Resistive Switching Characteristics in Solution-Processed Copper Oxide

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C: Physical Processes in Nanomaterials and Nanostructures

Resistive Switching Characteristics in SolutionProcessed Copper Oxide (CuO) by Stoichiometry Tuning x

Shania Rehman, Ji-Hyun Hur, and Deok-kee Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00432 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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The Journal of Physical Chemistry

Resistive Switching in Solution-Processed Copper Oxide (CuxO) by Stoichiometry Tuning Shania Rehman, Ji-Hyun Hur and Prof. Deok-kee Kim* Department of Electrical Engineering, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul 05006, Korea *E-mail: [email protected] Abstract

The control of resistive switching, in low cost solution-processed CuxO thin films, was demonstrated based on intentional manipulation of intrinsic point defects. Cu interstitials offered a unique way to create metallic Cu filament even in the absence of electrochemically active Cu top electrode. The concentration of these Cu interstitials was controlled by annealing the CuO films at low temperature (300oC) in Ar environment with different oxygen contents. By varying the oxygen content, profound effect was observed on the resistivity of CuxO thin films, which in turn controlled the memory windows in different devices. Annealing at 0% O2 atmosphere created abundant cationic defects, which resulted in poor switching behavior. With the addition of 20% oxygen and increased annealing time, transition of CuO to Cu2O, determined by X-ray diffraction (XRD) and Raman spectroscopy, resulted in deterministic increase in on/off ratio by 3 orders of magnitude and improved endurance. On increasing the oxygen content above 20%, switching behavior was degraded. Increasing the oxygen content reduces the cationic defects which cause hindrance in the formation of filament responsible for switching behavior. Grain boundaries seems to play a vital role in controlling the variation in SET and RESET voltages. We found that precise control on the switching properties can be attained by modulating the Cu interstitials in these CuxO devices.

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Keywords: copper oxide, resistive switching, solution-process, stoichiometry 1. Introduction Conducting bridge random access memory (CBRAM) is considered an important candidate for next-generation non-volatile memory because of its simple device architecture, good scalability, low-voltage operation, and fast switching speed [1–6]. Recently, various oxides have been investigated for their possible application in CBRAM devices such as Ta2O5 [7, 8], SiO2 [9], ZrO2 [10], GeOx [11, 12], Cu-Te/Al2O3 [13]. Of these materials, CuxO has great potential to be used in CBRAM applications due to its strong affinity with complementary metal oxide semiconductor (CMOS) process [14]. Copper oxide has been the subject of intensive research for the past two decades because of its remarkable optical, electrical, thermal and magnetic properties [15]. It has gained considerable interest for its multiple applications in memory devices, solar cells, transistors, photo-catalytic and electrochemical applications, and gas sensors [16-24]. In particular, the two main copper oxide phases, which are cupric (CuO) and cuprous (Cu2O) oxide, are considered among the most important p-type semiconductors with a band gap Eg in the range of 1.7−2.2 eV [25-27,]. Several efforts have been devoted to improve the performance of CBRAM by improving the fabrication methods of the switching layer [28 - 31]. One of the most important factors in development of CBRAM is the synthesis of the switching (active) layer. Transition metal oxide layers have been synthesized by vacuum deposition systems. Although the vacuum-based deposition methods have gained much popularity because of several benefits associated with them, their high fabrication cost and large-area device uniformity undoubtedly restrict their application [32]. Solution-process is emerging because of its advantages over the vacuum-based ones, including printability, possibility of large-area fabrication, and low-cost characteristics. In this regard, solution-processed CBRAM may be considered an alternative processing 2 ACS Paragon Plus Environment

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technique for future low-cost memory technology. CBRAM consists of an ion-conducting insulating layer sandwiched between an electrochemically active (silver or copper) and an inert electrode [33]. It is reported that resistive switching behavior has been observed in Ag-rich Ag2S thin films on applying voltage bias [34]. Resistive switching behavior is attributed to excess Ag filament formation in Ag2S thin films. Switching behavior showed strong dependence on different compositions of Ag and S in Ag2S thin films. Although excess Ag leads to the formation of filament, it also decreased the resistivity of Ag2S films, which resulted in a reduction of on/off ratio in switching devices. This concept of excess Cu in CuxO thin films to create Cu filament can be utilized instead of using Cu as top electrode. However, some modification in the synthesis phenomena may be required to obtain good switching characteristics to make these solutionprocessed CuxO thin films applicable in low cost memory technology. In this context, a solution-based method offers the possibility of tuning stoichiometry of CuxO, as the point defect concentration (Cu interstitials (Cui), Cu vacancies (Vcu), O interstitials (Oi) and O vacancies (Vo) derived from molecular precursor solutions, are strongly dependent on the annealing conditions and the precursor solution concentration [35 - 37]. The two more important native defects

playing an influential role in controlling the switching

characteristics are Vo [38, 39] and Cui [40-42]. Taking advantage of dependence of cationic defect concentration of solution-processed CuxO thin films on annealing conditions, concentration of cationic defects can be varied in CuxO thin films. Mostly solution-processed CuxO thin film grows in polycrystalline structure without annealing [43-45]. However the crystal quality can be improved by thermal annealing [44]. The main objective of this report is to study the effect of different amounts of Cu interstitials in CuxO thin films on the characteristics of CuxO device such as endurance, retention, and on/off ratio. For this purpose, CuxO thin films were annealed in different atmospheres by modulating the oxygen and Ar ratio to balance the composition of CuxO. The 3 ACS Paragon Plus Environment


effect of different oxygen contents in the mixture of oxygen and argon environment on the structural and electrical properties of the CuxO thin films and their electrical characteristics were investigated.

2. Results and discussion

XRD pattern of the as-deposited CuO film showed only CuO phase without any additional peaks of sub-oxides (Cu2O or Cu4O3) or Cu (Figure S1). The peaks in Figure S1 were matched with the reference: CuO monoclinic phase (JCPDS no. 80-1916; a = 4.692 Å, b = 3.428 Å, c = 5.137 Å, β = 99.546°). Figure 1(a) shows XRD patterns of the CuO thin films annealed at different atmospheres. The peaks were indexed to the references: CuO monoclinic phase (JCPDS no. 80-1916; a = 4.692 Å, b = 3.428 Å, c = 5.137 Å, β = 99.546°), Cu2O cubic phase (JCPDS no. 78-2076; a = 4.267 Å), and Cu cubic phase (JCPDS no. 851326; a = 3.615 Å). The CuO phase was observed in the XRD pattern of 0%O-20M sample. The XRD pattern of 0%O-20M sample represented a deteriorated crystal structure, which is indicated by the low intensity XRD peaks in Figure 1(a). Upon introducing the 20% oxygen in the Ar atmosphere in 20%O-40M sample, intensity of the CuO peaks increased slightly, indicating the slight improvement in crystal structure. However, a minute peak of Cu2O also emerged. For the 20%O-40M sample, increase in annealing time lead to the emergence of Cu2O peaks without a CuO peak. When increasing the annealing time further from 40 minutes to 60 minutes in 0%O-60M sample, Cu peaks with a small peak of Cu2O were observed. Upon increasing the concentration of oxygen from 20 % to 40% and 100% in 40%O-20M and 100%O-20M samples, the dominant phase was again CuO but with slightly different peak intensities for (110), (-111), (111) and (-202). Increase in intensities and decrease in FWHM of (110), (-111), (111) and (-202) peaks with an increase in oxygen concentration seemed to be due to the improved crystal structure and a decrease in the 4 ACS Paragon Plus Environment

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number of defects (oxygen vacancies and Cu interstitials) on increasing the O2 concentration. Figure. 1(a) indicates that annealing CuO under oxygen-deficient environment transforms it to Cu2O and Cu depending on the annealing conditions through the phase change from CuO to Cu2O caused by the oxygen desorption as follows [46]: 4 → 2 +

(1)

To study the dynamics of the phase transformation, Raman spectroscopy was performed as well. Figure 1(b) shows the Raman spectrum of all the samples annealed in different atmospheres. Peaks at 298 cm-1, 345 cm-1 and 632 cm-1 belong to CuO phase [47, 48]. Peaks at 218 cm-1 and 621 cm-1 correspond to Cu2O phase [49-52]. An additional peak at 521 cm-1 is attributed to Cu4O3, which is the chemical and structural intermediate of CuO and Cu2O [53]. CuO follows the sequential way to change its oxidation state from +2 to 0; i.e.

→ → → . In this regard, Cu4O3 and Cu2O are sub-oxide phases but Cu2O is the stable phase and Cu4O3 is the metastable state. Cu4O3 can be described as originating from the CuO structure by removal of oxygen atoms [54]. The spectrum of the 0%O-20M sample exhibited bands corresponding to CuO and Cu4O3. Therefore, it can be ascertained that the composition of films obtained with this annealing condition corresponds to CuO. However, this spectrum also contained an additional peak related to the presence of an intermediate phase showing that process of phase transformation had started in the sample. The obtained spectrum in the 20%O-20M sample contained CuO peaks with slightly higher intensities. This slight increase in intensity might be attributed to minutely reduced number of defects upon the addition of 20% O2. In the 20%O-40M sample where the annealing time was increased to 40 mins, the spectrum exhibited variation that reflects changes in chemical composition caused by diffusion of oxygen. Emergence of new peaks at 218 cm-1 and 621 cm-1 confirmed that CuO has been transformed into Cu2O by annealing in Ar and O2 environment for 40 mins. When increasing the percentage of oxygen to 40% in sample 40%O-20M, CuO peaks with a small peak of Cu2O were observed. Upon increasing the 5 ACS Paragon Plus Environment


percentage to 100% oxygen, 100%O-20M sample showed CuO peaks with higher intensity. Raman active modes in CuxO is known to involve only the movement of oxygen [53]. So the decrease in intensity of Raman peaks can be associated with oxygen vacancy creation on decreasing the oxygen ratio. Figure 2(a) shows the surface roughness and the average grain size of the thin films as a function of oxygen concentration in the mixture of Ar and O2., which was extracted from AFM measurements in Figure S2. The grain size of the CuxO thin films increased with an increase in the annealing time. The average grain sizes of the CuxO thin films in the samples 0%O-20M, 20%O-20M, 20%O-40M, 20%O -60M, and 40%O-20M after annealing were 5, 3.9, 16, 8, 4.9 and 6 nm, respectively. The increment of the grain size is mainly attributed to the increase of the surface energy at high temperature with increase in annealing time, which is consistent with previous reports [55]. However, the grain size of 20%O-60M was smaller as compared to 20%O-40M despite of the longer annealing time. This anomalous behavior can be related to desorption of oxygen and transformation of Cu2O phase to Cu as determined by XRD and Raman spectroscopy. The root-mean-square (RMS) roughness values of the corresponding thin films were 11, 10, 23, 15, 10 and 12 nm, respectively. Overall, the surface of the annealed films was comprised of non-uniformly distributed and tightly packed grains. Figure 2 (b) shows the sheet resistance measurements for the CuxO films prepared under various conditions. Sheet resistance of CuxO was strongly dependent upon the annealing conditions. There was a slight change in the sheet resistance as the oxygen content increased from 0% to 20% but 20%O-40M sample exhibited a high sheet resistance by more than three orders of magnitude as compared to 0%O-20M. The 20%O-60M sample annealed for 60 mins showed 50 Ω/sq sheet resistance, which was lower by eight order of magnitude than 20%O-40M sample. However, 40%O-20M and 100%O-20M showed sheet resistance comparable to 0%O-20M and 20%O-20M. This change in sheet resistance on varying the annealing conditions can be explained by the XRD results in Figure 1(a). 0%O-20M and 6 ACS Paragon Plus Environment

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20%O-20M samples belong to CuO phase and value of sheet resistance obtained for both samples matched with reported data of sheet resistance of CuO phase [56]. Increase in sheet resistance of 20%O-60M sample is associated with the transformation of CuO to Cu2O and it is known that Cu2O has higher resistivity than CuO [57]. The decrease in sheet resistance of 20%O-60M is also the result of phase transformation from Cu2O to Cu. Figure 3(a) shows PL spectra of CuxO samples annealed in different atmospheres. Each spectrum was resolved with two Gaussian functions. The black solid-line corresponds to the experimental data and the red dashed-line corresponds to the sum of fitted two Gaussian functions. Photoluminescence spectroscopy (PL) is a technique that can identify the defects introduced by intrinsic or extrinsic impurity and defect concentration as well. PL emission is typically observed as a broad peak which is further de-convoluted into different peaks for detailed analysis. The broad emission bands at 574 - 581 nm and 685-713 nm are associated with singly and doubly ionized oxygen vacancies in CuO, respectively [58, 59]. The emission bands at 771 nm and 842 are associated with doubly ionized oxygen vacancies and singly ionized oxygen vacancies in Cu2O, respectively [60-64]. In the case of CuxO thin films, the PL emission band shifted its peak to higher energy with an enough oxygen concentration. The luminescence intensity of each band showed strong dependence on oxygen content as shown in Figure 3(b). In PL spectroscopy, the area of the curve is proportional to the concentration of defects. So the concentration of oxygen vacancies was determined by dividing the area of respective peak (singly or doubly ionized oxygen vacancy) with sum of area of all de-convoluted peaks obtained after fitting. With an increase in oxygen concentration from 0% to 20% during annealing, the intensity of doubly ionized oxygen vacancies peak decreased from 80% to 68% in 20 min annealed sample and singly ionized oxygen vacancies peak increased from 20% to 32%, as is shown in Figure 3(b). With a further increase in oxygen concentration to 100%, the intensity of doubly ionized



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oxygen vacancies peak decreased from 68% to 45% and the intensity of singly ionized oxygen vacancies increased from 32% to 55%. Doubly ionized oxygen vacancies originate because of annealing at low oxygen pressure environment as follows: → + •• + 2 + ,

(2)

where •• is doubly positively charged oxygen vacancy. With the increase in oxygen concentration, the amount of doubly ionized oxygen vacancies decreased and singly ionized oxygen vacancies increase as shown in Figure 3(b). However, the overall concentration of oxygen vacancies decreased with the increase in oxygen content. Figure 4 shows the IV characteristics of Au/CuxO/ITO devices annealed at different atmospheres; (a) 20%O-20M, (b) 20%O-40M, (c) 20%O-60M, (d) 40%O-20M, and (e) 100%O-20M. The devices were operated with a compliance current of 100 µA. The direction of voltage sweep was clockwise as shown in Figure 4. When a DC voltage sweep was applied from 0 to -1 V and back to 0 V, as shown in Figure 4(a), a sudden increase of current was observed at ∼-0.7 V. The resistance change from pristine high resistance state (HRS) to LRS is defined as the “SET” process. The corresponding applied voltage is called as SET voltage (VSET). Subsequently, when the voltage bias was swept from 0 to 1 V and back to 0 V, the resistance of the device transferred back to HRS, which is called the “RESET process,” and the corresponding voltage is called the RESET voltage (VRESET). In the 0% oxygen environment, increased structural imperfections lead to high leakage current and poor endurance (not shown here). To reduce the leakage current, different amounts of oxygen (20%, 40%, 60% and 100%) were introduced during annealing. When the oxygen content was increased to 20% as shown in Figure 4(a), stable resistive switching was observed with an on/off ratio of 101. When the annealing time was increased from 20 mins to 40 mins, the on/off ratio was increased to 104 as shown in Figure 4(b). The origin of 8 ACS Paragon Plus Environment

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difference in resistance ratio refers to the density of defects and phase of CuxO. The switching was attributed to the formation of Cu filaments in CuxO solid electrolyte by the redox reaction. This increase in on/off ratio was attributed to the presence of Cu2O. Upon increasing the oxygen content to 40% (Figure 4(d)), resistive switching behavior was degraded because of reduced number of defects such as Cu+ interstitials. When the oxygen content is increased to 100% (Figure 4(e)), CuO becomes more insulative and no resistive switching is observed. The physical nature of each device state can be analyzed by measuring the resistance state of CuxO as a function of temperature. The resistance vs. temperature measurements for the LRS of 20%O-40M device is presented in Figure 5(a). The resistance increased linearly with increases in temperature, which indicates the metallic nature of the filament. The slope of the resistance vs. temperature was 33 Ω/K as determined by fitted line, which corresponds to the temperature coefficient of resistance (TCR) of 1.7 × 10–3 K–1 at room temperature by

the relation = (1 + ( − ), where Ro is the LRS at room temperature. A similar value of TCR of Cu nanowires (190 nm) was reported (α = 1.6 × 10–3 K–1) [65]. The diameter of Cu filament was estimated to be 171 nm by using the following relation [66]: D =

!"#$%&'( )*+,-

,

(3)

where L is the thickness of CuxO solid electrolyte (200 nm), ρfilament is the resistivity of Cu filament (2.3 mΩ·cm) [67], and RLRS is the resistance of LRS at room temperature (20 kΩ). Figure 5(b) shows the variation in SET and RESET voltages for 20%O-20M and 20%O-40M device. When the Ar concentration was 100%, there were many defects such as Cu interstitials and oxygen vacancies in the structure that could cause more leakage current. At 20% oxygen concentration (Figure 4(a)), the number of oxygen vacancies and Cu interstitials was reduced but still optimum to cause resistive switching in the device. In Figure



5(b), the variation of SET voltages for 20%O-20M was higher than that for 20%O-40M, which was explained using the migration through grain boundaries in the following. Figure 6 shows the grain boundaries and the schematic diagrams for Cu filament formation through the grain boundaries for (a) 20%O-20M and (b) 20%O-40M device, respectively. Annealing was performed to introduce Cu interstitials into the structure. Under the negative bias voltage on Au electrode, Cu interstitials will be attracted to Au electrode, where the excess of electrons will cause reduction of Cu ions. These Cu ions will be arranged in the form of filament that grows from cathode (Au) to anode (ITO), preferably along the grain boundaries, which will lead to LRS. The creation of Cu filament from Cu interstitials has been observed in other reports [39-41]. When applying the positive bias on Au electrode, the rupture of Cu filament occurs due to oxidation of Cu atoms, which will lead to HRS. For a 20%O-20M device, the active layer was still in CuO phase after annealing, as identified by XRD and Raman spectroscopy. The smaller grain and increased number of grain boundaries caused the easy migration of oxygen ions (responsible for creation of more Cu interstitials). However, this ease in migration through grain boundaries could be the reason for the large variation in SET voltage as shown in Figure 5(b). Increasing the annealing time to 40 minutes led to the transformation of CuO to Cu2O, as identified by XRD and Raman spectroscopy in Figure 1. As the CuO transforms to Cu2O, it was easier for the Cu+ ions to reduce to Cu metal as compared to Cu++ ions, which reduce to Cu+ first and then to Cu metal filament formation. The high resistance of Cu2O and ease in transformation to Cu ions caused the increase in on/off ratio of the device and lower SET voltage as compared to 20%O-20M device as shown in Figure 5(b). The reduced number of grain boundaries as shown in the Figure 6(b) could be the reason for smaller variation in SET voltage of 20%O40M device. As the annealing time was increased further, Cu phase in addition to Cu2O was identified by XRD and Raman spectroscopy in Figure 1, which caused poor resistive switching as shown 10 ACS Paragon Plus Environment

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in Figure 4(c). As the oxygen concentration was increased to 40% in the mixture of Ar and O2, the number of defects reduced, which caused poor resistive switching as shown in the Figure 4(d). The degradation in switching behavior seems to be due to the different phases of 20%O-40M (Cu2O) and 40%O-20M (CuO) as shown in Figure 2(e), rather than the physical mechanism change from filamentary to interface switching behavior. For the case of interface type resistive switching, the current flowing through the film is determined by the barrier height at the interface between the solid electrolyte and electrode. Resistive switching is characterized by the increase (HRS) or the decrease (LRS) of barrier height at solid electrolyte/metal interface. Generally, one interface has Ohmic behavior and the other has Schottky behavior in order to obtain resistive switching in interface type resistive switching behavior [68, 69]. The electron affinity ‘.’ of CuO, and work functions ‘/ ’of Au and ITO are 1.78 eV [70, 71], 5.2 - 5.4 eV, and 4.4 eV [72], respectively. Using these values, barrier height ‘/1 ’ at ITO/CuO interface is 2.62 eV and Au/CuO interface is 3.42 eV. In this case, both interfaces have Schottky barrier. Secondly, Au is an inert electrode and does not seems to play a role in determining the switching behavior by making an interfacial layer. However, ITO/CuO interface can be expected to play a role in determining resistive switching. From previous reports, it can be anticipated that ITO act as a reservoir of defects such as oxygen vacancies and ions [73]. Under the negative bias on ITO, Vo and Cui are attracted toward ITO. In the CuO/ITO interface region, carriers, e.g. electrons can be released (trapped) from the trapping centers (oxygen vacancies) under the voltage bias and can make interface layers narrower (thicker) and result in LRS (HRS), respectively. As Cui is less likely to be trapped at ITO interface, it is very difficult to expect the role of Cui in interface type resistive switching.



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To study in detail, different conduction mechanisms (Schottky emission and space charge limited) were fitted to the IV data of 40%O-20M device. The relation of current density with electric field is defined in Schottky emission as follows [74]: 2 = 3 exp 8 ∗

=> D ?@AB AC

9:;< EF

G,

(4)

where ‘J’ is the current density, ‘A*’ is the effective Richardson constant, ‘T’ is the absolute temperature, ‘q’ is the electronic charge ‘/1 ’ is the Schottky barrier height, ‘E’ is the electric field across the dielectric, ‘k’ is the Boltzmann’s constant, ‘εo’ is the permittivity in vacuum, and ‘εr’ is the optical dielectric constant. The IV data fitting (Figures S3 and S4) did not match with Schottky barrier conduction mechanism. Instead, it matched well with space charge limited current (SCLC), which is described as follows [74]: I ,

(5)

where ‘I’ is the current and ‘V’ is the voltage. Physical mechanism is still expected to be the filamentary instead of interface type switching. The change in resistance state and on/off ratio seems to be due to the different phases of 20%O-40M (Cu2O) and 40%O-20M (CuO) as shown in Figure 2(e). Upon increasing the oxygen content to 100%, CuO became more insulative as compared to 40%O-20M and no resistive switching was observed as shown in Figure. 4(e). The 100%O-20M sample demonstrated bipolar exponential characteristics. Similar behavior had been observed previously [75]. These exponential IV characteristics were explained according to mixed ionic electronic conduction (MIEC) theory. Upon applying the electric field, Cu ions, which are not sufficient to induce resistive switching, moved towards cathode. As a result, the anodic region has more vacant Cu lattice points, and the cathodic region has more Cu interstitials ions. So one side will behave like p-type and the other side will behave like n-type resulting in bipolar exponential characteristics. 12 ACS Paragon Plus Environment

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The retention of the 20%O-40M devices was measured at 27℃ and 85℃ by applying reading bias of +0.2 V as shown in Figure. 7(a). The on/off ratio was maintained at 104 with no significant degradation in resistance after 104 sec. The endurance of the 20%O-40M device is presented in Figure. 7(b). The LRS and HRS were stable for 200 cycles with a resistance ratio (RHRS/RLRS) of about 104. The results from endurance and retention data indicate the solution processed CuxO can be a good candidate for nonvolatile memory.

3. Conclusion

Resistive switching and bipolar diode behavior was demonstrated in CuxO thin films synthesized by a low cost solution-method. It was demonstrated that the resistive switching characteristics of these films were highly sensitive to the concentration of Cu interstitials. Defect engineering was performed in these CuxO thin films by annealing them in the mixture of Ar and O2 environment by modulating O2 content. The resistivity of CuxO thin films was strongly dependent on the oxygen content in the annealing atmosphere and annealing time. Phase transitions of CuxO thin films after annealing in different atmospheres was studied by XRD and Raman spectroscopy. The defect concentration was analyzed with PL spectroscopy. The 100% Ar annealing environment lead to large number of defects, which was not good for stable resistive switching behavior. With the addition of 20% oxygen, resistive switching behavior was improved. Phase transition of CuO to Cu2O was observed with an increase in annealing time with same annealing atmosphere. This transition resulted in an increase in the on/off ratio of the device by 3 orders of magnitude. Upon further increasing the oxygen content, the resistive switching behavior was degraded because of reduced number of defects. In the case of 100% oxygen, the resistive switching behaviour was not observed due to the decrease in Cui. The number of grain boundaries, determined by AFM, were considered to be responsible for the variation in SET and RESET voltages in different devices. The results



from endurance and retention data showed that the solution-processed CuxO can be a good candidate for nonvolatile memory.

4. Experimental Details 0.01 M copper (II) isopropoxide (Alfa Aesar) was dissolved in 200 ml isopropyl alcohol solution in a three-neck distillation flask at room temperature by magnetic stirring (500 rpm for 30 mins). Acid hydrolysis was performed by adding 0.01 M acetic acid dissolved in 3 ml nitric and sulfuric acid (99.9%, Alfa Aesar) in the flask and stirred for two hours. CuO nanoparticle power with an average diameter of 30 nm was obtained by decompression concentration using rotary evaporator at 80 ℃ . CuO nanoparticles were dispersed in methyl isobutyl ketone (MIBK) solution by using the wet milling dispersion process. 2 weight % silicate (SSK330T01, Ranco) was used as binder. Glass substrates were cleaned with acetone, isopropyl alcohol, and deionized water in an ultra-sonicator for 20 minutes and dried using a nitrogen gun subsequently. The 100-nmthick ITO was deposited on the glass substrate by sputtering. The CuO solution was filtered through 18 mm polytetrafluoroethylene (PTFE) syringe filter and then spin coated on the ITO at 3000 rpm for 40 sec. After spin coating, the thin films were placed on hot plate at 100 oC to remove the solvent. Subsequently, the as-deposited films were placed in a tubular furnace (Thermo Scientific Lindberg/Blue M Tubular Furnace) to perform annealing. The samples were annealed at different conditions by varying the composition of Ar and O2 environment in order to modulate the quantity of defects. The first sample (0%O-20M) was annealed in 100% Ar environment for 20 mins. The second (20%O -20M), third (20%O -40M) and fourth (20%O-60M) samples were annealed in 80% Ar and 20% O2 environment for 20, 40 and 60 mins, respectively. The fifth sample (40%O-20M) was annealed in 60% Ar and 40% O2 environment for 20 mins. The sixth sample (100%O-20M) was annealed at 100% O2 14 ACS Paragon Plus Environment

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environment for 20 mins. Finally, gold was sputter deposited as top electrode using a shadow mask of 30 µm × 30 µm. The phase and crystal structure of the CuxO thin films were determined by Bruker D8 Discover X-ray diffractometer (XRD) using CuKα (1.5406 Å) radiation operating at 40 kV, 40 mA (θ–2θ configuration). In order to study the phase dynamics, Raman spectra were obtained using a Renishaw micro-spectrometer with a laser wavelength of 514 nm at room temperature. The spot size was ~1 µm and the power was maintained at ~1.0 mW to reduce the heating effects. The VEECO Dimension 3100 atomic force microscopy (AFM) was used for surface roughness and grain size measurements. AFM measurement was done by using tapping mode on the scan area of 1 µm × 1 µm. The photoluminescence (PL) spectra were collected at room temperature by exciting at 514 nm with a He-Cd laser. The electrical characteristics were measured using an Agilent B1500 semiconductor characterization system at room temperature.

Acknowledgements This research was supported by Basic Research Program (2016R1D1A1B01009537), Nano Material Technology Development Program (2015M3A7B7045496) and Indo-Korea Joint Program of Cooperation in Science and Technology (2014K1A3A1A19067299) through the National Research Foundation of Korea (NRF) of Korea funded by the Ministry of Science, ICT & Future Planning. This research was supported by the MOTIE (Ministry of Trade, Industry & Energy (10080581) and KSRC (Korea Semiconductor Research Consortium) support program for the development of the future semiconductor device.

Supporting Information The supporting information is available free of charge on the ACS publications website (http://pubs.acs.org) 15 ACS Paragon Plus Environment


References: [1] Waser, R.; Aono, M. Nanoionics-based Resistive Switching Memories. Nat. Mater. 2007, 6, 833. [2] Pan, F.; Gao, S.; Chen, C.; Song, C.; and Zeng, F. Recent Progress in Resistive Random Access Memories: Materials, Switching Mechanisms, and Performance. Mater. Sci. Eng. R. Rep. 2014, 83, 1–59. [3] Waser, R.; Dittmann, R.; Staikov, G.; Szot, K. Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges. Adv. Mater. 2009, 21, 2632. [4] Yang, Y.; and Lu, W. Nanoscale Resistive Switching devices: Mechanisms and Modeling. Nanoscale 2013, 5, 10076–10092 [5] Yang, J. J.; Strukov, D. B.; and Stewart, D. R. Memristive Devices for Computing. Nat. Nanotechnol. 2013, 8, 13–24. [6] Yanagida, T.; Nagashima, K.; Oka, K.; Kanai, M.; Klamchuen, A.; Park, B. H.; Kawai, T. Scaling Effect on Unipolar and Bipolar Resistive Switching of Metal oxides. Sci. Rep. 2013, 3, 1657.


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Figure captions Figure 1: (a) XRD patterns of CuxO films on Si substrate annealed in the mixture of Ar and O2 atmosphere with different oxygen concentration varying from 20% to 100% (b) Raman spectra of CuxO thin films on Si substrate annealed in the mixture of Ar and O2 by modulating the concentration of oxygen from 20% to 100%.


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Figure 2: (a) Variation in grain size and roughness as a function of O2 concentration in the mixture of Ar and O2 annealing atmosphere. (b) Variation of sheet resistance of Cu O films as x

a function of O concentration in the mixture of Ar and O annealing atmosphere. 2

2

Figure 3: (a) PL spectroscopy of CuxO films on Si substrate annealed in the mixture of Ar and O2 atmosphere with different oxygen concentration varying from 20% to 100% (b) Histogram showing variation of oxygen vacancies with different O2 concentrations in the mixture of Ar and O2. Figure 4: IV characteristics of Au/CuxO/ITO devices with different annealing conditions. (a) 20%O-20M, (b) 20%O-40M, (c) 20%O-60M, (d) 40%O-20M, and (e) 100%O-20M. Figure 5: (a) LRS of 20%O-40M device as a function of temperature for calculation of thermal coefficient of resistivity. (b) Variation in VSET/VRESET of 20%O-20M and 20%O-40M sample. Figure 6: (a) Schematic drawing of the 20%O-20M device presenting smaller grains and abundant grain boundaries, extracted from AFM images shown parallel to schematic diagram. Cu nano-filament precipitates (red circles) are present at the grain boundaries showing the ease in migration in the device. (b) Schematic drawing of the 20%O-40M device presenting larger grains and reduced number of grain boundaries, extracted from AFM images shown parallel to schematic diagram. Figure 7: (a) Retention data of 20%O-40M device in the LRS (hollow circles and hollow o

squares) and HRS (solid circles and solid squares) at room temperature and 85 C. (b) Endurance of 20%O-40M device showing 200 cycles of stable reversible switching between LRS (red circles) and HRS (black squares).



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+V

+V

(b)

(a)



10

20%O-40M

20%O-40M

10

9

10

10

9

10

8

10

7

10

6

5

10

5

4

10

4

8

7

10

Resistance ()

10

Resistance ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 34 of 34

o

HRS (27 C) o HRS (85 C) o LRS (27 C) o LRS (85 C)

6

10 10 10

1

10

2

3

10

10

4

HRS LRS

0

10

Time (sec)

50

100 150 Switching Cycles

(b)

(a)


200

Resistive Switching Characteristics in Solution-Processed Copper Oxide

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