Characterization of Nanomechanical, Ferroelectric ... - ACS Publications

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Characterization of Nanomechanical, Ferroelectric, and Piezoelectric Properties by Nanoindentation and Piezoresponse Force Microscopy of PbTiO3 Thin Films Juan Ramos-Cano,† Mario Miki-Yoshida,† André Marino Gonçalves,‡ José Antônio Eiras,‡ Jesús González-Hernández,† José Antonio Rodríguez-López,† Patricia Amézaga-Madrid,† and Abel Hurtado-Macías*,† †

Centro de Investigación en Materiales Avanzados S.C., and Laboratorio Nacional de Nanotecnología, Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua, C.P. 31109 México ‡ Physics Department, Universidade Federal de São Carlos, Rod, Washington Luiz Km 235, CEP 13565-905 São Carlos, São Paulo, Brazil ABSTRACT: Lead titanate PbTiO3 films were deposited onto Pt(111)/Ti/SiO2/Si(100) substrates by the aerosol assisted chemical vapor deposition (AA-CVD) process followed by thermal annealing at 550 °C for 12 h. Piezoresponse force microscopic observations revealed a nano laminar domain structure in the PbTiO3 grains, and we attribute the lamellar domains to 180 and 90° ferroelectric domains. The polarization-switching mechanism of the domains in the representative PbTiO3 film under an external electric fields has also been studied, and it was revealed that a large-area polarization switching is usually accompanied by the appearance of a new direction of 90° domains in order to reduce the stress in the grains. Finally, nanomechanical properties such as the elastic modulus and hardness were evaluated by nanoindentation using the continuous stiffness measurement technique.

1. INTRODUCTION Ferroelectric thin films have attracted major attention in recent years because of their applications in integrated ferroelectric devices such as nonvolatile memories,1 ultrasonic sensors, and infrared detectors.2,3 In the past decade, research on ferroelectric thin films has particularly been focused on PbTiO3, BaTiO3, lead zironate titanate (PZT), and BaSrTi1−xO3.4,5 Lead-based ferroelectric thin films have been used lately in the electronic industry due to the fact that they can be easily integrated into electronic or optoelectronic devices such as piezoelectric transductors and high-sensibility pyroelectric sensors.6−16 Lead titanate (PbTiO3) is not the exception, with a perovskite type structure and singular properties such as spontaneous polarization, high Curie temperature (490 °C), large tetragonality, relatively low permittivity, and large pyroelectric coefficient.17,18 Several methods, physical and chemical, are used to deposit ferroelectric PbTiO3 thin films including the sol−gel and hydrothermal methods,19 radiofrequency magnetron sputtering,20 pulsed laser ablation,21 and chemical vapor deposition (CVD).22,23 There are a few studies on PbTiO3 thin film synthesis by the CVD method, but there are scarce reports on synthesis of PbTiO3 thin films by the aerosol assisted chemical vapor deposition (AA-CVD) technique.24 AA-CVD represents a good route to the synthesis of thin films, and it has been proven to be a simple, reproducible, and inexpensive method to produce metallic oxide films. The deposition rate, composition, thickness, microstructure, and other important parameters of the films can be easily controlled.25−29 Some characterization techniques have been used in the study of ferroelectric domains, such as optical birefringence, © 2013 American Chemical Society

transmission electron microscopy (TEM), chemical etching, and piezoresponse force microscopy (PFM).30−32 Among them, PFM has been proven to be one of the most powerful techniques in the study of ferroelectric domains due to its high resolution and ability for dynamic switching.33−35 In the piezoresponse mode, an ac voltage applied to the conductive tip of a PFM, working as a moving top electrode, causes the oscillation of electric dipoles in ferroelectric domains. They oscillate with the same frequency as the applied voltage. The amplitude and phase of this oscillation can be detected by the PFM tip as the first-harmonic signal through the tip−sample interaction. After being processed by a lock-in amplifier, this signal is used to image the piezoresponse of the ferroelectric domains. The contrast in the images of amplitude and phase, therefore, illustrates the local polarization in the ferroelectric materials. In this work we have used the PFM technique to analyze the nano laminar domains in the grains of the representative PbTiO3 film. The polarization-switching mechanism of the domains under external electric fields has also been studied, and it was revealed that a large-area polarization switching is usually accompanied by the appearance of a new direction of 90° domains in order to reduce the stress in the grains. Finally, nanomechanical properties such as elastic modulus and hardness were evaluated by nanoindentation using the continuous stiffness measurement technique. Received: Revised: Accepted: Published: 14328

April 9, 2013 September 6, 2013 September 16, 2013 September 16, 2013 dx.doi.org/10.1021/ie401134m | Ind. Eng. Chem. Res. 2013, 52, 14328−14334

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL DETAILS PbTiO3 thin films were deposited onto Pt(111)/Ti/SiO2/ Si(100) substrate (2.5 × 2.5 cm2) by the AA-CVD technique; details of the experimental setup and synthesis conditions have been reported previously.24,27,28 Several preliminary optimization tests were performed to fix deposition parameters such as substrate temperature, concentration of starting solution, carrier gas flux, and nozzle−substrate distance. In this work, PbTiO3 thin films were deposited at 400 °C and the other synthesis conditions were the same as those reported previously.24 After the synthesis, samples were annealed at 550 °C for 12 h, in a tubular Lindberg/Blue M furnace, Model STF54454C. A Pbrich atmosphere was used, by placing PbO powder around the sample. Five segments were applied to stabilize the ferroelectric perovskite phase (see Figure 1). Segments S1 and S2 were used

was calibrated using a standard fused silica sample. Test parameters were the following: the constants of area function were C0 = 24.08, C1 = −179.336, C2 = 6721.28, C3 = −24 409.2, and C5 = 18 691.8. A Berkovich diamond indenter with a tip radius of 20 ± 5 nm, depth limit of 170 nm, strain rate of 0.05 s−1, and harmonic displacement and frequency of 1 nm and 75 Hz, respectively, was used. Residual indentation of samples was recorded by a AFM NanoVision system attached to the nanoindenter system.

3. RESULTS AND DISCUSSION The crystalline structure of the PbTiO3 thin film was evaluated by GIXRD. Figure 2a shows that the crystalline phase of

Figure 1. Temperature ramps of the thermal annealing performed on PbTiO3 films.

Figure 2. (a) GIXRD diffractograms recorded with 0.5° of PbTiO3 thin film. (b) The inset shows a zoom in of 42.5 < 2θ < 47.5° that clearly shows the tetragonality of the tetragonal structure.

to decompose and eliminate possible organic residues from the precursor. Steps S3 and S4 correspond to the high temperature annealing. The heating and cooling rate was 3 °C min−1. The crystalline structure of the PbTiO3 thin film was analyzed by grazing incidence X-ray diffraction (GIXRD) in a Panalytical X-Pert system. The patterns were obtained using Cu Kα radiation at 40 kV and 35 mA. The diffracted beam path included a graphite flat crystal monochromator. The grazing incidence angle was fixed at 0.5°, whereas the scanning angle 2θ was varied between 20 and 75°, with a step size of 0.02°. Cross-sectional studies were performed to determine the thickness of the sample. High-resolution transmission electron microscopy (HRTEM) of the film’s cross section was performed with a JEOL JEM-2200FS system operated at 200 kV. Samples for HRTEM were prepared using a JEOL JEM9320 focused ion beam (FIB) system operated at 30 kV. Domain imaging, switching, and piezoelectric hysteresis were investigated by piezoresponse force microscopy (PFM), in a commercial atomic force microscope (AFM) Shimadzu SPM 9600 with an internal lock-in amplifier. The PFM was operated with an ac voltage amplitude of 4.0 Vp−p and frequency of 35 kHz, far below the resonance of the cantilever, applied between the bottom electrode and the conductive tip during imaging. Nanomechanical properties such as the hardness (H) and elastic modulus (E) of PbTiO3 representative thin film were evaluated by means of nanoindentation using the continuous stiffness measurement (CSM) method, employing a NanoIndenter G200 coupled with a DCM II head. The equipment

PbTiO3 film corresponds to a tetragonal perovskite-type structure according to JCPDS card no. 01-078-0299. Figure 2b shows clearly the development of the characteristic (002) and (200) peaks corresponding to the parameters c and a of tetragonal perovskite phase. These are in the range 42.5−47.5° 2θ.36 Moreover, TEM analysis was used to measure the thickness of the films and to corroborate the desired crystal structure. Figure 3a shows a film thickness of about 480 nm, and it is observed that the coating of PbTiO3 has good homogeneity. To determine the phase of the PbTiO3 thin film, the SAED pattern, obtained from the region marked with a red box in Figure 3a, was indexed as shown in Figure 3b. The crystallographic analysis of the planar spacing correspond to the (211), (100), and (111) planes of the PbTiO3 perovskite phase observed in the [0 −1 1] zone axis (JCPDS card no. 01078-0299). In order to evaluate the ferroelectric domain structure, switching domains and piezoelectricity, a PFM technique coupled to the AFM system was used. Figure 4a shows the AFM topographic image in contact mode of the PbTiO3 representative film, whereas Figure 4b shows the image of the piezoresponse phase in vertical PFM (V-PFM), where it is possible observe a 90° ferroelectric domain structure. In the V-PFM measurements, we are measuring the vertical component of the polarization, so white means the region with maximum intensity of polarization in the +Z direction (+Pz), black means the maximum intensity 14329

dx.doi.org/10.1021/ie401134m | Ind. Eng. Chem. Res. 2013, 52, 14328−14334


Article

Figure 3. (a) Low-magnification bright field STEM micrograph of the cross section of PbTiO3 thin film. (b) Selected area electron diffraction (SAED) pattern obtained from the region marked with a red box and zone axis [0,−1,1].

by the applied voltage is bigger than the sample coercive field, the ferroelectric domains will switch and the entire region will change color. Figure 5 shows the evolution of the relative reversed domains during domain polarization switching after application of the voltage indicated in the marked region. To achieve this process, a voltage of ±12 V was used, starting at −12 V in the first half of the scanning image area and ending at +12 V (see Figure 5a). Again we repeat the test in reversed order as shown in Figure 5b). It can be easily seen how polarization switches from one domain to another. In the case of the grains enclosed by the dotted line, the domains switch from the 180°−c domain to 90°−c/a domain. The appearance of the new nanolamellar 90°−c/a domain wall during polarization switching in the presence of an external electric field is believed to minimize the stress induced in the grains. By positioning the tip in a small region (typically 1 nm), it is possible to obtain a piezohysteresis curve. This is achieved by applying pulses of dc voltage and measuring the “remanent” piezoresponse of the region that corresponds to the remanent polarization. Figure 6a,b exhibits the position of the tip during the measurement. In the piezohysteresis curve (see Figure 6c), no conclusions can be made about the values of the d33 coefficient. This is actually in an arbitrary scale. Kalinin et al.37 showed that quantitative measurements of the d33 coefficients need a lot of conditions, including knowing the crystalline orientation of the grain, which becomes an almost impossible task in polycrystalline media. In this hysteresis we applied 9 V dc to pole the entire scanned region, as shown in Figure 6b. Then we start scanning a region of 1 nm in the grain, applying pulses of about 4 s, starting from −0.5 to −9 V with 0.5 V step, then starting from 0.5 to 9 V with 0.5 V step, and measuring the piezoresponse after each pulse. As in this hysteresis we measure (qualitatively) the remanent polarization, it is common to not measure the interval maximum voltage to zero (in our case, ±9 V to 0). In order to calculate the hardness (H) and elastic modulus (E) of PbTiO3 thin films, the elastic stiffness (S) of the contact must be known. Traditionally, S is determined from the slope of the load−displacement data acquired during the unloading process.38 However, such an approach is used to determine S (and thus H and E) only at the maximum penetration depth. The CSM option enables a continuous measurement of S during loading, and not just at the point of initial unloading.39 This is accomplished by superimposing a small oscillation on the primary loading signal and analyzing the resulting response

Figure 4. (a) AFM micrograph corresponding to topography (deflection signal) of the tested PbTiO3 polycrystalline thin film. (b) Image of the piezoresponse phase of the ferroelectric domains. (c) Zoom of the single grain, in which the domain configuration can be seen more clearly. The symbols ⊙, ⊗, and → represent the polarization vector directions in +Z (Pz), −Z (−Pz), and in the plane (P0), respectively.

of polarization in the −Z direction (−Pz), and gray regions mean zero polarization in the Z direction (P0) and, in consequence, polarization in the in-plane direction. Figure 4c is a zoom of Figure 4b that is shown in the red box. In Figure 4c the polarization vectors are represented, showing a possible configuration of the ferroelectric nanodomain structure of a single grain. The 90°−c/a domain wall separates regions with perpendicular directions of polarization vectors. In the PFM image (Figure 4c) this domain wall can be indicated as a wall separating regions with out-of-plane polarization “+Pz (⊙)” or “−Pz (⊗)” and regions with in-plane polarization “P0 (→)”. In PFM is also possible to imprint a ferroelectric polarization bounded by a region a few nanometers wide by applying a dc voltage to the tip while scanning. If the electric field generated 14330



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Figure 5. Evolution of relative reversed domains during domain polarization switching after applying the voltage indicated in the marked region: (a) starting with +12.5 V and ending at −12.5 V; (b) inverse process.

Figure 6. (a, b) AFM topography and images of the piezoresponse phase of position of the tip during piezohysteresis measurement respectively. (c) Piezohysteresis curve of the PbTiO3 nanocrystal marked.

frame stiffness (Kf), stiffness of the support springs (Ks), and mass (m) are known. With a continuous measurement of S, hardness, and elastic modulus, a continuous function of surface penetration can be obtained. Therefore, the CSM option is especially useful for evaluating films on substrates, where the mechanical properties change as a function of surface penetration. Figure 7a,b shows the behavior of the hardness (H) and elastic modulus (E) versus the displacement into the surface (h) of the representative PbTiO3 film. In Figure 7a,b H and E

of the system by means of a frequency-specific amplifier. The equation to determine the S in a CSM test is the following: −1

S=

1 F0 Z0

2

cos φ − (K s − mω )

−

1 Kf

(1)

In a CSM experiment, the excitation frequency (ω) is set. The displacement amplitude (Z0), phase angle (φ), and excitation amplitude (F0) are measured if the machine parameters load14331



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Figure 7. (a, b) Elastic modulus, E, and hardness, H, as a function of displacement into surface of the representative PbTiO3 film, respectively. (c) Squared stiffness over load versus displacement into surface.

in penetration depth or displacement into the surface, h, while values of S2/P between 20 and 170 nm are practically constant. On the other hand, the pop-ins are clearly seen in Figure 7d, in the characteristic curve load versus displacement into the surface, which occurred at low load at 10−25 nm (see the zoom in Figure 7d). Its effect is due to the switching domains of ferroelectric transition.

show the same behavior in general, leading to average values of H = 9.34 ± 1.66 GPa and E = 168.67 ± 8.31 GPa. These values were taken in the range 25−170 nm of the displacement into the surface. The values between 0 and 25 nm are inconsistent due to the use of a Berkovich tip with a curvature radius of 20 ± 5 nm and, on the other hand, switching domains of ferroelectric transition. This effect can be seen clearly in Figure 7c, where squared stiffness over the load (S2/P) is a function of h. This chart is used to see the effect of the tip radius, compliance of the loading frame, and area function; i.e., the equipment is properly calibrated. We observe erratic values from 0 to 20 nm

4. CONCLUSIONS Uniform, relatively homogeneous, and well-adhered PbTiO3 thin films with a tetragonal perovskite-type structure were 14332



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(12) Bao, D.; Yao, X.; Wakiya, N.; Shinozaki, K.; Mizutani, N. Crystallization and Optical Properties of Sol-gel-derived PbTiO3 Thin Films. Mater. Sci. Eng. 2002, B94, 269. (13) Byun, Ch.; Jang, J. W.; Lee, B. W. Correlation between the domain structure and ferroelectricity in PbTiO3 thin films. Mater. Lett. 1998, 34, 308−311. (14) Bao, D.; Yao, X.; Shinozaki, K.; Mizutani, N. Growth and electrical properties of Pb(Zr,Ti)O3 thin films by a chemical solution deposition method using zirconyl heptanoate as zirconium source. J. Cryst. Growth 2003, 259, 352−357. (15) Uchino, K. Ferroelectric Devices; Marcel Dekker Inc.: New York, 2000. (16) Tomashpol’skii, Yu. Ya.; Rybakova, L. F.; Lunina, T. V.; Fedoseeva, O. F.; Prutchenko, S. G.; Men’shikh, S. A. Ferroelectric Lead Zirconate Titanate Films Prepared by Spray Pyrolysis of Carboxylate Solutions. Inorg. Mater. 2001, 37, 500−507. (17) Harjuoja, J.; Kosola, A.; Putjonen, M.; Niinisto, L. Atomic Layer Deposition and Post-deposition Annealing of PbTiO3 Thin Films. Thin Solid Films 2006, 496, 346−352. (18) Park, Y. I.; Kim, C. E.; Lee, H. W. Effects of Catalyst and Solvent on PbTiO3 Fibers Prepared From Triethanolamine Complexed Titanium Isopropoxide. J. Sol-Gel Sci. Technol. 1999, 14, 149− 162. (19) Wang, D. G.; Chen, C. Z.; Ma, J.; Liu, T. H. Lead-based Titanate Ferroelectric Thin Films Fabricated by a Sol-gel Technique. Appl. Surf. Sci. 2008, 255, 1637−1645. (20) Xu, Z.; Chan, W. Preparation and Electrical Properties of Highly (1 1 1) Oriented Antiferroelectric PLZST Films by Radio Frequency Mangnetron Sputtering. Acta Mater. 2007, 55, 3923−3928. (21) Lisca, M.; Pintilie, L.; Alexe, M.; Teodorescu, C. M. Thickness Effect in Pb(Zr0.2Ti0.8)O3 Ferroelectric Thin Films Grown by Pulsed Laser Deposition. Appl. Surf. Sci. 2006, 252, 4549−4552. (22) Byun, K.; Lee, W. Effects of Pulsed Plasma on Low Temperature Growth of Pb-based Ferroelectric Films and Direct Liquid Injection Metalorganic Chemical Vapor Deposition. Curr. Appl. Phys. 2007, 7, 113−117. (23) Koo, J. M.; Kim, S.; Shin, S.; Park, Y.; Lee, J. K. Influence of Purge Gas on the Characteristics of Lead-zirconium-titanate Thin Films Prepared by Metalorganic Chemical Vapor Deposition. Ceram. Int. 2008, 34, 1003−1006. (24) Ramos-Cano, J.; Hurtado-Macías, A.; Antúnez-Flores, W.; Fuentes-Cobas, L.; González-Hernández, J.; Amézaga-Madrid, P.; Miki-Yoshida, M. Synthesis by Aerosol Assisted Chemical Vapor Deposition and Microstructural Characterization of PbTiO3 Thin Films. Thin Solid Films 2013, 531, 179−184. (25) Patil, P. S. Versatility of Chemical Spray Pyrolysis Technique. Mater. Chem. Phys. 1999, 59, 185−198. (26) Perednis, D.; Gauckler, L. J. Thin Films Deposition Using Spray Pyrolysis. J. Electroceram. 2005, 14, 103−111. (27) Bouchikhi, A. E.; Philippot, E.; Et-tabirou, M. PZT Thin Films Preparation by Pyrosol Process. Moroccan J. Condens. Matter 2007, 8, 69. (28) Amézaga-Madrid, P.; Antúnez-Flores, W.; Monárrez-García, I.; González-Hernández, J.; Martínez-Sánchez, R.; Miki-Yoshida, M. Synthesis, Structural Characterization and Optical Properties of Multilayered Yttria-Stabilized ZrO2 Thin Films Obtained by Aerosol Assisted Chemical Vapour Deposition. Thin Solid Films 2008, 516, 8282−8288. (29) Amézaga-Madrid, P.; Antúnez-Flores, W.; González-Hernández, J.; Sáenz-Hernández, J.; Campos-Venegas, K.; Solís-Canto, O.; Ornelas-Gutiérrez, C.; Vega-Becerra, O.; Martínez-Sánchez, R.; MikiYoshida, M. Microstructural Properties of Multi-nano-layered YSZ Thin Films. J. Alloys Compd. 2010, 495, 629−633. (30) Gruverman, A.; Auciello, O.; Ramesh, R.; Tokumoto, H. Scanning Force Microscopy of Domain Structure in Ferroelectric Thin Films: Imaging and Control. Nanotechnology 1997, 8, A38. (31) Ganpule, C. S.; Nagarajan, V.; Li, H.; Ogale, A. S.; Steinhauer, D. E.; Aggarwal, S.; Ramesh, R.; Wolf, P. D. Role of 90° Domains in Lead Zirconate Titanate Thin Films. Appl. Phys. Lett. 2000, 77, 292.

obtained on a Pt(111)/Ti/SiO2/Si(100) substrates by aerosol assisted CVD technique, followed by a thermal treatment at 550 °C. Piezoresponse force microscopic observation revealed a nano laminar domain structure in the PbTiO3 grains. The polarization switching of the particular grains occurred by switching domains from the 180°−c domain to the 90°−c/a domain. In the piezohysteresis curve no conclusions can be made about the values of the d33 coefficient. This is actually in an arbitrary scale. Kalinin et al.35 show that quantitative measurements of the d33 coefficients need a lot of conditions, including knowing the crystalline orientation of the grain, which becomes an almost impossible task in polycrystalline media. Nanomechanical properties were evaluated by means of nanoindentation using the CSM technique, obtaining E = 168.67 ± 8.31 GPa and H = 9.34 ± 1.66 GPa. The CSM is a very sensitive technique that allowed the effect of the switching domains on the ferroelectric transition in the first values of displacement into the surface (0−25 nm) by the presence of pop-ins.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +52 (614) 4394885. Fax: +52 (614) 4391130. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Oscar Solis Canto, Wilber Antunez, Carlos Ornelas and Enrique Torres from Centro de Investigación en Materiales Avanzados (CIMAV) for all the technical support. Financial support from CONACyT (Project 102181) and SEPCONACYT project no 2008-1-106655 is gratefully acknowledged.

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REFERENCES

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