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Low temperature-grown KNbO thin films and their application to piezoelectric nanogenerators and self-powered ReRAM device Tae-Ho Lee, Hyun-Gyu Hwang, Seonghoon Jang, GUNUK WANG, Seongbeom Han, Dong-Hwee Kim, Chong-Yun Kang, and Sahn Nahm ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11519 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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ACS Applied Materials & Interfaces

Low temperature-grown KNbO3 thin films and their application to piezoelectric nanogenerators and selfpowered ReRAM device Tae-Ho Lee, † Hyun-Gyu Hwang,‡ Seonghoon Jang, ‡ Gunuk Wang, ‡ Seongbeom Han, ‡ DongHwee Kim, ‡ Chong-Yun Kang,‡,§ and Sahn Nahm*,†,‡ † Department of Materials Science and Engineering, Korea University, 145 Anam-ro, Seongbukgu, Seoul, 02841, Republic of Korea ‡ Department of Nano Bio Information Technology, KU-KIST Graduate School of Converging Science and Technology, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul, 02841, Republic of Korea. § Electronic Materials Center, KIST, 39-1 Hawolkok-dong, Seongbuk-gu, Seoul 137-791, Republic of Korea

Keywords: Low temperature-grown KNbO3, Nanocrystal, Nanogenerator, ReRAM, Selfpowered

1 ACS Paragon Plus Environment


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Abstract

Amorphous KNbO3 (KN) film containing KN nanocrystals was grown on TiN/SiO2/Si substrate at 350oC. This KN film showed a dielectric constant (εr) and piezoelectric strain constant (d33) of 43 and 80 pm/V at 10 V, respectively, owing to the existence of KN nanocrystals. Piezoelectric nanogenerators (PNGs) were fabricated using KN films grown on the TiN/Polyimide/PET substrates. PNG fabricated with the KN film grown at 350oC showed an open-circuit output voltage of 2.5 V and a short-circuit current of 70 nA. The KN film grown at 350oC exhibited a bipolar resistive switching behavior with good reliability characteristics that can be explained by the formation and rupture of the oxygen vacancy filaments. The KN resistive random access memory device powered by the KN PNG also showed promising resistive switching behavior. Moreover, the KN film shows good biocompatibility. Therefore, the KN film can be used for self-powered biomedical devices.

Introduction KNbO3 (KN) is a promising material for various electronic devices, because it has excellent piezoelectric and nonlinear optical properties, high optical transparency, and a large index of refraction in the visible range.1–9 In particular, interest in KN ceramics for their application to lead-free piezoelectric multilayer actuators has increased, because they exhibited large electric field induced strain with high Curie temperature.10 The KN single crystals have also been investigated, because they show excellent nonlinear optical properties.11,12 However, it was very difficult to obtain good quality KN single crystal using classical methods, owing to their nonstoichiometric nature, inhomogeneity, and cracking.13–15 Recently, the hydrothermal method


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has been used to synthesize the KN single crystals, but the size of the single crystal is unsatisfactory.16–19 Epitaxial KN thin films were extensively studied for their applications to nonlinear optical and electro-optical devices.20,21 They were usually grown on single crystal substrates by various growing methods.22–25 However, K2O easily evaporated during the growing processes and formed secondary phases, and thus it was difficult to grow the KN thin films with stoichiometric compositions.26 Therefore, K2O-excess or dual targets were used to grow stoichiometric KN thin films.27 However, since the composition of the target was different from that of the film, it was difficult to control the composition of the KN thin film.26 The stoichiometric KN films were obtained by annealing the amorphous KN films grown at room temperature (RT) under K2O atmosphere, to compensate the evaporated K2O.26 Their electrical properties, dielectric constant (εr), and piezoelectric strain constant (d33) were also reported.26 However, the growth processes were complicated, and thus it was difficult to use this method for practical applications. Investigations of the piezoelectric and triboelectric nanogenerator have attracted considerable attention.28,29 In particular, the output power density of the piezoelectric nanogenerators (PNG) is influenced by d332.30 Therefore, piezoelectric materials are required to have a large d33 value for their application to PNG. The Pb(Zr,Ti)O3 (PZT)-based piezoelectric materials have been used for PNG because they have a large d33 value.31 However, as they contain more than 60 wt% PbO, the PZT-based materials can cause environmental problems. Therefore, the PNGs fabricated using lead-free piezoelectric materials have attracted significant attention.28 The (Na0.5K0.5)O3 (NKN)-based piezoelectric materials have been generally investigated as lead-free piezoelectric materials because of their excellent piezoelectric properties.32,33 Furthermore, the PNGs have been fabricated using NKN film.34 However, the d33



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value of NKN film has been reported as 75 pm/V and that of KN film is 125 pm/V, indicating that a PNG fabricated by a KN film should produce a large output power, which can be used for self-powered devices without any environmental problems.26 Recently, investigations of the artificial synapse using resistive random access memory (ReRAM) device have increased.35–43 Moreover, it has been reported that (Na0.5K0.5)NbO3 thin films grown at low temperature exhibited promising ReRAM and artificial synaptic properties.44– 46

It is thus interesting to investigate the ReRAM properties of KN film. Furthermore, since KN

film has a large d33, KN thin film can be used as a PNG that can power ReRAM devices. Therefore, in this work, KN films were grown at low temperatures (≤ 350oC) to prevent K2O evaporation, and their electric, dielectric, and piezoelectric properties were investigated. In particular, KN films grown at 350oC were used to fabricate the PNG and ReRAM device. The KN ReRAM device, which was powered by the KN PNG, showed promising ReRAM properties with good biocompatibility, and thus the KN film can be used to fabricate self-powered ReRAM devices, which could be used for artificial synapses. In addition, for the application to PNG, the film should be grown on a flexible polymer substrate, such as Polyimide (PI), which is stable at low temperatures (≤ 350oC). Therefore, it was important to grow the film at a low temperature for application to the PNG. Moreover, the major phase of the KN film used in this work was an amorphous phase because the KN films were grown at low temperature.

Experimental Procedure The 36 nm–thick KN films were grown on TiN/SiO2/Si (TiN–Si) substrates at various temperatures between RT and 350oC, using RF sputtering. A KN ceramic target of 2-inch diameter was used for the sputtering, and it was produced by the conventional solid state


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method.9 The KN films were deposited in a vacuum chamber with the sputtering power of 80 W under an atmosphere of argon and oxygen gases mixture with the ratio of 4:1. The total pressure of ambient gas was 10 mTorr. Pt was deposited on the KN films using conventional DC sputtering (Emitech K550, USA) to form the top electrode. The Pt top electrodes were patterned using a metal mask to form discs of various diameters. The 300 nm–thick KN films were grown on flexible TiN/PI (TiN–PI) substrates for the synthesis of KN PNGs using an RF sputtering method, as shown in Figure S1a of the Supporting Information 1 under the same conditions used for the fabrication of the ReRAM device. Moreover, the KN/TiN/PI was attached on the PET at RT after the growth of KN film on the TiN/PI substrate to fabricate the KN PNG (see Figure S1b of the Supporting Information 1). A conductive fabric tape was attached on the KN/TiN–PI device to form the top electrode. The KN films were polled by applying a DC electric field of 200 kV/cm for 1 h. The structural properties of the KN films were examined using X-ray diffraction (XRD: Rigaku D/max–RC, Tokyo, Japan), scanning electron microscopy (SEM: Hitachi S–4300, Tokyo, Japan), and high-resolution transmission electron microscopy (HRTEM; Tecnai G2 TF 30ST, FEI, USA). Image processing software (Gatan Digital Micrograph 3.x) was used to automate the fast Fourier transformation (FFT) calculation. The interface between the KN film and TiN electrode was investigated using Auger electron spectroscopy (AES: Physical Electronics PHI 700 Auger Spectroscope). Atomic force microscopy (AFM: Park system XE–100, Korea) and current AFM (CAFM) were used to measure the surface roughness of the KN films and the current of KN/TiN–Si films, respectively. The Pt/KN/TiN-Si ReRAM device was used to measure the XPS spectra and the thickness of the KN film was 36 nm. The Pt top electrode and KN film were removed during the XPS measurement. The chemical binding energies of the N1s



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and O1s were measured by X–ray Photoelectron Spectroscopy (XPS: ULVAC–PHI X–tool, Japan). The current-voltage (I–V) characteristics, DC-sweep endurance, and retention properties of the KN ReRAM were measured by source-meter (Keithley 2400 USA). A current compliance of 4 mA was used to set the current limit, in order to prevent complete dielectric breakdown. The εr and tan δ values were measured by precision LCR meter (Agilent 4285A, Santa Clara, CA) in the frequency range 75.0 kHz – 1.0 MHz. The d33 value was measured using piezoelectric force microscopy (PFM: Park system XE–100, Korea). Stress was applied to the PNGs using a bending tester (ZBT–200, Zeetech, Korea). The open-circuit voltages and short-circuit currents of the PNGs were measured by nano–voltage meter (Keithley 2182A: input impedance of > 10 GΩ, delta mode, low noise measurements at high speeds, USA) and a pico–ampere meter (Keithley 6485 5–1/2 digit pico–ammeter: digital filter, up to 1000 readings/s, and 10 fA resolution, USA), respectively. The calculation processes for the strain developed in the KN film are shown in Figure S2 in the Supporting Information 2. The strain rate is the strain divided by the time for which the strain is applied to the PNG. Moreover, Figure S3 in the Supporting Information 3 shows the measurement setups for the electric outputs of the PNG. For biocompatibility test of KN film grown at 350oC, human dermal fibroblasts (HDFs) were seeded into 24–well cell culture plates, with 3 x 104 cells per well. Cells were cultured in Dulbecco’s Modified Eagle’s Medium (Corning) supplemented with 10 % FBS (Hyclone), penicillin (10,000 units/ml) and 10,000 μg/ml streptomycin (Gibco) for 7 days. The viability of the HDFs was evaluated by Live/Dead assay kit (Invitrogen). Calcein–AM (0.5 μl) and ethidium homodimer–1 (1 μl), which stain the live cells green and the dead cells red, respectively, were dissolved in PBS (1 ml). This mixture was added in cell culture wells and incubated at RT for 30


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mins, and the stained cell images were collected by confocal laser scanning microscopy (Nikon A1R).

Results and discussion Figure 1a shows the XRD patterns of the KN films grown on the TiN–Si substrates at various temperatures. Reflections for the crystalline KN phase were not observed, indicating that the amorphous KN phase was formed in the films grown at temperatures below 350oC. It was not possible to grow the KN film on the TiN–Si substrate at temperature higher than 350oC, because the TiN electrode becomes unstable at high temperature (≥ 400oC) with the increased resistivity. The inset of Figure 1a shows a cross–sectional SEM image of a KN film grown at 350oC. A 36 nm–thick KN film was grown well on the TiN electrode with the continuous interface between the KN film and TiN electrode. Figure 1b shows the AES depth profile of the KN film grown on the TiN–Si substrate at 350oC. No diffusion of Ti or N ions into the KN film, or of K+ or Nb5+ ions into the bottom TiN electrode, was detected, indicating the formation of the chemically sharp KN/TiN interface. Therefore, the 36 nm–thick KN film was well formed on the TiN–Si substrate. Moreover, KN film grown at 350oC shows a smooth surface and dense microstructure, as shown in Figure S4a–d of the Supporting Information 4. For detailed analysis of the microstructure of the KN films, HRTEM analysis was conducted on the KN films grown at 300 and 350oC, as shown in Figure 2a,b, respectively. Pure amorphous KN phase was developed in the film grown at 300oC, and the FFT pattern calculated from the image, which is shown in the inset of Figure 2a, confirms the formation of pure amorphous KN phase in this film. The amorphous KN phase was also developed in the KN film grown at 350oC, but a small amount of the KN nanocrystal, which is identified by the circle in Figure 2b, was



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observed in this KN film. The size of the KN nanocrystal is approximately 5.0 nm. The inset of Figure 2b shows the FFT pattern calculated from the HRTEM image of the nanocrystal. The reflections shown in this figure can be indexed as the reflections of the (001) and (110) planes of the crystalline KN phase. The HRTEM results indicated that the pure amorphous phase was formed in the KN films grown at low temperatures (≤ 300oC), while the amorphous KN phase began to crystalize in the KN film grown at 350oC. Figure 3a shows εr and tan δ values of the KN films grown at various temperatures. The KN films grown at low temperatures (≤ 300oC) show low εr values of 22 – 23, owing to the formation of the amorphous KN phase, and the variation of the εr value with respect to measuring frequency can be negligible. However, the KN film grown at 350oC shows an increased εr value of 43, which can be explained by the presence of KN nanocrystals in the amorphous KN film. All the films exhibit a low tan δ of 1.0% at 100 kHz that increased slightly with the increase of frequency, while the KN film grown at 350oC showed a small tan δ of 4.0% at 1.0 MHz. The crystalline KN film showed a very large εr value of 884 with a slightly large tan δ of 6.71% at 100 kHz, and the KN ceramics showed an εr value of 540.9,26 Therefore, it is considered that the εr value of the KN film grown at 350oC is much smaller than that of the crystalline KN film and the KN ceramics. The d33 value of the KN films grown at 300 and 350oC were also measured, as shown in Figure 3b,c, respectively. The amorphous KN film shows a saturated d33 value of approximately 16 pm/V at 10 V (see Figure 3b). On the other hand, the KN film grown at 350oC, which has the KN nanocrystals, exhibits an increased d33 value of approximately 80 pm/V at 10 V, probably owing to the presence of KN nanocrystals. The d33 values of the crystalline KN film and the KN ceramics were reported as approximately 125 pm/V


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and 109 pC/N, respectively, indicating that the KN film grown at 350oC exhibited a relatively large piezoelectric property with a high d33 value. The 300 nm–thick KN films were grown on the TiN–PI substrates at various temperatures to fabricate the PNGs. Figure 4a,b show the open-circuit output voltages and the short–circuit output currents of the PNGs fabricated using the KN films grown at various temperatures. The KN PNG was bent using a bending machine with strain and strain rate of 0.76 % and 0.79 % s-1, respectively, which were the maximum strain and strain rate obtained by the bending machine. Negative and positive voltages (or currents) were obtained when the PNG was bent and released, respectively. The PNG fabricated by the KN films grown at the low temperatures (≤ 300oC) exhibited low output voltage (≤ 1.0 V) and output current (≤ 20 nA). The increased output voltage and output current of 2.5 V and 70 nA, respectively, were obtained from the PNG fabricated using the KN film grown at 350oC. Similar results were obtained when the output voltage and current were measured along the reverse directions, as shown in Figure S5a,b of Supporting Information 5. Moreover, the output voltages and output currents of the KN PNG did not change (or slightly increased) after 3,000 times of operation, as shown in Figure 4c,d. Therefore, the KN PNG has good reliability properties. In addition, the output voltage and output currents were measured at various strain and strain rate, as shown in Figure S6a–d of the Supporting Information 6, and they increased with the increase of the strain and strain rate. In general, the electric output energy density generated by the piezoelectric materials is proportional to d332 of the piezoelectric materials.29 Therefore, the increased output voltage and output current of the PNG fabricated by the KN film grown at 350oC can be explained by the larger d33 value of this KN film. In addition, the matching resistance of the KN PNG was approximately 15 MΩ and a maximum output power of 106 nW was obtained at 15 MΩ under a



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strain of 0.76 % and a strain rate of 0.79 % s-1, as shown in Figure S7c of Supporting Information 7. Moreover, the transferred charge of the KN PNG was also measured under a strain of 0.76 % and a strain rate of 0.79 % s-1, and a maximum charge of 5 nC was obtained, as shown in Figure S7d of Supporting Information 7. Figure 5a–d show the I–V curves of the KN films grown on the TiN–Si substrates at various temperatures, and Pt was used as a top electrode of these devices. Typical bipolar switching curves were obtained from all of the KN films. Electroforming process was required to obtain the bipolar switching curves of the KN film, and - 8.0 V was applied to the Pt top electrode for the electroforming process. Set and reset voltages for the KN film grown at RT were small, and increased with the increase of the growth temperature. The KN film grown at 350oC shows the increased set and reset voltages of - 2.1 and 2.1 V, respectively. The resistance ratio between the high resistance state (HRS) and low resistance state (LRS) is very small for the KN films grown at temperature lower than 350oC. However, the KN film grown at 350oC shows an increased resistance ratio of approximately 10 at 0.25 V, owing to the decreased current in HRS (see Figure 5d). Moreover, the KN film grown at 350oC has good switching uniformity and reliability characteristics, as shown in Figure S8a–e of Supporting Information 8. The KN film has a small resistance ratio between HRS and LRS, and similar set and reset voltages compared with those of the NKN film. The large resistance ratio of the NKN film was maintained up to 200 times per measurement.41 For the KN film, although a small resistance ratio between the HRS and LRS was observed, it was well maintained after 1000 times per measurement, as shown Figure S8c and S8d in Supporting Information 8. Therefore, it is assumed that the KN film has better reliability properties than the NKN film. However, more work is required to determine if the KN film is superior to the NKN film.


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The current conduction mechanism of the Pt/KN/TiN–Si device in HRS and LRS with the KN film grown at 350oC was investigated. Figure 6a shows the resistances of the KN films with the various sizes of the top electrode that were measured at LRS and HRS. The RLRS does not change with the increase of the size of the top electrode, indicating that the current at LRS can be explained by the localized conducting filament mechanism.47 On the other hands, the RHRS decreased with the increase of the size of the top electrode. Therefore, the current of the KN film in HRS cannot be explained by the localized conducting filament mechanism.48 The current of the dielectric thin film is generally explained by Schottky emission (SE), Poole–Frankel emission (PFE), and space charge-limited-conduction (SCLC) mechanisms. Figure 6b shows the I–V curve of the Pt/KN/TiN–Si in HRS, and a positive voltage was applied to the Pt top electrode to obtain this I-V curve. Figure S9a–c of Supporting Information 9 show that the SE and PFE mechanisms cannot explain the current in the KN film at HRS. However, the current of KN film grown at 350oC can be explained by the SCLC mechanism, because as Figure 6b shows, the slope of the I-V curve is close to 1.0 for low electric field, and changes to approximately 2.0 for high electric field. For the SCLC conduction mechanism, the interface between TiN electrode and KN film needs to be conductive.49 Therefore, it is considered that many defects, such as oxygen vacancies, are formed in the KN film near the KN/TiN interface after the set process, and decreased the barrier height of the KN/TiN interface, resulting in the formation of quasiconductive KN/TiN interface. Figure S9d of Supporting Information 9 shows that the SCLC mechanism can also explain the I–V curve in HRS before the set process, which was obtained when the negative voltage was applied to the Pt top electrode. Therefore, the Pt/KN interface in HRS is also considered to be conductive after the forming process. The current mechanism of the LRS was also investigated, as shown in Figure 6b. The slope of the I-V curve is close to 1.0,



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indicating that the current in the KN film in LRS can be explained by the metallic ohmic conduction mechanism. For detailed investigation of the resistive switching mechanism of the Pt/KN/TiN–Si ReRAM device with the KN film grown at 350oC, the TEM/EDS line scanning was conducted at the KN/TiN interface in HRS and LRS, as shown in Figure 7a,b, respectively. A chemically sharp interface was formed between the KN film and TiN electrode in HRS (see Figure 7a). On the other hand, increased oxygen concentration was observed in the TiN electrode near the KN/TiN interface in LRS, indicated by the arrow in Figure 7b. Therefore, it is considered that during the set process, a positive bias was applied to the TiN bottom electrode, and the oxygen ions in KN film, which have effective negative charge, moved into the TiN electrode, and increased the oxygen concentration in TiN electrode in LRS, as shown in Figure 7b. Moreover, it can be suggested that TiOxNy phase could be formed at the KN/TiN interface. HRTEM analysis was also conducted on the KN/TiN interface in HRS and LRS, as shown in Figure 7c,d. In HRS, the amorphous KN film was well grown on the TiN electrode (see Figure 7c). However, for the KN film in LRS, the TiOxNy nanocrystals were formed at the KN/TiN interface indicated by the red circles in Figure 7d. The FFT pattern was calculated from the HRTEM lattice image of the TiOxNy nanocrystal, and the reflections shown in this FFT pattern were identified as the (011) reflection of the TiO0.34N0.74 phase, as shown in the inset of Figure 7d. Therefore, it can be concluded that the oxygen ion interacted with the TiN electrode, and formed the crystalline TiO0.34N0.74 phase at the KN/TiN interface in LRS. Furthermore, it is considered that a large amount of oxygen vacancies existed in the KN film in LRS, and these oxygen vacancies formed the conductive filaments in the KN film, and behaved as the conducting path of the electrons in LRS. Therefore, the switching mechanism of the Pt/KN/TiN–Si ReRAM device can be explained


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by the formation and rupture of the oxygen vacancy filaments in the KN film. In addition the XPS results shown in Figure S10a–f of Supporting Information 10 confirmed the presence of the TiOxNy phase at the KN/TiN interface. The in situ CAFM analysis was conducted on the KN film in HRS and LRS, which was grown at 350oC, to investigate the presence of the conductive filaments in KN film in LRS. Figure 8a,b show three-dimensional current maps measured at 1.0 V for the KN film in HRS and LRS, respectively, and the size of the area is approximately 1.0 x 1.0 μm2. Current was not detected from the KN film in HRS (see Figure 8a), but a current of 12 nA was detected in a specific area of the KN film in LRS indicated by I in Figure 8b. The I–V curves were also measured at areas I and II, which are shown in Figure 8b, using CAFM. A bipolar switching curve was obtained from position I, as shown in Figure 10c but the current was not detected in position II (see Figure 8d). Since a large current of 12 nA and a bipolar switching curve were only obtained from the specific area I of the KN film in LRS, it is considered that a conductive oxygen vacancy filament existed in this specific point I, and behaved as the current path in LRS. Therefore, it can be concluded that the oxygen vacancies filaments formed in the specific areas in the KN film, and behaved as the current path of KN film in LRS. Furthermore, the resistive switching mechanism of the Pt/KN/TiN–Si ReRAM device can be explained by the formation and rupture of the oxygen vacancies filaments in KN film. The KN film grown at 350oC on the TiN–PI substrate can be used for the PNG, and this KN film when grown on TiN–Si substrate can also be used for the ReRAM devices. Therefore, it is considered that the KN ReRAM device can be powered by the KN PNG. The KN ReRAM was connected to the KN PNG, as shown in Figure 9a, and the currents of the KN ReRAM device in LRS and HRS were obtained by applying the negative and positive pulses generated from the



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KN PNG, which was operated manually by hand. Negative and positive pulses were produced when the KN PNG was bent and released, respectively, as shown in Figure 9a. The current of the ReRAM devices were measured at 0.1 V by pulse generator. The KN PNG was operated by hand with an average strain and strain rate of 1.52 % and 1.82 % s-1, respectively, and the output voltages generated by KN PNG ranged between 2 to 3 V (or – 2 to – 3 V), as shown in Figure 9b, and they are large enough to operate the KN ReRAM device. Figure 9c shows the endurance characteristic of the KN ReRAM powered by the KN PNG. The KN ReRAM powered by the KN PNG exhibited a stable resistive switching behavior, with an RHRS and RLRS ratio of 10. In addition, the resistance switching characteristics of the KN ReRAM were investigated under various bending conditions of the KN PNG, as shown in Figure S11a–f of Supporting Information 11. The KN PNG should be bent with a strain and strain rate greater than 0.76 % and 0.79 % s-1, respectively, to operate the KN ReRAM. Therefore, this results show that the KN film grown at low temperature can be used for self-powered ReRAM devices. Lastly, the biocompatibility of the KNbO3 films grown on the sapphire substrate was evaluated, to confirm that KNbO3 film is a suitable material for implantable biomedical devices. Human dermal fibroblasts (HDFs) derived from the dermis of human skin were seeded on the KNbO3 films, and cultured for 7 days. Figure 10a,b show DIC and fluorescence confocal microscope images of these cells on the cell culture plate and KNbO3 film, respectively, where green and red colors indicate live and dead cells, respectively. Even after 7 days, 99 % of the cells were still alive on the KNbO3 films, and their morphology was similar to healthy control cells, as shown in Figure S12a,b in Supporting Information 12. These results demonstrate that the KNbO3 films are biocompatible, and can be used for biomedical devices. However, more


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work is required to demonstrate that the KN PNG can be operated in vivo for the application to implantable devices.

Conclusions Pure amorphous phase was formed in the KN films grown on the TiN-Si substrate at temperatures lower than 350oC, and the KN nanocrystal began to form in the film grown at 350oC. The εr and d33 values of the KN films grown at low temperature (≤ 300oC) were small at 22 and 16 pm/V, respectively. Increased εr and d33 values of 43 and 80 pm/V, respectively, were obtained from the KN film grown at 350oC, probably due to the existence of the KN nanocrystals. The KN PNG fabricated using the KN film grown on TiN-PI substrate at 350oC exhibited a large open-circuit output voltage (~ 2.5 V) and short-circuit output current (~ 70 nA) under the stain and strain rate of 0.76 % and 0.79 % s-1, respectively. The KN film grown on the TiN–Si substrate at 350oC exhibits a typical bipolar switching curve after the forming process, and it showed good reliability characteristics. During the set process, the oxygen ions moved into the TiN electrode, and the oxygen vacancies in KN film formed the conducting filaments. The bipolar resistive switching behavior of the Pt/KN/TiN–Si ReRAM device can be explained by the formation and rupture of the oxygen vacancy filaments. Finally, the KN ReRAM device, which was operated by the KN PNG, exhibited good resistive switching behavior, with an RHRS and RLRS ratio of 10. Furthermore, the KN film shows good biocompatibility. Therefore, the KN film grown at 350oC can be used for self–powered biomedical devices.



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Figure 1. (a) XRD patterns of the KN films grown on the TiN–Si substrate at various temperatures, and (b) AES depth profiles of KN film grown at 350oC. The inset of Figure 1(a) is the SEM image of the KN film grown at 350oC.


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Figure 2. HRTEM lattice images of the KN films grown at (a) 300oC, and (b) 350oC. Insets of these figures show the FFT patterns calculated from the corresponding high-resolution image.



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Figure 3. (a) The εr and tan δ of the KN films grown at various temperatures. The d33 versus V curves of the KN films grown at (b) 300oC, and (c) 350oC.


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Figure 4. (a) The open-circuit output voltages, and (b) the short-circuit output currents of the PNGs fabricated using the KN films grown at various temperatures. (c) The open-circuit output voltages, and (d) the short–circuit output currents of the KN PNG continuously measured up to 3,000 times.



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Figure 5. The I–V curves of the KN films grown on the TiN–Si substrates at various temperatures, and Pt was used as the top electrode of the devices: (a) RT, (b) 200oC, (c) 300oC, and (d) 350oC.


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Figure 6. (a) Resistances of the Pt/KN/TiN-Si devices with various sizes of the top electrode in LRS and HRS, and (b) the I–V curves of the KN film grown at 350oC in LRS and HRS.



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Figure 7. TEM/EDS line scanning of the KN/TiN interface in (a) HRS, and (b) LRS and HRTEM lattice images taken from the KN/TiN interface in (c) HRS, and (d) LRS.


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Figure 8. Three-dimensional current maps measured by CAFM for the KN film grown at 350°C in the (a) HRS, and (b) LRS. I–V curves taken from the areas (c) I, and (d) II shown in (b).



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Figure 9. (a) Schematic diagram of KN ReRAM operated by KN NG. (b) The open-circuit output voltage of the KN PNG operated by hand, with an average strain and strain rate of 1.52 % and 1.82 % s-1, respectively. (c) Endurance property of the KN ReRAM device operated by the KN PNG.


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Figure 10. DIC and fluorescent confocal images of the human dermal fibroblasts cultured for 7 days (a) in control cell culture plate, and (b) on KNbO3 film grown on the sapphire substrate.

ASSOCIATED CONTENT The following files are available free of charge. 1. Structure of the KN/TiN/PI/PET PNG 2. Calculation of the strain developed in the KN PNG 3. Measurement setup for the application of mechanical energy to the KN PNG 4. Structural and chemical analysis on the surface of the KN films 5. Electrical output characteristics measured along the reverse direction for KN PNG synthesized with KN film grown at 350oC



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6. Open-circuit voltages and short-circuit currents of KN PNG measured at various strains and strain rates 7. Output properties of the KN nanogenerator measured using external loads 8. Stability and reliability properties of the Pt/KN/TiN-Si ReRAM grown at 350oC 9. Leakage current mechanism of the KN film in HRS grown at 350oC 10. XPS spectra obtained from the KN/TiN interface 11. Resistance switching characteristics under various bending conditions 12. Biocompatibility of the KNbO3 films (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +82-2-928-3584

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2017R1A2B4007189). We thank the KU-KIST graduate school program of Korea University


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