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TiO2 nanorods composite film by increased oxygen vacancy reservoir
Materials Science in Semiconductor Processing 108 (2020) 104907
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Improved resistive switching behavior of multiwalled carbon nanotube/ TiO2 nanorods composite film by increased oxygen vacancy reservoir Navaj Mullani a, Ijaz Ali a, Tukaram D. Dongale b, *, Gun Hwan Kim c, Byung Joon Choi d, Muhammad Abdul Basit a, Tae Joo Park a, ** a
Department of Materials Science & Chemical Engineering, Hanyang University, Ansan, 15588, South Korea Computational Electronics and Nanoscience Research Laboratory, School of Nanoscience and Biotechnology, Shivaji University, Kolhapur, 416004, India Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon, 34114, South Korea d Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul, 01811, South Korea b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Memristive device MWCNTs TiO2 Nanocomposites Resistive switching memory
The non-linear nature in the current-voltage relationship and good resistive switching characteristics were demonstrated with the help of TiO2 nanorods-functionalized multiwalled carbon nanotube (fMWCNT) composite grown by the low-cost hydrothermal method. The composites were characterized by X-ray diffraction, scanning electron microscopy, Raman, photoluminescence, and X-ray photoelectron spectroscopy to investigate the structural, morphological, and chemical composition of composite films. The resistive switching characteristics of the TiO2-fMWCNT nanocomposites were found to be strongly dependent on the fMWCNT concentration. The enhanced switching performance is associated with the surface nanostructure and chemical composition of the nanocomposites. Owing to the hierarchical rutile TiO2 nanorods and opportune fMWCNT content, the nano composite based device with 0.03 wt % fMWCNT exhibited the best resistive switching performance with good endurance and retention non-volatile memory properties. Interestingly, with the optimized stoichiometric composition and operation conditions, forming-free, low operational voltage, self-rectifying like properties have been simultaneously achieved, which are some of the prerequisites for next-generation memory devices. In addition to this, the double-valued charge-magnetic flux nature of the developed devices was demonstrated. The experimental current-voltage characteristics are well-matched with the Ohmic and Schottky conduction mechanisms.
1. Introduction The von Neumann architecture comprises a design having a central processing unit and a memory as separate units. These two units are connected by a single channel that carries instructions, data from memory to processor, which creates as von Neumann bottleneck [1]. However, the modern computing systems have become more complexed therefore, von Neumann bottleneck produces a significant effect on data processing. Because of this, real parallel processing is not achieved by von Neumann architecture and is unable to perform memory and logic operation in a single unit [2]. Furthermore, conventional computers work on a digital system by using a binary number system but the human memory, cognition, learning and logic operations are carried out by means of analog signal processing [3]. The binary system only uses two
numbers 0 and 1, that is, true or false for information processing. However, the conventional computing architectures facing challenges, including the heat wall, the memory wall, and it leads to the end of Moore’s law. Given this, it is predicted that the conventional circuit technology and components are inappropriate and unsuitable for future computing paradigms. New kind of techniques such as liquid cybernetic systems could be a better solution for the next generation zettascale computing applications [4]. Eventually, limitations of conventional computers and system were observed, which drew the attention of re searchers towards a system which has the potential to overcome these problems and execute the system operation systematically. The device having the ability to co-locate memory and compute in the same phys ical device is the next big thing in the computing and mem ristor/memristive is considered to be a potential candidate for the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.D. Dongale), [email protected] (T.J. Park). https://doi.org/10.1016/j.mssp.2019.104907 Received 9 November 2019; Received in revised form 20 December 2019; Accepted 23 December 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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in-memory and neuromorphic computing applications. Developments in memristor technology may provide an alternative path that enables the fabrication of hybrid non-volatile memory systems and three dimensional (3D) crossbar arrays. Due to its long retention time, consistency over large endurance cycles and non-volatile switch ing, proved its potential to be used in memory and logic devices [5]. In recent years, researchers are trying to find out new ways to replace the conventional von Neumann architecture and in this scenario, the memristor has been cropping up in many devices and circuitry. The memristor has been utilized to catalyze the memory devices owing to its unique structural, chemical, electrical properties, since after its first experimental realization at Hewlett-Packard laboratories in 2008 based on TiO2 [6]. Furthermore, memristive devices have been also used in the neuromorphic systems and logic circuits [7]. Before the discovery of practical memristor/memristive device, Prof. Leon Chua has predicted and put forward a fourth fundamental circuit element, based on the nonlinear circuit theory and the firm mathematical foundation [8]. Furthermore, the extended class of memristor has been postulated by Chua and Kang, called as memristive devices family in 1976 based on the concrete mathematical formulations [9]. In recent years, different materials and deposition techniques have been explored by many re searchers for the development of memristors or memristive devices. In particular, chalcogenides, polymers, metal oxides, ferrites and recently the complementary metal-oxide-semiconductor (CMOS) compatible materials like SiN and SiO2 have been utilized to develop the active layer of memristor/memristive devices [10–12]. Among them, the polymers and chalcogenides have been limited due to high-temperature melting and phase transformation issues [13] which could be delayed their utility in future logic and memory devices; whereas the CMOS compatible materials demand the high-cost physical deposition tech niques with highly equipped and controlled environment. Due to these reasons, low-cost chemical synthesis techniques has been showing its potential to fabricate the memory devices and its application [14]. The metal oxide/polymer nanocomposites gaining attention to fabricating flexible resistive switching devices due to their uniform nanocomposites formation ability and room temperature fabrication process. Recently, Chiolerio et al. have provided an in-depth fabrication process of ZnO/ polymer/colloidal suspensions based devices, which featuring the low power resistive switching operations [15]. The metal oxide memristive devices have intrinsic and favorable properties towards memristive behavior. The active layer deposition of metal oxides like TiO2, ZnO, HfO2, SnO2, WO3, NiO [16–21], etc. are CMOS compatible materials, making them suitable candidates to develop high-performance resistive switching memory and electronic synaptic devices. The hydrothermal technique has been utilized to grow the hierar chical nanostructures of different dimensions as well as nanomaterials with different and desired shapes and sizes in low-cost processing. In addition to this, 1D memristor or memristive devices fabricated via hydrothermal technique has been attracted great attention due to its excellent charge carrier transportation through straight conducting paths which may behave differently than branched conducting paths based devices and could be applicable for high-density memory storage application due to its superior electrical characteristics at low power consumption [21]. Moreover, the low powered memristor/memristive devices can be used to couple with the living neurons, thereby the end system can emulate the neuromorphic properties with fewer energy consumptions similar to the natural neurobiological process [22]. However, at the microscopic level, the energy required for vacancy formation and diffusion or creation of a conducting path is critical to determining numerous key device properties like forming voltage, switching voltage, endurance, retention, etc. The conventional resistive memory devices needed an additional electroforming step and selector devices for the proper operation. The former issue increases the power consumption while the later issue increases the complexity in the crossbar array configuration [16]. The electroforming-free resistive memory devices could be a possible solution for the low power memory
devices while selector-less resistive memory devices could be developed by engineering the self-rectification property. In view of this, the development of a resistive switching cell that simultaneously shows electroforming free operation and self-rectification is a demanding task of the hour. In this work, we have adopted a hydrothermal technique owing to its ability to grow desired nanostructures of the TiO2 with tunable stoichiometry and possible to make easy metal oxide-carbon composite films with enhanced resistive switching properties. There are many reports available in the literature for the development of TiO2 thin film memristive devices by using the different precursors and methods [23,24]. However, there are still many ways to incorporate the carbon nanostructures into the lattice structures of the metal oxide to improve the device quality and electrical behavior. In order to influence the desired device properties, the TiO2-functionalized multiwalled car bon nanotube (fMWCNT) composite films were synthesized with mod ifications in deposition techniques and with different ratios of MWCNTs. The electrical and compositional characterizations were also studied in detail to confirm the developed devices can be potentially applicable for the resistive switching memory devices. The criteria for the identifica tion of the extended class of memristor device i.e. memristive device, is also illustrated with the help of double valued charge-magnetic flux characteristics. The device conduction mechanisms are also obtained by fitting the experimental data to the Ohmic and Schottky conduction models. 2. Experimental section 2.1. Materials The pristine commercial MWCNTs having a diameter of 15 nm were purchased from Excellent Mecha Power company (South Korea), Tita nium Isopropoxide (TTIP, C12H28O4Ti, 98%) were purchased from Sigma Aldrich, which was used as a Ti precursor. In order to maintain the acidic condition of a reaction solution, concentrated hydrochloric acid (HCl, extra pure grade (37%) Daejung chemicals, South Korea) was used. Sodium dodecyl sulfate was used as a surfactant during the deposition of TiO2 and TiO2-MWCNT composite films, which act as a dispersant in the followed deposition process. However, the solvents and reagents like acetone, ethanol, and Isopropyl alcohol (IPA, Daejung Chemicals) were used to clean the glassware and substrates. All commercially available chemicals were used without any additional modifications during the synthesis. 2.2. Ultrasonic cleaning treatment of fluorine-doped tin oxide (FTO) substrate We have used fluorine-doped tin oxide (FTO) (~10 Ω/sq.) as a bot tom electrode in this work as shown in Fig. 1. The adopted ultra-sonic cleaning treatment is as follows: Initially, the FTO substrate was cleaned with double distilled water and acetone (1:1) and then kept inclined into the wall of the beaker vertically in the ultrasonic bath for 30 min, followed by washing with distilled water and drying. The re sistivity of the 3 � 1 cm-sized FTO after cleaning is 6–7 Ω/sq. 2.3. Covalent functionalization of MWCNTs by using acid treatment MWCNTs were functionalized by using the acid reflux method re ported elsewhere [25]. In the typical process, the commercial MWCNTs were refluxed in a mixture of H2SO4:HNO3 (3:1 volume ratio) at 100 � C for 3 h. Then, the content was cooled, centrifuged with ethanol and distilled water, and washed by using double distilled water to maintain its neutral pH. Further, it was dried at 70–80 � C in the vacuum oven and immediately dispersed in water using sonicator. The functionalized MWCNTs are abbreviated as a fMWCNTs in the rest of the paper. 2
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Fig. 1. Schematic representation of device fabrication flow.
2.4. Hydrothermal deposition of TiO2 films
(diameter: ~200 μm, optical microscope image is shown in the right corner of Fig. 1) metal shadow mask was kept on the active layer during deposition. In the Ar environment, the process pressure is about ~1.1 � 10 2 at 200 W was achieved and Al top electrode was deposited at optimized process time and conditions for all devices. Fig. 1 showing the schematic representation of device fabrication flow. Further, all elec trical measurements were recorded by using the semiconductor parameter analyzer (Keithley, 4200) and memristor characterization system (ArC ONE). In addition to this, the films were characterized by Xray diffractometer (Rigaku, Dmax 2500), Scanning electron microscope (SEM, HITACHI, S4800) facilitated with energy dispersive X-ray spec troscopic (EDS) analyzer), X-ray electron spectroscopy (XPS) using Al Kα as the X-ray source (Thermo Scientific, VG ESCALAB 220i) with an analysis angle of 45� , photoluminescence (HORIBA-Lab RAMHR), Raman (NSL–FN–532, LASER wavelength: 532 nm) and Fourier trans form infrared spectroscopy (FT-IR) (Thermo Fisher Scientific, NICOLET iS10) to know the crystalline structure, phase identification, morpho logical details, chemical composition and chemical states of the syn thesized material. Cross-sectional SEM images and ImageJ software determined the thickness of all active layers.
Vertically aligned TiO2 nanorod array and TiO2-fMWCNT were deposited by adopting the low-temperature hydrothermal method, as shown in Fig. 1. The hydrothermal method is mainly depending on the material growth temperature, the pressure created in an autoclave and the creation of growth centers on the substrate. In the typical deposition process, we have prepared a stoichiometric chemical bath by taking the 1:1 volume ratio of (HCl þ H2O) with 0.5 ml of SDS as a surfactant and stirred for 10 min. TTIP (0.5 ml) was added dropwise into the above solution during continuous stirring, upon addition of titanium precursor again the whole solution was stirred for 30 min. The HCl was used to take control over the hydrolysis of Ti precursor (TTIP) in the presence of water, whereas the associated reaction mechanism was explained in Fig. 1. Upon stirring, the obtained transparent reaction solution was transferred into the Teflon lined hydrothermal autoclave having 50 ml capacity and FTO was immersed vertically into the reaction bath. The inner Teflon liner was capped and placed inside stainless steel autoclave with firm tightening. The autoclave was kept inside a hot air oven at 160 � C for 3 h for the reaction. After cooling down to room temperature, the adherent TiO2 film was washed multiple times with deionized water to remove the unreacted impurities. The prepared sample was abbreviated as bare TiO2. The reaction is carried out by the heterogeneous nucleation mechanism. To enhance the device properties, fMWCNTs were used to make the TiO2-CNT composites by using a similar method. A similar recipe was carried out for the deposition of TiO2-CNT composite films, only 0.01, 0.03 and 0.05 wt % of fMWCNTs were sonicated into 3 ml of deionized water and added after titanium precursor addition steps into the above-mentioned process. The fMWCNTs-assisted samples were abbreviated as TC0.01, TC0.03, and TC0.05 (wt %) respectively. Finally, all films were annealed at 350 � C temperature for 1 h to get the desired phase of TiO2.
3. Results and discussion The phase identification and crystalline structure of the hydrother mally deposited TiO2 nanorods and TiO2-fMWCNTs composite films were matched and indexed to rutile TiO2 with tetragonal nanocrystalline structure. Fig. 2 represents the XRD pattern of pure TiO2 and TiO2fMWCNTs composite films. The obtained XRD data is well-matched with standard values of JCPDS card no. 01-089-8304, indicating TiO2 is formed in the rutile phase. Whereas, the slight variation in peak strength and the increase in average crystallite size were observed upon fMWCNTs addition. The peaks appeared at 2θ angle of 70.06, 76.36, and 79.85 in TC0.03 sample and 2θ angle of 70.08, 76.81, and 79.85 in TC0.05 sample were absent in bare TiO2 and TC 0.01 sample. The average crystallite size of TiO2, TC0.01, TC0.03, and TC0.05 was 6.14, 6.75, 6.43, 6.81 nm respectively, calculated by using the standard Debye-Scherrer formula. The most intense peaks with amplitude and position, full width half maximum (FWHM), microstrain density (ε) and dislocation density (δ) were calculated and summarized in Table 1. The results of Table 1 suggested that the crystallographic changes appeared in the TiO2 after the addition of fMWCNTs. The FWHM of the
2.5. Fabrication and characterizations of TiO2-fMWCNT memristive devices We have fabricated Al/TiO2/FTO film memristive device, and to enhance the device conduction properties, TiO2-fMWCNT composites were also deposited as an active layer. For the bottom electrode, FTO was used, whereas the Al top electrode (thickness: ~70 nm) was deposited by using the DC magnetron sputtering. The circular-shaped 3
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All SEM images showing the uniform coverage of TiO2 nanorods with a strong adherent and pinhole-free growth structure. The appearance of TiO2 nanorods was originated from the hydrolysis and condensation reaction mechanism of the prepared bath under the action of an opti mized growth temperature and growth centers created during the hy drothermal process [24]. It was observed that the thickness of the TiO2 films varies depending upon the concentrations of fMWCNTs in the TiO2, as shown in Fig. S1 (supplementary information). The bare TiO2 film shows higher thickness (3.42 μm) whereas, fMWCNTs based devices show a lower thickness. The device based on TC0.03 sample shows the lowest thickness (1.69 μm) among all devices. In addition to this, the energy-dispersive X-ray spectroscopy (EDS) profile of each sample is shown in Fig. S2 (supplementary information). The EDS profile results suggested that the absence of Al penetration through the nanorods, suggesting the active layer plays a vital role in the resistive switching than the top Al electrode. The diameter of the synthesized TiO2 nanorods was calculated by ImageJ software, and the distribution of each sample was shown in Fig. S3 (supplementary information). The average nano rods diameter was found to be 140.49 (bare TiO2), 233.44 (TC0.01), 187.47 (TC0.03), 252.71 nm (TC0.05), respectively. The average diameter of composite thin films was found to be higher than the bare TiO2 thin film. This may be due to the incorporation of the fMWCNTs in the TiO2. To reveal the incorporation of fMWCNTs in nanorods, higher magnification SEM imaging (Fig. S4a) was done which affirmed the uniformly distributed presence of fMWCNTs in TC0.03. EDS (Fig. S4b) and elemental area maps equally endorsed the successful incorporation of fMWCNTs owing to the well spread C signals (Fig. S4c) along with those for Ti and O. The pinched hysteresis is a fingerprint characteristic of resistive switching devices or more generally memristive devices [26]. In the present work, we have varied the active layer composition by making the composite of TiO2-fMWCNTs to alter the properties of bare TiO2. Fig. 4a is revealing the low-voltage induced resistive switching charac teristics of the Al/TiO2/FTO device and also the devices made with the different concentrations of fMWCNTs. The metal oxide-fMWCNTs composite leads to the increase in the conductivity, as evidenced by the I–V results. Interestingly, no forming voltage was applied to get the resistive switching in all developed devices. It indicates that the devices are operated on the forming-free principle. In the typical resistive memory device, an initial high forming bias needed to get the resistive switching. However, devices with higher forming voltage specification increase the complexity of memory architecture and the power budget of the system [27]. Therefore, devices with forming-free operations need to be fabricated to reduce system complexity. The obtained pinched hys teresis loops in the I–V plane are in accordance with memristive device theory. It is interesting to note that all the devices switch at the sym metric SET and RESET voltage (�1 V), also called bipolar resistive
Fig. 2. XRD patterns of (a) Bare TiO2 (b) TC0.01 (c) TC0.03 and (d) TC0.05 samples. All the diffraction peaks correspond to the rutile phase of TiO2.
composite thin films was found to be lower than bare TiO2. A similar kind of pattern was observed in the case of the microstrain and dislo cation density properties. Both these properties show lower values than bare TiO2. A minor change in the rutile TiO2 peaks with minimum changes into the FWHM values confirming the crystallinity of the sam ples was maintained after the addition of fMWCNTs. The lattice distor tions of composite films were found to be very small as compared to the bare TiO2, owing to the lower values of the microstrain and dislocation density. The surface morphology of films was analyzed with the help of SEM. Fig. 3 represents the surface morphologies of bare TiO2 and nano composite films fabricated with different concentrations of fMWCNTs.
Table 1 Peak positions with amplitude and position, FWHM, Microstrain, and dislocation density of TiO2 composite films. Sample code
Peak (hkl)
Position (2θ)
Amplitude (a.u.)
FWHM (Degree)
Microstrain Density(10 3) (line 2m 4)
Dislocation Density (nm 2)
Bare TiO2
110 101 111 211 110 101 111 211 110 101 111 211 110 101 111 211
27.49 36.18 41.17 54.37 27.33 36.02 41.1 54.37 27.33 36.02 41.26 54.21 27.33 36.02 41.26 54.22
587.2 1508.3 719.1 1093.1 532.5 1911.9 861.6 1160.3 535.1 1397.4 436.1 519.1 502.7 1641.5 543.28 640.42
0.02349
5.872
0.0265
0.02192
5.325
0.0219
0.02262
5.351
0.0241
0.02070
5.027
0.0215
TC0.01
TC0.03
TC0.05
4
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Fig. 3. SEM plane-view images of (a) Bare TiO2, (b) TC0.01, (c) TC0.03 and (d) TC0.05 composites.
switching, which has potential application in the resistive memory de vices. It is observed that the TC0.03 device shows suitable resistive switching property and excellent hysteresis area (82.30 μW). The lower switching voltage and current could result in low power consumption during the resistive switching process. In fact, higher switching voltage and current (>5 V and > few mA) are not advisable for low power and high-density memory application. Furthermore, biological synapse consumes very less energy (1–10 fJ) per events, therefore, resistive switching based synaptic devices should be operated on the biological energy scale [28]. In the present case, the optimized device (TC0.03) consumes 3.527 mW/μm2 and 0.495 mW/μm2 of power during the SET and RESET process, respectively. The uneven loops during positive and negative biases were observed for all devices, showing the self-rectifying characteristics. This kind of property is beneficial to develop selector-less memory devices [28]. In the crossbar type memory struc ture, the well-known sneak current problem is addressed by stacking the selector device layer on the resistive memory layer. However, the fabrication of a selector device with highly non-linear rectifying I–V characteristics and combine them with the memory layer is a very difficult task in the vertically integrated structure [29]. In view of this, a device with self-rectifying characteristic and intrinsic resistive switching property could lead to the small feature size, low power consumption and high storage density [30]. The rectifying behavior of CNT assisted devices may be due to the influence of oxygen vacancies near the Al/TiO2 interface during the resistive switching process and different work functions of the active layer and top Al contact [31]. At the device level, considerable perceptions of memristive switching have been acquired in recent years, largely due to the development of advanced characterization tools that can probe the processes and drives switching into the devices. However, it is also crucial to the techno logical development of any device is an understanding of the funda mental and underlying processes that govern its operation. The memristive devices are the extended class of memristor devices, which are based on the mathematical treatments proposed by Chua and Kang [8]. Theoretical prediction related to an extended class of memristor devices is helping to resolve the haziness in the naming of the device, which is either a memristor or memristive device. The theoretical de scriptions suggested that the two-terminal circuit element must show
passivity in the I–V plane and single-valued non-linear charge-magnetic flux characteristics [32]. To classify the developed devices, we have calculated the time domain flux, time-domain charge and charge-magnetic flux behavior of the developed devices, as shown in Fig. 4b, c, and d, respectively. The device charge and flux calculation details were systematically explained in the ref. [33]. This kind of property is entirely relying on the shape and size of the obtained hys teresis loop, resistive switching behavior, and nature of the symme tric/asymmetric set and reset voltages [32]. The memristive behavior can be used for more accurate characterization and modeling of resistive switching devices with rectifying effects and complex switching features [17]. The present results suggested that the TC0.03 device shows a higher charge magnitude than other devices. The nature of charge-magnetic flux characteristic of all devices appears as a double valued. The single-valued charge-magnetic flux characteristic is a pre requisite for a memristor device [5]; on the other hand, double valued charge-magnetic flux characteristic represents the memristive behavior. Because of this, all the developed devices in the present work emulating the properties of a memristive device. To confirm the obtained electrical analysis, the devices were char acterized by physio-chemical techniques. At the outset, Raman spectra of all the samples were recorded. The Raman spectroscopy is a powerful technique used for the superficial detection and confirmation of com posite nanostructures. Raman spectra of the bare TiO2, TiO2-fMWCNT composites, and fMWCNTs are shown in Fig. 5a and b. Raman spectrum of bare TiO2 and TiO2-fMWCNT composites shows the characteristic peaks at 448.03 and 611.05 cm 1 corresponding to the Eg and A1g modes of vibrations, respectively [34]. Furthermore, the characteristics peaks of D and G bands of carbon nanotubes are also obtained in all nanocomposite films. These peaks of Eg and A1g modes of vibrations also confirms the existence of the rutile phase TiO2, which is in good agreement with the obtained XRD phase indexing. Raman spectrum of fMWCNTs was also recorded separately, which shows two bands, namely, the D band and G band. The D band is an indicative disorder in the graphitic structure at 1355.02 cm 1, whereas the G band (charac teristic ordered graphitic structure) at 1600.01 cm 1 corresponds to ordered sp2 hybridization of MWCNTs. The intensity ratio (ID/IG ¼ 0.99) for fMWCNT is revealing the presence of acidic functional moieties on 5
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Fig. 4. (a) I–V characteristics of bare TiO2 and TiO2-fMWCNT active layered film memristive devices. (b) Time-domain magnetic flux, (c) Time-domain charge, and (d) Charge-magnetic flux behavior of all memristive devices. (arrows representing the HRS to LRS and LRS to HRS transition). The arrows indicate the resistive switching direction.
the surface of fMWCNTs with the conversion of the carbon atoms hy bridization, further evidenced in core-level spectra of C1s. To further confirm the activation of MWCNTs, the FT-IR spectra were also recorded separately and shown in Fig. 5c. In the FT-IR spectrum of the covalent functionalized MWCNTs, peaks observed at 1726 cm 1, which confirms the successful functionalization of oxygen moieties [35]. The –OH stretching of adsorbed H2O and the surface hydroxyl group in fMWCNTs is observed at 3400-3500 cm 1 [35]. Raman spectrum of all TiO2-fMWCNTs composite films showing the characteristic peaks of rutile TiO2 along with D (1354.9 cm 1) and G (1568.32 cm 1) bands of MWCNTs [34], which categorically revealed the existence of MWCNTs inside the prepared nanocomposites. However, the characteristic peak of the FTO glass substrate has also appeared at approximately 230 cm 1 during the detection of samples. The slight decrease in the peak in tensity, with a small shifting in the peak positions of all Raman bands for nanocomposite, has been observed. It is due to the increase in the crystallite size and intrinsic nature of rutile TiO2 which gives favor to accommodate the carbon nanostructures into their host lattice [36]. Further, it is seen that hydrothermal growth of TiO2 and TiO2-fMWCNTs composite film followed by thermal annealing of all devices shows improved resistive switching characteristics due to a large number of oxygen vacancies and superior crystallinity of the composite film. The improved resistive switching is related to the increased oxygen va cancies which have also been evidenced by obtained photo-luminescence (PL) spectra. Fig. 6a shows the measured PL spectra of TiO2 and TiO2-fMWCNTs film by using the photoexcitation
wavelength of 365 nm; where the intensity of the near band edge (NBE) emission and deep-level emission (DLE) peak is increased in TiO2-fMWCNTs composite film compared to pure TiO2 film. From the graph, it is seen that the TC 0.03 device shows the highest PL intensity. The increase of the DLE peak is due to the large number of fMWCNTs were accommodated into the network of TiO2 nanorods and hence efficiently increase the number of free oxygen vacancies that can work as vacancy reservoir during the switching. The emission around 417 and 488 nm is correlated to the surface oxygen vacancies, which is in good arrangement with the absorption spectrum. The charge state of the va cancies can drop one or two electrons to the valence band and the depletion region [37,38], which can encourage considerable variation in the bandgap which leads to increased electronic transport. Furthermore, the PL spectra demonstrate that the composite films have quite effi ciently created the vacancy pool and improved the crystalline structure of the film which consequently improves the device performance. Moreover, in order to know the different chemical states of the sur face elements the optimized nanocomposite thin film sample was analyzed by XPS. Fig. 6b, representing the full survey spectrum of TC0.03, in which all the peaks of all elements appear which are present into the nanocomposite, namely Ti, O and C, which has been properly detected at their characteristics binding energy levels. Furthermore, high-resolution core-level spectra of (a) Ti2p, (b) O1s, and (c) C1s ele ments have been represented into Fig. S5 (supplementary information). The Ti2p spectra consist of two peaks at 458.73eV and 464.44eV which are corresponding to Ti2p1/2 and Ti2p3/2 states respectively. The 6
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Fig. 5. (a) Raman spectra of bare TiO2 and TiO2-fMWCNT composite films. (b) Raman spectra of pure fMWCNTs showing the confirmation of fMWCNTs by showing its ideal ID/IG ratio. (c) FT-IR Spectra of fMWCNTs.
Fig. 6. (a) PL spectra of bare TiO2 and TiO2-fMWCNT composite films, where the intensity of the NBE emission and DLE peak increased for TiO2-fMWCNT composite film compared to pure TiO2 film. (b) XPS survey, (c) Ti 2p, and (d) O 1s core-level spectra of TC0.03 composite.
difference in binding energy (5.71eV), attributable to the presence of Ti4þ state in an octahedral arrangement of rutile TiO2. Furthermore, the high-resolution core-level spectra of O 1s showing a significant peak at ~530.61eV is showing the presence of lattice oxygen bonded with Ti species in Ti–O–Ti manner; further deconvolution of peaks is showing the broader peak at 532.04eV which are identical to the O–C bonding in TiO2-fMWCNTs nanocomposite [38]. Whereas in contrast to C1s spectra, – C bonds of the intense peak at 284.47 which is of graphitic C– fMWCNTs, whereas former peaks of C–O also appeared at 285.27 owed to the covalent functionalization of carbon nanotubes results in chemical defect and instinctive surface groups. However, there is an absence of any other peaks ~ at 281eV, confirming that, there is no doping of carbon elemental species [39], in nanocomposites that are dominantly present in fMWCNTs. In the present case, TC0.03 composite device shows the enhanced I–V characteristic due to the maximum number of oxygen vacancies present in the device. From the electrical behavior of the device, it is also observed that the top electrode and active layer interface also playing a
crucial role in the resistive switching process for both bare and fMWCNTs composite devices. The different amount of oxygen vacancies has generated near the Al/TiO2 interfaces during the switching, which results in the good resistive switching property. Further, the resistive switching can be modified due to various micro/nanostructure and modulated energy barrier at the Al/TiO2-fMWCNT interface. Therefore, the current is gradually increased or decreased according to the polarity of an external stimulus. The results suggested that fMWCNTs composite devices generate higher oxygen vacancies. The presence of a higher concentration of oxygen vacancies leading to a decrease in energy bar rier at the Al/TiO2 interface; therefore, a higher magnitude of the cur rent is observed for fMWCNTs composite devices. Furthermore, fMWCNTs improves the conductivity of the metal oxide. The inclusive effect resulted in the better electrical characteristics of TiO2-fMWCNT memristive devices. This kind of strategy of well-aligned growth of nanorods with increased oxygen vacancies gives rise to the non-linear electrical properties. It could presumably help to implement such a de vice in the fabrication of 1 transistor-1 resistor structured devices, which 7
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Fig. 7. (a) Endurance and (b) retention characteristics of TC0.03 memristive device.
could be a new approach to tackle the sneak path issue of resistive switching random access memory in the crossbar arrays structure [24]. The non-volatile memory effect of the developed device was evaluated by measuring the endurance and retention characteristics. In order to evaluate the device reliability and non-volatile memory properties, TC0.03 composite device was characterized with repetitive ON/OFF sweeping operations. Obtained by repetitive ON/OFF sweeping opera tion, the endurance cycling test results are shown in Fig. 7a. The re sistances of the low resistance state (LRS) (ROFF) and high resistance state (HRS) (RON) are measured at the read voltage of 0.25 V and write voltage of � 1 V. The ROFF/RON ratio of the present case was 9.02, which is sufficient for efficient memory applications. The resistive switching in TC0.03 composite devices is reproducible with cyclic tests of sequential 103 cycles. The retention property of memory was conducted for an extended period to evaluate the data storage capability and stability in resistive states, as shown in Fig. 7b. The TC0.03 composite device shows good retention characteristics over 103 s without degradation in resis tance states (LRS and HRS). The statistical analysis of endurance data suggested that the LRS and HRS have mean resistance equals to 10.20 kΩ and 92.07 kΩ, respectively. The standard deviation of LRS and HRS is
1.38 kΩ and 3.49 Ω, respectively. Furthermore, statistical data sug gested that the LRS (13.52%) has a higher coefficient of variation than HRS (3.79%). The improvement in the non-volatile memory properties of the device can be engineered through the development of the exact vertical growth of nanorod array which will strongly take control over the ionic and electronic transports by reducing the local temperature and joule heating processes present into the device. Until now, the limitation on cyclic performance is still inadequate due to the lack of enough vacancy migration to switch the device from HRS to LRS. In the case of chemical approach, the vacancy migration can be expected to be improved by tuning the various parameters such as extent growth of nanorods, the stoichiometric ratio of metal interstitials and oxygen ions, and by obtaining the desired crystalline size. In order to investigate the conduction mechanism of the TC0.03 composite device, the I–V characteristics were plotted in a log-log scale, as shown in Fig. 8a and b. Additionally, slopes of low, high, and entire voltage regions were calculated and provided in Fig. 8a and b. It is immensely important to understand the conduction mechanism of the HRS than the LRS, because, in most of the cases, LRS follows Ohmic conduction mechanism. In the present case, low and high voltage
Fig. 8. (a) Positive and (b) negative bias Log-Log I–V characteristics of TC0.03 memristive device. The Ohmic conduction mechanism fitting results of (c) positive and (d) negative bias I–V data (entire voltage region or LRS). Schottky conduction mechanism fitting results to (e and g) low and (f and h) high voltage region I–V data (positive and negative bias HRS data). 8
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regions represent the HRS whereas, LRS is represented in terms of the entire voltage region. Furthermore, the experimental I–V data is fitted with the best model available in the literature, as shown in Fig. 8c–h. It is observed that the low and high voltage region I–V data is well fitted with the Schottky conduction model with good-adjusted R2 fitting values. In addition to this, the entire voltage region is well fitted to the Ohmic conduction model. Considering the fitting results, it seems that the conduction of the device is due to the combined effect of Schottky and Ohmic conduction mechanisms. In particular, it is seen that after the voltage stressing to the developed devices, the energy barrier lowering could take place at the Al/TiO2 interface. At a sufficient voltage, elec trons from the top electrode move into the conduction band of TiO2. Furthermore, existing traps present in the TiO2 could be filled by the electrons at a sufficient voltage. These filled traps and oxygen ions/va cancies give rise to the current in the device under the influence of external electrical stimulus.
Acknowledgments This work was supported by the Human Resources Development Program (No. 20174030201830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104907. References [1] H.A.D. Nguyen, J. Yu, L. Xie, M. Taouil, S. Hamdioui, D. Fey, Memristive devices for computing: beyond CMOS and beyond von Neumann, IEEE/IFIP Int. Conf. VLSI Syst. VLSI-Soc. 1 (2017), https://doi.org/10.1109/VLSI-SoC.2017.8203479. [2] K.H. Kim, S. Gaba, D. Wheeler, J.M. Cruz-Albrecht, T. Hussain, N. Srinivasa, W. Lu, A functional hybrid memristor crossbar-array/CMOS system for data storage and neuromorphic applications, Nano Lett. 12 (2012) 389–395, https://doi.org/ 10.1021/nl203687n. [3] S.H. Jo, T. Chang, I. Ebong, B.B. Bhadviya, P. Mazumder, W. 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4. Conclusions Oxygen vacancy induced resistive switching performance of TiO2fMWCNTS composite films were successfully fabricated on the FTO substrate through the low-cost hydrothermal method. The films were mainly composed of rutile TiO2 pinhole-free nanorods with enhanced surface charged oxygen vacancies. The Raman and photoluminescence studies are demonstrating the switching behavior is enhanced due to the presence of fMWCNTs and oxygen vacancies compare to the bare device. Moreover, the nanostructure, surface energy, and Al/TiO2 interface are also the main factors influencing the device properties. The forming free bipolar resistive switching with a memristive hysteresis loop is observed in all devices at lower operating voltage. Furthermore, 0.03 wt% fMWCNTs TiO2 composite device showing the self-rectifying property with excellent hysteresis area (82.30 μW), showing its another high storage density memory device related potential. The 1D surface struc ture of nanocomposites, chemical composition and surface vacancy reservoir consequently leading to the large hysteresis loops. The device charge-flux behavior shows the double-valued relation, which matched with the definition of memristive devices. The non-volatility of the de vice resuming the developed device can switch up to 103 cycles and can retain the data over 103 s. The charge transport behavior is in good agreement with the Ohmic and Schottky conduction mechanisms. The enhanced transport properties in such devices can be obtained through the simple fabrication method of such composites, whereas the further improvement can be pulled out by making the devices with modulated ratio of metal interstitials and oxygen vacancies and well-aligned growth of meticulous 1D nanostructure which would be the new strat egy to fabricate low-cost nonvolatile memory devices for the future electronic prototypes and systems. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Navaj Mullani: Conceptualization, Methodology, Writing - original draft, Software. Ijaz Ali: Investigation, Methodology. Tukaram D. Dongale: Visualization, Formal analysis. Gun Hwan Kim: Validation. Byung Joon Choi: Formal analysis, Writing - review & editing. Muhammad Abdul Basit: Writing - review & editing, Validation. Tae Joo Park: Supervision, Project administration, Funding acquisition, Resources.
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Improved resistive switching behavior of multiwalled carbon nanotube/ TiO2 nanorods composite film by increased oxygen vacancy reservoir Navaj Mullani a, Ijaz Ali a, Tukaram D. Dongale b, *, Gun Hwan Kim c, Byung Joon Choi d, Muhammad Abdul Basit a, Tae Joo Park a, ** a
Department of Materials Science & Chemical Engineering, Hanyang University, Ansan, 15588, South Korea Computational Electronics and Nanoscience Research Laboratory, School of Nanoscience and Biotechnology, Shivaji University, Kolhapur, 416004, India Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon, 34114, South Korea d Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul, 01811, South Korea b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Memristive device MWCNTs TiO2 Nanocomposites Resistive switching memory
The non-linear nature in the current-voltage relationship and good resistive switching characteristics were demonstrated with the help of TiO2 nanorods-functionalized multiwalled carbon nanotube (fMWCNT) composite grown by the low-cost hydrothermal method. The composites were characterized by X-ray diffraction, scanning electron microscopy, Raman, photoluminescence, and X-ray photoelectron spectroscopy to investigate the structural, morphological, and chemical composition of composite films. The resistive switching characteristics of the TiO2-fMWCNT nanocomposites were found to be strongly dependent on the fMWCNT concentration. The enhanced switching performance is associated with the surface nanostructure and chemical composition of the nanocomposites. Owing to the hierarchical rutile TiO2 nanorods and opportune fMWCNT content, the nano composite based device with 0.03 wt % fMWCNT exhibited the best resistive switching performance with good endurance and retention non-volatile memory properties. Interestingly, with the optimized stoichiometric composition and operation conditions, forming-free, low operational voltage, self-rectifying like properties have been simultaneously achieved, which are some of the prerequisites for next-generation memory devices. In addition to this, the double-valued charge-magnetic flux nature of the developed devices was demonstrated. The experimental current-voltage characteristics are well-matched with the Ohmic and Schottky conduction mechanisms.
1. Introduction The von Neumann architecture comprises a design having a central processing unit and a memory as separate units. These two units are connected by a single channel that carries instructions, data from memory to processor, which creates as von Neumann bottleneck [1]. However, the modern computing systems have become more complexed therefore, von Neumann bottleneck produces a significant effect on data processing. Because of this, real parallel processing is not achieved by von Neumann architecture and is unable to perform memory and logic operation in a single unit [2]. Furthermore, conventional computers work on a digital system by using a binary number system but the human memory, cognition, learning and logic operations are carried out by means of analog signal processing [3]. The binary system only uses two
numbers 0 and 1, that is, true or false for information processing. However, the conventional computing architectures facing challenges, including the heat wall, the memory wall, and it leads to the end of Moore’s law. Given this, it is predicted that the conventional circuit technology and components are inappropriate and unsuitable for future computing paradigms. New kind of techniques such as liquid cybernetic systems could be a better solution for the next generation zettascale computing applications [4]. Eventually, limitations of conventional computers and system were observed, which drew the attention of re searchers towards a system which has the potential to overcome these problems and execute the system operation systematically. The device having the ability to co-locate memory and compute in the same phys ical device is the next big thing in the computing and mem ristor/memristive is considered to be a potential candidate for the
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.D. Dongale), [email protected] (T.J. Park). https://doi.org/10.1016/j.mssp.2019.104907 Received 9 November 2019; Received in revised form 20 December 2019; Accepted 23 December 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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in-memory and neuromorphic computing applications. Developments in memristor technology may provide an alternative path that enables the fabrication of hybrid non-volatile memory systems and three dimensional (3D) crossbar arrays. Due to its long retention time, consistency over large endurance cycles and non-volatile switch ing, proved its potential to be used in memory and logic devices [5]. In recent years, researchers are trying to find out new ways to replace the conventional von Neumann architecture and in this scenario, the memristor has been cropping up in many devices and circuitry. The memristor has been utilized to catalyze the memory devices owing to its unique structural, chemical, electrical properties, since after its first experimental realization at Hewlett-Packard laboratories in 2008 based on TiO2 [6]. Furthermore, memristive devices have been also used in the neuromorphic systems and logic circuits [7]. Before the discovery of practical memristor/memristive device, Prof. Leon Chua has predicted and put forward a fourth fundamental circuit element, based on the nonlinear circuit theory and the firm mathematical foundation [8]. Furthermore, the extended class of memristor has been postulated by Chua and Kang, called as memristive devices family in 1976 based on the concrete mathematical formulations [9]. In recent years, different materials and deposition techniques have been explored by many re searchers for the development of memristors or memristive devices. In particular, chalcogenides, polymers, metal oxides, ferrites and recently the complementary metal-oxide-semiconductor (CMOS) compatible materials like SiN and SiO2 have been utilized to develop the active layer of memristor/memristive devices [10–12]. Among them, the polymers and chalcogenides have been limited due to high-temperature melting and phase transformation issues [13] which could be delayed their utility in future logic and memory devices; whereas the CMOS compatible materials demand the high-cost physical deposition tech niques with highly equipped and controlled environment. Due to these reasons, low-cost chemical synthesis techniques has been showing its potential to fabricate the memory devices and its application [14]. The metal oxide/polymer nanocomposites gaining attention to fabricating flexible resistive switching devices due to their uniform nanocomposites formation ability and room temperature fabrication process. Recently, Chiolerio et al. have provided an in-depth fabrication process of ZnO/ polymer/colloidal suspensions based devices, which featuring the low power resistive switching operations [15]. The metal oxide memristive devices have intrinsic and favorable properties towards memristive behavior. The active layer deposition of metal oxides like TiO2, ZnO, HfO2, SnO2, WO3, NiO [16–21], etc. are CMOS compatible materials, making them suitable candidates to develop high-performance resistive switching memory and electronic synaptic devices. The hydrothermal technique has been utilized to grow the hierar chical nanostructures of different dimensions as well as nanomaterials with different and desired shapes and sizes in low-cost processing. In addition to this, 1D memristor or memristive devices fabricated via hydrothermal technique has been attracted great attention due to its excellent charge carrier transportation through straight conducting paths which may behave differently than branched conducting paths based devices and could be applicable for high-density memory storage application due to its superior electrical characteristics at low power consumption [21]. Moreover, the low powered memristor/memristive devices can be used to couple with the living neurons, thereby the end system can emulate the neuromorphic properties with fewer energy consumptions similar to the natural neurobiological process [22]. However, at the microscopic level, the energy required for vacancy formation and diffusion or creation of a conducting path is critical to determining numerous key device properties like forming voltage, switching voltage, endurance, retention, etc. The conventional resistive memory devices needed an additional electroforming step and selector devices for the proper operation. The former issue increases the power consumption while the later issue increases the complexity in the crossbar array configuration [16]. The electroforming-free resistive memory devices could be a possible solution for the low power memory
devices while selector-less resistive memory devices could be developed by engineering the self-rectification property. In view of this, the development of a resistive switching cell that simultaneously shows electroforming free operation and self-rectification is a demanding task of the hour. In this work, we have adopted a hydrothermal technique owing to its ability to grow desired nanostructures of the TiO2 with tunable stoichiometry and possible to make easy metal oxide-carbon composite films with enhanced resistive switching properties. There are many reports available in the literature for the development of TiO2 thin film memristive devices by using the different precursors and methods [23,24]. However, there are still many ways to incorporate the carbon nanostructures into the lattice structures of the metal oxide to improve the device quality and electrical behavior. In order to influence the desired device properties, the TiO2-functionalized multiwalled car bon nanotube (fMWCNT) composite films were synthesized with mod ifications in deposition techniques and with different ratios of MWCNTs. The electrical and compositional characterizations were also studied in detail to confirm the developed devices can be potentially applicable for the resistive switching memory devices. The criteria for the identifica tion of the extended class of memristor device i.e. memristive device, is also illustrated with the help of double valued charge-magnetic flux characteristics. The device conduction mechanisms are also obtained by fitting the experimental data to the Ohmic and Schottky conduction models. 2. Experimental section 2.1. Materials The pristine commercial MWCNTs having a diameter of 15 nm were purchased from Excellent Mecha Power company (South Korea), Tita nium Isopropoxide (TTIP, C12H28O4Ti, 98%) were purchased from Sigma Aldrich, which was used as a Ti precursor. In order to maintain the acidic condition of a reaction solution, concentrated hydrochloric acid (HCl, extra pure grade (37%) Daejung chemicals, South Korea) was used. Sodium dodecyl sulfate was used as a surfactant during the deposition of TiO2 and TiO2-MWCNT composite films, which act as a dispersant in the followed deposition process. However, the solvents and reagents like acetone, ethanol, and Isopropyl alcohol (IPA, Daejung Chemicals) were used to clean the glassware and substrates. All commercially available chemicals were used without any additional modifications during the synthesis. 2.2. Ultrasonic cleaning treatment of fluorine-doped tin oxide (FTO) substrate We have used fluorine-doped tin oxide (FTO) (~10 Ω/sq.) as a bot tom electrode in this work as shown in Fig. 1. The adopted ultra-sonic cleaning treatment is as follows: Initially, the FTO substrate was cleaned with double distilled water and acetone (1:1) and then kept inclined into the wall of the beaker vertically in the ultrasonic bath for 30 min, followed by washing with distilled water and drying. The re sistivity of the 3 � 1 cm-sized FTO after cleaning is 6–7 Ω/sq. 2.3. Covalent functionalization of MWCNTs by using acid treatment MWCNTs were functionalized by using the acid reflux method re ported elsewhere [25]. In the typical process, the commercial MWCNTs were refluxed in a mixture of H2SO4:HNO3 (3:1 volume ratio) at 100 � C for 3 h. Then, the content was cooled, centrifuged with ethanol and distilled water, and washed by using double distilled water to maintain its neutral pH. Further, it was dried at 70–80 � C in the vacuum oven and immediately dispersed in water using sonicator. The functionalized MWCNTs are abbreviated as a fMWCNTs in the rest of the paper. 2
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Fig. 1. Schematic representation of device fabrication flow.
2.4. Hydrothermal deposition of TiO2 films
(diameter: ~200 μm, optical microscope image is shown in the right corner of Fig. 1) metal shadow mask was kept on the active layer during deposition. In the Ar environment, the process pressure is about ~1.1 � 10 2 at 200 W was achieved and Al top electrode was deposited at optimized process time and conditions for all devices. Fig. 1 showing the schematic representation of device fabrication flow. Further, all elec trical measurements were recorded by using the semiconductor parameter analyzer (Keithley, 4200) and memristor characterization system (ArC ONE). In addition to this, the films were characterized by Xray diffractometer (Rigaku, Dmax 2500), Scanning electron microscope (SEM, HITACHI, S4800) facilitated with energy dispersive X-ray spec troscopic (EDS) analyzer), X-ray electron spectroscopy (XPS) using Al Kα as the X-ray source (Thermo Scientific, VG ESCALAB 220i) with an analysis angle of 45� , photoluminescence (HORIBA-Lab RAMHR), Raman (NSL–FN–532, LASER wavelength: 532 nm) and Fourier trans form infrared spectroscopy (FT-IR) (Thermo Fisher Scientific, NICOLET iS10) to know the crystalline structure, phase identification, morpho logical details, chemical composition and chemical states of the syn thesized material. Cross-sectional SEM images and ImageJ software determined the thickness of all active layers.
Vertically aligned TiO2 nanorod array and TiO2-fMWCNT were deposited by adopting the low-temperature hydrothermal method, as shown in Fig. 1. The hydrothermal method is mainly depending on the material growth temperature, the pressure created in an autoclave and the creation of growth centers on the substrate. In the typical deposition process, we have prepared a stoichiometric chemical bath by taking the 1:1 volume ratio of (HCl þ H2O) with 0.5 ml of SDS as a surfactant and stirred for 10 min. TTIP (0.5 ml) was added dropwise into the above solution during continuous stirring, upon addition of titanium precursor again the whole solution was stirred for 30 min. The HCl was used to take control over the hydrolysis of Ti precursor (TTIP) in the presence of water, whereas the associated reaction mechanism was explained in Fig. 1. Upon stirring, the obtained transparent reaction solution was transferred into the Teflon lined hydrothermal autoclave having 50 ml capacity and FTO was immersed vertically into the reaction bath. The inner Teflon liner was capped and placed inside stainless steel autoclave with firm tightening. The autoclave was kept inside a hot air oven at 160 � C for 3 h for the reaction. After cooling down to room temperature, the adherent TiO2 film was washed multiple times with deionized water to remove the unreacted impurities. The prepared sample was abbreviated as bare TiO2. The reaction is carried out by the heterogeneous nucleation mechanism. To enhance the device properties, fMWCNTs were used to make the TiO2-CNT composites by using a similar method. A similar recipe was carried out for the deposition of TiO2-CNT composite films, only 0.01, 0.03 and 0.05 wt % of fMWCNTs were sonicated into 3 ml of deionized water and added after titanium precursor addition steps into the above-mentioned process. The fMWCNTs-assisted samples were abbreviated as TC0.01, TC0.03, and TC0.05 (wt %) respectively. Finally, all films were annealed at 350 � C temperature for 1 h to get the desired phase of TiO2.
3. Results and discussion The phase identification and crystalline structure of the hydrother mally deposited TiO2 nanorods and TiO2-fMWCNTs composite films were matched and indexed to rutile TiO2 with tetragonal nanocrystalline structure. Fig. 2 represents the XRD pattern of pure TiO2 and TiO2fMWCNTs composite films. The obtained XRD data is well-matched with standard values of JCPDS card no. 01-089-8304, indicating TiO2 is formed in the rutile phase. Whereas, the slight variation in peak strength and the increase in average crystallite size were observed upon fMWCNTs addition. The peaks appeared at 2θ angle of 70.06, 76.36, and 79.85 in TC0.03 sample and 2θ angle of 70.08, 76.81, and 79.85 in TC0.05 sample were absent in bare TiO2 and TC 0.01 sample. The average crystallite size of TiO2, TC0.01, TC0.03, and TC0.05 was 6.14, 6.75, 6.43, 6.81 nm respectively, calculated by using the standard Debye-Scherrer formula. The most intense peaks with amplitude and position, full width half maximum (FWHM), microstrain density (ε) and dislocation density (δ) were calculated and summarized in Table 1. The results of Table 1 suggested that the crystallographic changes appeared in the TiO2 after the addition of fMWCNTs. The FWHM of the
2.5. Fabrication and characterizations of TiO2-fMWCNT memristive devices We have fabricated Al/TiO2/FTO film memristive device, and to enhance the device conduction properties, TiO2-fMWCNT composites were also deposited as an active layer. For the bottom electrode, FTO was used, whereas the Al top electrode (thickness: ~70 nm) was deposited by using the DC magnetron sputtering. The circular-shaped 3
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All SEM images showing the uniform coverage of TiO2 nanorods with a strong adherent and pinhole-free growth structure. The appearance of TiO2 nanorods was originated from the hydrolysis and condensation reaction mechanism of the prepared bath under the action of an opti mized growth temperature and growth centers created during the hy drothermal process [24]. It was observed that the thickness of the TiO2 films varies depending upon the concentrations of fMWCNTs in the TiO2, as shown in Fig. S1 (supplementary information). The bare TiO2 film shows higher thickness (3.42 μm) whereas, fMWCNTs based devices show a lower thickness. The device based on TC0.03 sample shows the lowest thickness (1.69 μm) among all devices. In addition to this, the energy-dispersive X-ray spectroscopy (EDS) profile of each sample is shown in Fig. S2 (supplementary information). The EDS profile results suggested that the absence of Al penetration through the nanorods, suggesting the active layer plays a vital role in the resistive switching than the top Al electrode. The diameter of the synthesized TiO2 nanorods was calculated by ImageJ software, and the distribution of each sample was shown in Fig. S3 (supplementary information). The average nano rods diameter was found to be 140.49 (bare TiO2), 233.44 (TC0.01), 187.47 (TC0.03), 252.71 nm (TC0.05), respectively. The average diameter of composite thin films was found to be higher than the bare TiO2 thin film. This may be due to the incorporation of the fMWCNTs in the TiO2. To reveal the incorporation of fMWCNTs in nanorods, higher magnification SEM imaging (Fig. S4a) was done which affirmed the uniformly distributed presence of fMWCNTs in TC0.03. EDS (Fig. S4b) and elemental area maps equally endorsed the successful incorporation of fMWCNTs owing to the well spread C signals (Fig. S4c) along with those for Ti and O. The pinched hysteresis is a fingerprint characteristic of resistive switching devices or more generally memristive devices [26]. In the present work, we have varied the active layer composition by making the composite of TiO2-fMWCNTs to alter the properties of bare TiO2. Fig. 4a is revealing the low-voltage induced resistive switching charac teristics of the Al/TiO2/FTO device and also the devices made with the different concentrations of fMWCNTs. The metal oxide-fMWCNTs composite leads to the increase in the conductivity, as evidenced by the I–V results. Interestingly, no forming voltage was applied to get the resistive switching in all developed devices. It indicates that the devices are operated on the forming-free principle. In the typical resistive memory device, an initial high forming bias needed to get the resistive switching. However, devices with higher forming voltage specification increase the complexity of memory architecture and the power budget of the system [27]. Therefore, devices with forming-free operations need to be fabricated to reduce system complexity. The obtained pinched hys teresis loops in the I–V plane are in accordance with memristive device theory. It is interesting to note that all the devices switch at the sym metric SET and RESET voltage (�1 V), also called bipolar resistive
Fig. 2. XRD patterns of (a) Bare TiO2 (b) TC0.01 (c) TC0.03 and (d) TC0.05 samples. All the diffraction peaks correspond to the rutile phase of TiO2.
composite thin films was found to be lower than bare TiO2. A similar kind of pattern was observed in the case of the microstrain and dislo cation density properties. Both these properties show lower values than bare TiO2. A minor change in the rutile TiO2 peaks with minimum changes into the FWHM values confirming the crystallinity of the sam ples was maintained after the addition of fMWCNTs. The lattice distor tions of composite films were found to be very small as compared to the bare TiO2, owing to the lower values of the microstrain and dislocation density. The surface morphology of films was analyzed with the help of SEM. Fig. 3 represents the surface morphologies of bare TiO2 and nano composite films fabricated with different concentrations of fMWCNTs.
Table 1 Peak positions with amplitude and position, FWHM, Microstrain, and dislocation density of TiO2 composite films. Sample code
Peak (hkl)
Position (2θ)
Amplitude (a.u.)
FWHM (Degree)
Microstrain Density(10 3) (line 2m 4)
Dislocation Density (nm 2)
Bare TiO2
110 101 111 211 110 101 111 211 110 101 111 211 110 101 111 211
27.49 36.18 41.17 54.37 27.33 36.02 41.1 54.37 27.33 36.02 41.26 54.21 27.33 36.02 41.26 54.22
587.2 1508.3 719.1 1093.1 532.5 1911.9 861.6 1160.3 535.1 1397.4 436.1 519.1 502.7 1641.5 543.28 640.42
0.02349
5.872
0.0265
0.02192
5.325
0.0219
0.02262
5.351
0.0241
0.02070
5.027
0.0215
TC0.01
TC0.03
TC0.05
4
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Fig. 3. SEM plane-view images of (a) Bare TiO2, (b) TC0.01, (c) TC0.03 and (d) TC0.05 composites.
switching, which has potential application in the resistive memory de vices. It is observed that the TC0.03 device shows suitable resistive switching property and excellent hysteresis area (82.30 μW). The lower switching voltage and current could result in low power consumption during the resistive switching process. In fact, higher switching voltage and current (>5 V and > few mA) are not advisable for low power and high-density memory application. Furthermore, biological synapse consumes very less energy (1–10 fJ) per events, therefore, resistive switching based synaptic devices should be operated on the biological energy scale [28]. In the present case, the optimized device (TC0.03) consumes 3.527 mW/μm2 and 0.495 mW/μm2 of power during the SET and RESET process, respectively. The uneven loops during positive and negative biases were observed for all devices, showing the self-rectifying characteristics. This kind of property is beneficial to develop selector-less memory devices [28]. In the crossbar type memory struc ture, the well-known sneak current problem is addressed by stacking the selector device layer on the resistive memory layer. However, the fabrication of a selector device with highly non-linear rectifying I–V characteristics and combine them with the memory layer is a very difficult task in the vertically integrated structure [29]. In view of this, a device with self-rectifying characteristic and intrinsic resistive switching property could lead to the small feature size, low power consumption and high storage density [30]. The rectifying behavior of CNT assisted devices may be due to the influence of oxygen vacancies near the Al/TiO2 interface during the resistive switching process and different work functions of the active layer and top Al contact [31]. At the device level, considerable perceptions of memristive switching have been acquired in recent years, largely due to the development of advanced characterization tools that can probe the processes and drives switching into the devices. However, it is also crucial to the techno logical development of any device is an understanding of the funda mental and underlying processes that govern its operation. The memristive devices are the extended class of memristor devices, which are based on the mathematical treatments proposed by Chua and Kang [8]. Theoretical prediction related to an extended class of memristor devices is helping to resolve the haziness in the naming of the device, which is either a memristor or memristive device. The theoretical de scriptions suggested that the two-terminal circuit element must show
passivity in the I–V plane and single-valued non-linear charge-magnetic flux characteristics [32]. To classify the developed devices, we have calculated the time domain flux, time-domain charge and charge-magnetic flux behavior of the developed devices, as shown in Fig. 4b, c, and d, respectively. The device charge and flux calculation details were systematically explained in the ref. [33]. This kind of property is entirely relying on the shape and size of the obtained hys teresis loop, resistive switching behavior, and nature of the symme tric/asymmetric set and reset voltages [32]. The memristive behavior can be used for more accurate characterization and modeling of resistive switching devices with rectifying effects and complex switching features [17]. The present results suggested that the TC0.03 device shows a higher charge magnitude than other devices. The nature of charge-magnetic flux characteristic of all devices appears as a double valued. The single-valued charge-magnetic flux characteristic is a pre requisite for a memristor device [5]; on the other hand, double valued charge-magnetic flux characteristic represents the memristive behavior. Because of this, all the developed devices in the present work emulating the properties of a memristive device. To confirm the obtained electrical analysis, the devices were char acterized by physio-chemical techniques. At the outset, Raman spectra of all the samples were recorded. The Raman spectroscopy is a powerful technique used for the superficial detection and confirmation of com posite nanostructures. Raman spectra of the bare TiO2, TiO2-fMWCNT composites, and fMWCNTs are shown in Fig. 5a and b. Raman spectrum of bare TiO2 and TiO2-fMWCNT composites shows the characteristic peaks at 448.03 and 611.05 cm 1 corresponding to the Eg and A1g modes of vibrations, respectively [34]. Furthermore, the characteristics peaks of D and G bands of carbon nanotubes are also obtained in all nanocomposite films. These peaks of Eg and A1g modes of vibrations also confirms the existence of the rutile phase TiO2, which is in good agreement with the obtained XRD phase indexing. Raman spectrum of fMWCNTs was also recorded separately, which shows two bands, namely, the D band and G band. The D band is an indicative disorder in the graphitic structure at 1355.02 cm 1, whereas the G band (charac teristic ordered graphitic structure) at 1600.01 cm 1 corresponds to ordered sp2 hybridization of MWCNTs. The intensity ratio (ID/IG ¼ 0.99) for fMWCNT is revealing the presence of acidic functional moieties on 5
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Fig. 4. (a) I–V characteristics of bare TiO2 and TiO2-fMWCNT active layered film memristive devices. (b) Time-domain magnetic flux, (c) Time-domain charge, and (d) Charge-magnetic flux behavior of all memristive devices. (arrows representing the HRS to LRS and LRS to HRS transition). The arrows indicate the resistive switching direction.
the surface of fMWCNTs with the conversion of the carbon atoms hy bridization, further evidenced in core-level spectra of C1s. To further confirm the activation of MWCNTs, the FT-IR spectra were also recorded separately and shown in Fig. 5c. In the FT-IR spectrum of the covalent functionalized MWCNTs, peaks observed at 1726 cm 1, which confirms the successful functionalization of oxygen moieties [35]. The –OH stretching of adsorbed H2O and the surface hydroxyl group in fMWCNTs is observed at 3400-3500 cm 1 [35]. Raman spectrum of all TiO2-fMWCNTs composite films showing the characteristic peaks of rutile TiO2 along with D (1354.9 cm 1) and G (1568.32 cm 1) bands of MWCNTs [34], which categorically revealed the existence of MWCNTs inside the prepared nanocomposites. However, the characteristic peak of the FTO glass substrate has also appeared at approximately 230 cm 1 during the detection of samples. The slight decrease in the peak in tensity, with a small shifting in the peak positions of all Raman bands for nanocomposite, has been observed. It is due to the increase in the crystallite size and intrinsic nature of rutile TiO2 which gives favor to accommodate the carbon nanostructures into their host lattice [36]. Further, it is seen that hydrothermal growth of TiO2 and TiO2-fMWCNTs composite film followed by thermal annealing of all devices shows improved resistive switching characteristics due to a large number of oxygen vacancies and superior crystallinity of the composite film. The improved resistive switching is related to the increased oxygen va cancies which have also been evidenced by obtained photo-luminescence (PL) spectra. Fig. 6a shows the measured PL spectra of TiO2 and TiO2-fMWCNTs film by using the photoexcitation
wavelength of 365 nm; where the intensity of the near band edge (NBE) emission and deep-level emission (DLE) peak is increased in TiO2-fMWCNTs composite film compared to pure TiO2 film. From the graph, it is seen that the TC 0.03 device shows the highest PL intensity. The increase of the DLE peak is due to the large number of fMWCNTs were accommodated into the network of TiO2 nanorods and hence efficiently increase the number of free oxygen vacancies that can work as vacancy reservoir during the switching. The emission around 417 and 488 nm is correlated to the surface oxygen vacancies, which is in good arrangement with the absorption spectrum. The charge state of the va cancies can drop one or two electrons to the valence band and the depletion region [37,38], which can encourage considerable variation in the bandgap which leads to increased electronic transport. Furthermore, the PL spectra demonstrate that the composite films have quite effi ciently created the vacancy pool and improved the crystalline structure of the film which consequently improves the device performance. Moreover, in order to know the different chemical states of the sur face elements the optimized nanocomposite thin film sample was analyzed by XPS. Fig. 6b, representing the full survey spectrum of TC0.03, in which all the peaks of all elements appear which are present into the nanocomposite, namely Ti, O and C, which has been properly detected at their characteristics binding energy levels. Furthermore, high-resolution core-level spectra of (a) Ti2p, (b) O1s, and (c) C1s ele ments have been represented into Fig. S5 (supplementary information). The Ti2p spectra consist of two peaks at 458.73eV and 464.44eV which are corresponding to Ti2p1/2 and Ti2p3/2 states respectively. The 6
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Fig. 5. (a) Raman spectra of bare TiO2 and TiO2-fMWCNT composite films. (b) Raman spectra of pure fMWCNTs showing the confirmation of fMWCNTs by showing its ideal ID/IG ratio. (c) FT-IR Spectra of fMWCNTs.
Fig. 6. (a) PL spectra of bare TiO2 and TiO2-fMWCNT composite films, where the intensity of the NBE emission and DLE peak increased for TiO2-fMWCNT composite film compared to pure TiO2 film. (b) XPS survey, (c) Ti 2p, and (d) O 1s core-level spectra of TC0.03 composite.
difference in binding energy (5.71eV), attributable to the presence of Ti4þ state in an octahedral arrangement of rutile TiO2. Furthermore, the high-resolution core-level spectra of O 1s showing a significant peak at ~530.61eV is showing the presence of lattice oxygen bonded with Ti species in Ti–O–Ti manner; further deconvolution of peaks is showing the broader peak at 532.04eV which are identical to the O–C bonding in TiO2-fMWCNTs nanocomposite [38]. Whereas in contrast to C1s spectra, – C bonds of the intense peak at 284.47 which is of graphitic C– fMWCNTs, whereas former peaks of C–O also appeared at 285.27 owed to the covalent functionalization of carbon nanotubes results in chemical defect and instinctive surface groups. However, there is an absence of any other peaks ~ at 281eV, confirming that, there is no doping of carbon elemental species [39], in nanocomposites that are dominantly present in fMWCNTs. In the present case, TC0.03 composite device shows the enhanced I–V characteristic due to the maximum number of oxygen vacancies present in the device. From the electrical behavior of the device, it is also observed that the top electrode and active layer interface also playing a
crucial role in the resistive switching process for both bare and fMWCNTs composite devices. The different amount of oxygen vacancies has generated near the Al/TiO2 interfaces during the switching, which results in the good resistive switching property. Further, the resistive switching can be modified due to various micro/nanostructure and modulated energy barrier at the Al/TiO2-fMWCNT interface. Therefore, the current is gradually increased or decreased according to the polarity of an external stimulus. The results suggested that fMWCNTs composite devices generate higher oxygen vacancies. The presence of a higher concentration of oxygen vacancies leading to a decrease in energy bar rier at the Al/TiO2 interface; therefore, a higher magnitude of the cur rent is observed for fMWCNTs composite devices. Furthermore, fMWCNTs improves the conductivity of the metal oxide. The inclusive effect resulted in the better electrical characteristics of TiO2-fMWCNT memristive devices. This kind of strategy of well-aligned growth of nanorods with increased oxygen vacancies gives rise to the non-linear electrical properties. It could presumably help to implement such a de vice in the fabrication of 1 transistor-1 resistor structured devices, which 7
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Fig. 7. (a) Endurance and (b) retention characteristics of TC0.03 memristive device.
could be a new approach to tackle the sneak path issue of resistive switching random access memory in the crossbar arrays structure [24]. The non-volatile memory effect of the developed device was evaluated by measuring the endurance and retention characteristics. In order to evaluate the device reliability and non-volatile memory properties, TC0.03 composite device was characterized with repetitive ON/OFF sweeping operations. Obtained by repetitive ON/OFF sweeping opera tion, the endurance cycling test results are shown in Fig. 7a. The re sistances of the low resistance state (LRS) (ROFF) and high resistance state (HRS) (RON) are measured at the read voltage of 0.25 V and write voltage of � 1 V. The ROFF/RON ratio of the present case was 9.02, which is sufficient for efficient memory applications. The resistive switching in TC0.03 composite devices is reproducible with cyclic tests of sequential 103 cycles. The retention property of memory was conducted for an extended period to evaluate the data storage capability and stability in resistive states, as shown in Fig. 7b. The TC0.03 composite device shows good retention characteristics over 103 s without degradation in resis tance states (LRS and HRS). The statistical analysis of endurance data suggested that the LRS and HRS have mean resistance equals to 10.20 kΩ and 92.07 kΩ, respectively. The standard deviation of LRS and HRS is
1.38 kΩ and 3.49 Ω, respectively. Furthermore, statistical data sug gested that the LRS (13.52%) has a higher coefficient of variation than HRS (3.79%). The improvement in the non-volatile memory properties of the device can be engineered through the development of the exact vertical growth of nanorod array which will strongly take control over the ionic and electronic transports by reducing the local temperature and joule heating processes present into the device. Until now, the limitation on cyclic performance is still inadequate due to the lack of enough vacancy migration to switch the device from HRS to LRS. In the case of chemical approach, the vacancy migration can be expected to be improved by tuning the various parameters such as extent growth of nanorods, the stoichiometric ratio of metal interstitials and oxygen ions, and by obtaining the desired crystalline size. In order to investigate the conduction mechanism of the TC0.03 composite device, the I–V characteristics were plotted in a log-log scale, as shown in Fig. 8a and b. Additionally, slopes of low, high, and entire voltage regions were calculated and provided in Fig. 8a and b. It is immensely important to understand the conduction mechanism of the HRS than the LRS, because, in most of the cases, LRS follows Ohmic conduction mechanism. In the present case, low and high voltage
Fig. 8. (a) Positive and (b) negative bias Log-Log I–V characteristics of TC0.03 memristive device. The Ohmic conduction mechanism fitting results of (c) positive and (d) negative bias I–V data (entire voltage region or LRS). Schottky conduction mechanism fitting results to (e and g) low and (f and h) high voltage region I–V data (positive and negative bias HRS data). 8
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regions represent the HRS whereas, LRS is represented in terms of the entire voltage region. Furthermore, the experimental I–V data is fitted with the best model available in the literature, as shown in Fig. 8c–h. It is observed that the low and high voltage region I–V data is well fitted with the Schottky conduction model with good-adjusted R2 fitting values. In addition to this, the entire voltage region is well fitted to the Ohmic conduction model. Considering the fitting results, it seems that the conduction of the device is due to the combined effect of Schottky and Ohmic conduction mechanisms. In particular, it is seen that after the voltage stressing to the developed devices, the energy barrier lowering could take place at the Al/TiO2 interface. At a sufficient voltage, elec trons from the top electrode move into the conduction band of TiO2. Furthermore, existing traps present in the TiO2 could be filled by the electrons at a sufficient voltage. These filled traps and oxygen ions/va cancies give rise to the current in the device under the influence of external electrical stimulus.
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4. Conclusions Oxygen vacancy induced resistive switching performance of TiO2fMWCNTS composite films were successfully fabricated on the FTO substrate through the low-cost hydrothermal method. The films were mainly composed of rutile TiO2 pinhole-free nanorods with enhanced surface charged oxygen vacancies. The Raman and photoluminescence studies are demonstrating the switching behavior is enhanced due to the presence of fMWCNTs and oxygen vacancies compare to the bare device. Moreover, the nanostructure, surface energy, and Al/TiO2 interface are also the main factors influencing the device properties. The forming free bipolar resistive switching with a memristive hysteresis loop is observed in all devices at lower operating voltage. Furthermore, 0.03 wt% fMWCNTs TiO2 composite device showing the self-rectifying property with excellent hysteresis area (82.30 μW), showing its another high storage density memory device related potential. The 1D surface struc ture of nanocomposites, chemical composition and surface vacancy reservoir consequently leading to the large hysteresis loops. The device charge-flux behavior shows the double-valued relation, which matched with the definition of memristive devices. The non-volatility of the de vice resuming the developed device can switch up to 103 cycles and can retain the data over 103 s. The charge transport behavior is in good agreement with the Ohmic and Schottky conduction mechanisms. The enhanced transport properties in such devices can be obtained through the simple fabrication method of such composites, whereas the further improvement can be pulled out by making the devices with modulated ratio of metal interstitials and oxygen vacancies and well-aligned growth of meticulous 1D nanostructure which would be the new strat egy to fabricate low-cost nonvolatile memory devices for the future electronic prototypes and systems. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Navaj Mullani: Conceptualization, Methodology, Writing - original draft, Software. Ijaz Ali: Investigation, Methodology. Tukaram D. Dongale: Visualization, Formal analysis. Gun Hwan Kim: Validation. Byung Joon Choi: Formal analysis, Writing - review & editing. Muhammad Abdul Basit: Writing - review & editing, Validation. Tae Joo Park: Supervision, Project administration, Funding acquisition, Resources.
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