Structural and Electrical Characterizations of PbTiO3 Thin Films ...

Journal of The Electrochemical Society, 153 共11兲 F260-F265 共2006兲

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0013-4651/2006/153共11兲/F260/6/$20.00 © The Electrochemical Society

Structural and Electrical Characterizations of PbTiO3 Thin Films Grown on LaNiO3-Buffered Pt/Ti/SiO2 /Si Substrates by Liquid Phase Deposition Ming-Chi Hsu,a Yu-Ming Sun,a Ing-Chi Leu,b and Min-Hsiung Hona,c a

Department of Materials Science and Engineering, National Cheng Kung University, Tainan, 701 Taiwan Department of Materials Science and Engineering, National United University, Miao-Li, 360 Taiwan c Da-Yeh University, Da-tsuen, Chang-hua, 515 Taiwan b

Ferroelectric PbTiO3 共PTO兲 thin films were successfully deposited on the LaNiO3 共LNO兲 buffered Pt/Ti/SiO2 /Si substrates by the liquid phase deposition method. The LNO layer served as both the bottom electrode and seeding layer for the PTO films. The structure, morphology, and electrical properties of the films were investigated by analytical techniques and electrical measurements. X-ray diffraction revealed that the as-deposited amorphous precursor films were decomposed and crystallized into perovskite structure after annealing at 650°C. Scanning electron microscopy showed that the thin films were smooth, dense, and crack-free with a grain size of ⬃200 nm. The mechanism of liquid phase deposition for PTO is proposed. The room temperature dielectric constant and dielectric loss of the PTO films, measured at kHz are 96.8 and 0.09, respectively, for the film with 200 nm thickness as annealed at 650°C for 1 h. The capacitor exhibits a hysteresis loop with a remanent polarization of 2.1 ␮C/cm2 and a coercive field of 33.4 kV/cm, respectively. © 2006 The Electrochemical Society. 关DOI: 10.1149/1.2349279兴 All rights reserved. Manuscript submitted December 23, 2005; revised manuscript received July 20, 2006. Available electronically September 14, 2006.

Ferroelectric thin films have been developed rapidly in recent years because of their potential applications including high volume capacitors, infrared detectors, actuators, and nonvolatile memories etc.1-3 Among many ferroelectric materials, lead titanate systems such as PbTiO3, PbZrTiO3, PbLaTiO3 . . . . are desirable for these applications due to the large piezoelectric, pyroelectric, high dielectric constant, and large remanent polarization.4,5 The PbTiO3 thin films were successfully prepared using a number of techniques including radio-frequency 共rf兲 sputtering,6 ion beam sputtering,7 metallorganic chemical vapor deposition 共MOCVD兲,8 pulse laser ablation,9 and chemical solution deposition 共CSD兲.10 However, the production of thin films by physical methods requires special equipment and often leads to difficulties in controlling the stoichiometry of this multicomponent oxide system. The CSD processing has attracted increasing interest due to its simplicity, low cost, and ease in achieving stoichiometry. This process, however, required repeated coatings to obtain the desired thickness. Furthermore, the precursors and the solvents used to prepare the sol are always highly reactive toward moisture and harmful to human body. Recently, a novel wet process called liquid phase deposition 共LPD兲11,12 has been developed for the preparation of metal oxide thin film by hydrolysis of metal-fluoro complex and the F− consumption reaction with boric acid or aluminum from aqueous solution. Using this method, both the complex process and expensive facilities are not required. In addition, because this method is performed in a homogeneous mixing aqueous system, it is easy to synthesize multicomponent oxide thin films on various kinds of substrate with large areas and complex morphologies. Until now, simple oxides, such as TiO2, FeOOH, and some perovskite oxides have been developed.13-17 However, to the best of our knowledge, no research about the fabrication and the electric properties of relative ferroelectric lead-based titanate thin films by LPD method has been reported. It is well known that the fabrication of the bottom electrode for a ferroelectric thin film capacitor is important in device application. Platinum is the most common material used as a bottom electrode in thin film capacitors. However, a Pt bottom electrode faces a lot of challenges to its practical applications because of its large lattice mismatch with the perovskite films, the formation of the hillocks from Pt layer which will lead to electrical shorts, and the unsatisfactory performance of the capacitors against fatigue.18-20 Recently, using the conductive oxide electrodes, such as La0.5Sr0.5CoO3

共LSCO兲,21 RuO2,22 SrRuO3,23 and LaNiO3 共LNO兲24 instead of conventional Pt electrode has been considered to be an excellent alternative for solving these problems. In this paper, we first extended the LPD process for fabricating high quality PbTiO3 共PTO兲 ferroelectric thin films on the conductive LaNiO3 共LNO兲 seeding layers which were spin coated on the conventional Pt/Ti/SiO2 /Si electrodes. The LNO is a perovskite-type metallic oxide with a lattice parameter of 3.84 Å, which matches well with ferroelectric thin films such as PbTiO3 and BaTiO3.25 In addition, its high conductivity and large reduction for the fatigue problem in lead-based thin films make LNO as a favorable candidate for electrode in the fabrication of ferroelectric memories or other applications.26,27 The structures and the PTO growth mechanisms are characterized and studied by several analysis techniques and the electric properties are also described. Through this research, such a facile method may provide a new aspect for preparing the lead based titanate ferroelectric thin films. Experimental The LNO thin films were deposited on the Pt/Ti/SiO2 /Si substrate by chemical solution process with lanthanum nitrate and nickel acetate as the starting materials which were dissolved in 2-methoxyethanol and then mixed at a ratio of La:Ni = 1:1.24 The final solution was clear and stable with concentration of 0.4 M. Film deposition was accomplished by a spin-coating technique at 4000 rpm for 30 s. The films were dried on a hot plate at 100°C for 10 min and then pyrolyzed at 350°C for 10 min. The process was repeated to achieve the desired thickness ⬃150 nm before crystallizing at 700°C for 1 h. The PTO precursor thin films were then grown on the LNO substrates by LPD method at 30°C. First, a homogeneous precursor solution was prepared by mixing 25 mM ammonium hexafluorotitanate 关共NH4兲2TiF6兴, 25 mM lead nitrate 关Pb共NO3兲2兴, and 75 mM boric acid 共H3BO3兲 at the pH of the solution of 2.75 as adjusted by nitric acid addition. Then, the LNO substrates were immersed in this precursor solution vertically to prevent the aggregated particles which formed homogeneously in solutions from accumulating on the substrates. After growth, the substrates were rinsed carefully in distilled water and then dried by clean nitrogen. For Fourier transform infrared 共FT-IR兲 and thermogravimetric analysis-differential thermal analysis 共TG-DTA兲, the powder samples were also prepared and annealed under the same reaction conditions as that for the thin films.

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Figure 1. TGA snd DSC plots of PTO powder prepared by LPD.

Samples were analyzed by X-ray diffraction 共XRD兲 analysis 共Rigaku D/max 2500 V diffractometer with Cu K␣ radiation兲. The morphology and the thickness were examined using scanning electron microscopy 共Philip XL-40, FEG兲. X-ray photoelectron spectroscopy 共XPS兲 was measured on a VG Scientific ESCALAB 250 system with Al source. All the binding energies were referenced to the C 1s peak at 284.6 eV of the surface adventitious carbon. FTIR spectroscopy 共Perkin-Elmer Spectrum One兲 was performed as a resolution of 4 cm−1 with KBr as a reference. TGA and DTA were performed using a TG-DTA instrument 共Setsys Evolution 16/18, Setaram, France兲, under an air flow with a heating rate of 5°C/min from room temperature to 1000°C. For the electrical measurement, top Pt electrodes of 200 ⫻ 200 ␮m were deposited by sputtering. The dielectric properties were measured using an impedance analyzer 共HP4284兲 and the polarization-electric field 共P-E兲 hysteresis loops were obtained with a RT66A testing unit connected with a high-voltage interface. Results and Discussion The thermolytic behavior of PTO precursor powder obtained by LPD method is shown in Fig. 1. The major weight loss of about 8.7% from room temperature to 350°C can be attributed to the evaporation of physically adsorbed water and other species. The weight loss between 380 to 580°C 共about 4.6%兲 is due to the thermal decomposition of the intermediate complex containing NH+4 and F− in the PTO precursor powders.28 The weight loss of about 1.5% from 600 to 730°C can be assigned to the dehydration of the PTO precursor. The exothermal peak at 750°C in the DTA curve is observed without weight loss, which can be attributed to the crystallization of PTO precursor. Typical FTIR spectra of the PTO precursor powders before and after annealing at 750°C are shown in Fig. 2. For the as-deposited precipitate powder 共Fig. 2a兲, a clear band at around 536 cm−1 is characteristic of the Ti–F stretching mode.16 A broad band at around 820 cm−1 suggests the presence of –Ti–O chains.29 Bands at 1625 30 and 1400 cm−1 are due to adsorbed water and residual NO−1 3 , re−1 spectively. The broad bands at around 3000–3800 cm are assigned to the O–H vibration of water and Ti–OH and N–H vibration mode for NH+4 ions.31 After calcining at 750°C 共Fig. 2b兲, the intensities of O–H, N–H, NO−1 3 , and Ti–F decrease. The presence of the absorption peaks at 581 and 716 cm−1 clearly show typical metal-oxide bonds in perovskite structure which are ascribed to 关TiO6兴2− octahedral.32 The above observations are in good agreement with the thermolytic behavior shown in Fig. 1. For the as-deposited PTO powders, the main components are hydrolysis products of titanium fluorite complex, water, and some impurities such as NH+4 and NO−3

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Figure 2. FTIR spectra of precipitate powders 共a兲 as-deposited and 共b兲 after annealing at 750°C.

ions from the treatment solution. After heat-treatment, the impurities almost disappear and the hydrolysis products and fluorine species are decomposed. Finally, the PTO precursors are crystallized to the perovskite PTO. Figure 3a shows the XRD profiles for the LNO substrate and Fig. 3b shows the PTO films deposited on LNO bottom electrodes after annealing at different temperatures for 1 h in air. The 2␪ diffraction angles and relative intensities of the XRD peaks of the LNO thin films are found to agree with those of the pseudocubic phase of LNO perovskite. The LNO films obtained have a good crystallinity with random orientation, and no other peak is observed except the diffraction peaks of Pt substrate and LNO thin films. In the Fig. 3b, the as-deposited films are amorphous until 450°C, as no obvious diffraction peak of PTO is detected. After annealing at 550°C for 1 h, as-deposited PTO precursors start to crystallize to perovskite phase and a small amount of pyrochlore phase which transforms into perovskite phase after annealing at 650°C for 1 h. The PTO exhibits a good crystallinity and no evidence of preferred orientation or no other secondary phase can be detected. Comparing with the TGDTA curve, it can be seen that PTO thin films crystallize on the LNO-buffered substrates at an annealing temperature as low as 650°C. This should be attributed to the different surface-interface interactions during solid formation. It was reported in the literature10,33 that a certain extent of Pb loss would happen as a result of its high vapor pressure during the annealing process. That is why an excess amount of lead 共5 ⬃ 10 mol %兲 is always used in the fabrication of the sol to compensate the lead loss in the thermal treatment process. However, an interesting phenomenon could be observed. In this process, although the starting molar ratio of Pb/Ti in the reacting solution was 1:1, no diffraction peaks corresponding to titania and pyrochole phase could be detected after annealing at 650°C, which confirms that a small amount of Pb element was lost during the annealing process. From XRD analysis results, it is proposed that the as-prepared PbTiO3 precursor film deposited on the LNO substrate should be a compound with stable Pb–Ti–O bonds, rather than a mixture of precipitated Pb- and Ti-containing species. Then this as-prepared precursor solid is transformed to the crystalline PbTiO3 by annealing at 650°C. By the above results, the chemical reactions of LPD-PTO thin films can be proposed as the following chemical equilibriums 关TiF6兴2− + nH2O 关TiF6−n共OH兲n兴2− + nHF

关1兴

Pb关TiF6兴 + nH2O Pb关TiF6−n共OH兲n兴 + nHF

关2兴

H3BO3 + 4HF BF−4 + H3O+ + 2H2O

关3兴


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Figure 3. XRD patterns of 共a兲 the LNO thin film on a Pt/Ti/SiO2 /Si substrate and 共b兲 the as-deposited PTO thin film on the LNO substrate and after annealing for 1 h in air at 450, 550, and 650°C.

Pb关TiF6−n共OH兲n兴 → PbTiO3 + 共n − 3兲H2O + 共6 − n兲HF

关4兴

In the liquid phase deposition process, metal oxide or hydroxide thin films are formed by means of a hydrolysis equilibrium reaction of a metal-fluoro complex ion and an F− consuming reaction 共Eq. 1兲. In this treatment solution for deposition, a hydrolysis reaction of 关TiF6兴2− ion in aqueous solution is presumed. Then, Pb2+ ions was

added into this treatment solution, the Pb2+ ion could be coordinated with intermediate hydrolyzed species to give species Pb关TiF6−n共OH兲n兴共Eq. 2兲. Then the hydrolysis equilibrium is shifted to the right side by addition of boric acid as the F− scavenger 共Eq. 3兲.16 Following the F− release, reaction proceeds leading to PTO precursor film formation. Finally, PTO precursor is decomposed and crystallized into perovskite PTO after annealing 共Eq. 4兲. To realize the chemical states of Pb, Ti, and O in the as-deposited PTO thin film and that after annealing, the samples were further characterized by the XPS technique. For the as-deposited thin film, XPS results suggest that only Pb, Ti, O, C, and F are present 共Fig. 4a兲. The C 1s peak appearing near 284.6 eV is attributed to contamination. The exist of F 1s peak located at 685 eV suggests that the as-deposited film contains a certain quantity of fluorine which comes from the intermediate hydrolyzed species of Pb关TiF6−n共OH兲n兴 which disappears completely after annealing at 650°C. At the same time, the PTO precursor thin film is decomposed and transformed into crystalline perovskite PTO. A highresolution XPS analysis was performed to estimate the peak energies more accurately for the elements of Pb, Ti, and O. Curve-fitting was performed on the high-resolution spectra to extract the peak energies. The curve fitting results were generated by the subtraction of a Shirley background, followed by decomposition calculations using Gaussian-Lorentzian mixed function. Figure 4b shows the peakfitted high resolution spectrum of lead doublet 共Pb 4f7/2 and Pb 4f5/2兲 for as-deposited sample and that after annealing. As shown in the figure, for the as-deposited sample, the Pb 4f7/2 peak is symmetric and narrow, and its binding energy is 140.2 eV. This detected binding energy is higher than the possible species Pb共NO3兲2 共139.1 eV兲 and PbO 共138.9 eV兲34 existed in the as-deposited films. This higher chemical shift of lead could therefore be proposed as the result of the Pb关TiF6−n共OH兲n兴 complex. After annealing at 650°C, the location of Pb 4f7/2 peak changes apparently and exhibits two binding states: one at 136.5 eV and the other at 138.5 eV, associated with different Pb species present in the sample. The first peak at lower binding energy is assigned to metallic lead, and at the higher energy corresponds to the lead in the perovskite lattice.35,36 As presented in Fig. 4c, the Ti 2p3/2 peak at 459.8 eV demonstrates that most titanium is bonded as Ti-F, following the hydrolysis of the Pb关TiF6−n共OH兲n兴 complex in the as-deposited thin films. After annealing at 650°C, the precursor is decomposed and crystallized, so the binding energy of Ti 2p3/2 decreases to 458.3 eV, indicating that Ti is present in the form of perovskite PTO.35 In Fig. 4d, the high-resolution scanning spectrum of O 1s for the as-deposited films differs considerably from that after annealing. The O 1s peak from the as-deposited films is actually two peaks whose binding energies are 531.3 and 533.8 eV, respectively. This result together with the preceeding results concerning Pb and Ti suggest that near the surface, most oxygen is in the form of a titanium hydroxide complex, surface absorbed oxygen, and H2O. The H2O disappears completely during annealing and the binding energies move to 530.2 and 531.8 eV because of the PTO and the surface absorbed oxygen.35,36 After Ar+ sputtering for 1 min, the surface absorbed oxygen disappears completely, and the XPS spectrum of the O 1s peak is likely to be the contribution of perovskite PTO. Figure 5a and b show the SEM surface micrographs of the PTO thin films deposited on the LNO surface before and after annealing at 650°C, respectively. The observation of the as-deposited film indicates a very smooth and dense surface morphology. Although the particles are somewhat aggregated, the particles are clearly observed as globular shape with particle diameter ranging from 5–10 nm. After heat-treatment, the surface morphology of the thin films changes significantly due to the phase transformation from amorphous to perovskite and the growth of PTO crystallites. Large grains with 200 nm are observed for the PTO films deposited on the LNO substrate. In this study, the LNO substrate is not only used as the bottom electrode but also provides nucleation site for the formation of the PTO grains, thus decreasing the nucleation energy for the



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Figure 4. 共a兲 Wide-scan XPS spectra of PTO thin film surface before and after annealing at 650°C and 共b兲 high-resolution XPS spectra of the Pb 4f region of the as-deposited PTO thin film and after annealing at 650°C. 共c兲 The Ti 2p region of the as-deposited PTO thin film and after annealing at 650°C. 共d兲 The O 1s region of the as-deposited PTO thin film, after annealing at 650°C and Ar+ sputtering.

perovskite phase. Therefore, the average grain size in the PTO film on the LNO substrate is much larger than those prepared on the Pt substrate.37,38 Figure 5c shows the cross section of the multilayer film of PTO/LNO/Pt/Ti/SiO2 /Si with the PTO thickness of about 200 nm at deposited rate with 70 nm/h at 30°C. Abrupt boundaries are observed, suggesting a high quality interface can be obtained. Figure 6 illustrates the variation of dielectric constant and dielectric loss for a 200 nm film, as measured at room temperature as a function of frequency in the range from 10 Hz to 105 Hz. The dielectric constant decreases from 170 to 72 with increasing frequency from 10 to 105 Hz. The dielectric loss, which shows a minimum value at ⬃10 kHz, is ⬃0.072. This film has a little dielectric constant and dielectric loss frequency dispersion, and are comparable to several values reported for PbTiO3 thin films prepared by other methods.2,39,40 In general, the capacitance-voltage 共C-V兲 curves based on lowsignal measurements have been used for the assessment of ferroelectricity. Figure 7 displays the dependence of the capacitance of the Pt/PTO/LNO capacitor as a function of bias voltage at 100 kHz. A normal two-peak 共butterfly兲 shape is observed, indicating the ferroelectric behavior of the PTO thin film at room temperature. The difference between the heights of the peaks may be attributed to the difference between the interfaces of the top and the bottom elec-

trodes. Furthermore, it can be seen that the center of C-V curves is not located at zero bias field, which suggests the existence of internal electric fields by space charge, asymmetric distribution of trapped charges at the interface between the thin film and the electrodes.41 The capacitance rises from 148 to 170 pF as the bias voltage is increased from −8 to 8 V. Figure 8 plots polarization against electric field 共P-E兲共the hysteresis loop兲 of the PTO thin films on the LNO/Pt/Ti/SiO2 /Si substrate. The hysteresis loop was measured in virtual ground mode at room temperature. The measured remanent polarization and coercive field are 2.1 ␮C/cm2 and 33.4 KV/cm, respectively, in a maximum applied field of 125 KV/cm field. The leakage current becomes considerable when applied voltage is increased in the P-E measurement, which may result in an inflation of the remanent polarization. Conclusions We show the first experimental evidence for the preparation of high-quality PTO thin films on LNO-buffered Pt/Ti/SiO2 /Si substrates by the LPD method. The as-deposited thin films are amorphous and mainly composed of hydrolysis products of titanium fluorite complex. Perovskite PTO thin films with high quality could be obtained after annealing at 650°C. The XRD, XPS, SEM, and elec-


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Figure 6. The variation of dielectric constant and dielectric loss for the PTO thin films deposited on the LNO buffered Pt/Ti/SiO2 /Si substrates annealed at 650°C.

Figure 7. C-V characteristics for the PTO thin film.

Figure 5. SEMs of 共a兲 the as-deposited PTO thin film obtained for 3 h deposition, 共b兲 after annealing at 650°C in air, and 共c兲 cross-sectional SEM of 共b兲.

trical measurements elucidate the characteristics of this multilayer composite film. The desired stoichiometric composition is easily achieved from the composition of the treatment solution, without the need for regarding the lead loss during the postannealing process. The multilayer films exhibit a good crystallinity and smooth surfaces. The PTO capacitor has a dielectric constant of 96.8 and a dielectric loss of 0.09 at 1 kHz. A ferroelectric hysteresis loop with a remanent polarization 共Pr兲 of 2.1 ␮C/cm2 and a coercive field 共Ec兲 of 33.4 kV/cm is obtained at an applied electric field of 125 kV/cm. These results reveal that LPD method can be an alternative to prepare PTO thin films with good electrical and microstructural qualities from a stable and nonhazardous aqueous solution of the required chemical composition.

Figure 8. Polarization-electric field 共P-E兲 hysteresis loop for the PTO thin film.


Journal of The Electrochemical Society, 153 共11兲 F260-F265 共2006兲 Acknowledgments The authors thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under contract no. 93-2216-E-006-039. National Cheng Kung University assisted in meeting the publication costs of this article.

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