Realization of Selector‐Memory Bi‐Functionality with Self‐Current Regulation Utilizing Poly‐Crystalline Based GST Electrolyte for Memristor Hardware Development

Conductive bridge random‐access memory (CBRAM) are two terminal devices that offer excellent switching performance. In addition, CBRAM shows various switching modes, including volatile threshold switching (TS) and nonvolatile threshold switching (N‐TS). These properties expand its applications to memory, selector, biological synapses, and neurons. However, due to the uncontrollable behavior of stochastic switching between TS and N‐TS in CBRAM devices, a novel approach is needed to improve the switching performance of CBRAM. Moreover, conventional devices that have different stacking between TS and N‐TS increase fabrication cost and worsen the device yield. Here, the selector‐memory bi‐functionality with self‐current regulation effect of Ag‐inserted Ge2Sb2Te5 (GST) thin films is demonstrated. Selector‐memory bi‐functionality, having TS behavior with an adjustable on/off current, with confined conductive filaments (CFs) improves the uniformity and reduces the fabrication cost by implementing TS/N‐TS in a single stack. From the material analysis, it becomes evident that confined Ag‐based CFs within GST films are key factors for realizing selector‐memory bi‐functionality. The selector‐memory bi‐functionality is achieved through the reaction of Ag metal cations with non‐bonded Te atoms in GST film depending on field polarity. These results suggest that the Ag‐inserted GST film contributes to the development of large‐scale nonvolatile memory and neuromorphic application.


Introduction
Resistive switching random access memory (RRAM) has been intensively studied for two-terminal devices, such as nonvolatile memory and bioinspired neuromorphic applications.The RRAM resistance can be changed by applying an electrical voltage by means of the formation and disruption of nanoscale conductive filaments (CFs) with relatively high conductance.CFs can originate from anion vacancies in the metaloxide material (oxide-based RRAM) or from active metal cations ionized by the electrode in conductive bridge random access memory (CBRAM).CBRAM has been widely studied because of its large on/off ratio, [1] fast program speed, [2][3][4] low operation current, [5][6][7] multilevel capability, [8,9] neuromorphic application, [10] and easy integration at the back end of a logic process. [11]The formation and disruption of CFs play a key role in CBRAM operation based on the ionic migration of highly mobile active metal cations, such as Ag or Cu.
When a positive external bias is applied to the Ag electrode, the Ag electrode is oxidized, and Ag cations are injected into the electrolyte. [12]Once Ag cations are reduced in the electrolyte, the conductive filament forms.
Volatile resistive switching, also known as threshold switching (TS), is commonly observed in CBRAMs.TS devices exhibit a low threshold voltage (<1 V) and rapid response speed (approximately on the nanosecond scale). [13,14]Unlike nonvolatile threshold switching (N-TS) in CBRAM devices for memory operation, the TS device turns off rapidly when the external bias is removed, increasing the resistance of the cell and creating unique temporal conductance evolution dynamics.In the case of N-TS, the rupture of the CF is caused by the out-diffusion of metallic atoms to the electrolyte medium or the application of a reverse (REV) bias to push away metal cations from the inert electrode. [3,15]Meanwhile, the TS characteristics are determined by atomic clustering to minimize the CF-electrolyte interface energy. [16,17]The tunability of TS and N-TS could provide a powerful method that enables cross-point arrays with nonvolatile memory and fast selection devices in the same configuration.In addition, the changeable properties of TS and N-TS mimic the short-term and longterm plasticity of biological synapses, realizing bio-inspired neuromorphic applications, respectively. [18,19][22] The compliance current during CF formation determines the size of the CF, which relies on a surface-limited self-diffusion mechanism. [23]However, this compliance current method for controlling the TS and N-TS characteristics is difficult to apply because of its stochastic phenomena, lack of reality for different stacking between TS and N-TS for memory devices, and neuromorphic applications using crossbar array structure.Novel approaches are required to overcome problematic stochastic behaviors between TS and N-TS.
[26][27][28] Alloys and compounds composed of Ge 2 Sb 2 Te 5 (GST) chalcogenides are major materials used in phase change random access memory [29][30][31] and optical rewritable disks.[34][35] Moreover, the migration and diffusion dynamics of Ag for CF formation in chalcogenides are beneficial because of the low average coordination number [36] and a large number of non-bonded Te anionic defects that are instrumental in Ag migration in the chalcogenide matrix. [24]Considering this point, chalcogenidebased CBRAM devices have been developed wherein the formation and rupture of metal-assisted conductive filaments occur within chalcogenides. [24,37]However, chalcogenide films are generally made of very soft materials, such as tin (low Young's modulus), and are easily deformed (high Poisson's ratio).In addition, the large thermal expansion coefficient of chalcogenide films makes them vulnerable to void formation within the film during device fabrication or switching operation. [38]These floppy properties of chalcogenide films lead to high operation current and poor reliability of chalcogenide-based CBRAMs.Charles et al. reported novel approaches to show the two distinct TS or N-TS switching modes under the same compliance current using a Ge 3.5 Sb 1.0 Te 5.5 layer and an Ag top electrode (TE). [39]However, it shows drawbacks such as high operation current (≈10 mA) and reliability issues.To realize low power and stable operation in chalcogenide-based CBRAMs, the number of Ag cations participating in Ag-based CF formation needs to be controlled.
On the other hand, CBRAMs have some reliability issues, such as a large dispersion of the set/reset voltage and the endurance and retention.The large variation in the set/reset voltage is caused by stochastic metal cation transport through an amorphous 3D electrolyte and the difficulty in controlling the number of Ag cations in the electrolyte.To reduce these large dispersions of set/reset voltage, confined filament structures to restrict ion movement to 1D have been proposed, such as an additional thin layer that acts as a buffer layer, an epitaxial layer, or a nanopore structure. [40,41]However, these methods require a high-temperature or complex anodizing process that has complementary metal-oxide-semiconductor compatibility issues.In this respect, chalcogenide materials have the advantage of easily transforming the crystalline phase through deposition or postannealing processes. [34,35]The crystalline phase is thermodynamically stable, and the grain boundaries between crystalline phases can act as effective confined filament structures.
In this work, a device that has Ag sandwiched by two GST thin films was developed to restrict Ag-based CF formation with adjustable selector-memory bi-functionality, which involves typical TS modulation with on/off current and hold properties.Two different samples (annealed and unannealed) were prepared following the post-annealing process to elucidate the effect of the crystallization process on selector-memory bi-functionality.It was found that the selector-memory bi-functionality could be controlled easily by the forward (FWD) and REV bias in the Agsandwiched crystalline GST film structures.In the crystalline GST structure, the Ag metal cations migrate along grain boundaries and corners between crystalline phases, resulting in a small on/off-current variation and tunable conductance according to the REV bias.Detailed physical property and electrical analyses were performed to reveal the mechanism of TS with varying on/off current and hold properties.The experimental results show that the Ag-inserted structure between crystalline GST thin films can improve the stability of chalcogenide CBRAMs, which can be used in future multilevel crossbar memory applications and non-von Neumann architecture systems with low fabrication cost and high device yield.

Results and Discussion
A W/GST/Ag/GST/W device on the SiO 2 /Si substrate was fabricated (the detailed information for the fabrication is described in the Experimental Section).A schematic diagram and scanning electron microscopy image of a fabricated crossbar-type device are shown in Figure 1a.The inset image of Figure 1a is a magnified image of the 5 × 5 μm 2 active area.Structural characterization of the GST layer using X-ray diffraction (XRD) was performed in the conventional -2 mode from 25°to 70°, as shown in Figure 1b.The GST/Ag/GST sample on the SiO 2 /Si substrate without the rapid thermal annealing (RTA) process (denoted as the "As-DEP sample") showed that the deposited As-DEP sample was in the amorphous phase.After annealing at 200°C for 10 min (denoted as the "RTA sample"), the diffraction pattern of the sample shows peaks at 29.8°(200) and 42.6°(220), indicating face-centered cubic (FCC) phase structures.Additionally, another peak appears at 39.8°(106), corresponding to hexagonal (HEX) phase structure.The higher intensity of three peaks in the RTA sample compared to the As-DEP sample suggests the presence of smaller grains in GST film. [42]Figure 1c,d show energydispersive spectroscopy (EDS) line scan and mapping images of the RTA sample.The GST composition ratio without Ag was confirmed to be close to Ge:Sb:Te = 2:2:5 (Ge, 22.2%; Sb, 23.9%; Te, 53.9%), as shown in Figure 1c.Moreover, the very high Ag atomic concentration (Ag, 40%) in the RTA sample is also confirmed in Figure 1c.In Figure S1 (Supporting Information), the high-angle annular dark-field (HAADF) image of the RTA sample confirms the presence of the Ag layer in the entire GST/Ag/GST structure.Moreover, the non-specific peak for the Ag layer in Figure 1d is likely due to Ag diffusion into the GST layer during fabrication and the RTA process, which is facilitated by the high diffusivity of Ag (The HAADF image and EDS mapping were taken in the same area).As shown in Figure S2 (Supporting Information), we conducted the X-ray photoelectron spectroscopy (XPS) depth profile analysis on both As-DEP and RTA samples to identify the cause of Ag diffusion.the Ag atomic concentration within the GST/Ag/GST stack of the As-DEP sample is higher than that of the RTA sample.Because the As-DEP sample did not experience the RTA process, the Ag diffusion is limited com-pared to the RTA sample.Figure 1e shows the cross-sectional scanning transmission electron microscopy (STEM) image of the W/GST/Ag/GST/W sample after the RTA process, where distinctive crystalline regions, denoted as yellow circles in the GST layers, were observed.Crystalline regions formed during RTA at 200 °C for 10 min.In the inset of Figure 1e, a fast Fourier transform micrograph in the typical area shows the crystalline phase with a stripe-spot pattern.XPS was also performed to analyze the bonding nature of GST/4-nm-thick Ag/GST structure as shown in Figure 2. Figure 2a represents the XPS spectra of Ge 2p.The Ge 2p 3/2 peak and Ge 2p 1/2 are located at 1218.9 and 1250 eV, respectively.The Ge 2p 3/2 peak at 1218.9 eV originated from Ge─Te bonding. [43]The positions of the two peaks are not changed with respect to the RTA process.Therefore, the presence of Ag─Ge bonds is ruled out.In Figure 2b, the binding energy of the Sb 3d 5/2 peak shifts from 528.8 eV of the As-DEP sample to 528.4 eV of the RTA sample, which is related to the Sb─Te bond.46] In a similar manner, the Te 3d 5/2 peak related to metallic Te 3d 5/2 in Figure 2c decreases after the RTA process, shifting from 573.1 to 572.65 eV.The peak for 3d 3/2 , which indicates the existence of the Te─Te bond and Ge-Te bond, also decreased from 583 to 582.75 eV.The peaks shift to the lower binding energy clearly after the RTA process is responsible for the formation of the Ag─Te bond. [47]In the case of the Ag 3d peak in Figure 2d, however, the Ag 3d 5/2 peak increases from 368 to 368.2 eV, indicating the formation of Ag-Te bonds. [48]The typical TS behaviors of the two samples obtained from the As-DEP and RTA devices are shown in Figure 3a.FWD bias voltages were applied to the TE, whereas the bottom electrode (BE) was grounded without a compliance current.Unlike other conventional CBRAM devices, the forming process of the As-DEP and RTA devices was not observed during the first DC sweep, which is a significant advantage for actual applications in terms of variation control and simple circuitry.It is believed that Ag diffused extensively into the upper and lower GST layers, even at room temperature.For the As-DEP device, the device resistance remained high in the low-voltage region (off-state).When the voltage exceeded the threshold voltage (V th ), as indicated by arrow 1, the current abruptly jumped to a high current level, which is a self-limited on-current with no external current compliance (on-state).Self-limiting characteristics occurred after V th owing to the presence of a limited number of  Ag atoms of CF in the GST layer.Figure S3 (Supporting Information) shows DC I-V sweeps of GST/8-nm-thick Ag/GST and 40nm-thick GST/Ag TE stacks to confirm the effect of the thickness of the inserted Ag layer and Ag TE.Unlike the As-DEP device (GST/4-nm-thick Ag/GST stack), both samples show typical bipolar resistive switching (RS) without self-limiting characteristics.The large amount of Ag atoms from the 8-nm-thick Ag layer or Ag TE leads to the bipolar RS.From these results, the formation of Ag-based CFs can be restricted by inserting a 4-nm-thick Ag layer between the GST films.When the applied voltage is smaller than the hold voltage (V hold ), the device spontaneously switches back to the off state.This holding characteristic is clear evidence that the device is operating as a selector.When the applied voltage is lower than V hold , the external electric field is insufficient to maintain the Ag-based CF.The selector characteristics with respect to V th and V hold are also shown in the triangle pulse measurement as shown in Figure S4 (Supporting Information).This measurement was conducted using a 2 μs rising/falling pulse waveform.The Ag-based CF rapidly diffuses into the GST by spontaneous relaxation in a short time.To confirm the effect of the annealing process, the RTA sample was measured in the same manner.Although the device also shows selector characteristics, the on/off current is increased, and V th and V hold are reduced.This may be related to the different film morphologies of amorphous GST and crystalline GST and, consequently, the different cell kinetics, including cation mobilities and redox rates.Moreover, the increase in the on/off current is related to the bulk conduction of the GST films. [25]The crystalline state of the GST film has a much lower resistance than the amorphous state owing to p-type semiconducting behavior with a large hole concentration by Ge and Sb va-cancies in the FCC and HEX phases. [49]Although this could be a drawback for selector device applications, it improves the uniformity of Ag-based CBRAM devices, as represented by the blurred lines in Figure 3a.Consecutive DC FWD sweeps were performed for both the As-DEP and the RTA devices.Even though the As-DEP device had a large off-current distribution (median, μ, 44.9 nA; standard deviation, , 55.5 nA; and coefficient of variation, /μ, 1.24), the RTA devices displayed a small off-current distribution (μ, 1.77 μA; , 0.138 μA; and /μ, 0.08).This implies that Ag-based CFs were generated and dissolved randomly in the amorphous GST films.However, the RTA sample exhibited a notably small TS distribution compared with the As-DEP device in Figure 3a.Considering the dispersion of Ag metal atoms in the crystalline GST layer, it is believed that Ag metal atoms are preferentially present at the grain corners, junctions, and boundaries because they are well-known sites of nucleation. [37]The deviceto-device variation is also confirmed as shown in Figure S5 (Supporting Information).Each device performed 10 I-V sweeps to derive the distribution of off-current.The distinct 10 RTA devices show small cycle-to-cycle variations as well as device-to-device variations.Figure 3b shows the bipolar I-V sweeps for the RTA device.The device operated as a selector, as indicated by arrow 1, during the FWD sweep.After the REV sweep up to −1.5 V (arrow 2), a different selector property was observed, as indicated by arrow 3. The off-current at 0.2 V decreased in the FWD sweep after the REV sweep voltage was applied, exhibiting hold characteristics.These results indicate that the RTA device can function as a selector and memory device.It is noted that we observed selector property only before the REV sweep, while the device has both selector and memory properties after the REV sweep.To explore the selector-memory bi-functionality, bipolar DC sweep measurements were performed using the RTA device, as shown in Figure 4a.The FWD-only sweep (denoted as 1) is for the selector operation between 0 and 0.5 V, and the FWD sweep after the REV sweep (denoted as 2 to 4) is for the memory operation.REV sweeps of 2 to 4 were performed up to −0.5, −1.0, and −1.5 V, respectively.Compared with the FWD-only sweep, the off-current of the FWD sweep after the REV sweep (denoted as 2) decreased from 1.55 to 0.57 μA.After the REV sweep, the RTA device retained its selector properties in the lower current window.This indicates that the distribution of Ag metal cations in the GST layer was modified by the REV bias DC sweep.Ag metal cations may migrate around the TE because of the REV bias, resulting in a lower off-current during a small FWD bias.For further investigation of the possibility of multilevel memory properties of the RTA sample, the voltage at the end of the REV sweep increases from −1.0 to −1.5 V, respectively (denoted as 3 to 4), and 3 to 4 in Figure 4a shows the I-V curves according to the range of the REV sweeps.As the range of the REV sweeps increases, the off-current of the sample continues decreasing.This implies that a sufficiently high electric field moves a large number of Ag metal atoms toward the TE. [37]Moreover, the on-current level at 0.5 V also decreases with the larger REV sweeps.This result suggests that the large number of Ag metal atoms condensed near the TE makes Ag-based CF formation through the GST layer difficult or it reduces the number of Ag atoms in CFs within the GST layer.The on/off currents at 0.5 and 0.2 V, respectively, have been extracted according to the number of REV sweeps, as shown in Figure 4b.The on/off ratio between the first and the third REV sweeps differs by approximately one order of magnitude.This observation indicates that the W/GST/Ag/GST/W sample with the RTA process device exhibits stable multilevel performance (see Figure S6, Supporting Information of bipolar DC sweep measurements on the RTA device for the on/off-current reduction and the on/off-current increase, Supporting Information).The As-DEP device shows large distributions, including current variation and abnormal transition, making it impossible to demonstrate multilevel performance (Figure S8, Supporting Information).We guess that the large variation and abnormal transition of the As-DEP device stem from the lack of Ag-Te bonds within the GST/Ag/GST stack and the amorphous phase of the GST layer in the As-DEP device.An FWD DC sweep cycling test of the RTA device was conducted after the REV sweeps.Figure 4c shows the off-current at 0.2 V of 100 FWD sweeps.The RTA device exhibited reliable performance with a clear memory window in a range of REV sweeps (see Figure S9, Supporting Information for the entire DC sweeps, Supporting Information).Thus, the W/GST/Ag/GST/W sample with the RTA process showed selector-memory bi-functionality.Because of the crystalline phase of the GST layer in the switching medium, the Ag-based CFs are confined at the grain boundaries, and selector-memory bi-functionality can be easily controlled by the applied bias polarity.Furthermore, Figure 4d shows a hold voltage/current box plot extracted from the FWD DC sweeps as a function of the REV sweep ranges.The large REV sweep range makes it difficult to form Ag-based CF owing to Ag metal migration to the TE; the smaller Ag atoms are involved in CF formation from the TE to the BE.Consequently, the hold voltage/current increases/decreases with an increased REV sweep range. [13,23]igure 5a,b show the negative/positive write pulse dependence of the RTA device resistance for different pulse voltages/pulse widths.Before the pulse dependence test was performed, the RTA device underwent an opposite-polarity DC sweep to adjust the initial resistance state.Figure 5a shows the dependence of negative pulse voltage/pulse width on the RTA device response.When using the same pulse voltage, a longer pulse duration results in an increase in resistance.Similarly, applying a higher voltage pulse with the same pulse width also leads to an increase in resistance.This result implies that Ag migration is related to pulse voltage and pulse width.Although Figure 5b shows a slight resistance decrease for the positive write pulse at a 1 μs pulse width, an abrupt resistance decrease for the positive write pulse dependence at 10 μs and 100 μs pulse widths.One possible scenario for the abrupt decrease in resistance during a positive write pulse is the clustering of Ag cations beneath the TE due to a prior REV DC sweep before the positive write pulse test.After applying an electric field of sufficient pulse width to dissolve Ag clusters and migrate Ag cations, an abrupt change in resistance occurred, forming Ag-based CFs. [36]Figure 5c shows the stable multilevel retention characteristics without considerable resistance degradation for 1000 s.Each multilevel state was achieved using -2 V/1 ms, +2 V /1 μs, and +2 V/100 μs pulses before retention measurement.Because the off-current of the lowest level of states (denoted by the orange line) tends to increase due to the Ag atom migration from the TE to BE by the concentration gradient diffusion process. [36]Based on its retention characteristics, the RTA device can hold several conductance states.To investigate the reliability of the RTA devices, an endurance test was performed with −2 V/1 ms and +2 V/100 μs pulses.As shown in Figure 5d, the device achieved a robust endurance of up to 10 5 switching cycles.We also present long-term plasticity by using the multilevel property of the RTA sample for analog computing device as shown in Figure S10 (Supporting Information).The device exhibits long-term potentiation/depression under consecutive 200 identical pulses with a read voltage of 0.2 V.The conductance can be gradually increased by 200 positive pulses (±2.5 V/100 μs).
Through the mechanism of selector-memory bi-functionality and multilevel operation by movement of Ag metal atoms between electrodes, the entire switching process can be explained as schematically shown in Figure 6.The virgin state after the RTA process is shown in Figure 6a.Owing to the high mobility of Ag metal and the high ion conductivity of the GST film, Ag metal atoms were diffused into the GST layer during fabrication.Because crystalline GST films are p-type semiconductors and contain a large number of nonbonded Te anionic defects, Ag atoms combine with free holes to form Ag metal cations (Ag + h = Ag + ). [36]The Ag metal cations with high mobility react with the non-bonded Te atoms and migrate along the grain boundaries to the equivalent sites.When a positive bias is applied to the TE, Ag metal cations tend to form Ag-based CF by recombining with electrons from the BE, resulting in the V th and on-state of the selector (Ag + + e = Ag).When the applied bias is reduced down to V hold , the electric field is not sufficient to sustain the Ag-based CF.Owing to the nonstoichiometry of the GST film, the Ag-based CF rapidly diffuses into the GST layer, resulting in the off state of the selector. [50]The highly mobile Ag metal cations undergo reactions with non-bonded Te atoms, decreasing the number of defect sites.Therefore, the decrease in the density of defect states by Ag metal cations would increase the band gap of the GST films near the BE, resulting in the asymmetry bipolar I-V characteristic as shown in Figures S6 and S7 (supporting information). [51]However, after the REV sweep, as shown in Figure 6d, Ag metal cations become crowded near the TE by the external field.In this situation, the formation of Ag-based CF becomes difficult with the same applied bias.The multilevel characteristics and lowering of the current of the device by REV bias strongly indicate this situation.It is noted that the device shows both selector and memory behaviors.

Conclusion
An approach that provides reliable selector-memory bifunctionality using an Ag-inserted crystalline GST CBRAM structure was demonstrated.Because of the crystalline structure of the GST film as an electrolyte, selector-memory bifunctionality could be easily controlled by the FWD or REV applied bias, and multilevel conductance was confirmed by the retention characteristics.These findings provide compelling evidence for Ag-inserted crystalline GST cells, which are important to realize feasible Ag-based CBRAMs, low variations, and reliable device performance.The entire mechanism of the observed selector-memory bi-functionality can be explained by a model based on the typical Ag-based TS theory that the reactions of Ag metal cations with nonbonded Te atoms in the GST layer can be attributed to the migration and formation of Ag-based CF under an electric field.This Ag-inserted crystalline GST CBRAM device provides not only new possibilities for the application of chalcogenide materials, but also integration using the back-endof-line process for nonvolatile memory and non-von Neumann architectures due to the low-temperature process.

Experimental Section
Device Fabrication: The metal-insulator-metal crossbar-type device was fabricated on a 250-nm set oxidized Si substrate.After conventional photolithography and lift-off processes, a 50-nm-thick W BE was deposited from the W target using a magnetron sputtering system.Then, a 20-nmthick GST layer was deposited by magnetron sputtering with a power of 80 W in RF mode at room temperature.After GST film deposition, a 4nm-thick Ag layer for limited CF formation within the GST film was deposited using the electron beam evaporation method, followed by 20-nmthick GST and 50-nm-thick W TE deposition using the same process as for the previous deposition.To form a crystalline-phase GST film, one of the fabricated devices was subjected to RTA for 10 min at 200 °C under a highvacuum atmosphere.During the entire fabrication process, including the deposition and photolithography, the process temperature was carefully controlled to be less than 30 °C to prevent GST crystallization, except for the dedicated thermally annealed sample.
Characterization and Measurement: X-ray diffraction (XRD) of the device was conducted using RIGAKU Smart Lab (Cu K  line,  = 1.5406Å).Energy-dispersive spectroscopy and scanning transmission electron microscopy images were conducted using HD-2300A by Hitachi.The bonding nature of the GST/Ag/GST stack was characterized using X-ray photoelectron spectroscopy (Nexsa G2 by Thermo Scientific).The Nexsa G2 system, equipped with a micro-focus monochromator X-ray source (Al K, 1486.6 eV) and an anode operated at 24.5 W and 15 kV, was used to conduct the XPS experiments.The scan area was 100 μm × 100 μm, and a pass energy of 58.70 eV was employed for the depth profile, utilizing the MAGCIS dual-mode ion source.Depth profile XPS study was performed by etching the surface by Ar ion beam for ≈15 s repeatedly, and XPS spectra were taken after every etching cycle.The I-V characterization in this work was conducted with a Keithley 4200A SCS Parameter Analyzer.

Figure 1 .
Figure 1.a) Schematic diagram of the W/GST/Ag/GST/W stacked device structure and SEM image of the crossbar structure.b) XRD spectra clearly showing the amorphous phase of the As-DEP device and poly-crystalline phase of the RTA sample.c) EDS line scan between GST/Ag/GST area of the RTA sample.d) EDS mapping of GST/Ag/GST area of the RTA sample; The Ag was observed in the entire GST/Ag/GST structure in the Ag element map denoted by turquoise.This is likely due to Ag diffusion into the GST layer during fabrication and the RTA process, which is facilitated by the high diffusivity of Ag. e) Cross-sectional STEM image of the RTA sample.Yellow circles represent crystalline phases.The inset shows the selected area diffraction patterns, which clearly display the crystalline phases.

Figure 3 .
Figure 3. a) The 10 I-V sweeps of the As-DEP and RTA samples.The inset shows off-current distributions at 0.2 V of As-DEP and RTA devices.b) The I-V sweep for bipolar characteristics of the RTA device.The off-current decreases during an FWD sweep after a REV sweep voltage is applied.

Figure 4 .
Figure 4. a) I-V characteristics of FWD-only sweep and after REV sweeps in the RTA sample.b) Reduction of the on/off current according to the number of REV DC cycles.c) 100 DC FWD endurance characteristics of FWD-only sweep and after REV sweeps of the RTA device.d) Hold voltage/current according to REV sweep range.

Figure 5 .
Figure 5. a,b) The negative/positive write pulse dependence of the RTA devices resistance after having different pulse voltages/pulse widths.c) Retention characteristic of the RTA sample at various conductances at room temperature.d) Endurance of RTA device over 10 5 switching cycles.Each cycle is composed of negative write pulses with −2 V, 1 ms and positive write pulses with 2 V, 100 μs.

Figure 6 .
Figure 6.Schematic representation of the selector-memory bi-functionality mechanism.a) Virgin state.b) FWD sweep operation over V th and c) under V hold .d) After REV sweep operation.e) FWD sweep operation over V th after the REV sweep operation and f) under V hold .