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Morphology control of ferroelectric lead titanate thin films prepared by electrostatic spray deposition
Thin Solid Films 458 (2004) 71–76
Morphology control of ferroelectric lead titanate thin films prepared by electrostatic spray deposition H. Huang*, X. Yao, X.Q. Wu, M.Q. Wang, L.Y. Zhang Electronic Materials Research Laboratory, Xi’an Jiao Tong University, Xi’an 710049, PR China Received 11 October 2002; received in revised form 28 December 2002; accepted 26 November 2003
Abstract Ferroelectric PbTiO3 thin films of various surface morphologies (dense or highly porous) were prepared on Si(111) and Pty TiySi(111) substrates by electrostatic spray deposition (ESD). Substrate temperature was the key factor affecting the film morphology, and the appropriate substrate temperature was a little higher than the boiling point of the solvent in precursor. The PbTiO3 films prepared by repeated depositionypyrolysis cycles were porous due to the preferential deposition of charged droplets. Modified ESD was developed to densify the porous films by repairing the cracks and pores in the porous films. The dielectric constant and the dissipation factor of the PbTiO3 film at 100 kHz were 177 and 0.025, respectively. A model was proposed to illustrate the deposition process of the films. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 77.84.-s; 81.15.Pq; 78.55.Mb Keywords: Deposition process; Surface morphology; Ferroelectric properties; Growth mechanism
1. Introduction Lead titanate, PbTiO3, is a typical ferroelectric material with a large spontaneous polarization, and relatively small dielectric constant. Potential applications in piezoelectric, photo-electronics, and non-volatile voltage devices have brought much attention to the method of preparation of PbTiO3 thin films. Lots of methods such as metalorganic chemical vapor deposition w1,2x, rf magnetron sputtering w3x, sol–gel w4x, hydrothermal method w5x, and spray pyrolysis w6x have been used to prepare PbTiO3 thin films. In the past few years, a novel spray pyrolysis technique, electrostatic spray deposition (ESD), was employed to prepare TiO2 w7x, ZrO2 w8x, SnO2 w9x, MgO w10x, ZnS w11x, CdS w12x, LiCoO2 w13x and LaCoO3 w14x thin films. In this technique, precursor solution was atomized into charged droplets in several microns by the high voltage applied on spraying nozzle. These charged droplets were attracted to the heated substrate by electrostatic force, and then deposited on the substrate to form thin film. ESD is an economical *Corresponding author. Tel.: q86-82668794; fax: q86-82668794. E-mail address: [email protected] (H. Huang).
and effective deposition method with the advantages of simple set-up, single precursor and defect reparation w15x. However, due to the evaporation and preferential deposition of charged droplets in ESD, many films prepared by ESD were not very smooth and dense w13– 15x. In this article, porous PbTiO3 thin films were prepared by ESD. A modified ESD process was used to smooth and densify the porous thin films. The dielectric properties of the as-deposited thin films were measured. 2. Experimental details The ESD set-up used to deposit PbTiO3 thin films is schematically shown in Fig. 1. PbTiO3 thin films were deposited in the shielded quartz chamber. Two coaxial quartz tubes were employed in this study. Oxygen gas flowed through the outer tube to carry the droplets as well as to cool the spraying nozzle. While the inner one was 0.4 mm in inner diameter, and was used as spraying nozzle. When high positive voltage was applied on the conductive precursor, strong electrostatic field was established between nozzle and grounded substrate holder.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.304
H. Huang et al. / Thin Solid Films 458 (2004) 71–76
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Table 1 Typical deposition parameters of PbTiO3 thin films by ESD Precursor Substrate Distance between nozzle and substrate Precursor feeding rate Substrate temperature High voltage applied O2 flow rate Deposition time Pyrolysis temperature Pyrolysis time Final annealing temperature Annealing time
Fig. 1. Schematic diagram of ESD set-up.
Lead acetate trihydrate, Pb(OAc)2.3H2 O, and tetrabutyl titanate, wCH3(CH2)3Ox4 Ti, dissolved in 2-ethoxy ethanol were used as the precursor. The precursor solution was fed into the nozzle by peristaltic pump, and was atomized into fine droplets in the strong electrostatic field. These droplets deposited efficiently on heated substrate by Coulomb force. Typical deposition parameters used in this study were summarized in Table 1. The depositionypyrolysis cycle was repeated to increase the thickness of the film. In each cycle, film was deposited at 150 8C for 20 min, and heated to 330 8C for 20 min, and then cooled to 150 8C for the next deposition. Finally, the as-deposited film was annealed at 550 8C for 60 min. Although prescribed film thickness could be obtained by repeated depositions, but asdeposited films were porous. In order to prepare dense thick films, the ESD process was modified. In the modified ESD, additional depositions at room temperature for 2 min were carried out at each end of pyrolysis process. After deposition, the left precursor was characterized by infrared spectroscopy (Nicolet, MAGNA-IR 760) in the wave number range from 400 to 4000 cmy1. The phase compositions of the films were characterized using a Rigaku-Dymax-2400 X-ray diffractometer (XRD) with Cu Ka radiation. The morphologies and crosssection of the as-deposited films were observed by KYKY 2800 scanning electron microscopy (SEM) and atomic force microscopy (AFM) (Digital Instrument, Nanoscope III). A sharp step was prepared on the film by wet etching, and was used to measure the thickness of the film by AFM using contact mode. To evaluate the films electrically, a platinum bottom electrode and gold top electrode (1 mm in diameter) were deposited by sputtering. Dielectric properties of the films were tested using a HP4192A impedance analyzer.
0.01 M Si(111), PtyTiySi 25 mm 0.045 mlymin 150 8C q12 kV 280 sccm 20 min 330 8C 20 min 550 8C 60 min
and shape of the peaks were not changed, and no new peaks were found by spectroscopy either. It indicated that high positive voltage applied on the precursor hardly changed the molecule structure of precursor, and no new species were synthesized. However, the transmittance of the precursor was affected apparently by high voltage, and increased continuously with increasing spraying time, which might result from the decrease of precursor concentration with the increasing spraying time. Fig. 3 shows XRD patterns of PbTiO3 thin films deposited at 100, 150 and 200 8C by one cycle. Thin films deposited at deposition temperature lower than 150 8C were perovskite PbTiO3, and no pyrochlore phase was found. While there was some pyrochlore phase in the film deposited at 200 8C. The SEM micrographs of PbTiO3 thin films deposited at different substrate temperatures by one cycle are shown in Fig. 4. The films deposited at substrate temperature lower than 150 8C were cracked, while those deposited at substrate temperature higher than 200 8C were agglomerates of tiny particles (Fig. 4b and c). The film deposited at 150 8C was dense (Fig. 4a). At low substrate temperature, solvent evaporating rate was relatively low, and cracks were easy to occur in the
3. Results and discussion Fig. 2 shows the infrared spectroscopy of the left precursors by spraying for different times. The position
Fig. 2. Infrared spectroscopy of the left precursors by spraying for (a) 0 min, (b) 20 min and (c) 40 min.
H. Huang et al. / Thin Solid Films 458 (2004) 71–76
Fig. 3. XRD patterns of PbTiO3 thin films prepared by one cycle at (a) 100 8C, (b) 150 8C and (c) 200 8C.
films. When deposition was carried out at higher substrate temperature, crusts or dry powders might form during the transportation of droplets, which would affect the microstructure and adhesion of the films. Therefore, substrate temperature was a key factor affecting the surface morphologies of thin films prepared by ESD. It
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was found in our study that 150 8C was the ideal substrate temperature for preparing dense PbTiO3 thin films. Moreover, 150 8C was slightly higher than the boiling point, 131 8C, of 2-ethoxy ethanol. Therefore, in order to prepare dense films, the appropriate substrate temperature should be a little higher than the boiling point of the solvent in precursor, which was as same as the results issued by Chen et al. w7x. Fig. 5 shows the AFM micrograph of the PbTiO3 thin film prepared at 150 8C for 20 min. The grain size of the films was approximately 45 nm. The surface of the film was not very smooth, and some small juts were observed. The thickness of the PbTiO3 film prepared by one cycle measured by AFM was 350 nm. In usual sol–gel processes such as spin-coating or dip-coating, once a film has reached a critical thickness prior to densification, cracks will occur. The same result was also observed in the films prepared by ESD. When the deposition time was prolonged from 20 to 40 min, the obtained PbTiO3 film was cracked. Therefore, repeated depositionypyrolysis cycles were used to increase film thickness. The surface morphology of the PbTiO3 film prepared by two cycles is shown in Fig. 6a. The
Fig. 4. SEM micrographs of PbTiO3 thin films prepared by one cycle at (a) 150 8C, (b) 250 8C and (c) 350 8C.
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H. Huang et al. / Thin Solid Films 458 (2004) 71–76
Fig. 5. AFM micrograph of PbTiO3 thin film prepared at 150 8C by one cycle.
grains in the film formed three-dimensionally connected porous framework. With the deposition cycles increased to three, the pore size became smaller, and the rims of the pores grew thicker (as shown in Fig. 6b). Fig. 6c shows the SEM micrograph of PbTiO3 thin film prepared by the modified ESD with two cycles. The pores and cracks in the porous film prepared by normal deposition (Fig. 6a) were filled up by the additional deposition at room temperature. The film was relatively smooth and dense with tiny particles approximately 1.2 mm imbedded in it. Furthermore, the imbedded particles in the film could confine the stress relief associated with constrained densification and crystallization, so the film was not easy to crack. The cross SEM micrograph (Fig. 7) showed that the thickness of this film was 800 nm. The electrical properties of the PbTiO3 thin film prepared by modified ESD were measured. The capacitance variation with the dc bias voltage (C–V curve) measured at 100 kHz showed the ‘single butterfly loops’
Fig. 6. SEM micrographs of PbTiO3 thin film prepared by (a) two cycles and (b) three cycles.
Fig. 7. SEM micrographs of (a) the surface and (b) the cross-section of PbTiO3 thin film prepared by modified ESD.
H. Huang et al. / Thin Solid Films 458 (2004) 71–76
Fig. 8. C–V curve of PbTiO3 thin film prepared by modified ESD measured at 100 kHz.
(Fig. 8), which proved the typical ferroelectric nature of PbTiO3 thin film. The C–V curve was not symmetric along zero bias voltage, which was due to the ionic defects and interfaces existing in the film. Fig. 9 shows the frequency variation of the dielectric constants and the dissipation factor of the film measured at room temperature in the frequency range of 5–100 kHz. The dielectric constant decreased slightly with the increasing frequency, while dissipation factor increased with the increasing frequency. Specially, the dielectric constant and dissipation factor at 100 kHz were 177 and 0.025, respectively. The dielectric constant was larger than 42 obtained from PbTiO3 film prepared by sol–gel w16x, and was close to 210 obtained from PbTiO3 film prepared by spray pyrolysis w17x.
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Fig. 9. Variation of dielectric constant and the dissipation factor with frequency for PbTiO3 thin film prepared by modified ESD.
dense structure (illustrated in Fig. 10) was proposed. In ESD, precursor was atomized into tiny droplets in the strong electrostatic field, and the charged droplets were attracted to the grounded substrate by electrostatic force. As the deposition temperature was a little higher than the boiling point of solvent, the droplets were still wet when depositing on the substrate. The droplets spread on the surface of the substrate, and discharged by transferring the charges to the grounded substrate (Fig. 10a). The discharge proceeded slowly on Si substrate and insulating PbTiO3 thin film, so it affected the
4. Deposition model of ESD In sol–gel, films will become porous when the films begin to crystallize before densification. Brinker et al. w18x presented a theory coupling the large deformation of the solid skeleton to capillary pressure in the interstitial liquid to predict the course of drying of dip-coated porous gel coatings. Kaneko et al. w19x found that the rough surface of ZnFe2O4 thin films deposited by spray pyrolysis could be attributed to the solution properties with increasing pH. Choy et al. w15x thought that porous coating could be achieved by tailoring a combination of heterogeneous chemical reaction near the vicinity of the heated substrate and homogenous gas phase reactions to occur. Chen et al. w20x proposed a possible formation mechanism for the porous structure prepared by ESD, but the influence of electrostatic field and charges in droplets, which were the main characters of ESD, were neglected in the mechanism. According to the results and observations in our studies, a possible formation model of PbTiO3 thin films prepared by ESD was brought out, and a transformation mechanism of PbTiO3 thin film from porous surface to
Fig. 10. A formation model for PbTiO3 thin films deposited by ESD. (a) Droplets transport and evaporation; (b) Thin layer formed; (c) Formation of juts and preferential landing of droplets; (d) Juts growth and connection; (e) Droplets deposition at room temperature; (f) Film surface smoothed by modified deposition at room temperature.
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morphology of the film. Negative charges were induced on the surface of layer in the strong electrostatic field. The distribution of induced charges was not uniform on the surface of the layer, and the induced charges concentrated more at the places where the curvature was greater. Therefore, small charge-concentrated juts were formed during deposition (Fig. 10b). The charged droplets were attracted more strongly to the juts than to smooth surface, which leaded to preferential landing on the juts (Fig. 10c). With the deposition going on, the juts became larger and were linked with each other (Fig. 10d), thus the obtained film was porous. When additional deposition at room temperature was added at the end of pyrolysis process, wet charged droplets were absorbed into the cracks and the pores by strong Coulomb force and capillary force, and filled them up (Fig. 10e). Therefore, cracks and pores formed previously in the film were repaired by the subsequently arriving droplets, and the porous surface of the films became smooth (Fig. 10f). From the SEM micrographs of PbTiO3 thin films prepared by ESD, we could see these stages clearly. Some juts and cracks were observed in the PbTiO3 thin film prepared at 150 8C for 20 min (Fig. 4a). The film prepared by two cycles was very porous (Fig. 6a), which was due to the preferential deposition on curved juts. As the depositionypyrolysis cycles increased to three, the juts grew larger, and pores in the film became smaller (Fig. 6b). When 2-min deposition at room temperature was added to each end of the pyrolysis process, the cracks and pores in the film were repaired. Therefore, the surface of the film was smoothed by modified ESD (Fig. 6c). 5. Conclusions Ferroelectric PbTiO3 thin films were prepared by ESD. It was found that substrate temperature was the key factor affecting the film morphology. The appropriate substrate temperature was a little higher than the boiling point of the solvent in precursor. The surface morphologies of PbTiO3 thin films could be controlled to be highly porous or dense by adjusting the deposition parameters. The pores and cracks in the porous films could be repaired by modified ESD process, thus the
surface of the porous film became smooth and dense. The dielectric constant and the dissipation factor of PbTiO3 thin film at 100 kHz were 177 and 0.025, respectively. A formation mechanism for the transformation from porous surface to dense structure was proposed according to the SEM micrographs of PbTiO3 thin films. Acknowledgments This work is sponsored by the 973 National High Technology Program of China under Contract No. 2002CB613305 and International Cooperation Research Program between China and Israel. References w1x G.R. Bai, H.L.M. Chang, C.M. Foster, Z. Shen, D.J. Lam, J. Mater. Res. 9 (1994) 156. w2x Y.F. Chen, J.X. Chen, L. Shun, T. Tao, P. Li, N.B. Ming, L.J. Shi, J. Crystal Growth 146 (1995) 624. w3x C.M. Wu, T.J. Hong, T.B. Wu, J. Mater. Res. 12 (1997) 2158. w4x J. Zhang, L. Wang, J. Gao, Z. Song, X. Zhu, C. Lin, L. Hou, E. Liu, J. Crystal Growth 197 (1999) 874. w5x W.S. Cho, M. Yoshimura, J. Mater. Res. 12 (1997) 833. w6x A.R. Raju, C.N.R. Rao, Appl. Phys. Lett. 66 (1995) 896. w7x C.H. Chen, E.M. Kelder, J. Schoolman, Thin Solid Films 342 (1999) 35. w8x J.D. Vyas, K.L. Choy, Mater. Sci. Eng. A277 (2000) 206. w9x H. Gourari, M. Lumberras, R. Van Landschoot, J. Schoonman, Sensor. Actutor B58 (1999) 365. w10x S.G. Kim, J.Y. Kim, H.J. Kim, Thin Solid Films 376 (2000) 110. w11x L.W. Lenggoro, K. Okuyama, J.F. Mora, N. Tohge, J. Aerosol Sci. 31 (2000) 121. w12x B. Su, K.L. Choy, Thin Solid Films 359 (2000) 160. w13x C.H. Chen, E.M. Kelder, J. Schoonman, J. Mater. Chem. 6 (1996) 765. w14x C.H. Chen, E.M. Kelder, J. Schoonman, J. Electrochem. Soc. 144 (1997) L289. w15x K.L. Choy, Surf. Eng. 16 (2000) 465. w16x M.L. Calzada, J. Mendiola, F. Carmona, P. Ramos, R. Sirera, Mater. Res. Bull. 31 (1996) 413. w17x P. Murugavel, R. Sharma, A.R. Raju, C.N.R. Rao, J. Phys. D: Appl. Phys. 33 (2000) 906. w18x R.A. Cairncross, P.R. Schunk, K.S. Chen, S.S. Prakash, J. Samuel, A.J. Hurd, C.J. Brinker, Drying Tech. 15 (1997) 1815. w19x Z.B. Wu, M. Okuya, S. Kaneko, Thin Solid Films 385 (2001) 109. w20x C.H. Chen, E.M. Kelder, J. Schoonman, J. Mater. Sci. 31 (1996) 5437.
Morphology control of ferroelectric lead titanate thin films prepared by electrostatic spray deposition H. Huang*, X. Yao, X.Q. Wu, M.Q. Wang, L.Y. Zhang Electronic Materials Research Laboratory, Xi’an Jiao Tong University, Xi’an 710049, PR China Received 11 October 2002; received in revised form 28 December 2002; accepted 26 November 2003
Abstract Ferroelectric PbTiO3 thin films of various surface morphologies (dense or highly porous) were prepared on Si(111) and Pty TiySi(111) substrates by electrostatic spray deposition (ESD). Substrate temperature was the key factor affecting the film morphology, and the appropriate substrate temperature was a little higher than the boiling point of the solvent in precursor. The PbTiO3 films prepared by repeated depositionypyrolysis cycles were porous due to the preferential deposition of charged droplets. Modified ESD was developed to densify the porous films by repairing the cracks and pores in the porous films. The dielectric constant and the dissipation factor of the PbTiO3 film at 100 kHz were 177 and 0.025, respectively. A model was proposed to illustrate the deposition process of the films. 䊚 2003 Elsevier B.V. All rights reserved. PACS: 77.84.-s; 81.15.Pq; 78.55.Mb Keywords: Deposition process; Surface morphology; Ferroelectric properties; Growth mechanism
1. Introduction Lead titanate, PbTiO3, is a typical ferroelectric material with a large spontaneous polarization, and relatively small dielectric constant. Potential applications in piezoelectric, photo-electronics, and non-volatile voltage devices have brought much attention to the method of preparation of PbTiO3 thin films. Lots of methods such as metalorganic chemical vapor deposition w1,2x, rf magnetron sputtering w3x, sol–gel w4x, hydrothermal method w5x, and spray pyrolysis w6x have been used to prepare PbTiO3 thin films. In the past few years, a novel spray pyrolysis technique, electrostatic spray deposition (ESD), was employed to prepare TiO2 w7x, ZrO2 w8x, SnO2 w9x, MgO w10x, ZnS w11x, CdS w12x, LiCoO2 w13x and LaCoO3 w14x thin films. In this technique, precursor solution was atomized into charged droplets in several microns by the high voltage applied on spraying nozzle. These charged droplets were attracted to the heated substrate by electrostatic force, and then deposited on the substrate to form thin film. ESD is an economical *Corresponding author. Tel.: q86-82668794; fax: q86-82668794. E-mail address: [email protected] (H. Huang).
and effective deposition method with the advantages of simple set-up, single precursor and defect reparation w15x. However, due to the evaporation and preferential deposition of charged droplets in ESD, many films prepared by ESD were not very smooth and dense w13– 15x. In this article, porous PbTiO3 thin films were prepared by ESD. A modified ESD process was used to smooth and densify the porous thin films. The dielectric properties of the as-deposited thin films were measured. 2. Experimental details The ESD set-up used to deposit PbTiO3 thin films is schematically shown in Fig. 1. PbTiO3 thin films were deposited in the shielded quartz chamber. Two coaxial quartz tubes were employed in this study. Oxygen gas flowed through the outer tube to carry the droplets as well as to cool the spraying nozzle. While the inner one was 0.4 mm in inner diameter, and was used as spraying nozzle. When high positive voltage was applied on the conductive precursor, strong electrostatic field was established between nozzle and grounded substrate holder.
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.11.304
H. Huang et al. / Thin Solid Films 458 (2004) 71–76
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Table 1 Typical deposition parameters of PbTiO3 thin films by ESD Precursor Substrate Distance between nozzle and substrate Precursor feeding rate Substrate temperature High voltage applied O2 flow rate Deposition time Pyrolysis temperature Pyrolysis time Final annealing temperature Annealing time
Fig. 1. Schematic diagram of ESD set-up.
Lead acetate trihydrate, Pb(OAc)2.3H2 O, and tetrabutyl titanate, wCH3(CH2)3Ox4 Ti, dissolved in 2-ethoxy ethanol were used as the precursor. The precursor solution was fed into the nozzle by peristaltic pump, and was atomized into fine droplets in the strong electrostatic field. These droplets deposited efficiently on heated substrate by Coulomb force. Typical deposition parameters used in this study were summarized in Table 1. The depositionypyrolysis cycle was repeated to increase the thickness of the film. In each cycle, film was deposited at 150 8C for 20 min, and heated to 330 8C for 20 min, and then cooled to 150 8C for the next deposition. Finally, the as-deposited film was annealed at 550 8C for 60 min. Although prescribed film thickness could be obtained by repeated depositions, but asdeposited films were porous. In order to prepare dense thick films, the ESD process was modified. In the modified ESD, additional depositions at room temperature for 2 min were carried out at each end of pyrolysis process. After deposition, the left precursor was characterized by infrared spectroscopy (Nicolet, MAGNA-IR 760) in the wave number range from 400 to 4000 cmy1. The phase compositions of the films were characterized using a Rigaku-Dymax-2400 X-ray diffractometer (XRD) with Cu Ka radiation. The morphologies and crosssection of the as-deposited films were observed by KYKY 2800 scanning electron microscopy (SEM) and atomic force microscopy (AFM) (Digital Instrument, Nanoscope III). A sharp step was prepared on the film by wet etching, and was used to measure the thickness of the film by AFM using contact mode. To evaluate the films electrically, a platinum bottom electrode and gold top electrode (1 mm in diameter) were deposited by sputtering. Dielectric properties of the films were tested using a HP4192A impedance analyzer.
0.01 M Si(111), PtyTiySi 25 mm 0.045 mlymin 150 8C q12 kV 280 sccm 20 min 330 8C 20 min 550 8C 60 min
and shape of the peaks were not changed, and no new peaks were found by spectroscopy either. It indicated that high positive voltage applied on the precursor hardly changed the molecule structure of precursor, and no new species were synthesized. However, the transmittance of the precursor was affected apparently by high voltage, and increased continuously with increasing spraying time, which might result from the decrease of precursor concentration with the increasing spraying time. Fig. 3 shows XRD patterns of PbTiO3 thin films deposited at 100, 150 and 200 8C by one cycle. Thin films deposited at deposition temperature lower than 150 8C were perovskite PbTiO3, and no pyrochlore phase was found. While there was some pyrochlore phase in the film deposited at 200 8C. The SEM micrographs of PbTiO3 thin films deposited at different substrate temperatures by one cycle are shown in Fig. 4. The films deposited at substrate temperature lower than 150 8C were cracked, while those deposited at substrate temperature higher than 200 8C were agglomerates of tiny particles (Fig. 4b and c). The film deposited at 150 8C was dense (Fig. 4a). At low substrate temperature, solvent evaporating rate was relatively low, and cracks were easy to occur in the
3. Results and discussion Fig. 2 shows the infrared spectroscopy of the left precursors by spraying for different times. The position
Fig. 2. Infrared spectroscopy of the left precursors by spraying for (a) 0 min, (b) 20 min and (c) 40 min.
H. Huang et al. / Thin Solid Films 458 (2004) 71–76
Fig. 3. XRD patterns of PbTiO3 thin films prepared by one cycle at (a) 100 8C, (b) 150 8C and (c) 200 8C.
films. When deposition was carried out at higher substrate temperature, crusts or dry powders might form during the transportation of droplets, which would affect the microstructure and adhesion of the films. Therefore, substrate temperature was a key factor affecting the surface morphologies of thin films prepared by ESD. It
73
was found in our study that 150 8C was the ideal substrate temperature for preparing dense PbTiO3 thin films. Moreover, 150 8C was slightly higher than the boiling point, 131 8C, of 2-ethoxy ethanol. Therefore, in order to prepare dense films, the appropriate substrate temperature should be a little higher than the boiling point of the solvent in precursor, which was as same as the results issued by Chen et al. w7x. Fig. 5 shows the AFM micrograph of the PbTiO3 thin film prepared at 150 8C for 20 min. The grain size of the films was approximately 45 nm. The surface of the film was not very smooth, and some small juts were observed. The thickness of the PbTiO3 film prepared by one cycle measured by AFM was 350 nm. In usual sol–gel processes such as spin-coating or dip-coating, once a film has reached a critical thickness prior to densification, cracks will occur. The same result was also observed in the films prepared by ESD. When the deposition time was prolonged from 20 to 40 min, the obtained PbTiO3 film was cracked. Therefore, repeated depositionypyrolysis cycles were used to increase film thickness. The surface morphology of the PbTiO3 film prepared by two cycles is shown in Fig. 6a. The
Fig. 4. SEM micrographs of PbTiO3 thin films prepared by one cycle at (a) 150 8C, (b) 250 8C and (c) 350 8C.
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Fig. 5. AFM micrograph of PbTiO3 thin film prepared at 150 8C by one cycle.
grains in the film formed three-dimensionally connected porous framework. With the deposition cycles increased to three, the pore size became smaller, and the rims of the pores grew thicker (as shown in Fig. 6b). Fig. 6c shows the SEM micrograph of PbTiO3 thin film prepared by the modified ESD with two cycles. The pores and cracks in the porous film prepared by normal deposition (Fig. 6a) were filled up by the additional deposition at room temperature. The film was relatively smooth and dense with tiny particles approximately 1.2 mm imbedded in it. Furthermore, the imbedded particles in the film could confine the stress relief associated with constrained densification and crystallization, so the film was not easy to crack. The cross SEM micrograph (Fig. 7) showed that the thickness of this film was 800 nm. The electrical properties of the PbTiO3 thin film prepared by modified ESD were measured. The capacitance variation with the dc bias voltage (C–V curve) measured at 100 kHz showed the ‘single butterfly loops’
Fig. 6. SEM micrographs of PbTiO3 thin film prepared by (a) two cycles and (b) three cycles.
Fig. 7. SEM micrographs of (a) the surface and (b) the cross-section of PbTiO3 thin film prepared by modified ESD.
H. Huang et al. / Thin Solid Films 458 (2004) 71–76
Fig. 8. C–V curve of PbTiO3 thin film prepared by modified ESD measured at 100 kHz.
(Fig. 8), which proved the typical ferroelectric nature of PbTiO3 thin film. The C–V curve was not symmetric along zero bias voltage, which was due to the ionic defects and interfaces existing in the film. Fig. 9 shows the frequency variation of the dielectric constants and the dissipation factor of the film measured at room temperature in the frequency range of 5–100 kHz. The dielectric constant decreased slightly with the increasing frequency, while dissipation factor increased with the increasing frequency. Specially, the dielectric constant and dissipation factor at 100 kHz were 177 and 0.025, respectively. The dielectric constant was larger than 42 obtained from PbTiO3 film prepared by sol–gel w16x, and was close to 210 obtained from PbTiO3 film prepared by spray pyrolysis w17x.
75
Fig. 9. Variation of dielectric constant and the dissipation factor with frequency for PbTiO3 thin film prepared by modified ESD.
dense structure (illustrated in Fig. 10) was proposed. In ESD, precursor was atomized into tiny droplets in the strong electrostatic field, and the charged droplets were attracted to the grounded substrate by electrostatic force. As the deposition temperature was a little higher than the boiling point of solvent, the droplets were still wet when depositing on the substrate. The droplets spread on the surface of the substrate, and discharged by transferring the charges to the grounded substrate (Fig. 10a). The discharge proceeded slowly on Si substrate and insulating PbTiO3 thin film, so it affected the
4. Deposition model of ESD In sol–gel, films will become porous when the films begin to crystallize before densification. Brinker et al. w18x presented a theory coupling the large deformation of the solid skeleton to capillary pressure in the interstitial liquid to predict the course of drying of dip-coated porous gel coatings. Kaneko et al. w19x found that the rough surface of ZnFe2O4 thin films deposited by spray pyrolysis could be attributed to the solution properties with increasing pH. Choy et al. w15x thought that porous coating could be achieved by tailoring a combination of heterogeneous chemical reaction near the vicinity of the heated substrate and homogenous gas phase reactions to occur. Chen et al. w20x proposed a possible formation mechanism for the porous structure prepared by ESD, but the influence of electrostatic field and charges in droplets, which were the main characters of ESD, were neglected in the mechanism. According to the results and observations in our studies, a possible formation model of PbTiO3 thin films prepared by ESD was brought out, and a transformation mechanism of PbTiO3 thin film from porous surface to
Fig. 10. A formation model for PbTiO3 thin films deposited by ESD. (a) Droplets transport and evaporation; (b) Thin layer formed; (c) Formation of juts and preferential landing of droplets; (d) Juts growth and connection; (e) Droplets deposition at room temperature; (f) Film surface smoothed by modified deposition at room temperature.
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morphology of the film. Negative charges were induced on the surface of layer in the strong electrostatic field. The distribution of induced charges was not uniform on the surface of the layer, and the induced charges concentrated more at the places where the curvature was greater. Therefore, small charge-concentrated juts were formed during deposition (Fig. 10b). The charged droplets were attracted more strongly to the juts than to smooth surface, which leaded to preferential landing on the juts (Fig. 10c). With the deposition going on, the juts became larger and were linked with each other (Fig. 10d), thus the obtained film was porous. When additional deposition at room temperature was added at the end of pyrolysis process, wet charged droplets were absorbed into the cracks and the pores by strong Coulomb force and capillary force, and filled them up (Fig. 10e). Therefore, cracks and pores formed previously in the film were repaired by the subsequently arriving droplets, and the porous surface of the films became smooth (Fig. 10f). From the SEM micrographs of PbTiO3 thin films prepared by ESD, we could see these stages clearly. Some juts and cracks were observed in the PbTiO3 thin film prepared at 150 8C for 20 min (Fig. 4a). The film prepared by two cycles was very porous (Fig. 6a), which was due to the preferential deposition on curved juts. As the depositionypyrolysis cycles increased to three, the juts grew larger, and pores in the film became smaller (Fig. 6b). When 2-min deposition at room temperature was added to each end of the pyrolysis process, the cracks and pores in the film were repaired. Therefore, the surface of the film was smoothed by modified ESD (Fig. 6c). 5. Conclusions Ferroelectric PbTiO3 thin films were prepared by ESD. It was found that substrate temperature was the key factor affecting the film morphology. The appropriate substrate temperature was a little higher than the boiling point of the solvent in precursor. The surface morphologies of PbTiO3 thin films could be controlled to be highly porous or dense by adjusting the deposition parameters. The pores and cracks in the porous films could be repaired by modified ESD process, thus the
surface of the porous film became smooth and dense. The dielectric constant and the dissipation factor of PbTiO3 thin film at 100 kHz were 177 and 0.025, respectively. A formation mechanism for the transformation from porous surface to dense structure was proposed according to the SEM micrographs of PbTiO3 thin films. Acknowledgments This work is sponsored by the 973 National High Technology Program of China under Contract No. 2002CB613305 and International Cooperation Research Program between China and Israel. References w1x G.R. Bai, H.L.M. Chang, C.M. Foster, Z. Shen, D.J. Lam, J. Mater. Res. 9 (1994) 156. w2x Y.F. Chen, J.X. Chen, L. Shun, T. Tao, P. Li, N.B. Ming, L.J. Shi, J. Crystal Growth 146 (1995) 624. w3x C.M. Wu, T.J. Hong, T.B. Wu, J. Mater. Res. 12 (1997) 2158. w4x J. Zhang, L. Wang, J. Gao, Z. Song, X. Zhu, C. Lin, L. Hou, E. Liu, J. Crystal Growth 197 (1999) 874. w5x W.S. Cho, M. Yoshimura, J. Mater. Res. 12 (1997) 833. w6x A.R. Raju, C.N.R. Rao, Appl. Phys. Lett. 66 (1995) 896. w7x C.H. Chen, E.M. Kelder, J. Schoolman, Thin Solid Films 342 (1999) 35. w8x J.D. Vyas, K.L. Choy, Mater. Sci. Eng. A277 (2000) 206. w9x H. Gourari, M. Lumberras, R. Van Landschoot, J. Schoonman, Sensor. Actutor B58 (1999) 365. w10x S.G. Kim, J.Y. Kim, H.J. Kim, Thin Solid Films 376 (2000) 110. w11x L.W. Lenggoro, K. Okuyama, J.F. Mora, N. Tohge, J. Aerosol Sci. 31 (2000) 121. w12x B. Su, K.L. Choy, Thin Solid Films 359 (2000) 160. w13x C.H. Chen, E.M. Kelder, J. Schoonman, J. Mater. Chem. 6 (1996) 765. w14x C.H. Chen, E.M. Kelder, J. Schoonman, J. Electrochem. Soc. 144 (1997) L289. w15x K.L. Choy, Surf. Eng. 16 (2000) 465. w16x M.L. Calzada, J. Mendiola, F. Carmona, P. Ramos, R. Sirera, Mater. Res. Bull. 31 (1996) 413. w17x P. Murugavel, R. Sharma, A.R. Raju, C.N.R. Rao, J. Phys. D: Appl. Phys. 33 (2000) 906. w18x R.A. Cairncross, P.R. Schunk, K.S. Chen, S.S. Prakash, J. Samuel, A.J. Hurd, C.J. Brinker, Drying Tech. 15 (1997) 1815. w19x Z.B. Wu, M. Okuya, S. Kaneko, Thin Solid Films 385 (2001) 109. w20x C.H. Chen, E.M. Kelder, J. Schoonman, J. Mater. Sci. 31 (1996) 5437.