Multi-level Storage Characteristics of MoSe2 Resistive Random Access Memory

Resistive Random Access Memory (RRAM) is a type of non-volatile memory (NVM) device that stores information by switching between high and low resistance values. It has attracted widespread attention due to its promising potential for miniaturization. In this study, molybdenum diselenide (MoSe2) was successfully synthesized via the hydrothermal method, and the RRAM was fabricated with MoSe2 as the resistance change layer. Furthermore, the MoSe2 samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results demonstrate that the prepared MoSe2 forms flower-like nanospheres assembled by nanosheets, with a particle size is about 200 nm. In addition, RRAM has a low operating voltage (< 1V), a high OFF/ON-state resistance ratio (> 102), a good endurance (103 cycles), and its resistance switching mechanism is dominated by the trap-controlled space charge limited current (SCLC) mechanism in the high resistance state (HRS) and by the ohmic mechanism in the low resistance state (LRS). Furthermore, multi-level storage is achieved by adjusting the compliance currents (Icc) and the stop voltage (Vstop).


Introduction
With the rapid development of computer technology, the Internet of things, and electronic products, the advancement of memory technology as a fundamental technology is imminent.Currently, flash memory faces challenges such as high energy consumption, low storage density, and slow operation speed, which hinder its ability to meet the new development requirements [1][2][3][4][5].As a result, it is extremely important to develop RRAM with good retention, fatigue resistance, fast reading and writing speed, high storage density, and multi-level storage [6][7].Moreover, the resistance change layer material of the RRAM determines the device's performance, making it an important focus of research.Currently, although metal oxides like CuO [8], Al2O3 [9], and TiO2 [10] are extensively researched, their limited flexibility hinders their suitability for use in flexible wearables.
Transition metal dichalcogenides (TMDs), such as MoS2 [11], MoSe2 [12], and MoTe2 [13], are a group of flexible two-dimensional layered materials similar to graphene.They have garnered significant interest because of their distinctive electrical, optical, and mechanical properties.As a TMDs material, MoSe2 has been reported as a RRAM medium material [14][15][16][17][18].In 2015, Yan [19] et al. connected MoSe2 nanorods with Au and Ag electrodes in the plane to fabricate a RRAM.The test results showed that the switching ratio was 50.The operating voltage was 2.6 V, and the cycle was 100 times.This is the first RRAM prepared with MoSe2 as the resistive layer.In 2016, Zhang [12] et al. deposited MoSe2 nanosphere arrays on fluorine-doped tin oxide (FTO) substrates, and the test results were unsatisfactory, with a switching ratio of only 12 and an operating voltage of 2.8 V.In 2017, Han [20] et al. fabricated RRAM based on MoSe2 hexagonal nanosheets, achieving a switching ratio of 70 and an operating voltage of 2.75 V. Additionally, the RRAM developed by Mao [21] et al. using spherical MoSe2 demonstrated a switching ratio of 35, an operating voltage of 5.2 V, and 100 cycles.It is important to note that the RRAM switches based on MoSe2 materials mentioned above are relatively small, have low tolerance, and require high operating voltages.The voltage requirement for flexible wearable devices is typically less than 1V.However, the operating voltage of most resistive random access memories with metal oxides and two-dimensional materials as resistive layers exceeds 1V, which hinders their application in wearable devices due to the need for a switching ratio greater than 10 2 and endurance higher than 10 3 .In this case, the method of implementing multi-level storage involves controlling the transition voltage or current-limiting value during the resistance transition process to achieve conductive filaments of varying sizes, thereby obtaining different resistance values.Furthermore, Wang [22] et al. prepared the Cu/HfO2:Cu/Pt device, which achieves a total of four resistance states by controlling the Icc value during the resistance transition process.Next, highresistance states with varying resistance values are achieved by controlling the Vstop.
In this study, the smaller size of MoSe2 was achieved by optimizing the material preparation process, leading to enhanced performance of Ag/MoSe2/Cu in terms of switching ratio, operating voltage, and tolerance.The resistive switching mechanism and multi-level storage characteristics of the device were studied.

Preparation of MoSe2
Flower-like nanospheres of MoSe2 were prepared using the hydrothermal method.A stoichiometric quantity of Na2MoO4 and Se powder was dissolved in 60 mL of hydrazine hydrate (N2H4•H 2O) and deionized water, and then transferred to a 100 mL Teflon-lined stainless steel autoclave.Afterward, the autoclave was sealed and maintained at 190℃ for 48h, and then cooled to room temperature naturally.Subsequently, a black precipitate was collected.After being washed with absolute ethanol and distilled water, the final product was dried in a vacuum chamber at 80°C for 8 h.

Preparation of Ag/MoSe2/Cu structured device
The fabrication process for the Ag/MoSe2/Cu device is as follows.First, 1 g of MoSe2 powder was weighed and added to 10 mL of N, N-dimethylformamide solvent.Secondly, the Ag layer was deposited as the bottom electrode on the glass substrates by thermal evaporation.The MoSe2 thin film was then deposited on Ag by vacuum filtration with a suspension of MoSe2.Finally, an active metal Cu top electrode was thermally evaporated and patterned into multiple circular pads with a diameter of 250 μm using a shadow mask.

Characterization of MoSe2
The X-ray powder diffraction (XRD) pattern of as-prepared MoSe2 is shown in figure 1a, and it closely matches the pattern of PDF No. 29-0914, indicating the successful synthesis of MoSe2.Other than that, the reflections at (002), (100), (103), and (113) correspond to the hexagonal phase of MoSe2.Moreover, the presence of MoSe2 is further confirmed through elemental analysis conducted using energy-dispersive X-ray spectroscopy (EDX).The EDX data in figure 1b confirms that the flower-like nanospheres contain only Mo and Se elements, without any other impurities.The atomic ratio of Mo:Se is approximately 1:2, which is close to the stoichiometric ratio of MoSe2.In addition, the chemical compositions of MoSe2 were investigated through X-ray photoelectron spectroscopy (XPS) analysis.The high-resolution spectra of Mo and Se elements are depicted in figure 1c and d, respectively.To be specific, Figure 1c contains 3D peaks corresponding to Mo with binding energies of 228.48 and 231.68 eV.The two peaks are related to the Mo 3d5/2 and Mo 3d3/2 respectively, ascribed to the Mo 4+ state in MoSe2.Figure 1d shows the Se3d doublet, corresponding to the 3d5/2 at a binding energy of 54.18 eV and 3d3/2 at a binding energy of 55.08 eV, attributed to the Se 2− state of MoSe2.
SEM and TEM images of the as-prepared MoSe2 nanosheets are shown in figure 1e and 2f, revealing a exhibits a three-dimensional flower-like structure with dimensions of approximately 200 nm.Other than that, high-resolution transmission electron microscopy (HRTEM) images are presented in figure 1h and 1g, further insight into the detailed structure of the as-prepared MoSe2.Furthermore, the HRTEM image reveals the three-dimensional flower-like structure of MoSe2, which is comprised of numerous two-dimensional MoSe2 nanosheets with thicknesses ranging from 6 to 8 molecular layers (see figure 1f).Furthermore, the HRTEM image in figure 1g shows that the observed interplanar distance between lattice fringes is 0.28 nm, which is consistent with the (100) planes of the hexagonal MoSe2 phase.

Electrical Characteristic and Charge Transport Mechanism
Figure 2a depicts the current-voltage (I-V) characteristics and a cross-sectional SEM image (inset) of the Ag/MoSe2/Cu device.A clear bipolar switching with bistable states, including a high resistance state (HRS) and a low resistance state (LRS), can be observed.Initially, the current increases progressively with the voltage as the RRAM is swept positively from 0 to 3 V.A sudden increase in current occurs at 0.74 V (stage II), indicating the transition of the RRAM from HRS to LRS, which is referred to as the "Set" process.Besides, the RRAM exhibits good stability in the LRS when the voltage is swept from stage III to stage IV, indicating nonvolatile memory properties.Moreover, applying a reverse voltage of -0.46V could restore the HRS, a process was referred to as the the "Reset" process.Figure 2b depicts the endurance analysis of the fabricated device at a reading voltage of approximately 50 mV.The resistance is approximately 13 Ω in the LRS (ON state) and 2×10 3 Ω in the HRS (OFF state), with an OFF/ON-state resistance ratio of up to ~10 2 .Other than that, the resistance of the LRS (ON state) and the HRS (OFF state) is nearly unchanged after approximately 10 3 cycles, demonstrating the highly stable nature of the resistive switching effects Moreover, the electrical characteristics of previously reported MoSe2 memory devices have also been separately compared with the MoSe2 memory device presented in this work, as shown in table 1 [23][24][25].To further investigate the conduction and switching mechanism, the typical I-V curves in the positive and negative voltage regions were presented and fitted on double-logarithm scale in figure 2c and 3d.In the HRS state, two linear relations are used to fit the logarithmic plot of current vs. voltage from 0 to 0.3 V and from 0.3 to 0.74 V give the slopes of 1 and 2.1, respectively (figure 2c).For small bias voltages, the number of thermally-generated charge carriers exceeds that of injected carriers, resulting in partial filling of traps, indicating the Ohmic behavior.For large bias voltages, the electric field across the device is sufficient to fill almost all traps with charge carriers, in accordance with the trap-filled limit current law (I∝V 2 ), i.e., the SCLC mechanism.In addition, the slope of the I-V curves in the LRS is ~1 that obeys Ohmic conduction, showing the formation of a localized conductive filament.Furthermore, the fitting results in figure 3d are similar to those in figure 3c, indicating the ohmic conduction mechanism at LRS and the SCLC at HRS.The SCLC conduction mechanism refers to an insulator conduction mechanism.This suggests that the conduction mechanism of the Ag/MoSe2/Cu device in the high resistance state mirrors the conduction behavior of the MoSe2 film itself in the resistance change layer.Additionally, the conduction behavior in the low resistance state indicates the formation of conductive filaments with metal characteristics in the device [25][26][27].
Figure 3 depicts a schematic structural diagram of the resistive switching mechanism.When a positive voltage is applied to the Ag electrode, the Ag atoms become positively charged and oxidized into Ag + , while as the voltage is exceedingly small.The electrons injected from one end of the Cu electrode are trapped by lattice defects at the edge of the material, as shown in figure 3a.As the voltage increases, the defect state gradually fills, and the injected electrons undergo a reduction reaction with Ag + moving toward the Cu electrode to generate Ag atoms.These Ag atoms gradually accumulate to form Ag conductive filaments, as shown in figure 3b.As the voltage is increased, the Ag atoms continuously accumulate, eventually forming a channel that connects the top electrode and the bottom electrode.At that time, the injected electrons transition into electrons participating in conduction, resulting in a sudden increase in the current, and the device switches to a low resistance state.The SET process is completed as shown in figure 3c.When a reverse voltage is applied, the Ag conductive filament will be oxidized to Ag + near the bottom electrode, causing the filament to break.As a result, the device will return to the high resistance state, completing the RSET process [28][29][30].

Multilevel Storage Characteristics
RRAM devices were tested under different Icc of 0.01 A, 0.05 A, and 0.1 A, resulting in four distinct resistance states.Besides, the distribution of these states is depicted in figure 4b, where distinct states can be clearly distinguished.Furthermore, the resistance of the device in the LRS decreased as Icc increased.Clearly, higher compliance currents lead to the formation of a stronger conductive filament, resulting in lower resistance values in LRS. Figure 4c illustrates the multi-level resistive switching behaviors with different reset Vstop (-3, -4, and -5V) and a compliance current of 0.05 A. It can be observed that the resistance of the OFF-state varies under different Vstop, the resistance of OFF-state is distinct for the same individual memory cell, as illustrated in figure 4d.It is noted that the higher stop resets voltages in the reset process, and the higher resistance values of HRS can be achieved under the same compliance current.Besides, the resistance of the high resistance state depends on the degree of filament breakage during the RESET process.As Vstop increases, the height of the barrier at the filament breakage also increases, indicating that it becomes more challenging for electrons to cross the Schottky barrier and participate in the conduction process.Therefore, the resistance of the high resistance state gradually increases [31][32][33].

Conclusion
Flower-like nanospheres of MoSe2 with a diameter of approximately 200 nm were synthesized using the hydrothermal method.A random-access memory (RRAM) was fabricated using MoSe2 as the resistive layer, Ag as the bottom electrode, and Cu as the top electrode.Subsequently, it was discovered that the RRAM exhibits a bipolar resistance switching phenomenon, as evidenced by the I-V resistance switching characteristic curve test, with a switching ratio of approximately 10 2 , a cycle number of about 10 3 , and the operating voltage of less than 1V.This discovery opens up the possibility of its future application in wearable devices.Furthermore, the fitting of I-V curves shows that the resistive switching behavior of Ag/MoSe2/Cu is primarily influenced by the Ag conductive filaments and the lattice defects at the material's edge.In addition, multi-level storage characteristics are achieved by adjusting the limiting current and the cut-off voltage, demonstrating the device's potential for multi-level storage.Furthermore, the phenomenon of multi-level storage is further explained based on the conductive filament theory.

Figure 2 .
Figure 2. (a) I-V curves and cross-sectional SEM image (inset) of the Ag/MoSe2/Cu device; (b) The resistance-cycle curve with a positive bias voltage of 50 mV; Double logarithmic plots of the I-V curves at the LRS and the HRS during the (c) positive and (d) negative voltage regions.

Figure 4 .
Figure 4. (a) Different Icc of I-V curve and (b) Statistics of HRS and LRS; (c) Different Vstop of I-V curve and (d) Statistics of HRS and LRS.

Table 1 .
Comparison of performance parameters of MoSe2 RRAM.