Current Applied Physics 14 (2014) 1707e1712 Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap Correlation between crystallinity and resistive switching behavior of sputtered WO3 thin films Thi Bang Tam Dao a, Kim Ngoc Pham a, Yi-Lung Cheng b, Sang Sub Kim c, Bach Thang Phan a, d, * a Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam Department of Electrical Engineering, National Chi-Nan University, Nan-Tou, Taiwan, ROC Department of Materials Science and Engineering, Inha University, Republic of Korea d Laboratory of Advanced Materials, University of Science, Vietnam National University, Ho Chi Minh City, Viet Nam b c a r t i c l e i n f o a b s t r a c t Article history: Received June 2014 Received in revised form 13 August 2014 Accepted 10 October 2014 Available online 18 October 2014 The as-deposited WO3 thin films were post-annealed at different temperatures (300 C and 600 C) in air to investigate a correlation between crystallinity and switching behavior of WO3 thin films Associating the results of XRD, FTIR, XPS and FESEM measurements, the annealing-caused crystallinity change contributes to the variation of the switching behaviors of the WO3 thin films The as-deposited WO3 films with low crystalline structure are preferred for random Ag conducting path, resulting in large switching ratio but fluctuating IeV hysteresis, whereas the annealed WO3 films with crystallized compact structure limits Ag conducting path, favoring the stable IeV hysteresis but small switching ratio It is therefore concluded that electrochemical redox reaction-controlled resistance switching depends not only on electrode materials (inert and reactive electrodes) but also on crystallinity of host oxide © 2014 Elsevier B.V All rights reserved Keywords: Resistive random access memory (ReRAM) WO3 thin films Electrochemical redox Crystallinity Annealing Introduction Recent research has demonstrated that resistive random access memory (ReRAM) is promising candidate for future non-volatile memories Oxide-based ReRAM structures exploit the functionality of capacitor structures where the oxide materials, such as ternary oxides (Cr-doped SrTiO3, Cr-doped SrZrO3, Pr0.7Ca0.3MnO3, etc.) [1e6], binary oxides (NiO, TiO2, CuOx, HfO2, ZrOx, ZnO, Nb2O5, Al2O3, WOx) [7e12] are sandwiched between two metal electrodes Even though these materials show promising properties, the involved switching mechanisms are still content of current research activities The study of the film structure-order is important in obtaining a clear understanding of its revealed switching properties For example, TiO2 has various crystalline phases and also various resistive switching characteristics have been observed in the amorphous, anatase, and rutile structures [13e18] Lee et al., also observed that the epitaxial binary oxide NiO shows bipolar switching while the polycrystalline NiO shows unipolar switching [19] Improved crystallinity with increasing ZnO layer thickness * Corresponding author Faculty of Materials Science, University of Science, Vietnam National University, Ho Chi Minh City, Vietnam E-mail address: pbthang@hcmus.edu.vn (B.T Phan) http://dx.doi.org/10.1016/j.cap.2014.10.009 1567-1739/© 2014 Elsevier B.V All rights reserved reduced the number of extended defects, which then reduced the number of available sites for conduction path formation despite the increased density of oxygen-related defects contributing to the path formation, resulting in an increased set voltage in the high resistive state [20] Shang et al., reported that bipolar resistive switching of WO3 thin films can be improved by in situ oxygen annealing, which is attributed to the decrease in the surface density states [21] Syu et al., shows that the resistance switching behavior of WOx - RRAM devices is unstable because the diverse oxidation state provided the stochastic conduction paths By introducing a silicon element, Si interfusion in WOx resistance switching layer can effectively localize the filament conduction paths to improve the resistance switching property [22] Jang et al., tuned the switching characteristics by changing the additional oxygen content (d) of the WO3ỵd oxide As the value of d varies, the switching becomes to be unstable [23] It is therefore suggested that in preparing oxide-based ReRAM devices, especially in device scaling, careful control of crystallinity would be important It is known that WO3 has many polymorphs, depending temperature and preparation conditions (monoclinic, triclinic, orthorhombic, hexagonal and tetragonal) [24] The high diversity of physical parameters (e.g., crystal structure and density) and chemical parameters (e.g., valence state of W ions and composition) makes the research more 1708 T.B.T Dao et al / Current Applied Physics 14 (2014) 1707e1712 complex and more interesting for the variation of switching properties From this point of view, in this study, we reported a correlation between crystallinity and switching behavior of sputtered WO3 thin films Experimental The WO3 thin films were fabricated using the DC sputtering technique, from metallic W targets on Pt/Ti/SiO2/Si substrates The deposition process of 300-nm-thick WO3 thin films was executed under the total pressure PArỵO2 of 103 Torr at 300 C, and the mixture ratio of oxygen to argon gas, PO2/PArỵO2, was xed at 90% Before the top electrode deposition, the as-deposited WO3 films were annealed at various temperatures (300 C and 600 C) in air for h During the deposition of the 100-nm-thick top electrode (Ag) in an argon environment at  10À3 Torr, a mask was used for top electrode patterning The crystalline phases of the thin films were characterized in qÀ2q mode by D8 Advance (Bruker) X-ray diffractometer (XRD) with Cu Ka radiation (l ¼ 0.154 nm) and Fourier transform infrared spectroscopy (FTIR) The surface morphologies of the films were obtained using scanning electron microscopy (FESEM) X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical state of the films Deconvolution of the XPS spectra included Shirley baseline subtraction was carried out using the least squares curve fitting program The profile of the peaks was taken as a Gaussian function Currentevoltage (IeV) measurements were carried out using a semiconductor characterization system (Keithley 4200 SCS) and probe station The IeV curves was obtained under voltage sweep mode with the V / e Vmax / V / ỵ Vmax / V voltage profile, sweep speed is normal mode and step voltage is 0.02 V The read voltage for endurance test is 0.5 V The Pt bottom electrode was biased and the Ag top electrode was grounded Results and discussion The XRD patterns of both the as-deposited WO3 and the annealed WO3 thin films are shown in Fig Representative diffraction peaks at 2q ¼ 22.87, 23.74 , 24.4 , 26.8 , 29.4 , 34.12 , and 49.99 can be clearly identified The as-deposited WO3 thin films have two visible peaks, the board peak at 2q ¼ 22.87 and the other peak at 2q ¼ 29.4 The 300 C e annealed WO3 thin films have four peaks at 2q ¼ 23.74 , 24.4 , 29.4 and 34.12 Among Fig XRD of (a) as-deposited WO3 film, (b) 300 C e annealed WO3 film, and (c) 600 C e annealed WO3 film those peaks, the intense peak locates at 2q ¼ 24.4 , while the intensity of peak at 2q ¼ 29.4 decreases There are six diffraction peaks, 2q ¼ 23.74 , 24.4 , 26.8 , 29.4 , 34.12 , 49.99 , observed from the 600 C e annealed WO3 thin films with two intense peaks at 2q ¼ 24.4 , 34.12 In order to classify the crystal type of those WO3 thin films from the above diffraction peaks, we investigated the card number LCPDS of Triclinic phase, Monoclinic, Orthohombic phase, Hexagonal phase, Tetragonal phase Based on the card number LCPDS, those mentioned diffraction peaks are characteristic peaks of (002), (020), (200), (120), (112), (220) and (400) planes of Monoclinic phases The board and low intensity of (002) peak around 2q ¼ 22.87, indicating that the as-deposited WO3 thin film is low crystallinity In contrast, the XRD patterns of the annealed WO3 thin films reveal the sharp and intense (200) peak at 24.4 , implying an improvement of the crystalline by the annealing treatment It is therefore noted that annealing the as-deposited WO3 thin films enhance significantly crystallinity The FTIR spectra of both the as-deposited WO3 and the annealed WO3 thin films are shown in Fig Tungsten oxide film comprises OeWeO grains or crystals with terminal W]O bonds on their boundaries The as-deposited WO3 films consist of bands at 621 cmÀ1, 669 cmÀ1, 730 cmÀ1, 954 cmÀ1 and 1109 cmÀ1 The annealed WO3 films at 300 C have bands at 621 cmÀ1, 730 cmÀ1, 954 cmÀ1 and 1109 cm-1 When the WO3 films were annealed up to 600 C, bands are found at 621 cmÀ1, 730 cmÀ1, 804 cmÀ1, 866 cmÀ1, 954 cmÀ1 and 1109 cmÀ1 The number of bands shows that the post-annealing treatment strongly affected the structure of WO3 thin film The bands at 621 cmÀ1 and 730 cmÀ1 exist in all investigated WO3 thin films However, these two bands are competitive intensity, the band at 621 cmÀ1 is dominant in the as-deposited WO3 thin films, whereas the band at 730 cmÀ1 become well-defined and comparative intensity in the 600 C -annealed WO3 thin films All the WO3 films show a band around 954 cm1, which has been assigned to the W6ỵ ẳ O stretching mode of terminal oxygen atoms possibly on the surfaces of the cluster or micro-void structures in the films [25] The visible board band centered at 669 cmÀ1 is ascribed to the low crystallinity material [26] With increasing annealing temperature, the peak at 669 cmÀ1 disappeares The peak at 730 cmÀ1 belongs to the stretching vibration of crystalline WO3 [26] The results indicate that the as-deposited WO3 thin films Fig FTIR spectroscopy of (a) as-deposited WO3 film, (b) 300 C e annealed WO3 film, and (c) 600 C e annealed WO3 film T.B.T Dao et al / Current Applied Physics 14 (2014) 1707e1712 1709 are partially crystallized This result is consistent to the XRD data with the board (002) peak and the sharp (112) peak With the 600 C e annealed WO3 thin films, both the sharp peaks at 730 cmÀ1 and 804 cmÀ1 are assigned as WeOeW stretching modes in WO6 octahedral units and WO4 tetrahedral units, characterizing the monoclinic phase [25,27e29] The shorter WeOeW bonds are responsible for the stretching mode at 804 cmÀ1, whereas the longer bonds are the source of the 730 cmÀ1 peak The peak at 866 cmÀ1 may be ascribed to WO3.nH2O [28] In summary, as the post-annealing temperature increases, the crystallinity of the film tends to improve Fig shows superimposed O 1s photoelectrons spectra of both the as-deposited WO3 and the annealed WO3 thin films The core level spectra of O 1s can be deconvoluted into two peaks corresponding to lattice oxygen/stoichiometric WO3 phase (LO, ~ 530 eV) and non-lattice oxygen/non-stoichiometric WO3Àx phase (NLO, ~ 531 eV) [30e32] Fig shows the XPS of the W 4f core level spectrum of both the as-deposited WO3 and the annealed WO3 thin films The W 4f7/2 and W 4f5/2 peaks of the W6ỵ ion were assigned to the peaks at around 36 eV and 37.9 eV These peaks coincide with literaturereported W 4f binding energies measured on similar WO3 thin films [30e36] In addition, the W 4f spectrum of all thin films present the clear shoulder at around 34.5 eV, assigned to W5ỵ Among those thin lms, only the 600 C e annealed WO3 thin films have an additional shoulder at lower binding energy (~33.1 eV), which is assigned to W4ỵ The lower valence states of W ions (W5ỵ and W4ỵ) indicate the presence of reduced WO3x phase Since oxygen vacancies exists in the films, the electronic near its adjacent W atoms increases, creating a larger screening, which lowers the 4f level binding energy The two peaks located at higher binding energies (~39.7 eV and 41.3 eV) are assigned to W5ỵ and W6ỵ of W5p3/ Since the as-deposited WO3 films were annealed in air, the annealing process just affected the crystallinity, not stoichiometry of films Therefore, oxygen vacancies exist in all the as-deposited WO3 and the annealed WO3 films Fig XPS spectrum of the W4f core level of (a) as-deposited WO3 film, (b) 300 C e annealed WO3 film, and (c) 600 C e annealed WO3 film Fig XPS spectrum of the O 1s core level of (a) as-deposited WO3 film, (b) 300 C e annealed WO3 film, and (c) 600 C e annealed WO3 film 1710 T.B.T Dao et al / Current Applied Physics 14 (2014) 1707e1712 The surface morphology of the WO3 thin films was examined by FESEM, as shown in Fig The surface structure of the as-deposited WO3 thin film is porous with unclear irregularly grains, whereas both the annealed WO3 thin films are non-porous surface morphology with clear grains Meanwhile, the morphology and the porosity of as-deposited films are greatly affected by the annealing temperature: the higher the annealing temperature is, the more visible the grain boundaries are and the compacter the structure is According to the XRD, FTIR, XPS and FESEM analyses, the crystallinity was improved as well as the grain become visibly as annealing temperature increasing Fig shows the IeV characteristics of the as-deposited Ag/WO3/ Pt device and the annealed Ag/WO3/Pt devices All devices showed the bipolar resistance switching It is worthwhile to point out that no forming process is necessary for activating the switching effect Based on the IeV hysteresis, the initial high-resistance state (HRS) was changed to a low-resistance state (LRS) as a negative bias (0 / Fig IeV characteristics of (a) as-deposited WO3 film, (b) 300 C e annealed WO3 film, and (c) 600 C e annealed WO3 film Fig FESEM images of (a) as-deposited WO3 film, (b) 300 C e annealed WO3 film, and (c) 600 C e annealed WO3 film e 1.5 V) applied to the Pt bottom electrode The device remained in the LRS for subsequently descending, and the LRS was progressively changed to the HRS only by a voltage sweep in the positive voltage region (0 / ỵ V) Among the investigated WO3 thin films, only IeV curves of 600 C e annealed WO3 thin films superimpose to T.B.T Dao et al / Current Applied Physics 14 (2014) 1707e1712 each other, which seems to be influenced by its high crystalline structure An endurance test has been carried out at reading voltage of 0.5 V, as shown in Fig All devices present a clear reversible switching for over 100 cycles The value of HRS of WO3 films decreases as increasing the annealing temperature The lower resistance at higher annealing temperature is the result of the improvement of crystallinity It is noted that the annealing treatment strongly affected a switching ratio As shown in Fig 8, the switching ratio of the as-deposited thin film is about 70, but the switching ratio down to 30 and for the thin films annealed at 300 C and 600 C, respectively Because the switching ratio is smaller than 10, the post-annealed thin films at 600 C cannot be applied for ReRAM, although this structure show the stable switching over hundreds of cycle The film annealed at 600 C shows higher crystallinity but its switching ratio is the lowest, which may be ascribed to the crystallized compact structure of the thin film Associating the results of XRD, FTIR, XPS and FESEM measurements, obviously, the annealing-caused microstructure change contributes to the variation of the switching behaviors of the WO3 thin films Since the IeV curve of the LRS in log e log scale shows a linear relationship between current and voltage (not shown here), in addition to the nature of electrode, a reactive Ag electrode and an inert Pt electrode along with the switching direction, it is suggested that switching mechanism in both the as-deposited and annealed WO3 thin films is controlled by the electrochemical redox reactions [37,38], which is explained as follows By applying a negative voltage at the Pt bottom electrode (a positive voltage at the Ag top electrode), an electrochemical reaction occurs in the anode (Ag), which oxidizes the Ag metal atoms to Ag ions, the metal ions Agỵ start from the top interface and easily drift through the as-deposited low crystalline WO3 films to connect the bottom electrode At the Pt cathode, an electrochemical reduction and an electro-crystallization of Ag occur This electrocrystallization process results in the formation of an Ag filament, which grow towards the Ag electrode As a result, the Ag filaments grow and connect the Ag top electrode, leading to HRS to LRS switching To reset the cell, a positive voltage is applied at the Pt bottom electrode (a negative switching voltage at the Ag top electrode), which leads to a dissolution of the Ag filament and LRS to HRS occurs As mentioned above, the as-deposited WO3 films 1711 Fig Switching ratio of as-deposited WO3 films and annealed WO3 films (300 C and 600 C) have more pores in the bulk than those WO3 films annealed at 300 C and 600 C Therefore, as numerous randomly Ag metallic path forms, resulting in fluctuating IeV hysteresis In the postannealed WO3 thin films, the denser structure could not offer extensive internal volume to conduct ions, the number of Ag conducting path is limited, resulting in stable IeV hysteresis and lower switching ratio In comparison, Syu et al., shows that the resistance switching behavior of WOx e RRAM devices is unstable because the diverse oxidation state of W ions (W6ỵ, W5ỵ, and W4ỵ) provided the stochastic W conduction paths [22] In their study, the WO3 thin films were sputtered at room temperature and the authors not reported the crystalline structure of the WOx thin films In general, WO3 thin films prepared at room temperature are amorphous, resulting in many voids for providing the stochastic conduction paths Our as-deposited WO3 thin films have low crystallinity with only two valence states of W ions (W6ỵ, W5ỵ) show the uctuating switching, which is consistent to Syu's report [22] However, our 600 C e annealed WO3 thin films have high crystallinity with many valence states of W ions (W6ỵ, W5ỵ, and W4ỵ) show the stable switching behavior (Figs and 8) or the stochastic Ag conduction paths are limited It is suggested that the stochastic Ag conduction paths are also controlled by the crystalline structure Conclusions The as-deposited WO3 thin films were post-annealed at different temperatures (300 and 600 C) in air to investigate the effects of crystallinity on switching behaviors of the films The asdeposited WO3 films are monoclinic phase with low crystallinity Annealing the films up to 600 C improve the crystallization The resistance switching mechanism is the Ag filament paths mediated by electrochemical redox reactions, in which the Ag conducting formation is influenced by the crystalline structure The electrochemical redox reaction depends on crystalline structure of WO3 thin films The as-deposited WO3 with low crystalline structure are preferred for large switching ratio but fluctuating IeV hysteresis, whereas the annealed WO3 with crystallized compact structure favor the stable IeV hysteresis but small switching ratio Acknowledgment Fig Endurance of (a) as-deposited WO3 film, (b) 300 C e annealed WO3 film, and (c) 600 C e annealed WO3 film This work is financially supported by Vietnam National University in HoChiMinh City under Grant B2013-18-02 1712 T.B.T Dao et al / Current Applied Physics 14 (2014) 1707e1712 References [1] A Beck, J.U Bednorz, C Gerber, Ch Rossel, D Windmer, Appl Phys Lett 77 (2000) 139 [2] R Waser, M Aono, Nat Mater (2007) 833 [3] B.T Phan, J Lee, Appl Phys Lett 93 (2008) 222906 [4] B.T Phan, J Lee, Appl Phys Lett 94 (2009) 232102 [5] B.T Phan, N.C Kim, J Lee, J Kor Phys Soc 54 (2009) 873 [6] B.T Phan, Taekjib Choi, A Romanenko, Jaichan Lee, Solid-State Electron 75 (2012) 43 [7] B.J Choi, D.S Jeong, S.K Kim, S Choi, J.H Oh, C Rohde, H.J Kim, C.S Hwang, K Szot, R Waser, B Reichenberg, S Tiedke, J Appl Phys 98 (2005) 033715 [8] K Jung, H Seo, Y Kim, H 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between crystallinity and switching behavior of sputtered WO3. .. as-deposited WO3 films consist of bands at 621 cmÀ1, 669 cmÀ1, 730 cmÀ1, 954 cmÀ1 and 1109 cmÀ1 The annealed WO3 films at 300 C have bands at 621 cmÀ1, 730 cmÀ1, 954 cmÀ1 and 1109 cm-1 When the WO3 films... film The bands at 621 cmÀ1 and 730 cmÀ1 exist in all investigated WO3 thin films However, these two bands are competitive intensity, the band at 621 cmÀ1 is dominant in the as-deposited WO3 thin films,