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Effect of precursor solution composition on lead titanate thin films prepared by a sol-gel method
ELSEVIER
Materials
Chemistry
and Physics 51 (1997)
147-151
Effect of precursor solution composition on lead titanate thin films prepared by a sol-gel method Yen-Chao Department
of Chemical
Engineering,
Lai ‘, Y.S. Gong, Chiapyng
National
Taiwan
Universig
Received21 December1996;revised
23
of Science
Lee *
and Technology,
Taipei 10572, TaiwaA, XOC
May 1997;accepted18June1997
Abstract PT films of different compositions were prepared by a sol-gel method and characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. Surface segregation of Pb,O, and the existence of TiOz were observed in the film deposlted by a Pb-rich precursor solution. PbTi,O, should exist in the film prepared with excess Ti in the precursor solution. Stoichiometric films were obtained from the Pb/Ti = 1 precursor solution. The growth mechanism which controls the film characterics is presented. 0 1997 Elsevier Science S.A. Kc>w,ords; Ferroelcctrics;PbTi03thin films; Sol-gel;Ramansprcuoscopy; XPS
1. Introduction Ferroelectric thin films have attracted considerable interest in recent years owing to their unique electrical and optical properties. Four deposition techniques have been used to prepare ferroelectric thin films: radio-frequency (rf) magnetron sputtering [ I-41, chemical vapor deposition (CVD) [5,6], ionized cluster beam deposition [7], and sol-gel growth [&lo]. In recent years, the sol-gel spin-on technique, with its merits of strict compositional control and low fabrication cost, has been utilized extensively to produce ferroelectric films primarily for memory applications [ 9, lo]. Since PbO has a relatively high vapor pressure, it can easily segregate to the surface and then evaporate. Therefore, the effect of lead oxide content of the target on the film composition was studied thoroughly, when the PbTiO, thin films were prepared by using the rf-magnetron sputtering method. Jaber et al. [ II] reported that evaporation of PbO competes, during growth and formation of PbTiO,, with reaction of PbO with TiOP. Castellano and Feinstein [ 121 observed Pb concentration decreased for substrate temperatures above 200°C. Krupanidhi et al. [ 131 also performed elemental analysis of films deposited from targets having different percentages of excess PbO. They found that the Pb/Ti ratio approached that of the desired material for films deposited from targets enriched with 10% excess PbO. However, the effa of lead * Correspondingauthor. ’ Presentaddress:Departmentof ChemicalEngineering,NationalTaipei University of Technology,Taipei,Taiwan, ROC. 0254-0584/97/$17.00Q 1997Elsevier ScienceS.A. All rights reserved P11SO254-0584(97)01985-S
content in the precursor solution on the composition of films prepared by the sol-gel method has never been reported. Raman spectroscopy is a non-destructive optical technique which can be used to characterize different phases. Several applications of laser Raman spectroscopy to the analysis of thin film compositions or phase transformations are reported [ 14-171. In this article, we employ the sol-gel method to prepare PbTi03 thin films with different lead contents in the precursor solution. The purpose of this work is to correlate the variation of film crystalline phase and chemical composition to the lead content of the sol-gel precursor solution.
2. Experimental We prepared the solution for film fabrication by the conventional sol-gel process, using lead acetate trihydrate and titanium n-butoxide as precursors, and 2-methoxyethanol as the solvent. Lead acetate trihydrate was dissolved in hot 2-methoxyethanol (70°C) at a 1:4 molar ratio. The solution was heated at 120°C for 20 min to allow the water of hydration to expel. The dehydrated solution was cooled to 90°C before a measured amount of titanium n-butoxide was added. The solution was reheated to 120°C and stirred for 5 min. Formamide ( 10 ~01%) was added to the solution; it is known to be a drying control chemical, which prevents cracking of the film. Precursor solutions of 2.0, 1.33, 1.0, 0.8 and 0.67 Pb/Ti ratios
118
Y.-C. Lai et al. /Murrrials
Chemistry
were prepared. Films of TiO, and lead oxides were prepared by the same procedure, except that lead acetate trihydrate and titanium n-butoxide were excluded, respectively. We used silicon wafers as substrates. An adhesion layer of 0.1 Frn Ti was grown using electron-beam evaporation after which a 0.1 pm thick layer of platinum was allowed to deposit. Pt was chosen because it resists oxidation. The substrate was spin-coated with the prepared solution. The rotation speed and the spin time were fixed at 3500 rpm and 10 s, respectively, to ensure that the film thickness was uniform on the substrate. After drying at room temperature for 2 h, the samples were preannealed at 300°C in air for 0.5 h. The films were then annealed in air for 1 h at 600°C and the thickness of the final film is about 1500 A. Raman spectra were recorded on a Renishaw RS-2000 spectrometer. XPS analyses were performed in a VG Microtech MT-500 spectrometer. The spectrometer was equipped with a hemispherical analyzer and all XPS data presented here were acquired using the Mg Ka X-rays ( 1253.6 eV) The electron take-off angle was maintained constant at 0”. The binding energies of various core levels were determined with respect to the adventitious C,, peak at 284.6 eV.
and Physics 51 11997) 147-151
;63 12.5
QJ)
509.0 389.9
:
/
A
3. Results and discussion Fig. 1 (a) shows the Raman spectrum for the PbTiO, thin film. The spectrum shows peaks at 283.3, 325.7, 498.4 and 601.5 cm-‘, which are characteristic of PbTiO? thin film [ I8 1. Fig. 1 (b) shows the Raman spectrum of the titania film. The appearance of Raman-active lines near 359.9,509.0 and 632.5 cm-’ is indicative of the anatase crystalline phase [ 17 1. In addition, a Raman spectrum from the lead oxide film is displayed in Fig. 1cc), A peak at 267.9 cm- ’ is due to the orthorhombic PbO and the peak appearing at 348.0 cm - ’ is attributed to tetragonal PbO, whereas peaks at 519.7, 543.0 and 591.7 cm- I are characteristic of Pb,Oj [ 191. Fig. 2(a)-(d) depicts a series of Raman spectra of a series of samples which were prepared with precursor solutions having different Pb/Ti ratios and annealed at 600°C for 1 h. Fig. 2(a) shows the Raman spectrum of the thinfilmprepared with a Pb/Ti=2 precursor solution. In addition to PbTiO,(PT) and Pb,O,, TiOz is clearly observed. Raman spectrum for the film prepared from a Pb/Ti = 1.25 precursor solution is shown in Fig. 2(b), which resembles that shown in Fig. 2 ( a) ; PbTiO,, Pb,Od and TiOz phases are all present, but peak intensities are different. In Fig. 2(a), the peak intensity of PbTi03 at 283.3 cm-’ is comparable with that of Pb,O, at 519.7 cm-‘. However, in Fig. 2(b) the dominant peak is at 283.3 cm-’ which is due to PbTiO, whereas the Pb,O, peak at 5 19.7 cm-’ becomes weaker. This variation of peak intensities indicates the decrease of Pb content in the film. Fig. 2(c) and (d) gives the Raman patterns for the thin films obtained from Ti-rich precursor solutions. When the precursor solution is prepared with a Pb/Ti ratio of 0.8 the
i’ 519.7
(cl
591.7
543
267.9
348.0
400
600
Delta
800
1000
wave number
(cm,])
Fig. 1. Raman spectra of thin films of: (a) stoichiometric and (4 iead oxides.
1200
PbTiO,,
i b) TiO,,
Raman spectrum of the film (Fig. 2(c) ) shows only the characteristic peaks of PT. In addition, when the Pb/Ti ratio is 0.67 in the precursor solution, TiOz exists simultaneously in two phases (Fig. 2(d)). Interestingly, as can be seen in Fig. 2(a) and (b) where Pb content is rich, TiOz can also be observed. However, Pb30, is absent when Ti is rich in the precursor solutions (Fig. 2(c) and (d) ) . Based on the Raman patterns. we interpret that the growth mechanism is governed by the formation of different species. A competition exists between the formationof PbTi03, Pb30, and TiO?. This competition is influenced by the free energy of formation A G and the Pb/Ti ratio. The free energy change AG of Pb30J, TiOz and PbTiO, is - 143.7, -212.6 and - 257.51 kcal mol- ’ [20], respectively. Therefore, according to free energy change A G, the tendency toward formation
Y.-C. Lcri et al. /Maierials
Citemisr~
and Physics 51 (1997)
147-151
119
Pb,O, PT
PT +
L
PbrO,
TiOz(A)
PT
R
(4
PT
(b)
PT+TiO,(A)
PbLh PT QJO<
\
d
,
400
I
,
600 Deltn
1000
800 HRW
number
Fig. 2. Raman spectra of thin films prepared
I
s
400
1200
Delta
(cm.‘)
with precursor
solutions
having:
of the phases varies as PbTiO, > TiOz > Pb30,. This indicates that when Pb is rich (Fig. 2(a) and (b)), PbTi03 phase should dominate, and the appearance of Pb304 is excepted. In addition, the appearance of TiO, is due to its superior tendency toward formation than Pb30S. On the contrary, when Ti is in excess (Fig. 2(c) and (d) ) only PbTi03 and TiO, exist, and Pb,O, disappears. The wide scan XPS spectra of all samples in the binding energy ranging from O-1000 eV indicate that no impurity is present in the spectrum, except carbon. The Pb,f,Ti,, and O,, core level spectra of the PT thin film are shown in Fig. 3 (a)(c), respectively. The peak of Pb4f(7,2j at 137.2 eV is observed in Fig. 3(a). The spin-orbit splitting A (4f(7/ 2) -4f( 5/2) ) is 4.9 eV. According to Fig. 3 (b) the binding energy of Ti2pt 3/2) is 457.5 eV and the spin-orbit splitting A(2p(3/2)-2p(1/2)) is5.9eV.Fig.3(~) revealsthato,, is an asymmetric peak, possibly composed of more than one component peak. A deconvolution method was applied, in which the O,, spectrum was assumed to comprise two overlapping peaks. Each component peak in the spectrum was fitted to a Gaussian-Lorentzian type distribution with lower binding energy (LBE) and high binding energy (HBE) peaks at 529.3 eV and 530.6 eV, respectively. The composite curve of the two components closely corresponds the experimental spectrum. Following Barr [ 211, the LBE and the HBE peaks can be assigned to the oxide and the hydroxide/
adsorbed oxygen species, respectively.
600
(a) PbiTi
wake
I
9
800
1000
number
= 2.0, (b) Pb/Ti = 1.33, (c) PbiTi
I
1
1200
(crn,l)
= 0.8, and (d) PbiTi
= 0.67.
Fig. 4 gives the core level spectra of the TiO, film. According to Fig. 4(a) the binding energy of Tilp( 3,2j is 458.5 eV and the spin-orbit splitting A (2p( 312) -2p( 112)) is 5.7 eV. Fig. 4(b) shows that the LBE and HBE peaks of O,, are at 529.6 and 53 1.O eV, respectively. Core level spectra of the film of lead oxides-are shown in Fig. 5. Inspection of the Pb,, peaks (Fig. 5(a)) shows that the peak comprises several doublets, ascribable to various lead oxides. The wider FWHM of the XPS spectrum of the O,, peak can also be ascribed to various lead oxides. The existence of Pb30A and PbO has been confirmed by the Raman spectrum (Fig. 1(c) ) . The corresponding binding energies are similar to those obtained by other researchers [ 22,231. It is worthwhile to point out that most of core level spectra of the films prepared with precursor solutions of different Pb/ Ti ratios resemble those of PbTiO, film shown previously in Fig. 3; neither TiOz nor PbO could be observed on the surface of any sample. In addition, Pb,O, was not present on films prepared from precursor solutions with Pb/Ti ratios less than 1.33. However, the Pbdr core level spectrum (Fig. 6) of the film prepared with the Pb/Ti = 1.33 precursor solution shows the existence of Pb,O, on the surface. Similar result was also obtained for the film prepared with the Pb/Ti = 2.0 precursor solution. The film compositions were estimated from the XPS spec-
tra of each component after normalizing the peak area with the respective relative sensitivity factor [ 241. The composi-
Y.-C. Lai
et 01. lhf~ttrrials
Chetnistty
und Physics
51 (1997)
-2%0
147-151
455
4.54
46s
Binding Ewgy
$312 457.5
/
is; ( \ (b> i
470
{et’)
Ti2,
\
1
I
i i
lccxw-
]
4cca 20%
531 i
o.2w922 %O I
452
454
456
458
460
.462
464
466
468'--
524
526
-
'L
528
530
'L. 532
_ 534
536 1
-370 , Bindii
Binding Er.crgy (ev)
Energy (cv)
Fig. 4. XPS spectra of the TiOl thin film after annealing at 600°C in air for 1 h: (a) in the region of the Ti,,, and (b) in the region of the O,,.
XlIT
iam0 I-
7
4l7R lmxl14wo-
Fig. 3. XPS spectra of the stoichiometric 600°C in air for 1 h: (a) in the region Tizp, and (c) in the region of the O,,.
PbTiO, thin film after annealing at of the Pbdr, (b) in the region of the
tions of the films obtained from five precursor solutions with different Pb/Ti ratios were systematically analyzed. The results are presented in Fig. 7. The dashed line indicates the values when the Pb/Ti ratios of the final film and the precursor solution are equal. When the Pb/Ti = 2.0 in the precursor solution, the PbiTi ratio of the film is 1.55, which is higher than 1.0 but less that 2,O. This result suggests that extensive evaporation of Pb must have occurred. The film prepared with a Pb/Ti = 1.33 precursor solution also shows excess of lead (Pb/Ti = 1.53). This can be explained by the surface segregation of Pb& (Fig. 6) and/or the existence of TiOz (Fig. 2(b)) because TiOz does not distribute over the surface.
Binding Energy (CL’)
-loOqL~
522
534
526
528
530
532
534
536
538
Au
Binding Energy (eV)
Fig. 5. XPS spectra of the lead oxides thin film after annealing at 600°C in air for 1 h: (a) in the region of the Pbji, and tb) in the region of the O,,.
Y.-C. LA
er al. /Moterink
Chwnisq
md
Phpics
51 (1997)
147-151
151
4. Conclasions~ Pb,,
-tiO
ii,
I;4
Ii6
138
140
Binding
Energy (cV)
142
I;4
146
145
Ii0
Fig. 6. Pb,r photoelectron peaks for the thin Film prepared with the Pb/ Ti = 1.33 precursor solution after annealing at 600°C in air for 1 h.
Standard Raman and XPS spectra of PbTiO,, TiO, and Pb,O, thin films prepared by the sol-gel method are presented. The film characteristics were significantly influenced by composition of precursor solution. Surface segregation of Pb30A and the existence of TiO, were observed in the film deposited by a Pb-rich precursor solution. PbTi,O, should exist in the film prepared with excess Ti in the precursor solution. Stoichiometric films were obtained from the Pb/ Ti = 1 precursor solution. Moreover, the growth mechanism is governed by the competition between formation of different species. This competition is influenced by the free energy of formation 4 G and the Pb/Ti ratio. References
I
2,Q t
,’
PbKi
Ratio of Precursor
,’
Solution
Fig. 7. Relation between the Pb/Ti ratio of final lilms and the Pb/Ti ratio of precursor solutions. The dashed line indicates the values when the Pb/Ti ratios of the final film and the precursor solution are equal.
Fig. 6 also shows that the stoichiometric PbTiO, film can be obtained when the Pb/Ti= 1 in precursor solution. This finding is also confirmed by the Raman spectrum (Fig. 1(a) ). It is very interesting to note that a film with Pb/ Ti = 0.8 could be prepared from the precursor solution with the same Pb/Ti ratio. The Raman spectrum (Fig. 2(c)) of the Pb/Ti = 0.8 film is the same as that of the Pb/Ti = 1.O film (Fig. 1(a) ) and neither Pb,O, nor TiOz could be identified. Chen et al. [25] and Iijima et al. [26] reportedthat films annealed at temperatures higher than 570°C showed a significant formation of PbTi,O, phase when Pb is deficient. This finding suggests that PbTi,O, which is indistinguishable from PbTiO, in the Raman spectrum and XPS should exist to explain the observations in Raman spectrum (Fig. 2(c) ) and film composition analysis (Pb/Ti = 0.8) when Pb is deficient in this study. Therefore, that the film prepared with the Pb/Ti = 0.67 precursor solution results in a higherPb/Ti ratio of 0.73 is due to the existence of both PbTi307 and TiO, (Fig. 2(d) ) in the film.
[ 11 K. Tanaka. Y. Higuma. K. Yokoyama, T. Nakagawa and Y. Hamakawa, Jpn. J. Appl. Phys., 15 (1976) 1381. [ 21 S.B. Krupanidhi. Maffei andM. Sayer, J. Appl. Phya., 51( 1983) 6601. [3] K. Sreenivas and M. Sayer, J. Appl. Phys., 61 ( 1988) 1483. [A] S.B. Krupanidhi and M. Sayer, J. Vat. Sci. Technol., A2 (198-t) 303. [5] S.G. Yoon and K.G. Kim, J. Electrochem. Sot., 135 ( 1988) 3138. [6] S.G. Yoon, I.D. Park. J.H. Choi and H.C. Kim. J. Vat. Sci. Technol.. A9 (1991) 381. [7] M. Huffman, T.S. Kalkur, L. Kammerdiner, R. Kwor, L.L. Levenson and M. Reeder, J. Vat. Sci. Technol., Al 1 ( 1993) 1406. [8] J.B. Blum and S.R. Gurkovich, J. Mater. Sci., 20 ( 1985) 1479. [9] K.D. Budd, S.K. Dey and D.A. Payne, Br. Cer. Proc., 36 (1985) 107. [ lo] S.K. Dey and R. Zuleeg, Proc. Symp. Integr. Ferroelectr., I (1989) 189. [ 1 I] B. Jaber, D. Remiens and B. Thierry, J. Appl. Phys., 79 ( 1996) 1182. [ 121 R.N. Castellano and L.G. Feiustein, J. Appl. Phys., 50 (1979) 4406. [ 131 S.B. Krupanidhi, N. Maffei, M. Sayer and K. El-Assal, J. Appl. Phys., 54 (1983) 6601. [ 141 P.V. Thomas, V. Ramakrishnan and V.K. Vaidyan, Thin Solid Films, 170 (1987) 35. [ 151 L.S. Hsu, R. Rujkorakam, J.R. Sites and C.Y. She, J. Appl. Phys., 59 (1986) 3175. [ 161 R. Vuppuladhadium, H.E. Jackson and J.T. Boyd, J. Appl. Phys.. 73 ( 1993) 7887. [ 171 G.J. Exarhos and M. Aloi, Thin Solid Films, 1931194 (1990) 12. [ 181 I. Taguchi, A. Pignolet, L. Wang, M. Proctor, F. Levy and P.E. Schmid, J. Appl. Phys., 73 ( 1993) 394. [ 191 K.R. Bullock, G.M. Trischan and R.G. Burrow, J. Electrochem. Sot., 130 (1983) 1283. 1201 F.T. Wall, Chemical Thermodynamic. W.H. Freeman and Company, San Francisco, 1974. [21] T.L. Barr, in N.S. McIntyre (ed.), Quantitative Surface Analysis of Materials, Americanity for Testing and Materials, Philadelphia, 1978, p. 83. 1221 L.R. Pederson. J. Electron Spectrosc. Relat. Phenom., 28 ( 1982) 203. [23] KS. Kim, T.J. O’Leary and N. Winograd, Anal. Chem., 35 ( 1973) 2213. [24] D. Briggs and M.P. Seah, PracticalSurface Analysis by Auger and XRay Photoelectron Spectroscopy, John Wiley & Sons, New York, 1983. [25] C. Chen, D.F. Ryder and W.A. Spurgon, J. Am. Ceram. Sot., 72 (1989) 1495. [26] K. Iijima, Y. Tomita, R. Takayama and I. Veda, J. Appl. Phys., 60 (1986) 361.
Materials
Chemistry
and Physics 51 (1997)
147-151
Effect of precursor solution composition on lead titanate thin films prepared by a sol-gel method Yen-Chao Department
of Chemical
Engineering,
Lai ‘, Y.S. Gong, Chiapyng
National
Taiwan
Universig
Received21 December1996;revised
23
of Science
Lee *
and Technology,
Taipei 10572, TaiwaA, XOC
May 1997;accepted18June1997
Abstract PT films of different compositions were prepared by a sol-gel method and characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. Surface segregation of Pb,O, and the existence of TiOz were observed in the film deposlted by a Pb-rich precursor solution. PbTi,O, should exist in the film prepared with excess Ti in the precursor solution. Stoichiometric films were obtained from the Pb/Ti = 1 precursor solution. The growth mechanism which controls the film characterics is presented. 0 1997 Elsevier Science S.A. Kc>w,ords; Ferroelcctrics;PbTi03thin films; Sol-gel;Ramansprcuoscopy; XPS
1. Introduction Ferroelectric thin films have attracted considerable interest in recent years owing to their unique electrical and optical properties. Four deposition techniques have been used to prepare ferroelectric thin films: radio-frequency (rf) magnetron sputtering [ I-41, chemical vapor deposition (CVD) [5,6], ionized cluster beam deposition [7], and sol-gel growth [&lo]. In recent years, the sol-gel spin-on technique, with its merits of strict compositional control and low fabrication cost, has been utilized extensively to produce ferroelectric films primarily for memory applications [ 9, lo]. Since PbO has a relatively high vapor pressure, it can easily segregate to the surface and then evaporate. Therefore, the effect of lead oxide content of the target on the film composition was studied thoroughly, when the PbTiO, thin films were prepared by using the rf-magnetron sputtering method. Jaber et al. [ II] reported that evaporation of PbO competes, during growth and formation of PbTiO,, with reaction of PbO with TiOP. Castellano and Feinstein [ 121 observed Pb concentration decreased for substrate temperatures above 200°C. Krupanidhi et al. [ 131 also performed elemental analysis of films deposited from targets having different percentages of excess PbO. They found that the Pb/Ti ratio approached that of the desired material for films deposited from targets enriched with 10% excess PbO. However, the effa of lead * Correspondingauthor. ’ Presentaddress:Departmentof ChemicalEngineering,NationalTaipei University of Technology,Taipei,Taiwan, ROC. 0254-0584/97/$17.00Q 1997Elsevier ScienceS.A. All rights reserved P11SO254-0584(97)01985-S
content in the precursor solution on the composition of films prepared by the sol-gel method has never been reported. Raman spectroscopy is a non-destructive optical technique which can be used to characterize different phases. Several applications of laser Raman spectroscopy to the analysis of thin film compositions or phase transformations are reported [ 14-171. In this article, we employ the sol-gel method to prepare PbTi03 thin films with different lead contents in the precursor solution. The purpose of this work is to correlate the variation of film crystalline phase and chemical composition to the lead content of the sol-gel precursor solution.
2. Experimental We prepared the solution for film fabrication by the conventional sol-gel process, using lead acetate trihydrate and titanium n-butoxide as precursors, and 2-methoxyethanol as the solvent. Lead acetate trihydrate was dissolved in hot 2-methoxyethanol (70°C) at a 1:4 molar ratio. The solution was heated at 120°C for 20 min to allow the water of hydration to expel. The dehydrated solution was cooled to 90°C before a measured amount of titanium n-butoxide was added. The solution was reheated to 120°C and stirred for 5 min. Formamide ( 10 ~01%) was added to the solution; it is known to be a drying control chemical, which prevents cracking of the film. Precursor solutions of 2.0, 1.33, 1.0, 0.8 and 0.67 Pb/Ti ratios
118
Y.-C. Lai et al. /Murrrials
Chemistry
were prepared. Films of TiO, and lead oxides were prepared by the same procedure, except that lead acetate trihydrate and titanium n-butoxide were excluded, respectively. We used silicon wafers as substrates. An adhesion layer of 0.1 Frn Ti was grown using electron-beam evaporation after which a 0.1 pm thick layer of platinum was allowed to deposit. Pt was chosen because it resists oxidation. The substrate was spin-coated with the prepared solution. The rotation speed and the spin time were fixed at 3500 rpm and 10 s, respectively, to ensure that the film thickness was uniform on the substrate. After drying at room temperature for 2 h, the samples were preannealed at 300°C in air for 0.5 h. The films were then annealed in air for 1 h at 600°C and the thickness of the final film is about 1500 A. Raman spectra were recorded on a Renishaw RS-2000 spectrometer. XPS analyses were performed in a VG Microtech MT-500 spectrometer. The spectrometer was equipped with a hemispherical analyzer and all XPS data presented here were acquired using the Mg Ka X-rays ( 1253.6 eV) The electron take-off angle was maintained constant at 0”. The binding energies of various core levels were determined with respect to the adventitious C,, peak at 284.6 eV.
and Physics 51 11997) 147-151
;63 12.5
QJ)
509.0 389.9
:
/
A
3. Results and discussion Fig. 1 (a) shows the Raman spectrum for the PbTiO, thin film. The spectrum shows peaks at 283.3, 325.7, 498.4 and 601.5 cm-‘, which are characteristic of PbTiO? thin film [ I8 1. Fig. 1 (b) shows the Raman spectrum of the titania film. The appearance of Raman-active lines near 359.9,509.0 and 632.5 cm-’ is indicative of the anatase crystalline phase [ 17 1. In addition, a Raman spectrum from the lead oxide film is displayed in Fig. 1cc), A peak at 267.9 cm- ’ is due to the orthorhombic PbO and the peak appearing at 348.0 cm - ’ is attributed to tetragonal PbO, whereas peaks at 519.7, 543.0 and 591.7 cm- I are characteristic of Pb,Oj [ 191. Fig. 2(a)-(d) depicts a series of Raman spectra of a series of samples which were prepared with precursor solutions having different Pb/Ti ratios and annealed at 600°C for 1 h. Fig. 2(a) shows the Raman spectrum of the thinfilmprepared with a Pb/Ti=2 precursor solution. In addition to PbTiO,(PT) and Pb,O,, TiOz is clearly observed. Raman spectrum for the film prepared from a Pb/Ti = 1.25 precursor solution is shown in Fig. 2(b), which resembles that shown in Fig. 2 ( a) ; PbTiO,, Pb,Od and TiOz phases are all present, but peak intensities are different. In Fig. 2(a), the peak intensity of PbTi03 at 283.3 cm-’ is comparable with that of Pb,O, at 519.7 cm-‘. However, in Fig. 2(b) the dominant peak is at 283.3 cm-’ which is due to PbTiO, whereas the Pb,O, peak at 5 19.7 cm-’ becomes weaker. This variation of peak intensities indicates the decrease of Pb content in the film. Fig. 2(c) and (d) gives the Raman patterns for the thin films obtained from Ti-rich precursor solutions. When the precursor solution is prepared with a Pb/Ti ratio of 0.8 the
i’ 519.7
(cl
591.7
543
267.9
348.0
400
600
Delta
800
1000
wave number
(cm,])
Fig. 1. Raman spectra of thin films of: (a) stoichiometric and (4 iead oxides.
1200
PbTiO,,
i b) TiO,,
Raman spectrum of the film (Fig. 2(c) ) shows only the characteristic peaks of PT. In addition, when the Pb/Ti ratio is 0.67 in the precursor solution, TiOz exists simultaneously in two phases (Fig. 2(d)). Interestingly, as can be seen in Fig. 2(a) and (b) where Pb content is rich, TiOz can also be observed. However, Pb30, is absent when Ti is rich in the precursor solutions (Fig. 2(c) and (d) ) . Based on the Raman patterns. we interpret that the growth mechanism is governed by the formation of different species. A competition exists between the formationof PbTi03, Pb30, and TiO?. This competition is influenced by the free energy of formation A G and the Pb/Ti ratio. The free energy change AG of Pb30J, TiOz and PbTiO, is - 143.7, -212.6 and - 257.51 kcal mol- ’ [20], respectively. Therefore, according to free energy change A G, the tendency toward formation
Y.-C. Lcri et al. /Maierials
Citemisr~
and Physics 51 (1997)
147-151
119
Pb,O, PT
PT +
L
PbrO,
TiOz(A)
PT
R
(4
PT
(b)
PT+TiO,(A)
PbLh PT QJO<
\
d
,
400
I
,
600 Deltn
1000
800 HRW
number
Fig. 2. Raman spectra of thin films prepared
I
s
400
1200
Delta
(cm.‘)
with precursor
solutions
having:
of the phases varies as PbTiO, > TiOz > Pb30,. This indicates that when Pb is rich (Fig. 2(a) and (b)), PbTi03 phase should dominate, and the appearance of Pb304 is excepted. In addition, the appearance of TiO, is due to its superior tendency toward formation than Pb30S. On the contrary, when Ti is in excess (Fig. 2(c) and (d) ) only PbTi03 and TiO, exist, and Pb,O, disappears. The wide scan XPS spectra of all samples in the binding energy ranging from O-1000 eV indicate that no impurity is present in the spectrum, except carbon. The Pb,f,Ti,, and O,, core level spectra of the PT thin film are shown in Fig. 3 (a)(c), respectively. The peak of Pb4f(7,2j at 137.2 eV is observed in Fig. 3(a). The spin-orbit splitting A (4f(7/ 2) -4f( 5/2) ) is 4.9 eV. According to Fig. 3 (b) the binding energy of Ti2pt 3/2) is 457.5 eV and the spin-orbit splitting A(2p(3/2)-2p(1/2)) is5.9eV.Fig.3(~) revealsthato,, is an asymmetric peak, possibly composed of more than one component peak. A deconvolution method was applied, in which the O,, spectrum was assumed to comprise two overlapping peaks. Each component peak in the spectrum was fitted to a Gaussian-Lorentzian type distribution with lower binding energy (LBE) and high binding energy (HBE) peaks at 529.3 eV and 530.6 eV, respectively. The composite curve of the two components closely corresponds the experimental spectrum. Following Barr [ 211, the LBE and the HBE peaks can be assigned to the oxide and the hydroxide/
adsorbed oxygen species, respectively.
600
(a) PbiTi
wake
I
9
800
1000
number
= 2.0, (b) Pb/Ti = 1.33, (c) PbiTi
I
1
1200
(crn,l)
= 0.8, and (d) PbiTi
= 0.67.
Fig. 4 gives the core level spectra of the TiO, film. According to Fig. 4(a) the binding energy of Tilp( 3,2j is 458.5 eV and the spin-orbit splitting A (2p( 312) -2p( 112)) is 5.7 eV. Fig. 4(b) shows that the LBE and HBE peaks of O,, are at 529.6 and 53 1.O eV, respectively. Core level spectra of the film of lead oxides-are shown in Fig. 5. Inspection of the Pb,, peaks (Fig. 5(a)) shows that the peak comprises several doublets, ascribable to various lead oxides. The wider FWHM of the XPS spectrum of the O,, peak can also be ascribed to various lead oxides. The existence of Pb30A and PbO has been confirmed by the Raman spectrum (Fig. 1(c) ) . The corresponding binding energies are similar to those obtained by other researchers [ 22,231. It is worthwhile to point out that most of core level spectra of the films prepared with precursor solutions of different Pb/ Ti ratios resemble those of PbTiO, film shown previously in Fig. 3; neither TiOz nor PbO could be observed on the surface of any sample. In addition, Pb,O, was not present on films prepared from precursor solutions with Pb/Ti ratios less than 1.33. However, the Pbdr core level spectrum (Fig. 6) of the film prepared with the Pb/Ti = 1.33 precursor solution shows the existence of Pb,O, on the surface. Similar result was also obtained for the film prepared with the Pb/Ti = 2.0 precursor solution. The film compositions were estimated from the XPS spec-
tra of each component after normalizing the peak area with the respective relative sensitivity factor [ 241. The composi-
Y.-C. Lai
et 01. lhf~ttrrials
Chetnistty
und Physics
51 (1997)
-2%0
147-151
455
4.54
46s
Binding Ewgy
$312 457.5
/
is; ( \ (b> i
470
{et’)
Ti2,
\
1
I
i i
lccxw-
]
4cca 20%
531 i
o.2w922 %O I
452
454
456
458
460
.462
464
466
468'--
524
526
-
'L
528
530
'L. 532
_ 534
536 1
-370 , Bindii
Binding Er.crgy (ev)
Energy (cv)
Fig. 4. XPS spectra of the TiOl thin film after annealing at 600°C in air for 1 h: (a) in the region of the Ti,,, and (b) in the region of the O,,.
XlIT
iam0 I-
7
4l7R lmxl14wo-
Fig. 3. XPS spectra of the stoichiometric 600°C in air for 1 h: (a) in the region Tizp, and (c) in the region of the O,,.
PbTiO, thin film after annealing at of the Pbdr, (b) in the region of the
tions of the films obtained from five precursor solutions with different Pb/Ti ratios were systematically analyzed. The results are presented in Fig. 7. The dashed line indicates the values when the Pb/Ti ratios of the final film and the precursor solution are equal. When the Pb/Ti = 2.0 in the precursor solution, the PbiTi ratio of the film is 1.55, which is higher than 1.0 but less that 2,O. This result suggests that extensive evaporation of Pb must have occurred. The film prepared with a Pb/Ti = 1.33 precursor solution also shows excess of lead (Pb/Ti = 1.53). This can be explained by the surface segregation of Pb& (Fig. 6) and/or the existence of TiOz (Fig. 2(b)) because TiOz does not distribute over the surface.
Binding Energy (CL’)
-loOqL~
522
534
526
528
530
532
534
536
538
Au
Binding Energy (eV)
Fig. 5. XPS spectra of the lead oxides thin film after annealing at 600°C in air for 1 h: (a) in the region of the Pbji, and tb) in the region of the O,,.
Y.-C. LA
er al. /Moterink
Chwnisq
md
Phpics
51 (1997)
147-151
151
4. Conclasions~ Pb,,
-tiO
ii,
I;4
Ii6
138
140
Binding
Energy (cV)
142
I;4
146
145
Ii0
Fig. 6. Pb,r photoelectron peaks for the thin Film prepared with the Pb/ Ti = 1.33 precursor solution after annealing at 600°C in air for 1 h.
Standard Raman and XPS spectra of PbTiO,, TiO, and Pb,O, thin films prepared by the sol-gel method are presented. The film characteristics were significantly influenced by composition of precursor solution. Surface segregation of Pb30A and the existence of TiO, were observed in the film deposited by a Pb-rich precursor solution. PbTi,O, should exist in the film prepared with excess Ti in the precursor solution. Stoichiometric films were obtained from the Pb/ Ti = 1 precursor solution. Moreover, the growth mechanism is governed by the competition between formation of different species. This competition is influenced by the free energy of formation 4 G and the Pb/Ti ratio. References
I
2,Q t
,’
PbKi
Ratio of Precursor
,’
Solution
Fig. 7. Relation between the Pb/Ti ratio of final lilms and the Pb/Ti ratio of precursor solutions. The dashed line indicates the values when the Pb/Ti ratios of the final film and the precursor solution are equal.
Fig. 6 also shows that the stoichiometric PbTiO, film can be obtained when the Pb/Ti= 1 in precursor solution. This finding is also confirmed by the Raman spectrum (Fig. 1(a) ). It is very interesting to note that a film with Pb/ Ti = 0.8 could be prepared from the precursor solution with the same Pb/Ti ratio. The Raman spectrum (Fig. 2(c)) of the Pb/Ti = 0.8 film is the same as that of the Pb/Ti = 1.O film (Fig. 1(a) ) and neither Pb,O, nor TiOz could be identified. Chen et al. [25] and Iijima et al. [26] reportedthat films annealed at temperatures higher than 570°C showed a significant formation of PbTi,O, phase when Pb is deficient. This finding suggests that PbTi,O, which is indistinguishable from PbTiO, in the Raman spectrum and XPS should exist to explain the observations in Raman spectrum (Fig. 2(c) ) and film composition analysis (Pb/Ti = 0.8) when Pb is deficient in this study. Therefore, that the film prepared with the Pb/Ti = 0.67 precursor solution results in a higherPb/Ti ratio of 0.73 is due to the existence of both PbTi307 and TiO, (Fig. 2(d) ) in the film.
[ 11 K. Tanaka. Y. Higuma. K. Yokoyama, T. Nakagawa and Y. Hamakawa, Jpn. J. Appl. Phys., 15 (1976) 1381. [ 21 S.B. Krupanidhi. Maffei andM. Sayer, J. Appl. Phya., 51( 1983) 6601. [3] K. Sreenivas and M. Sayer, J. Appl. Phys., 61 ( 1988) 1483. [A] S.B. Krupanidhi and M. Sayer, J. Vat. Sci. Technol., A2 (198-t) 303. [5] S.G. Yoon and K.G. Kim, J. Electrochem. Sot., 135 ( 1988) 3138. [6] S.G. Yoon, I.D. Park. J.H. Choi and H.C. Kim. J. Vat. Sci. Technol.. A9 (1991) 381. [7] M. Huffman, T.S. Kalkur, L. Kammerdiner, R. Kwor, L.L. Levenson and M. Reeder, J. Vat. Sci. Technol., Al 1 ( 1993) 1406. [8] J.B. Blum and S.R. Gurkovich, J. Mater. Sci., 20 ( 1985) 1479. [9] K.D. Budd, S.K. Dey and D.A. Payne, Br. Cer. Proc., 36 (1985) 107. [ lo] S.K. Dey and R. Zuleeg, Proc. Symp. Integr. Ferroelectr., I (1989) 189. [ 1 I] B. Jaber, D. Remiens and B. Thierry, J. Appl. Phys., 79 ( 1996) 1182. [ 121 R.N. Castellano and L.G. Feiustein, J. Appl. Phys., 50 (1979) 4406. [ 131 S.B. Krupanidhi, N. Maffei, M. Sayer and K. El-Assal, J. Appl. Phys., 54 (1983) 6601. [ 141 P.V. Thomas, V. Ramakrishnan and V.K. Vaidyan, Thin Solid Films, 170 (1987) 35. [ 151 L.S. Hsu, R. Rujkorakam, J.R. Sites and C.Y. She, J. Appl. Phys., 59 (1986) 3175. [ 161 R. Vuppuladhadium, H.E. Jackson and J.T. Boyd, J. Appl. Phys.. 73 ( 1993) 7887. [ 171 G.J. Exarhos and M. Aloi, Thin Solid Films, 1931194 (1990) 12. [ 181 I. Taguchi, A. Pignolet, L. Wang, M. Proctor, F. Levy and P.E. Schmid, J. Appl. Phys., 73 ( 1993) 394. [ 191 K.R. Bullock, G.M. Trischan and R.G. Burrow, J. Electrochem. Sot., 130 (1983) 1283. 1201 F.T. Wall, Chemical Thermodynamic. W.H. Freeman and Company, San Francisco, 1974. [21] T.L. Barr, in N.S. McIntyre (ed.), Quantitative Surface Analysis of Materials, Americanity for Testing and Materials, Philadelphia, 1978, p. 83. 1221 L.R. Pederson. J. Electron Spectrosc. Relat. Phenom., 28 ( 1982) 203. [23] KS. Kim, T.J. O’Leary and N. Winograd, Anal. Chem., 35 ( 1973) 2213. [24] D. Briggs and M.P. Seah, PracticalSurface Analysis by Auger and XRay Photoelectron Spectroscopy, John Wiley & Sons, New York, 1983. [25] C. Chen, D.F. Ryder and W.A. Spurgon, J. Am. Ceram. Sot., 72 (1989) 1495. [26] K. Iijima, Y. Tomita, R. Takayama and I. Veda, J. Appl. Phys., 60 (1986) 361.