistive switching in Bi0.9La0.1FeO3 thin films

The achievement of a reproducible and controlled deposition of partitioned Cu2O/CuO thin films by techniques compatible with ULSI processing like reactive magnetron sputtering has been reported as an outstanding challenge in the literature. This phase partitioning underlies their performance as reversible resistive memory switching devices in advanced microelectronic applications of the future. They are currently fabricated by thermal oxidation and chemical methods. We have used a combination of an understanding from plasma chemistry, thermo-kinetics of ions, and rf power variation during deposition to successfully identify a processing window for preparing partitioned Cu2O/CuO films. The production of a core rich Cu2O and surface rich Cu2O/CuO mixture necessary for oxygen migration during resistive switching is confirmed by XRD peaks, Fourier transform infra red Cu (I)-O vibrational modes, XPS Cu 2P3/2 and O 1S peak fitting, and a comparison of satellite peak ratio's in Cu 2P3/2 fitted peaks. We are proposing based on the findings reported in this paper that an XPS satellite peak intensity(Is) to main peak intensity ratio (Im) ≤ 0.45 as an indicator of a core rich Cu2O and surface rich Cu2O/CuO formation in our prepared films. CuO is solely responsible for the satellite peaks. This is explained on the basis that plasma dissociation of oxygen will be limited to the predominant formation of Cu2O under certain plasma deposition conditions we have identified in this paper, which also results in a core-rim phase partitioning. The deposited films also followed a Volmer-Weber columnar growth mode, which could facilitate oxygen vacancy migration and conductive filaments at the columnar interfaces. This is further confirmed by optical transmittance and band-gap measurements using spectrophotometry. This development is expected to impact on the early adoption of copper oxide based resistive memory electronic switching devices in advanced electronic devices of the future. The relative abundance of copper is another major complementary driver for the interest in copper oxide films.

We developed a one-transistor one-resistor cell composed of one TiO2-based resistive switching (RS) device and one ZnO-based thin-film transistor (TFT). We study the electric characteristics of each component individually, and their interplay when both work together. We explored the direct control of bipolar RS devices, using our TFTs to drive current in both directions. We also report striking power implications when we swap the terminals of the RS device. The target of our work is the introduction of RS devices in large-area electronic (LAE) circuits. In this context, RS devices can be beneficial regarding functionality and energy consumption, when compared to other ways to introduce memory cells in LAE circuits.