Self‐repairable, high‐uniform conductive‐bridge random access memory based on amorphous NbSe2

Conductive‐bridge random access memory (CBRAM) emerges as a promising candidate for next‐generation memory and storage device. However, CBRAMs are prone to degenerate and fail during electrochemical metallization processes. To address this issue, herein we propose a self‐repairability strategy for CBRAMs. Amorphous NbSe2 was designed as the resistive switching layer, with Cu and Au as the top and bottom electrodes, respectively. The NbSe2 CBRAMs demonstrate exceptional cycle‐to‐cycle and device‐to‐device uniformity, with forming‐free and compliance current‐free resistive switching characteristics, low‐operation voltage, and competitive endurance and retention performance. Most importantly, the self‐repairable behavior is discovered for the first time in CBRAM. The device after failure can recover its performance to the initially normal state by operating with a slightly large reset voltage. The existence of Cu conductive filament and excellent controllability of Cu migration in the NbSe2 switching layer has been revealed by a designed broken‐down point approach, which is responsible for the self‐repairable behavior of NbSe2 CBRAMs. Our self‐repairable and high‐uniform amorphous NbSe2 CBRAM may open the door to the development of memory and storage devices in the future.


| INTRODUCTION
The development of information technology has put forward higher and higher requirements for data storage and processing.2][3][4][5][6] ReRAM refers to a two-terminal device of resistance switched between a high resistance state (HRS) and low resistance state (LRS) by applying voltages, 7,8 owing to the formation/ rupture of conductive filaments (CFs) in resistive switching layer. 9,10Conductive-bridge random access memory (CBRAM) is the typical ReRAM, where the top electrode is an active metal (such as Ag, Cu, and Ni) and the bottom electrode is an inert metal (such as Au, Pt, and TiN). 11,12In CBRAM, when the top electrode is positively biased, the active metal undergoes a redox reaction to become metal ions, migrating into the resistive layer under the electric field to form the CF; when the top electrode is negatively biased, the CF ruptures. 13s a nonvolatile memory technology, CBRAM is expected to become an alternative to traditional memory and storage technologies (e.g., dynamic random access memory [DRAM] and flash memory). 14Compared to DRAM, which is volatile and has low storage density, CBRAM does not lose data when power is turned off and has a larger storage capacity.Compared to flash memory, CBRAM has faster data transfer speed and lower power consumption. 15,16In addition, CBRAM has a simple electrode/resistive switching layer/electrode structure, which is compatible with the complementary metal-oxidesemiconductor (CMOS) process.In these regards, CBRAM emerges as a promising candidate for next-generation memory and storage device.
In the last decade, extensive efforts have been made for CBRAM.However, the challenges still exist at present and need to be addressed.][19][20][21] During the electrochemical metallization (ECM) process, the CBRAM is prone to degenerate and fail, where the ratio of the HRS to LRS scales down and deviates from the allowed ranges.Also, the cycle-to-cycle (c2c) and deviceto-device (d2d) variations are generally evident in CBRAM devices, 22 and the key parameters, including set voltage (V set ), reset voltage (V reset ), and resistance values of HRS and LRS, always fluctuate during the endurance process.In previous reports, the migration of cations in the switching layer is random and difficult to control accurately, so stochastic characteristics are ubiquitous in CBRAM. 23Inevitably, failed devices become waste electronic products.The high-uniformity CBRAM with a longterm lifetime is an essential prerequisite for practical applications in the future.
It is well known that the stochastic characteristic mainly depends on the ability of the switching layer to control the migration of cations.To resolve the key issue, we propose the self-repairability strategy of CBRAMs.In previous studies, storage devices with multilayer NbSe 2 /HfO 2 23 and NbSe 2 /NbO x 24 materials were reported, where the random Ag ion migration can be controlled with the NbSe 2 buffer layer.Considering the strong in-plane and out-of-plane ionic transport features, in this study, amorphous NbSe 2 was designed as the resistive switching layer, and the CBRAM device was constructed with Cu and Au as the top and bottom electrodes, respectively.As expected, the NbSe 2 CBRAM has ultrahigh uniform behaviors under operation, with exceptional c2c and d2d uniformity.Amazingly, the NbSe 2 CBRAM device that fails after multiple cycles can recover its performance to the original state by operating with a larger reset voltage.It is the first time the demonstration of self-repairable behavior in CBRAM is attributed to the excellent cation diffusion control capability of the NbSe 2 switching layer based on our analyses.The selfrepairable nature provides a new solution for extending the service life of the CBRAM device.Our proposed amorphous NbSe 2 CBRAMs with self-repairable and high-uniform characteristics are believed to have great potential in the memory and storage fields.

| Device fabrication
NbSe 2 films were deposited by radio-frequency magnetron sputtering, using a high-purity (99.99%)NbSe 2 target.Silica glass and SiO 2 /Si wafers were used as substrates.All kinds of substrates were ultrasonically cleaned successively with acetone, ethanol, deionized water, and ethanol for 30 min and dried in flowing nitrogen gas before use.The films were deposited at 300°C with an Ar pressure of 2 Pa and sputtering power of 20 W for 60 min.Both the bottom (Au) and top (Cu) electrodes were grown by electron beam evaporation.A layer of 10 nm Ti was deposited between the Au and the substrate to increase their adhesion.A shadow mask was used to define the circular top electrode with a diameter of 300 μm.A 40 nm Au was deposited to protect Cu from oxidation.

| Materials and device characterization
The morphologies, thicknesses, and structures of the samples were measured by high-resolution transmission electron microscopy (HRTEM; FEI Talos F200X), X-ray diffraction (XRD; Empyrean 200895), atomic force microscope (AFM; Bruker Dimension Icon), and field emission scanning electron microscopy (FESEM; Hitachi S4800).The elemental distributions and chemical bonding states were investigated by X-ray photoelectron spectroscopy (XPS; Thermo Scientific K-Alpha), and energy dispersive X-ray spectrometer (EDS) attached to the transmission electron microscopy (TEM).The performances of NbSe 2 CBRAM devices were measured using a semiconductor parameter analyzer (Keysight 4200A).
3 | RESULTS AND DISCUSSION

| Material and device characterization
In our CBRAMs, amorphous NbSe 2 was adopted as the resistive switching layer.Figure 1A   Figure 1C shows the surface scanning electron microscopy (SEM) image of NbSe 2 layers, which is uniform, smooth, and flat, without any visible defects.The root-mean-square roughness of deposited-NbSe 2 films is identified to be 1.6 nm from AFM images (Supporting Information: Figure S2).XPS spectra of the Nb 3d and Se 3d of NbSe 2 film are shown in Figure 1E,F, respectively, which were calibrated with reference to the C 1s peak. 25The binding energy values for the most intense peaks of Nb 3d and Se 3d are found to be 207.1 eV (Nb 3d 5/2 ) and 55 eV (Se 3d 5/2 ), respectively.The atomic ratio of Nb to Se is calculated to be 1:1.89 in films, close to that in the NbSe 2 target.Figure 1D shows XRD patterns of NbSe 2 films deposited on SiO 2 substrates.Only one halo peak around 21°is observed for NbSe 2 films, which is the same as that of SiO 2 substrates, so it comes from the SiO 2 substrate. 26,27There are no diffraction peaks from crystalline NbSe 2 , meaning that the NbSe 2 film deposited by magnetron sputtering is amorphous.The fast Fourier transformation of the NbSe 2 film is displayed in the inset of Figure 1B, obviously having broadly diffuse diffraction rings.All the results reveal the amorphous nature of NbSe 2 film as the switching layer in CBRAM.

| High-uniform resistive switching performance
Figure 2A displays the I-V curve of the resistive switching characteristics of the NbSe 2 CBRAM device for 100 cycles.In the switching behavior, a cycle is divided into four steps.The initial device is in HRS, whose resistance is around 700 Ω.The first step is the set process (HRS to LRS), where the top electrode bias increases from 0 to 0.8 V, and the resistance of the device begins to gradually reduce from 700 to 70 Ω above 0.3 V; in the second step, the voltage bias decreases from 0.8 to 0 V, and the device maintains the LRS of ∼70 Ω; the third step is the reset processes (LRS to HRS), where the top electrode bias is scanned from 0 to −0.8 V, and the resistance of the device suddenly elevates from 70 to 700 Ω around −0.4 V; in the fourth step, top electrode bias is scanned from −0.8 to 0 V, and the device keeps the HRS of 700 Ω.Both the absolute values of V set (0.3 V) and V reset (−0.4 V) are below 0.5 V, which is lower than most of the reported CBRAM devices, meaning that our NbSe 2 CBRAM can be operated at low voltages.The low-voltage operation can reduce power consumption and allow the device to be used in more advanced CMOS processes.The resistive switching mechanism is thought to be the formation and fracture of Cu CFs.During the set process, the top electrode of the device is positively biased, and Cu loses electrons and becomes Cu 2+ , which migrates to the bottom electrode to form CFs under the electric field force, so the device switches from HRS to LRS.During the reset process, where the top electrode of the device is negatively biased, the Cu CF fractures, and the Cu ions migrate back to the top electrode, so the device switches from LRS to HRS.
The NbSe 2 CBRAMs exhibit compliance with currentfree (I cc -free) characteristics.It is not necessary to apply I cc to protect the device from breakdown during the set process, which is beneficial to the scaling and integration of devices.Also, the I cc -free characteristics indicate that the device can suppress the overshoot of the set current.This built-in capability is attributed to the ability of the resistive switching layer to control the migration of Cu ions.This Cu-migrated controllability is also reflected in the excellent reproducible resistive switching.As illustrated in Figure 2A, all loops (including the first loop) are almost coincident, which indicates that the key parameters (e.g., V set , V reset , HRS, LRS) of CBRAMs hardly fluctuate.We count the values of V set and V reset for 100 cycles and use the Gaussian curve fitting their distribution, as shown in Figure 2B.The calculated σ/μ values of V set and V reset are 0.044 and 0.032, respectively, where σ is the root-mean-square error and μ is the mean value.Our CBRAM devices demonstrate excellent c2c uniformity with high stability during operations.Supporting Information: Table S1 summarizes the σ/μ values reported in works of literature, 23,[28][29][30][31] which evidently highlights the uniformity performance of our NbSe 2 CBRAMs.Furthermore, we investigated the d2d variability of NbSe 2 CBRAM devices.Figure 2C exhibits the boxplot of HRS and LRS for six devices.The I-V curves of these devices for 100 cycles are displayed in Supporting Information: Figure S3.All the devices have almost the same HRS and LRS values, confirming the remarkable d2d uniformity.Compared with those previously reported devices, our NbSe 2 CBRAMs exhibit evidently high repeatability.
We evaluated the endurance characteristics using the AC voltage pulse mode.Figure 2D shows the distributions of HRS and LRS for 10,000 cycles.There is no evident degeneration during the long-term operation, demonstrating its competitive endurance performance.Figure 2E presents the retention characteristics of the device tested at 85°C.Retention characteristic refers to the ability of a device to remain in a certain resistive state without external bias force.The resistances of NbSe 2 CBRAMs in HRS and LRS are read by V read of 0.01 V per second.Different resistive states of the device can remain almost unchanged for 10 4 s at 85°C.Based on the Arrhenius law, 32 it means that the resistive states can maintain for 10 years at room temperature.
To investigate the resistive switching mechanism of NbSe 2 CBRAM, we converted I-V curves into log I-log V curves and further analyzed the dependence of current and voltage.Figure 2F shows the log I-log V plots under positive bias, with the log-log linear fitting curves for different regions.Generally, the conduction mechanism of HRS in CF-based CBRAM is consistent with space charge limited conduction (SCLC). 9,33In detail, in the low-voltage region, the voltage and current satisfy Ohm's law, and the slope of the log I-log V curve is 1; as the voltage increases, the voltage and current relationship meet Child's law, and the slope of the curve will increase to ∼2, which is called the trapunfilled SCLC region; when the voltage further increases, the current increases sharply with the voltage, and the slope of the curve increases to ∼5, which is called the trap-filled SCLC region.For the HRS of NbSe 2 CBRAM, the slope of the log I-log V fitting curve increases from 1.03 to 2.11 and then to 4.45 during the set process, demonstrating that the HRS of our device well conforms to the SCLC mechanism.For LRS of NbSe 2 CBRAM, the slope of the fitting curve is exactly 1, showing a complete Ohmic conduction mechanism, which is due to the formation of Cu CFs in the NbSe 2 resistive switching layer.

| Self-repairable behavior
It is well known that failure is inevitable for an electronic device.For CBRAMs, during the ECM process, the device will always experience failure after cycles.Here, the failure refers to that the ratio of the HRS to LRS scales down and deviates from the allowed ranges, and it is no longer sufficient to meet the practical operating requirements.Failed devices are generally not usable anymore.However, we discover for the first time the repairability of CBRAM devices.Figure 3A shows the I-V curves with 1000 cycles for a normal NbSe 2 CBRAM, where the stop voltages in the set and reset processes are 0.8 and −0.8 V, respectively.Indubitably, the normal device has excellent c2c repeatability.As more and more cycles progress, the deterioration of device behaviors will appear.Figure 3B exhibits the failure process of the NbSe 2 CBRAM.The red curves indicate that the device still maintains normal performance.Since then, the resistance of HRS decreases significantly, while the LRS only slightly decreases, resulting in a reduction in the HRS/ LRS ratio (black curves).Finally, the resistance ratio is reduced to no longer match technological requirements and the device fails.
In the experiment, we applied a larger stop voltage (−1.0 V) in the reset process on the failed NbSe 2 CBRAM.Surprisingly, we found an interesting phenomenon: the device could work again, which is a very important finding.It means that the NbSe 2 CBRAM device can be repaired.Figure 3C shows the self-repairable process of our NbSe 2 CBRAM device, where the stop voltages in the set and reset processes are 1.0 and −1.0 V, respectively.During repair operation, because a larger voltage is adopted, a compliance current of 0.004 A was applied to protect the device from breaking down.The I-V curve for failed devices is black.The positive half-axis of the black curve is the I-V curve during the set process (stop voltage of 1.0 V) for the failed device, where the device is still in failure with the low resistance of HRS.The negative halfaxis of the black curve is the I-V curve during the reset process (stop voltage of −1.0 V) for the failed device, where the device has been repaired after the reset process with the high resistance of HRS same as that of the normal one (red curve).The red curve is the I-V curve of the subsequent cycle, which is almost the same as that of the normal device before failure.The observation clearly reveals that the NbSe 2 CBRAM device has been well repaired just via a reset process with a larger stop voltage of −1.0 V.
To verify that the NbSe 2 CBRAM device can normally work after repair, we conducted another 1000 cycles on the repaired device, as shown in Figure 3D.Clearly, the device has returned to its original state completely.The repairable performance means that the device may temporarily "fail" after cycles, but the device performance can be repaired by an excessive reset process.Also, we demonstrate that the NbSe 2 CBRAM device can be repaired repeatedly (Supporting Information: Figure S4).To the best of our knowledge, it is the first time to identify the self-repairable behavior for CBRAMs.It is a simple and rational strategy to repair the NbSe 2 CBRAM device, which is very plausible for practical applications.

| Cu migration investigation
To further confirm the existence of CFs and explore the ability of the NbSe 2 switch layer to control Cu migration, focused-ion-beam (FIB) and HRTEM measurements were carried out.It is usually difficult to directly observe the CFs because of their random location and nanometer size. 34Different from the in situ TEM method, [35][36][37][38] we adopted a novel approach to find the Cu CFs in the NbSe 2 layer.In the cycle test, when the sweeping voltage increases to around 3 V, the device will be broken down.Figure 4A shows the optical microscope images of the device before and after being broken down.The circles in Figure 4A are the top electrodes of the CBRAM devices.
The top electrode of the device on the left is complete, while the top electrode of the broken-down device on the right side shows a black spot, which is pointed out by the red arrow.Figure 4B shows the SEM image of the broken-down spot with a diameter of 2 μm.The brokendown spot is thought to be the position where the current density is the largest during the breakdown process.When the device is in LRS, the position where the CF is formed is the point with the highest current density, so it can be reasonably speculated that the inside of the broken-down point is the position where the CF is formed.FIB was used to prepare the TEM sample (Figure 4C), and the cross-sectional TEM image of the broken-down spot is displayed in Figure 4D.More detailed FIB process and TEM images are shown in Supporting Information: Figures S5 and S6. Figure 4E,F exhibit the distributions of Cu and Nb elements.Comparing the HRTEM image with the EDS element mapping, we can confirm the existence of Cu CFs inside the NbSe 2 film, which is marked by the red line.The EDS line profile around the CFs is presented in Figure 4G.area of the CFs, while the presence of Nb element in this area decreases.
In Figure 4E, the distribution of Cu can be observed at the interface between the NbSe 2 switching layer and the bottom Au electrode, which is naturally attributed to the congregated Cu atoms after migrating to the bottom electrode during the formation-rupture process of CFs after numerous cycles.The residual Cu in the bottom electrode makes the resistance of HRS lower, resulting in a decrease in the on-off ratio, which is consistent with the analysis of electrical behaviors (Figure 3).As mentioned above, the failure of our NbSe 2 CBRAM is not permanent, which can be reset to the normal HRS.
To have a deeper understanding of this issue, we design the following investigation: choose a broken-down device and exert a reset (stop voltage of −1.0 V) process on it to obtain the just-repaired device for FIB and HRTEM measurements.Figure 5 shows the cross-section HRTEM image and the mapping of all the involved elements of the device.No Cu CF can be found in the just-repaired NbSe 2 CBRAM device after a reset process, which means that the CFs in the switching layer can be reset to the original state even for the broken-down device.Also, note that the aggregated Cu that migrated to the bottom Au electrode disappears completely in the just-repaired device, suggesting that the residual Cu at the interface between the switching layer and the bottom electrode during the ECM process can also be reset to the original state.For a clear comparison, Supporting Information: Figure S7 displays the HRTEM images of the normal device, the broken-down device, and the just-repaired device, where the location, migration, and recovery of Cu element during the processes can be readily observed.The NbSe 2 CBRAM demonstrates an excellent ability to control Cu migration.
Based on our aforementioned analysis, we propose a model to explain the self-repairable mechanism of the NbSe 2 CBRAM, as schematically illustrated in Figure 6.The initial state of a normal CBRAM device is exhibited in Figure 6A.During the set operation, under positive bias (0.8 V in our case), Cu loses electrons to become Cu ions.Under the action of electric field force, Cu 2+ migrates to the bottom electrode and forms Cu CFs (Figure 6B).During the reset operation, under negative bias (−0.8 V in our case), the formed Cu CF becomes Cu 2+ again and migrates back to the top electrode (Figure 6C).After thousands of repetitive cycles, some Cu cannot migrate back and a certain residue of Cu remains on the bottom electrode (Figure 6D,E), which makes the resistance smaller than the initial state even if the CF is ruptured.In the following set process, the Cu CFs form, possibly with a slightly fat shape (Figure 6F), in what follows the incomplete reset process occurs, and more Cu aggregated on the bottom electrode (Figure 6G).As the set and reset programs proceed, the resistance values of both HRS and LRS decrease, but the LRS reduces severely, so the HRS/LRS ratio scales down and the device fails finally.For the failed device, the set process can also be carried out (Figure 6H), but it cannot work normally since the parameter deviates from the allowed range.After the set process for the failed device, if a larger negative stop voltage (−1.0 V in our case) is applied to the top electrode, the residual Cu on the bottom electrode will migrate back to the top electrode (Figure 6I).The recovery procedure is fast and one set-reset process with large stop voltages is enough to complete this procedure in our case.This is a complete reset operation and the device returns to the near initial state (Figure 6J).The device can work well and normally again similar to a new one.The self-repairable behavior of NbSe 2 CBRAM provides a novel solution for the long-term service life of the device, which will greatly facilitate the development and prospect of CBRAM in the memory and storage field.

| CONCLUSION
In summary, we have elaborately developed selfrepairable and high-uniform NbSe 2 CBRAM with the Cu top and Au bottom electrodes.Amorphous NbSe 2 film is used as the resistive switching layer, which was deposited by magnetron sputtering with a smooth surface and homogeneous structure.The NbSe 2 CBRAM presents remarkable c2c and d2d uniformity, as well as forming-free and compliance current-free resistive switching characteristics.The self-repairable behavior was confirmed in the CBRAM device for the first time.The device that fails after numerous cycles can readily recover its performance to the initial state by a reset process with a large stop voltage.We have designed the broken-down point in the NbSe 2 CBRAM device, from which the migration of Cu and the formation of Cu CFs have been firmly demonstrated.The self-repairable behavior can be well explained by a proposed mechanism model with the control ability of the location, migration, and recovery of Cu in the NbSe 2 switching layer.Our amorphous NbSe 2 CBRAM devices with excellent characteristics may open the door for their practical application in the memory and storage fields.
Figure S1) exhibit that all the elements of Cu, Nb, Se, Au, Ti, and Si are present and distributed uniformly in their respective regions.The observations confirm that the NbSe 2 film and the fabricated NbSe 2 CBRAM have excellent structure characteristics, which are conducive to the application in the resistive memory device.F I G U R E 1 NbSe 2 CBRAM structure and characterization.(A) Schematic diagram of the fabricated NbSe 2 CBRAM.(B) Cross-sectional HRTEM image of the CBRAM with 100-nm-thickness NbSe 2 , the inset shows the fast Fourier transformation of the NbSe 2 film.(C) Surface SEM image of the prepared NbSe 2 film.(D) XRD spectra of the NbSe 2 film grown on silica glass (red), and silica glass substrate (blue).XPS spectra of (E) Nb 3d and (F) Se 3d.CBRAM, Conductive-bridge random access memory; HRTEM, high-resolution transmission electron microscopy; SEM, scanning electron microscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction.
Resistive switching performance of Cu/NbSe 2 /Au CBRAM.(A) Normal I-V curves (100 cycles).(B) Histograms of operating voltage distribution visualized using Gaussian curve fitting.(C) The average HRS and LRS resistance for six different devices.(D) The retention characteristics of the NbSe 2 device at HRS and LRS.(E) The endurance characteristic of the device under pulsed voltage mode.(F) I-V characteristics with x and y axes in logarithmic coordinates during the set process, with linear fitting curves to the curves for the HRS and LRS.CBRAM, Conductive-bridge random access memory; HRS, high resistance state; LRS, low resistance state.

F
I G U R E 3 Self-repairable performance.(A) I-V curve for 1000 cycles before failure.(B) I-V curve of the failed device.(C) Repair operation.(D) I-V curve for 1000 cycles after repair.

F
I G U R E 4 Cu conductive filament investigation.(A) Optical microscope images of the device before and after broken down.(B) SEM image of the broken-down spot.(C) TEM image of the HRTEM sample prepared by FIB process.(D) TEM image of the broken-down spot with Cu conductive filament marked by the red line.(E, F) EDS elemental mapping of Cu and Nb for (D).(G) EDS line profile for Cu and Nb around the conductive filaments.(H) The area of the line profile.EDS, energy dispersive X-ray spectrometer; FIB, focused-ion-beam; HRTEM, high-resolution transmission electron microscopy; TEM, transmission electron microscopy.
Figure 4H shows the area of the line profile.The atomic percentage of Cu element is significantly increased in the F I G U R E 5 Just-repaired device after breaking down.(A) TEM image and elemental mapping for Cu (B), Nb (C), Se (D).No Cu conductive filament can be found in the reset device.TEM, transmission electron microscopy.

F
I G U R E 6 Schematic model of the mechanism for failure and repair.(A) The initial device.(B) Normal set operation.(C) Normal reset operation.(D) Residual Cu on the bottom electrode after multiple resistive switching cycles.(E) More residual Cu on the bottom electrode and the device failed.(F) Set operation after failure.(G) Reset operation after failure.(H) Set operation after failure.(I) Repairing operation by a more negative voltage.(J) Repaired device.
shows the schematic diagram of the designed NbSe 2 CBRAM device, with a stack structure of Au/Cu/NbSe 2 /Au/Ti/SiO 2 /Si, where SiO 2 /Si is the substrate, Au above SiO 2 is the bottom electrode, Ti is used to strengthen the adhesion between Au and the substrate, Cu is the top electrode, and Au deposited on Cu is used to protect Cu from oxidation.The diameter of the top electrodes is 300 μm.Figure1Bdisplays the cross-sectional HRTEM image of a complete device.Layers from the bottom to the top are SiO 2 layer,