Excellent resistive switching performance in Cu-Se-based atomic

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Excellent resistive switching performance in Cu-Se-based atomic switch using lanthanide metal nanolayer at Cu-Se/AlO interface 2

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Hyunsuk Woo, Sujaya Kumar Vishwanath, and Sanghun Jeon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18055 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Applied Materials & Interfaces

Excellent resistive switching performance in CuSe-based atomic switch using lanthanide metal nanolayer at Cu-Se/Al2O3 interface Hyunsuk Woo a, Sujaya Kumar Vishwanath a and Sanghun Jeon+

Department of Applied Physics, Korea University, 2511, Sejongro, Sejong, 339-700, Korea Corresponding author: [email protected]

a

Authors contributed equally

ABSTRACT

The next-generation electronic society is dependent on the performance of nonvolatile memory devices, which has been continuously improving. In the last few years, many memory devices have been introduced. However, an atomic switch is considered to be a simple and reliable basis for next-generation nonvolatile devices. In general, atomic switchbased resistive switching is controlled by electrochemical metallization. However, excess ion injection from the entire area of the active electrode into the switching layer causes device nonuniformity and degradation of reliability. Here, we propose the fabrication of a highperformance atomic switch based on Cux-Se1−x and inserting lanthanide (Ln) metal buffer layers such as neodymium (Nd), samarium (Sm), dysprosium (Dy), or lutetium (Lu) between the active metal layer and electrolyte. Current-atomic force microscopy (I-AFM) results confirm that Cu ions penetrate through the Ln-buffer layer and form thin conductive filaments (CF) inside switching layer. Compared with the Pt/Cux-Se1−x/Al2O3/Pt device, the

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optimized Pt/Cux-Se1−x/Ln/Al2O3/Pt devices show improvement in on/off (102–107) resistance ratio, retention (10 years/85 °C), endurance (~10000 cycles) and uniform resistance state distribution.

Corresponding author: [email protected] KEYWORDS: Atomic Switch, Cu-Se, Lanthanide metal buffer, Low-power operating device. 1.INTRODUCTION

Low-power operation, poor endurance, and scaling limitations are the challenges encountered in case of silicon-based flash memory devices. To overcome these limitations, new memory devices (ferroelectric random access memory (FeRAM), resistive random access memory (ReRAM), magnetic random access memory (MRAM), etc.) have attracted interest from many research groups.1-4 However, ReRAM with electrochemically active metal electrodes such as Cu or Ag (also known as atomic switches) hold great promise for next-generation information storage device applications, owing to their nonvolatile properties, good scalability, and simple structure.5,6 A typical atomic switch has a metal-insulator-metal(M-IM) structure that consists of an active metal (e.g., Cu, Ag), an oxide-based solid electrolyte (e.g., HfO2,7 ZrO2,8 Al2O3,9-10 TiO2,11-12) as a resistive switching layer, and electrically inert metal (e.g., Pt, Au, W). When an electric filed is applied across the top and bottom electrodes, the atomic switch may be in either of two resistance states (high- and low-resistance states, or 1 and 0). These resistance states can be realized through the formation/decomposition of nanoscale conductive filaments (CF) inside the solid electrolyte based on ion injection and the electrochemical reaction of active electrodes.13 However, over-diffusion of the active metal ions (Cu2+ or Ag−) into the amorphous solid electrolyte leads to the formation of irregular filaments, resulting in nonuniformity of the resistance state and causing reliability


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issues.14-16 First, the ion injection into the solid electrolyte enables the formation of many coarse CF rather than a single thin CF, which adversely affects low-voltage driving. Second, owing to excessive ion injection, it is difficult to completely remove the CF from the solid electrolyte. In this way, the ions gradually accumulate after repeated switching cycles, leading to deterioration of the resistance state. Therefore, many efforts have been reported to solve the abovementioned problems of the atomic switch, including methods for controlling CF to improve the device performance by metal impurity dopants,17-20 electric field control,2122

using modified active electrodes,23-24 inserting metal buffer layers (Ti, Hf, Ta),25 and nano-

indent growth. 26 However, the methods for improving the RS characteristics of the atomic switch that have been developed by reducing the diffusion of ions through the modified active metal electrodes are receiving more interest.22-24 By using Cu-alloy as an active electrode in conductive bridge random access memory (CBRAM) devices, Goux et al. reported resistive switching behavior in a CuxTe1−x/Al2O3/Si cell.24 The content of tellurium (Te) was increased to adjust the filament size, the SET voltage was improved from 3.5 V to 2 V, and endurance was approximately 103 cycles. Thus, it was realized that ion diffusion be controlled directly using modified active electrodes.

In this report, we propose a system to suppress ion diffusion to the nanometer range by using Cux-Se1−x alloy and inserting a lanthanide (Ln) buffer layer between the active electrode and electrolyte. The locally oxidized ions from the Cux-Se1−x alloy active electrode are expected to be injected into the solid electrolyte layer through the Ln-buffer layer, rather than the global active electrode. Additionally, the Ln-buffer layer controls Cu ion-diffusion to reduce the randomness of filament formation and to control oxygen vacancies in the solid electrolyte. Here, we show that the controlled atomic switch forms a nanoscale filament by inserting four species of lanthanide metals viz., neodymium (Nd), samarium (Sm), dysprosium (Dy), and



lutetium (Lu) buffer layers. Current-atomic force microscopy (I-AFM) images confirmed that the Ln-buffer layer limits ions to a localized region at the Ln-buffer/Al2O3 interface. We fabricated Cux-Se1−x/Al2O3/Pt and Cux-Se1−x/Ln/Al2O3/Pt devices to investigate the ion suppression effect and oxygen control, respectively. As a result, significant enhancements were observed viz., increased resistance on/off ratio, low voltage consumption, long retention time, and endurance of more than 104 cycles without significant degradation.

2. EXPERIMENTAL SECTION 2.1 Fabrication of Cu-Se-based atomic switch by inserting lanthanide metals: A Ti (10 nm)/Pt (30 nm) bottom electrode was deposited on 300-nm SiO2 substrate by electron-beam (E-beam) evaporation. The Ti layer was deposited as an adhesion layer between the SiO2 and Pt metal. A 3-nm layer of Al2O3 was chosen as the switching layer. The amorphous aluminum oxide was deposited by H2O-based atomic layer deposition (ALD) at 300 °C, using a tri-methyl aluminum precursor. To avoid excessive diffusion of Cu ions into the switching layer, we inserted a 2.5-nm-thick lanthanide metal nanolayer prior to the 50-nm Cux-Se1−x active electrode deposition (lanthanide nanolayer thickness is shown in Figure S1, Supporting information). Cux-Se1−x active electrodes with different compositions (x = 0.45, 0.11, and 0.05) and lanthanide metal nanolayer were sequentially deposited on the Al2O3/Pt/Ti based substrate. The patterns were formed using a shadow mask having a size of 30 µm × 30 µm. All metals are deposited at the rate of 0.5 Å/s using same E-beam evaporator and the working pressure for the evaporation is 5 × 10-7 torr. 2.2 Electrical and chemical characterization: All electrical characterizations were performed by using an Agilent B1500A semiconductor parameter analyzer in I–V sweep mode (0.02 V/step). During the voltage sweep measurement, a positive bias was applied to the top


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electrode and the bottom electrode was grounded. In addition, a waveform generator/fast measurement module was used for the endurance testing. The chemical configurations of the lanthanide metal/Al2O3 interfaces were confirmed by depth profile X-ray photoelectron spectroscopy (XPS, K-alpha (Thermo-VG, U.K.)). 2.3 Current atomic force microscopy measurement: Device preparation is required for the IAFM measurements, in order to minimize the global formation of filaments in the electrolyte. A Pt electrode was deposited on the bottom electrode after SiO2 island patterning. An atomic switch must be fabricated in the island, which requires the formation of CF in the electrolyte by the SET process. The I-AFM measurement was performed with an XE-100 AFM (Park Systems, Korea), a DLPCA-200 low-noise current amplifier (Femto Messtechnik GmbH, Germany), and a CDT-ContR conductive tip (Nanosensors, Switzerland). After active electrode etching, the CDT-ContR scans the surface in contact mode AFM to map the topography of the sample surface. At the same time, current flowing between the tip and sample is measured to map the electrical properties, such as conductivity of the sample surface. Commonly, the current flowing between conductive tip and sample has very small magnitude and needs to be amplified by a current amplifier. The DLPCA-200 variable-gain low-noise current amplifier allows a variable measurable current. In the CF depth profile, the electrolyte was scraped by the cantilever after the top electrode etching and the current value of sample surface was read simultaneously. During the scanning, the force with which the conductive tip presses the sample is adjustable. This be should set to the appropriate set-point (~50 nN) and Z servo gain value (~1) in the program.

3. RESULT AND DISCUSSION

The resistive switching mechanism of the Cux-Se1−x-based atomic switch is illustrated



schematically in Figure 1a. The mechanism of the atomic switch is based on electrochemical reactions of Cu. Under the influence of a positive bias, the Cu ions migrate through the electrolyte, form CFs between the active electrode (AE) and bottom electrode (BE) by reduction, and the device enters the low-resistance state (LRS). However, when a negative bias is applied to the AE, the CFs in the electrolyte decompose by oxidation, and the device returns to the high-resistance state (HRS). During the atomic switch operation, the performance of the device is closely related to the control of Cu diffusion through the electrolyte. We observe the influence of Cux-Se1−x-based atomic switch device with three different ratios of Cux-Se1−x (0.05 < x < 0.45). Figure 1b shows the I–V characteristics of atomic switching devices based on three compositions of Cux-Se1−x. The 1st voltage sweep is shown as a black line, and the 10th sweep is shown as a red line. Different positive voltages (0–4 V) are applied on the top electrode (TE) in the 1st sweep with the same 100 µA compliance current (Icc) for all the devices. The Cux-Se1−x (x = 0.45)-based device shows an on/off resistance ratio of approximately 102 and operating voltage within 1.5 V. However, the Cux-Se1−x (0.05 < x < 0.11)-based devices require a higher voltage sweeping (V = 2.6–3.1 V) than Cux-Se1−x (x = 0.45)-based devices and showed unstable-switching owing to lower concentration of Cu in the AE. To determine the reliability of Cux-Se1−x based devices, AC cyclic endurance was verified, as shown in Figure 1c.


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Figure 1. (a) Schematic illustration of Cux-Se1−x–based atomic switch device (Cux-Se1−x /Al2O3/Pt/Ti/SiO2/Si) and operation mechanism, containing filament formation/decomposition. (b) I–V characteristics of Cux-Se1−xbased atomic switching devices with three compositions. The black line shows the 1st sweep and the red line shows the 10th sweep. (c) Cyclic endurance data under AC mode for atomic switching devices based on three compositions of Cux-Se1−x.

The performance of the Cu0.45-Se0.55-based device was maintained for more than 150 cycles (on/off: 102). However, the devices based on Cu concentrations in the range 0.05 < x < 0.11 showed less endurance, 20 and 80 cycles (on/off :105 and 104), respectively. These indicate the Cu diffusion or concentration strongly affects the resistive switching in a Cu-Se-based atomic switch. However, the optimal Cu0.45-Se0.55 based device showed poor RS performance owing to random growth of CF in Al2O3, as shown in Figure S2, Supporting Information. To control the over-growth of Cu, high-density lanthanide metals (Nd (7.01 g/cm3), Sm (7.53 g/cm3), Dy (8.55 g/cm3), and Lu (9.84 g/cm3)) are used as buffer layers. The lanthanide metal buffer layer (2.5 nm) was deposited between the active electrode and the Al2O3 layers, as depicted

in

Figure


2a.


Figure 2. (a) Schematic illustration of Cux-Se1−x-based atomic switch with lanthanide buffer layer (CuxSe1−x/Ln/Al2O3/Pt/Ti/ SiO2/Si). (b) Depth-profile XPS of Cu0.45-Se0.55 /Ln/Al2O3 /Pt structure. XPS spectra of the O1s region for (c) the Nd/Al2O3, (d) Sm/Al2O3 (e) Dy/Al2O3, and (f) Lu/Al2O3 interfaces.

Furthermore, Ln-metals can easily absorb large amounts of oxygen from the electrolyte oxide.27 The effect of the Ln-buffer was investigated in more detail using XPS depth profile analysis. The atomic concentration of the Pt/Cu-Se/Lu/Al2O3/Pt film and the chemical bonding of the Ln/Al2O3 interface (Ln: Nd, Sm, Dy, Lu) using a theta probe spectrometer with a monochromator Al Ka X-ray source (E = 1486 eV) at a base pressure of 4.8 × 10−9 mbar. The result of the destructive vertical depth profile of the Pt/Cux-Se1−x (x = 0.45)/Lu/Al2O3/Pt film are presented in Figure 2b, which shows the variation in element content concentration while etching from the top electrode. The composition of the mixed layer including both Cu and Se can be verified on the upper part of the film and the O1s peak was confirmed from the binding energy depth profile for the core elements (OH, Al-O, and Ln-O) in the Ln/Al2O3 interface. Similarly, Figures 2c–f displays the binding energy variation of the O1s peak at the Ln (Nd, Sm, Dy, Lu)/Al2O3 interface when the etching time is


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approximately 570 s. The Al-O bond increased, and the Ln-O bond gradually decreased in the order Nd, Sm, Dy, Lu, which indicates the degree of oxidation of the interface by absorbing oxygen from Al2O3 owing to the Ln metal. Now, the resistance switching of Cux-Se1−x /Ln/Al2O3/Pt can be explained by the high controllability of the redox rate of optimized Cu0.45-Se0.55 with the Ln-buffer and the bond formation of Ln-O at the interface. The reaction at the interface between Al2O3 and the Ln metal indicates oxygen deficiency. When a positive voltage is applied to the TE, oxygen ions in Al2O3 migrate to the oxygen lattice sites (i.e., oxygen vacancies (Vox)) at the interface of the Ln-buffer layer. As a result, many oxygen vacancies are trapped at the interface. Owing to the presence of oxygen vacancies, the mobility of Cu ions increases, and the ions move along the oxygen vacancies, 28-29. Along with oxygen deficiency, CF grow vertically to the extent of the oxygen defects, and relatively thin filaments are formed in the solid electrolyte owing to the Ln-buffer layer (as shown in fig 4a). Therefore, in this study, we investigate the morphology and the depth-dependent size of the CF formed in the Al2O3 by I-AFM measurement. The I-AFM measurement is one of the approaches to characterize an atomic switch. The shape and size of the filament formed inside Al2O3 are key factors in determining the switching characteristics. Following the Celano approach, we use low-noise current amplification and conductive tip in an AFM to scan the surface and detect the amplified current between the substrate and conductive tip. 30-34 To scan the filament inside Al2O3, the filament must be formed through a SET process on a single cell of an atomic switch before the I-AFM measurement.



Figure 3. (a) I-AFM planar observation obtained in the Nd buffer, Sm buffer, Dy buffer, Lu buffer /Al2O3 interfaces and current extracted at the red dotted line near the CF. (b) Schematic illustration of the I-AFM tomography procedure, the diamond coated cantilever is exploited to slice electrolyte at different heights of the CF after the remove of the Cu0.45-Se0.55/Lu buffer layer. (c) Depth profile of CF formation in four different Ln metal buffer devices.

Then, the AE with a 50-nm Cu-Se layer to shield Al2O3 is removed using an etching solution. After that, we used a diamond-coated tip to remove any residual metal on the top of the Al2O3. Next, the diamond-coated tip sets the optimized set point value (controls the force applied vertically), and scans the X and Y axes after specifying the scan area. This process is repeated to scratch the localized area on the surface. Then, the contact mode cantilever is replaced with a scan of the sample surface to measure the current value. Figure S3, Supporting Information shows the process flow diagram for the I-AFM measurement. We removed the AE/buffer layer and repeatedly scratched the local area of the surface at 0.5 nm/scan rate to verify the influence of the four types of lanthanide buffer layers. Figure 3a shows the comprehensive


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current profile results from the Al2O3 top surface after removal of the lanthanide buffer layer. The left part of the series in Figure 3a shows the first scanned mapping image (scan range: 300 nm × 300 nm) of four devices. The bright region represents the CF in Al2O3 and the 1st IAFM view clearly shows the planar range of the CF at the nearest interface of the lanthanidebuffer layer. The area of the CF is observed in the device, only in the local region of the lanthanide-buffer. Owing to the high density of Ln-metals (Nd: 7.01 g/cm3, Sm: 7.53 g/cm3, Dy 8.55 g/cm3, and Lu: 9.84 g/cm3) as well as the degree of oxidation of lanthanide metal layers (as shown in Figure 2 c-f), the size of CF decreases through the metal films in the order Nd to Lu. The right of the series in Figure 3a shows the current in the CF along the Xaxis profile of the mapped image. The diameters of the CF in all the devices were different, but the maximum current flowing through the filament was maintained at approximately 100 nA. Figure 3b shows the result of the comprehensive current depth profile of the device with a Lu-buffer layer. The shape and thickness of the filament along the Al2O3 layer can be identified, resulting in a profile obtained through nine scraped off layers and nine scans, consecutively. The areas of the filaments with the Lu-buffer layer are 610 nm2 (1st layer), 820 nm2 (3th layer), 956 nm2 (5th layer), 1550 nm2 (7th layer), and 2720 nm2 (9th layer), respectively. The mapping image extraction of the I-AFM measurement clearly shows the transmission path of the CF by the Lu buffer layer. In addition to this, the depth profile can verify that thin filament can be formed when Nd, Sm, Dy, and Lu layers are sequentially employed into the buffer layer, as shown as Figure 3c. To evaluate the I–V characteristics of lanthanide buffer layer-based devices, I–V measurements were performed at room temperature and the sweeping voltages (0→ +1.5 → 0 → −1.5 → 0 V) are applied to the TE, while the BE is grounded. The current compliance (Icc) is set to 100 µA for the positive sweep to avoid overgrown filaments and 100 mA for the negative sweep during the switching measurement. To investigate the influence of the Lanthanide buffer thickness on the bipolar switching,



which verified by sandwiching Lu buffer layer of 1nm, 2.5nm, and 5nm thickness, respectively. And 2.5nm was selected as the optimised buffer layer thickness. All the lanthanide buffer-based devices have a 3-nm Al2O3 layer and we added a 2.5-nm lanthanide buffer layer sandwiched between the AE and the Al2O3 layer. Another major role of the buffer layer is to form a localized filament by suppressing the CF’s formation of a global region from the Cu-Se AE. As previously reported for the CBRAM,34-36 a titanium (Ti) buffer layer or vacancy control layer was used to block the Cu ion injection and to supply electrons to reduce the activation energy.37

Figure 4. (a) Schematic illustration of Cu-Se/Ln-based atomic switch mechanism in SET and ReSET. (b) Direct current I–V bipolar switching characteristics (102 cycles) of Nd buffer-based atomic switch device (100 µA compliance current is applied in SET/RESET processes), (c) Sm buffer-based atomic switch device, (d) Dy buffer-based atomic switch device, (e) Lu buffer-based atomic switch device for 102 consecutive sweeps. All the I–V bipolar switching curves were measured at the voltage sweep rate of 40 mV/s.

Lanthanide metals have the characteristic that they rapidly oxidize and easily bond with


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oxygen. The Ln layer, like the Ti buffer layer, can serve as an electron supply layer. When lanthanide metal layers are deposited on the upper layer of Al2O3, oxidation occurs at the interface and a thin Ln oxide layer is formed. The SET and RESET mechanism of the CuSe/Ln-based atomic switch is shown in Figure 4a. Cho et al. mentioned that at the interface of the metal electrode and metal oxides, a short-range oxide phase layer will form depending on their oxygen affinity, and acts as oxygen scavenger.25 Figures 4b–e show the I–V curve for the four-lanthanide buffer-based devices, obtained for the actual 100 cyclic sweeps (DC endurance). These data clearly show the improvement in the resistive uniformity and operation voltage. The average on/off resistance ration is increased from Nd (105) to Lu (107) due to decreasing in leakage current from Nd (10-10A) to Lu (10-12A) respectively38 as shown in Figures 4b–e. This decreased leakage current attributed due to the oxygen affinity of the lanthanide metal decrease from the Nd to Lu. Figure 5a shows the cumulative probability distributions of the SET/RESET voltages obtained from the four types of Ln-buffer layerbased devices. Compared to devices without buffer layer (shown in Figure S1), Ln-bufferbased devices showed a lower CV value (SET/RESET: 9.7%/6.9% for Nd, 12.1%/6.6% for Sm, 12.6%/8.1% for Dy, and 20.5%/9.9% for Lu) and narrower distribution (SET voltage average value: 0.45 V for Nd, 0.55 V for Sm, 0.71 V for Dy, 0.81 V for Lu and RESET voltage average value: −0.87 V for Nd, −1.15 V for Sm, −1.28 V for Dy, and −1.34 V for Lu). Figure 5b shows the cumulative probability of the resistivity distribution state of Ln-bufferbased device. The HRS CV of Cux-Se1−x with Ln-buffer layer-based devices is reduced from 62.8% (device without buffer layer) to 57.7% (Dy buffer layer), 41.1% (Sm buffer layer), 37.6% (Lu buffer layer), and 32.3% (Nd buffer layer)). From these results, it is thermodynamically more advantageous to form the oxygen deficiency at the Ln/Al2O3 interface and to interrupt Cu injection from the Cu-Se active electrode. Thus, this affects the formation of thin CF in the electrolyte by the oxygen defect formed at the interface and



reduces the enthalpy for Cu decay. The Ln-buffer layer improves not only the I–V switching characteristics (operating voltage and on/off resistance ratio), but also the reliability of the Cu-Se-based atomic switches. The reliability evaluation of the Ln-buffer-based devices was determined by the cyclic endurance and data retention.

Figure 5. (a) Cumulative probability of SET/RESET voltage. The data were obtained from cycle to cycle for each device. (b) The resistance states (LRS/HRS) of each device are shown as accumulation probability curves. (c) AC Endurance performance of LRS/HRS resistance plotted as a function of the number of cycles for four different lanthanide metal buffer devices. (d) Retention characteristics of LRS and HRS for four different lanthanide metal buffer devices at 85 °C. (e) LRS retention characteristics of four different lanthanide metal buffer devices at 160 °C (f) Mean time to failure versus 1/kT graph (Arrhenius plot). The data retention time of LRS at different temperatures from 160 °C to 240 °C.

Figure 5c shows the HRS and LRS obtained for the AC endurance measurement of Lnbuffer-based devices. The applied pulses of the AC pulse measurement consist of SET pulse, read pulse, RESET pulse, and read pulse. Here, the pulse width and the rising/falling time are 500 µs and 1 µs, respectively. (SET voltage: 1.5 V and RESET voltage: 2 V). Additionally,


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the read voltage measured the current at 0.05 V after the SET/RESET voltage was applied. All of Ln-buffer-based devices could maintain write/erase repetition of minimum 3870 cycles, maximum 10059 cycles. Here, the endurance shows a high tendency in devices with a relatively low on/off resistance ratio (10059 cycles for Nd, 9200 cycles for Sm, 8030 cycles for Dy, 3870 cycles for Lu). We also determined the retention of the Cu-Se-based atomic switch device with a Ln-buffer layer; Figure 5d shows the on-current (open dotted line) and off-current (solid dotted line) distribution of all of Ln-buffer devices as a function of retention time at 85 °C. The retention failure time of the Ln-buffer devices was 104 s at 85 °C. Thus, the Ln-buffer layer can improve the retention time of Cu-Se-based by donating additional electrons to the Al2O3 electrolyte and suppressing the decomposition of CF. Figure 5e shows the retention time of the LRS condition measured at 160 °C in the Ln-buffer-based atomic switching device. Here, the failure time criterion was fixed to critical variation (△RLRS > 10) and the read voltage was 0.05 V. We confirmed that the retention failure time of the Nd-buffer layer was longer than the other devices at 160 °C, 200 °C, and 240 °C, respectively, (160 °C for Nd-buffer: 9 × 105 s, 200 °C for Nd-buffer: 2 × 104 s, 240 °C for Nd-buffer: 2 × 102 s). In addition, the LRS failure time was investigated under the same condition for the rest of Ln (Sm, Dy, Lu)-buffer devices. As a result, Nd, Sm, Dy-buffer based devices guarantee approximately 10 years of HRS/LRS retention, as shown in the Arrhenius plot in Figure 5f. This result is evidence that Ln-buffer based devices have stable resistive switching characteristics and reliability, as shown in Table 1. To the best of our knowledge, no reports on atomic switches to date have used lanthanide metals as buffer layers.

4. CONCLUSIONS

In this study, we optimized a Pt/Cu-Se/Al2O3/Pt-based atomic switch using Nd, Sm, Dy, Lu



buffer layer inserted at the Cu-Se/Al2O3 interface to suppress the Cu injection. We then verified the improvement in resistance switching performance and device stability by comparing devices with and without the Ln-buffer layer. The maximum on/off resistance ratio was 107 by inserting a Lu buffer layer and we confirmed the lowest operating voltages were 0.35 V (SET) and −0.7 V (RESET) by inserting a Nd buffer layer. The inhibition of Cu injection and controlling the oxygen deficiency of the electrolyte assisted the improvement of the electrical characteristics by insertion of a Ln-buffer layer at the Cu-Se /Al2O3 interface. The I-AFM measurement was carried out to observe the size of CF inside the Al2O3 layer by inserting Ln-buffer layer. The diameter of the CF was gradually decreased in order of Nd, Sm, Dy, and Lu buffer layers at the Cu-Se /Al2O3 interface. Finally, in the HRS/LRS retention time test, the Ln- buffer-based device was found to ensure 104 s of retention at 85 °C. From the LRS failure time test, Nd, Sm, and Dy buffer layer-based devices are expected to guarantee HRS/LRS retention for approximately 10 years at 85 °C. In addition, an improved AC endurance of 104 cycles was also achieved. Our results provide insight into the switching mechanism of an optimized Cu-Se-based atomic switch with a Ln-buffer, and confirm that it is a promising candidate for next-generation nonvolatile memory applications. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (NRF-2014R1A6A1030732 ,2015M3A7B7045496) as well as the Technology Innovation Program (10049163) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).

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ToC Figure

Table 1. Cu based atomic switch performance with various similar structures

Structure

Window

Endurance (Cycles)

Retention (s)

Cu-Se/Al2O3/Pt

102

150

---

>10000

10years @85oC

Operating Voltages (VSET/VRESET) 1.5/-1.2

10

5

Cu-Se/Sm/Al2O3/Pt

10

6

~9300

10years @85 C

0.55/-1.15

Cu-Se/Dy/Al2O3/Pt

106

~8100

10years @85oC

0.71/-1.28

Cu-Se/Lu/Al2O3/Pt Cu/Nonporous WO3−x/ITO Cu/TiO2/ITO

7

Cu-Se/Nd/Al2O3/Pt

Cu/ZrO2: Cu/Pt Cu/HfO2: Cu/Pt

10

o

o

Reference

0.45/-0.87 This Work

~4000

Excellent resistive switching performance in Cu-Se-based atomic

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