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About the defect structure in differently doped PZT ceramics: A temperature dependent positron lifetime study
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 9127–9131 www.elsevier.com/locate/ceramint
About the defect structure in differently doped PZT ceramics: A temperature dependent positron lifetime study K. Drogowskaa,b,n, M. Elsayedc,d, R. Krause-Rehbergc, A.G. Balogha b
a Institute of Materials Science, Technische Universität Darmstadt, Petersenstrasse 23, 64287 Darmstadt, Germany J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, Prague 182 23, Prague-8, Czech Republic c Institute for Physics, Martin-Luther-Universität Halle, Von-Danckelmann-Platz 3, 06120 Halle (Saale), Germany d Department of Physics, Faculty of Science, Minia University, 61519 Minia, Egypt
Received 5 December 2013; received in revised form 24 January 2014; accepted 27 January 2014 Available online 5 February 2014
Abstract Pure and doped PZT ceramics (PZT:La þFe, PZT:La, PZT:Gd, PIC 151 and with 0.1, 0.25, 0.5, 1.0 mol% Fe doped samples) have been examined by Positron Annihilation Lifetime Spectroscopy (PALS) in the range of temperatures between 150 and 375 K. It was found that the defect-related lifetime increased with increasing temperature, indicating vacancy-like defects. With increasing Fe doping, a loss of vacancy agglomerations was observed, as well as a weaker dependence of lifetime on temperature. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Implantation; B. Defects; D. Perovskites; D. PZT
1. Introduction High permittivity dielectric materials have achieved wide usage in recent years because of their superior properties. Due to their high dielectric constant, high piezoelectric coefficient and spontaneous polarization, they have been widely investigated for possible applications in microelectronics as capacitors, randomaccess memory (FeRAM and F-RAM), actuators for fine displacement systems and sensors [1,2]. The most common compositions that are currently extensively used are perovskite ferroelectric oxides with a general formula ABO3, e.g. Pb(Zr,Ti) O3 (PZT) [3–5]. Various modifications, such as doping or hydrogen charging can be performed in order to improve the piezoelectric properties or formation of new structure [6,7]. Dopants can replace the ions in the lattice and, depending on ion radii, they can substitute the A þ 2 (lead) and B þ 4 (titanium/ zirconium) sites, creating the A or B site vacancies for charge n Corresponding author at: J. Heyrovský Institute of Physical Chemistry, v.v. i., Academy of Sciences of the Czech Republic, Dolejškova 3, Prague 182 23, Prague-8, Czech Republic. Tel.: þ 420 266052113; fax: þ 420 286582307. E-mail address: [email protected] (K. Drogowska).
http://dx.doi.org/10.1016/j.ceramint.2014.01.127 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
compensation. High valence dopants (La þ 3, Nb þ 5) are donors due to electrons contribution, and their substitution to A and B sites, respectively, create the A vacancies. Conversely, ions like Na þ and Fe þ 3 act as an acceptor, and their substitution into the A and B sites, respectively, create the O-vacancies [8–10]. The kinetics of oxygen vacancies has been already investigated and described [11]. Doping with different elements changes the physical properties of PZT ceramics. Namely, the electromechanical properties of PZT ceramics can be improved by the use of additives of rare earth elements, e.g. the incorporation of La ions into PZT ceramics increases their dielectric constants. Moreover, also point defects influence the physical properties of materials and doping of ferroelectrics creates different defect structures. Hotpressed ferroelectric PZT ceramics are suitable for electrooptical applications due to their high transparencies. The transparency seems to rely on the defect structure caused by the incorporation of La2 þ ions into the perovskite lattice [12]. Therefore it is necessary to study the different defect concentrations and structures in PZT ceramics that are differently doped. Positrons are sensitive to the electrical potential of point defects and have proved to be a valuable nondestructive probe
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K. Drogowska et al. / Ceramics International 40 (2014) 9127–9131
for detection of vacancy-type defects. Since they react with negative ions, they are excellent tool to investigate the ionized materials. Due to weak binding energy (⪢1 eV), positrons are effectively trapped only at low temperatures. At higher temperatures, they are thermally detrapped. During the past decades, Positron Annihilation Lifetime Spectroscopy (PALS) has been intensively applied to characterize defects in various semiconductors [13]. However, in many cases it is difficult to conclude the defect type only from the annihilation parameters. The point defect complexes with a dipole moment have been taken into account to be of particular relevance both for aging and in relaxor ferroelectrics [14–16]. Relaxor behavior in La doped PZT has been ascribed to their presence [17] and a recent study of fatigue behavior deduced the existence of VB type defects from the activation energy associated with the fatigue induced response [18]. Both lead vacancy (VPb) and B-site vacancy (VB) in PZT were then observed. Positron trapping in B-site vacancies was inferred in PZT. In Fe doped PZT the fraction of positrons trapped in VPb-type defects compared to VB vacancies was decreased [19]. The positron annihilation technique operates the principle that positrons can be trapped during the diffusion by lattice defects. These techniques have specific sensitivity to open volume defects, varying in size from monovacancies to vacancy clusters. Positrons which are trapped by open volume defects alter their annihilation characteristics. The methods are well established for the study of metals and semiconductors [13,20], but application to oxide materials has been limited. Studies of variable energy positron annihilation spectroscopy have shown sensitivity to vacancy defects formed due to oxygen deficiency and to La doped PZT samples [21]. Previous PALS measurements on ceramic PZT samples gave evidence of a long defect lifetime attributed to lead vacancies [22]. A PALS study in La and Nb doped rhombohedral Pb(Zr0.6Ti0.4)O3 ceramics with a dominant defect lifetime in the region of 300 ps has also been reported [23]. After thermalization, positrons annihilate in the material from a state i with a lifetime τi and relative intensity Ii. This can be a delocalized state in the bulk lattice or a localized state in an open volume defect (e.g. vacancy). If the average lifetime τav ¼ ∑i I i τi is higher than the bulk lattice lifetime τb, where τb ¼ 1/λb (the annihilation rate) and it is characteristic for the material, it indicates that vacancy type defects are present. The rate of positron trapping into a vacancy defect, Kd, is proportional to the defect concentration (C). The constant of proportionality is the specific trapping coefficient μ. The positron capture by one defect type is described by the simple two-state trapping model, as follows (for more details see Ref. [13]): κd ¼ μC ¼ I 2
1 1 I2 1 1 ¼ τ1 τ2 I 1 τb τd
ð1Þ
It predicts two experimental lifetimes; the first one is reduced from the bulk lifetime by an amount that depends on the trapping rate of the defects, τ1 oτb, while the second one is the
characteristic defect component τ2 ¼ τd. If the two-state trapping model is applicable, the bulk lifetime can be calculated from the measured lifetimes and intensities, as follows: 1 I1 I2 ¼ þ τb τ1 τ2
ð2Þ
As the defect concentration increases, I2 can reach the saturation value, which means that all of positrons annihilated from the defects, and the sensitivity for defect concentration are lost. This leads to occurrence of trapping saturation. The defect concentration that the saturation occurs at depends on the values of both τb and τd. 2. Experimental The Pb(Zr0.46Ti0.54)O3 ceramic samples were prepared by a conventional solid state process from oxide precursors [24]. The samples were doped with other elements, i.e. La, Fe, Gd and both La þ Fe. The PIC 151 sample was produced by PI Ceramics. This ceramic is a modified lead zirconate–lead titanate material with high permittivity, high coupling factor and high piezoelectric charge constant. PZT:La þ Fe, PZT:La, PZT:Gd, PIC 151, PZT undoped and 0.1, 0.25, 0.5 and 1.0 mol% Fe doped samples were investigated by the PALS method. The positron lifetime measurements, in the temperature range of 120–425 K, were performed using a fast–fast coincidence spectrometer [13,25,26] with a time resolution of 225 ps (FWHM). 22NaCl was covered by a 2 mm thick aluminum foil on each side of the source and sandwiched between two identical samples. The activity of the used positron source was 1.11 MBq (30.0 μCi). Each measurement lasted 5 h what resulted with a lifetime spectrum of 4.0 106 coincidence counts. This high statistics is preferred when the computer code LT9.0 [27,28] is used for analyzing the lifetime spectra. The lifetime spectrum was analyzed as a sum of exponential decay components, nðtÞ ¼ ∑i I i exp ð t=τi Þ convoluted with a Gaussian describing the time resolution function of the spectrometer. The source correction of 13% (Al foil) and the time resolution were determined by measuring a semi-insulating GaAs reference sample, showing no positron traps. The spectra were analyzed with the two-state trapping model (one defect type) after source and background correction. For more details see Refs. [29,30]. 3. Results and discussion The results of the average positron lifetime in the function of temperature for differently doped PZT samples (PZT:La þ Fe, PZT:La, PZT:Gd and PIC 151) are illustrated in Fig. 1. The average lifetime increases with increasing temperature, indicating the detection of vacancy-like defects. Moreover, the defectrelated lifetime increases with increasing measurement temperature, as it is presented in Table 1. The presented data indicates that since the intensity I2 of the defect-related lifetime decreases, the concentration of the vacancy-like defects decreases.
K. Drogowska et al. / Ceramics International 40 (2014) 9127–9131
For this set of samples, τd at 150 K was found to be about 290 ps, what corresponds to single lead vacancy VPb or VPb–VO divacancy, or may be a mixture of them. This result cannot be separated from positron lifetime data alone because of the similar lifetime values of these types of defects [23,31]. As mentioned earlier, the longer lifetime component increases with increasing measurement temperature but its relative intensity decreases. This indicates the existence of defect agglomerations, e.g. double vacancies. PZT:La sample exhibits a higher defect lifetime value of 356 ps at 375 K; however, the lifetime in PZT:La þ Fe sample is less than that in PZT:La. This can be attributed to the charge transfer in the co-doped samples, which increases the electron density and reduces the positron lifetime. Due to the high defect concentration (related to I2) and the high mobility of oxygen vacancies, it is more likely that the defect lifetime is arising from a mixture of VPb and VPb–VO [23]. Fig. 2 shows the lifetime parameters, as deconvoluted from the measured lifetime spectra, versus the measurement temperature for an undoped Pb(Zr0.46Ti0.54)O3 sample. As it is shown, both τ1 and τ2 increase with increasing temperature, showing saturation behavior above 250 K. At the same time, I2 decreases down to the measurement temperature
Fig. 1. Average positron lifetime as a function of measuring temperature in differently doped PZT samples.
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Fig. 2. Temperature dependence of lifetime components and intensity of the defect-related component in an undoped PZT samples.
Fig. 3. Average positron lifetime as a function of the temperature in differently doped PZT:Fe samples.
Table 1 Temperature dependence of defect-related lifetimes and its relative intensities for differently doped PZT samples. Sample
Temperature (K)
Defect-related lifetime (ps)
I2 (%)
PIC 151
150 300 375 150 300 375 150 300 375 150 300 375
293 319 332 295 313 328 303 320 356 287 295 317
70.2 46.2 35.8 68.5 59.7 45.1 59.5 54 29 66.5 59.7 39.5
PZT:Gd
PZT:La
PZT:LaþFe
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Table 2 Temperature dependence of positron annihilation lifetimes and the intensity of the defect-related component in differently doped PZT:Fe samples. Sample
Temperature (K)
τ1 (ps)
τ2 (ps)
I2 (%)
PZT:Fe (0.10%)
150 200 280 325 150 200 280 325 150 200 280 325
166.2 188.4 191.3 191.2 208.2 214.7 217.1 218.3 215.0 216.0 216.0 216.3
215.3 219.8 236.6 244.4 – – – – – – – –
82.7 72.8 51.0 45.6 – – – – – – – –
PZT:Fe (0.25%)
PZT:Fe (0.50%)
of 250 K. τ1 and τ2 have saturation behavior with lifetimes of about 190 and 260 ps, respectively. Fig. 3 presents the temperature dependence of the average positron lifetime in undoped and doped with different Fe concentration PZT samples. An increase in the average lifetime with temperature is clearly shown for all of the samples, but this correlation is weaker in case of higher Fe concentrations. As it was published by Puff et al. [32], the changes in the mean lifetime is about 7 ps/100 K for undoped sample, and 2.5 ps/ 100 K for sample doped with 1 mol% Fe. Similar values were previously measured also for GaAs [33,34]. PZT ceramics with a band gap of 3.5 eV are ranked between high gap semiconductors and isolators; thus, they cannot be directly compared to GaAs. In this case, the weaker temperature dependence and the lower annihilation fraction, especially at higher temperatures for Fe-doped samples, may be explained as a consequence of the detrapping from extended Rydberg's states. Since the space charges in insulating or semiconducting materials are well known to distort the local electric field and affect high-field conduction, as well as other electrical properties [35], these results may be significant for further investigation of these materials. At low Fe doping concentration (0.1mol%), I2 decreases with increasing measurement temperature as shown in Table 2. But the defect lifetime (τ2) increases to values corresponding with the Pb vacancy agglomeration [22]. For higher Fe concentrations it was no longer possible to deconvolute the spectra into two lifetime components. A potential explanation could be that at higher Fe concentrations the lifetime will be shorter because of the stronger charge transfer effects and the computer code is not able to separate between the two lifetime values anymore. This is contrary to measurements on samples doped with La and Nb, where for doping concentrations higher than 0.5 mol% a trapping on Pb vacancies was still observed [23]. This behavior is in good agreement with our measurements for La, Gd and La þ Fe doped samples. With increasing Fe doping concentration it is clearly visible that the behavior of lifetime that was strongly correlated to the temperature for low-doping concentration becomes constant. The same observation was made by Puff et al. [32], where measurements have
been performed down to 10 K. In this region of temperatures, between 10 and 150 K, the similar slopes have been observed for undoped, as well as with 1 mol% Fe doped samples. These results reflect the changes in the charging state of defects and defects agglomerates. For higher temperatures and increase in Fe concentration, the average positron lifetime has resulted in the weakening of temperature dependence. This may be related to electrical charge of the Fe ions weakening the reaction between positrons and negative ions in the PZT ceramic. 4. Conclusions PALS measurements for PIC 151, PZT:Gd, PZT:La and PZT: Laþ Fe and PZT:Fe with different Fe concentrations have been performed. The increase of defect-related positron lifetime and decrease of relative intensity with increasing temperatures, indicating the existence of defect agglomerations, have been observed. For all of the samples, τd at 150 K is about 290 ps, what corresponds to the Pb vacancy or the VPb–VO divacancy, or their mixture. The lifetime of the co-doped sample was slightly lower than the lifetimes in the other samples. Doping with increasing Fe concentration results in weakening of the average lifetime dependence on temperature. The obtained lifetime values are lower than those for Pb vacancies (280–300 ps), suggesting the decrease of vacancy concentration. This behavior has not been observed for rare-earth elements doped samples. References [1] H. Haertling, Ferroelectric ceramics: history and technology, J. Am. Ceram. Soc. 82 (1999) 797. [2] S.M. Spearing, Materials issues in microelectromechanical systems (MEMS), Acta Mater. 48 (2000) 179. [3] S.L. Swartz, T.R. Shrout, W.A. Schulze, L.E. Cross, Dielectric properties of lead magnesium niobate ceramics, J. Am. Ceram. Soc. 67 (1984) 311. [4] N.A. Pertsev, G. Arlt, A.G. Zembilgotov, Prediction of a giant dielectric anomaly in ultrathin polydomain ferroelectric epitaxial films, Phys. Rev. Lett. 76 (1996) 1364. [5] B.G. Kim, S.M. Cho, T.Y. Kim, H.M. Jang, Giant dielectric permittivity observed in Pb-based perovskite ferroelectrics, Phys. Rev. Lett. 86 (2001) 3404.
K. Drogowska et al. / Ceramics International 40 (2014) 9127–9131 [6] A. Shafiei, C. Oprea, T. Troczynski, A. Bandyopadhyay, Investigation of the effects of high-pressure hydrogen on Pb(Zr,Ti)O3 (PZT) ceramics, J. Am. Ceram. Soc. 95 (2012) 782. [7] K. Drogowska, S. Flege, D. Rogalla, H.-W. Becker, E. Ionescu, N.-T. H. Kim-Ngan, A.G. Balogh, Hydrogen content analysis in hydrogencharged PZT ferroelectric ceramics, Solid State Ion. 235 (2013) 32. [8] Y. Xu, Ferroelectric Materials and Their Applications, Elsevier Science, New York, 1991. [9] F. Kulcsar, Electromechanical properties of lead titanate zirconate ceramics with lead partially replaced by calcium or strontium, J. Am. Ceram. Soc. 42 (1959) 343. [10] M. Takahashi, Effects of impurities on the mechanical quality factor of lead zirconate titanate ceramics, Japan. J. Appl. Phys. 10 (1971) 643. [11] S. Gottschalk, H. Hahn, S. Flege, A.G. Balogh, 18-Oxygen vacancy kinetics in ferroelectric PZT ceramics, J. Appl. Phys. 104 (2008) 114106. [12] S.-H. Lee, Ch.-B. Yoon, S.-B. Seo, H.-E. Kim, Effect of lanthanum on the piezoelectric properties of lead zirconate titanate–lead zinc niobate ceramics, J. Mater. Res. 18 (2003) 1765. [13] R. Krause-Rehberg, H.S. Leipner, Positron Annihilation in Semiconductors, Springer-Verlag, Berlin, 1999. [14] X. Ren, Large electric-field-induced strain in ferroelectric crystals by reversible domain switching, Nat. Mater. 3 (2004) 91. [15] S. Poykko, D.J. Chadi, Ab initio study of 1801 domain wall energy and structure in PbTiO3, Phys. Rev. Lett. 83 (1991) 1231. [16] E. Cockayne, B.P. Burton, Dipole moment of a Pb–O vacancy pair, Phys. Rev. B 69 (2004) 144116. [17] T.Y. Kim, H.M. Jang, The B-site vacancy as the origin of spontaneous normal-to-relaxor ferroelectric transitions in La-modified PbTiO3, Appl. Phys. Lett. 77 (2000) 3824. [18] C. Verdier, F.D. Morrison, D.C. Lupascu, J.F. Scott, Fatigue studies in compensated bulk lead zirconate titanate, J. Appl. Phys. 97 (2005) 024107. [19] D.J. Keeble, S. Singh, R.A. Mackie, M. Morozov, S. McGuire, D. Damjanovic, Cation vacancies in ferroelectric PbTiO3 and Pb(Zr,Ti) O3: a positron annihilation lifetime spectroscopy study, Phys. Rev. B 76 (2007) 144109. [20] P. Hautajarvi, Positrons in Solids, Springer, Berlin, 1979. [21] D.J. Keeble, B. Nielsen, A. Krishnan, K.G. Lynn, S. Madhukar, R. Ramesh, C.F. Young, Vacancy defects in (Pb,La)(Zr,Ti)O3 capacitors observed by positron annihilation, Appl. Phys. Lett. 73 (1998) 318.
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[22] R. Kagimura, D.J. Singh, Ab initio study of Pb antisite defects in PbZrO3 and Pb(Zr,Ti)O3, Phys. Rev. B 78 (2005) 174105. [23] S. Gottschalk, H. Hahn, A.G. Balogh, W. Puff, H. Kungl, M. J. Hoffmann, A positron lifetime study of lanthanum and niobium doped Pb(Zr0.6Ti0.4)O3, J. Appl. Phys. 96 (2004) 7464. [24] M. Morozov, D. Damjanovic, N. Setter, The nonlinearity and subswitching hysteresis in hard and soft PZT, J. Eur. Ceram. Soc. 25 (2005) 2483. [25] O.E. Mogensen, Positron Annihilation in Chemistry, Springer-Verlag, Berlin, Heidelberg, New York, 1995. [26] Y.C. Jean, P.E. Mallon, D.M. Schrader (Eds.), Principles and Application of Positron and Positronium Chemistry, World Scientific, Singapore, 2003. [27] J. Kansy, Microcomputer program for analysis of positron annihilation lifetime spectra, Nucl. Instrum. Methods Phys. Res. Sect. A 374 (1996) 235. [28] J. Kansy, LT for Windows, Version 9.0, Inst. of Phys. Chem. of Metals, Silesian University, Bankowa 12, PL-40-007 Katowice, Poland, March 2002, private communication. [29] M. Elsayed, V. Bondarenko, K. Petters, J. Gebauer, R. Krause-Rehberg, Vacancy generation during Cu diffusion in GaAs, J. Appl. Phys. 104 (2008) 103526. [30] G. Dlubek, J. Pionteck, M. Sniegocka, E.M. Hassan, R. Krause-Rehberg, Temperature and pressure dependence of the free volume in the perfluorinated polymer glass CYTOP: a positron lifetime and pressure– volume–temperature study, J. Polym. Sci. B: Polym. Phys. 45 (2007) 2519. [31] V.J. Ghosh, B. Nielsen, T. Friessnegg, Identifying open-volume defects in doped and undoped perovskite-type LaCoO3, PbTiO3 and BaTiO3, Phys. Rev. B 61 (2000) 207. [32] W. Puff, N. Balke, H. Kungl, M.J. Hoffmann, A.G. Balogh, Effect of Fedoping and electrical load on the defect structure of Pb(ZrxTi1 x)O3 ferroelectric ceramics, Phys. Status Solids C 10 (2007) 3839. [33] V. Bondarenko, Positron annihilation study of equilibrium point defects in GaAs (Ph.D. thesis), University Halle, 1994. [34] M.J. Puska, C. Corbel, R.M. Nieminen, Electron–positron Car–Parrinello methods: self-consistent treatment of charge densities and ionic relaxations, Phys. Rev. B 41 (1990) 9980. [35] H.L.W. Chan, W.K. Chan, Y. Chen, C.L. Choy, Thermal hysteresis in the resistivity of P(VDF-TrFE) copolymers, Ferroelectrics 196 (1997) 141.
CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 9127–9131 www.elsevier.com/locate/ceramint
About the defect structure in differently doped PZT ceramics: A temperature dependent positron lifetime study K. Drogowskaa,b,n, M. Elsayedc,d, R. Krause-Rehbergc, A.G. Balogha b
a Institute of Materials Science, Technische Universität Darmstadt, Petersenstrasse 23, 64287 Darmstadt, Germany J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, Prague 182 23, Prague-8, Czech Republic c Institute for Physics, Martin-Luther-Universität Halle, Von-Danckelmann-Platz 3, 06120 Halle (Saale), Germany d Department of Physics, Faculty of Science, Minia University, 61519 Minia, Egypt
Received 5 December 2013; received in revised form 24 January 2014; accepted 27 January 2014 Available online 5 February 2014
Abstract Pure and doped PZT ceramics (PZT:La þFe, PZT:La, PZT:Gd, PIC 151 and with 0.1, 0.25, 0.5, 1.0 mol% Fe doped samples) have been examined by Positron Annihilation Lifetime Spectroscopy (PALS) in the range of temperatures between 150 and 375 K. It was found that the defect-related lifetime increased with increasing temperature, indicating vacancy-like defects. With increasing Fe doping, a loss of vacancy agglomerations was observed, as well as a weaker dependence of lifetime on temperature. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: A. Implantation; B. Defects; D. Perovskites; D. PZT
1. Introduction High permittivity dielectric materials have achieved wide usage in recent years because of their superior properties. Due to their high dielectric constant, high piezoelectric coefficient and spontaneous polarization, they have been widely investigated for possible applications in microelectronics as capacitors, randomaccess memory (FeRAM and F-RAM), actuators for fine displacement systems and sensors [1,2]. The most common compositions that are currently extensively used are perovskite ferroelectric oxides with a general formula ABO3, e.g. Pb(Zr,Ti) O3 (PZT) [3–5]. Various modifications, such as doping or hydrogen charging can be performed in order to improve the piezoelectric properties or formation of new structure [6,7]. Dopants can replace the ions in the lattice and, depending on ion radii, they can substitute the A þ 2 (lead) and B þ 4 (titanium/ zirconium) sites, creating the A or B site vacancies for charge n Corresponding author at: J. Heyrovský Institute of Physical Chemistry, v.v. i., Academy of Sciences of the Czech Republic, Dolejškova 3, Prague 182 23, Prague-8, Czech Republic. Tel.: þ 420 266052113; fax: þ 420 286582307. E-mail address: [email protected] (K. Drogowska).
http://dx.doi.org/10.1016/j.ceramint.2014.01.127 0272-8842 & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
compensation. High valence dopants (La þ 3, Nb þ 5) are donors due to electrons contribution, and their substitution to A and B sites, respectively, create the A vacancies. Conversely, ions like Na þ and Fe þ 3 act as an acceptor, and their substitution into the A and B sites, respectively, create the O-vacancies [8–10]. The kinetics of oxygen vacancies has been already investigated and described [11]. Doping with different elements changes the physical properties of PZT ceramics. Namely, the electromechanical properties of PZT ceramics can be improved by the use of additives of rare earth elements, e.g. the incorporation of La ions into PZT ceramics increases their dielectric constants. Moreover, also point defects influence the physical properties of materials and doping of ferroelectrics creates different defect structures. Hotpressed ferroelectric PZT ceramics are suitable for electrooptical applications due to their high transparencies. The transparency seems to rely on the defect structure caused by the incorporation of La2 þ ions into the perovskite lattice [12]. Therefore it is necessary to study the different defect concentrations and structures in PZT ceramics that are differently doped. Positrons are sensitive to the electrical potential of point defects and have proved to be a valuable nondestructive probe
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K. Drogowska et al. / Ceramics International 40 (2014) 9127–9131
for detection of vacancy-type defects. Since they react with negative ions, they are excellent tool to investigate the ionized materials. Due to weak binding energy (⪢1 eV), positrons are effectively trapped only at low temperatures. At higher temperatures, they are thermally detrapped. During the past decades, Positron Annihilation Lifetime Spectroscopy (PALS) has been intensively applied to characterize defects in various semiconductors [13]. However, in many cases it is difficult to conclude the defect type only from the annihilation parameters. The point defect complexes with a dipole moment have been taken into account to be of particular relevance both for aging and in relaxor ferroelectrics [14–16]. Relaxor behavior in La doped PZT has been ascribed to their presence [17] and a recent study of fatigue behavior deduced the existence of VB type defects from the activation energy associated with the fatigue induced response [18]. Both lead vacancy (VPb) and B-site vacancy (VB) in PZT were then observed. Positron trapping in B-site vacancies was inferred in PZT. In Fe doped PZT the fraction of positrons trapped in VPb-type defects compared to VB vacancies was decreased [19]. The positron annihilation technique operates the principle that positrons can be trapped during the diffusion by lattice defects. These techniques have specific sensitivity to open volume defects, varying in size from monovacancies to vacancy clusters. Positrons which are trapped by open volume defects alter their annihilation characteristics. The methods are well established for the study of metals and semiconductors [13,20], but application to oxide materials has been limited. Studies of variable energy positron annihilation spectroscopy have shown sensitivity to vacancy defects formed due to oxygen deficiency and to La doped PZT samples [21]. Previous PALS measurements on ceramic PZT samples gave evidence of a long defect lifetime attributed to lead vacancies [22]. A PALS study in La and Nb doped rhombohedral Pb(Zr0.6Ti0.4)O3 ceramics with a dominant defect lifetime in the region of 300 ps has also been reported [23]. After thermalization, positrons annihilate in the material from a state i with a lifetime τi and relative intensity Ii. This can be a delocalized state in the bulk lattice or a localized state in an open volume defect (e.g. vacancy). If the average lifetime τav ¼ ∑i I i τi is higher than the bulk lattice lifetime τb, where τb ¼ 1/λb (the annihilation rate) and it is characteristic for the material, it indicates that vacancy type defects are present. The rate of positron trapping into a vacancy defect, Kd, is proportional to the defect concentration (C). The constant of proportionality is the specific trapping coefficient μ. The positron capture by one defect type is described by the simple two-state trapping model, as follows (for more details see Ref. [13]): κd ¼ μC ¼ I 2
1 1 I2 1 1 ¼ τ1 τ2 I 1 τb τd
ð1Þ
It predicts two experimental lifetimes; the first one is reduced from the bulk lifetime by an amount that depends on the trapping rate of the defects, τ1 oτb, while the second one is the
characteristic defect component τ2 ¼ τd. If the two-state trapping model is applicable, the bulk lifetime can be calculated from the measured lifetimes and intensities, as follows: 1 I1 I2 ¼ þ τb τ1 τ2
ð2Þ
As the defect concentration increases, I2 can reach the saturation value, which means that all of positrons annihilated from the defects, and the sensitivity for defect concentration are lost. This leads to occurrence of trapping saturation. The defect concentration that the saturation occurs at depends on the values of both τb and τd. 2. Experimental The Pb(Zr0.46Ti0.54)O3 ceramic samples were prepared by a conventional solid state process from oxide precursors [24]. The samples were doped with other elements, i.e. La, Fe, Gd and both La þ Fe. The PIC 151 sample was produced by PI Ceramics. This ceramic is a modified lead zirconate–lead titanate material with high permittivity, high coupling factor and high piezoelectric charge constant. PZT:La þ Fe, PZT:La, PZT:Gd, PIC 151, PZT undoped and 0.1, 0.25, 0.5 and 1.0 mol% Fe doped samples were investigated by the PALS method. The positron lifetime measurements, in the temperature range of 120–425 K, were performed using a fast–fast coincidence spectrometer [13,25,26] with a time resolution of 225 ps (FWHM). 22NaCl was covered by a 2 mm thick aluminum foil on each side of the source and sandwiched between two identical samples. The activity of the used positron source was 1.11 MBq (30.0 μCi). Each measurement lasted 5 h what resulted with a lifetime spectrum of 4.0 106 coincidence counts. This high statistics is preferred when the computer code LT9.0 [27,28] is used for analyzing the lifetime spectra. The lifetime spectrum was analyzed as a sum of exponential decay components, nðtÞ ¼ ∑i I i exp ð t=τi Þ convoluted with a Gaussian describing the time resolution function of the spectrometer. The source correction of 13% (Al foil) and the time resolution were determined by measuring a semi-insulating GaAs reference sample, showing no positron traps. The spectra were analyzed with the two-state trapping model (one defect type) after source and background correction. For more details see Refs. [29,30]. 3. Results and discussion The results of the average positron lifetime in the function of temperature for differently doped PZT samples (PZT:La þ Fe, PZT:La, PZT:Gd and PIC 151) are illustrated in Fig. 1. The average lifetime increases with increasing temperature, indicating the detection of vacancy-like defects. Moreover, the defectrelated lifetime increases with increasing measurement temperature, as it is presented in Table 1. The presented data indicates that since the intensity I2 of the defect-related lifetime decreases, the concentration of the vacancy-like defects decreases.
K. Drogowska et al. / Ceramics International 40 (2014) 9127–9131
For this set of samples, τd at 150 K was found to be about 290 ps, what corresponds to single lead vacancy VPb or VPb–VO divacancy, or may be a mixture of them. This result cannot be separated from positron lifetime data alone because of the similar lifetime values of these types of defects [23,31]. As mentioned earlier, the longer lifetime component increases with increasing measurement temperature but its relative intensity decreases. This indicates the existence of defect agglomerations, e.g. double vacancies. PZT:La sample exhibits a higher defect lifetime value of 356 ps at 375 K; however, the lifetime in PZT:La þ Fe sample is less than that in PZT:La. This can be attributed to the charge transfer in the co-doped samples, which increases the electron density and reduces the positron lifetime. Due to the high defect concentration (related to I2) and the high mobility of oxygen vacancies, it is more likely that the defect lifetime is arising from a mixture of VPb and VPb–VO [23]. Fig. 2 shows the lifetime parameters, as deconvoluted from the measured lifetime spectra, versus the measurement temperature for an undoped Pb(Zr0.46Ti0.54)O3 sample. As it is shown, both τ1 and τ2 increase with increasing temperature, showing saturation behavior above 250 K. At the same time, I2 decreases down to the measurement temperature
Fig. 1. Average positron lifetime as a function of measuring temperature in differently doped PZT samples.
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Fig. 2. Temperature dependence of lifetime components and intensity of the defect-related component in an undoped PZT samples.
Fig. 3. Average positron lifetime as a function of the temperature in differently doped PZT:Fe samples.
Table 1 Temperature dependence of defect-related lifetimes and its relative intensities for differently doped PZT samples. Sample
Temperature (K)
Defect-related lifetime (ps)
I2 (%)
PIC 151
150 300 375 150 300 375 150 300 375 150 300 375
293 319 332 295 313 328 303 320 356 287 295 317
70.2 46.2 35.8 68.5 59.7 45.1 59.5 54 29 66.5 59.7 39.5
PZT:Gd
PZT:La
PZT:LaþFe
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Table 2 Temperature dependence of positron annihilation lifetimes and the intensity of the defect-related component in differently doped PZT:Fe samples. Sample
Temperature (K)
τ1 (ps)
τ2 (ps)
I2 (%)
PZT:Fe (0.10%)
150 200 280 325 150 200 280 325 150 200 280 325
166.2 188.4 191.3 191.2 208.2 214.7 217.1 218.3 215.0 216.0 216.0 216.3
215.3 219.8 236.6 244.4 – – – – – – – –
82.7 72.8 51.0 45.6 – – – – – – – –
PZT:Fe (0.25%)
PZT:Fe (0.50%)
of 250 K. τ1 and τ2 have saturation behavior with lifetimes of about 190 and 260 ps, respectively. Fig. 3 presents the temperature dependence of the average positron lifetime in undoped and doped with different Fe concentration PZT samples. An increase in the average lifetime with temperature is clearly shown for all of the samples, but this correlation is weaker in case of higher Fe concentrations. As it was published by Puff et al. [32], the changes in the mean lifetime is about 7 ps/100 K for undoped sample, and 2.5 ps/ 100 K for sample doped with 1 mol% Fe. Similar values were previously measured also for GaAs [33,34]. PZT ceramics with a band gap of 3.5 eV are ranked between high gap semiconductors and isolators; thus, they cannot be directly compared to GaAs. In this case, the weaker temperature dependence and the lower annihilation fraction, especially at higher temperatures for Fe-doped samples, may be explained as a consequence of the detrapping from extended Rydberg's states. Since the space charges in insulating or semiconducting materials are well known to distort the local electric field and affect high-field conduction, as well as other electrical properties [35], these results may be significant for further investigation of these materials. At low Fe doping concentration (0.1mol%), I2 decreases with increasing measurement temperature as shown in Table 2. But the defect lifetime (τ2) increases to values corresponding with the Pb vacancy agglomeration [22]. For higher Fe concentrations it was no longer possible to deconvolute the spectra into two lifetime components. A potential explanation could be that at higher Fe concentrations the lifetime will be shorter because of the stronger charge transfer effects and the computer code is not able to separate between the two lifetime values anymore. This is contrary to measurements on samples doped with La and Nb, where for doping concentrations higher than 0.5 mol% a trapping on Pb vacancies was still observed [23]. This behavior is in good agreement with our measurements for La, Gd and La þ Fe doped samples. With increasing Fe doping concentration it is clearly visible that the behavior of lifetime that was strongly correlated to the temperature for low-doping concentration becomes constant. The same observation was made by Puff et al. [32], where measurements have
been performed down to 10 K. In this region of temperatures, between 10 and 150 K, the similar slopes have been observed for undoped, as well as with 1 mol% Fe doped samples. These results reflect the changes in the charging state of defects and defects agglomerates. For higher temperatures and increase in Fe concentration, the average positron lifetime has resulted in the weakening of temperature dependence. This may be related to electrical charge of the Fe ions weakening the reaction between positrons and negative ions in the PZT ceramic. 4. Conclusions PALS measurements for PIC 151, PZT:Gd, PZT:La and PZT: Laþ Fe and PZT:Fe with different Fe concentrations have been performed. The increase of defect-related positron lifetime and decrease of relative intensity with increasing temperatures, indicating the existence of defect agglomerations, have been observed. For all of the samples, τd at 150 K is about 290 ps, what corresponds to the Pb vacancy or the VPb–VO divacancy, or their mixture. The lifetime of the co-doped sample was slightly lower than the lifetimes in the other samples. Doping with increasing Fe concentration results in weakening of the average lifetime dependence on temperature. The obtained lifetime values are lower than those for Pb vacancies (280–300 ps), suggesting the decrease of vacancy concentration. This behavior has not been observed for rare-earth elements doped samples. References [1] H. Haertling, Ferroelectric ceramics: history and technology, J. Am. Ceram. Soc. 82 (1999) 797. [2] S.M. Spearing, Materials issues in microelectromechanical systems (MEMS), Acta Mater. 48 (2000) 179. [3] S.L. Swartz, T.R. Shrout, W.A. Schulze, L.E. Cross, Dielectric properties of lead magnesium niobate ceramics, J. Am. Ceram. Soc. 67 (1984) 311. [4] N.A. Pertsev, G. Arlt, A.G. Zembilgotov, Prediction of a giant dielectric anomaly in ultrathin polydomain ferroelectric epitaxial films, Phys. Rev. Lett. 76 (1996) 1364. [5] B.G. Kim, S.M. Cho, T.Y. Kim, H.M. Jang, Giant dielectric permittivity observed in Pb-based perovskite ferroelectrics, Phys. Rev. Lett. 86 (2001) 3404.
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