APPLIED PHYSICS LETTERS
VOLUME 78, NUMBER 18
30 APRIL 2001
Dielectric loss and defect mode of SrTiO3 thin films under direct-current bias Chen Ang,a) L. E. Cross, Zhi Yu, Ruyan Guo, and A. S. Bhalla Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802
Jian Hua Hao Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802
共Received 2 November 2000; accepted for publication 5 March 2001兲 The dielectric behavior of SrTiO3 thin films prepared by the pulsed-laser deposition technique on SrTiO3 single-crystal substrates is studied under dc electric field. A high dielectric constant maximum max(⬃2280) and a low-loss tan ␦ (⬃0.001) are obtained. Compared with the observation in SrTiO3 single crystals, an additional dielectric loss peak with frequency dispersion is observed around 150 K 共at 1 kHz兲. With increasing dc bias, the peak is suppressed and finally disappears at ⬃350 kV/cm; however, the temperature at which the peak occurs is independent of electric field. The possible physical mechanism of the peak is briefly discussed. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1367299兴
Currently, one of the most studied dielectric materials for tunable microwave devices1,2 is the perovskite SrTiO3. 3–6 The incipient ferroelectric SrTiO3 shows a high nonlinear electric field effect at low temperatures and a reasonably low dielectric loss tan ␦ (⬃10⫺4 ). 3,4 However, it is also found that when SrTiO3 is made as a thin film, the dielectric loss is increased by more than an order of magnitude (tan ␦ ⬃10⫺2 ). 5–7 The understanding of the physical nature of the dielectric loss in SrTiO3, especially under high dc electric field is highly desirable. However, studies of the dielectric loss under high dc electric field for SrTiO3 single crystals and thin films are scarce.8 Recently, with the progress in the preparation process of thin films, especially, by pulsed-laser deposition, highquality thin films have been prepared with high max(⬃⭓2000) and loss tan ␦ (⬃⭐10⫺3 ). 9 This indicates that thin films have become very promising candidates for practical applications. However, it is reported that for these thin films, some additional dielectric loss peaks appear around 70 and 150 K.9,10 Obviously, understanding of this additional loss peak is important. In this letter, we report on the dielectric behavior of SrTiO3 thin films deposited on SrTiO3 single-crystal substrates by pulsed-laser deposition. Under high dc electric fields up to 350 kV/cm, it is found that a noticeable dielectric loss peak with frequency dispersion is present around 150 K 共at 1 kHz兲. The peak is suppressed and finally disappeared with increasing dc bias, but the temperature at which the dielectric loss peak occurs does not change. This implies that the dielectric loss peak is a ‘‘defect mode’’ 共or ‘‘impurity mode’’兲, which might relate to the soft mode behavior of the host lattice of SrTiO3. SrTiO3 thin-film samples with a thickness of 1 m were prepared by the pulsed-laser deposition technique on SrTiO3 single-crystal substrates. The samples adopt a parallel-plate capacitor structure, i.e., Au/SrTiO3 thin-film/SrRuO3 /SrTiO3 a兲
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single-crystal substrate. First, a thin layer SrRuO3 layer was deposited on the SrTiO3 single-crystal substrate as a bottom electrode, then the SrTiO3 thin film was deposited on the SrRuO3 layer, and finally, a layer of gold was deposited on the SrTiO3 thin film as a top electrode. The complex dielectric permittivity was measured using a HP 4284A LCR meter with an ac field of 100 mV/m. The temperature dependence of the dielectric properties was measured in a cryostat system 共ADP202 Cryostat兲 in the temperature range 12–300 K, and under a vacuum of better than 10⫺4 Torr. The dc voltage was applied to the sample and a blocking circuit was adopted to separate the high dc voltage from the LCR meter. The typical dielectric behavior for the SrTiO3 thin-film samples is shown in Fig. 1共a兲. It can be seen that increases continuously with decreasing temperature, attaining the
FIG. 1. 共a兲 Temperature dependence of and tan ␦ for the SrTiO3 thin-film sample at 0.1, 1, and 10 kHz 共tan ␦ peaks: from left to right; ⑀: the data overlap each other兲. The inset shows the temperature dependence of the relaxation rate 共circles: experimental data; solid line: the Arrhenius law兲. 共b兲 Fit to the Barrett relation 共circles: experimental data; solid curve: the Barrett relation兲.
0003-6951/2001/78(18)/2754/3/$18.00 2754 © 2001 American Institute of Physics Downloaded 26 Apr 2001 to 130.203.164.215. Redistribution subject to AIP copyright or licence, see http://ojps.aip.org/aplo/aplcr.jsp
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Appl. Phys. Lett., Vol. 78, No. 18, 30 April 2001
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FIG. 3. Temperature dependence of and tan ␦ 共the inset兲 for the SrTiO3 thin film under different dc electric fields from 0 to 350 kV/cm at 10 kHz.
FIG. 2. Temperature dependence of and tan ␦ as a function of frequency for the SrTiO3 thin-film sample under different dc electric fields from 10 to 80 kV/cm at 0.1, 1, and 10 kHz 共tan ␦ peaks: from left to right; : the data overlap each other兲.
value 2280 at 12 K. As shown in Fig. 1共b兲, the could be fitted well to the Barrett relation11 ⫽C/ 关共 T 1 /2兲 coth共 T 1 /2T 兲 ⫺T 0 兴 ,
共1兲
with T 1 ⫽128 K, T 0 ⫽26.4 K, and C⫽8.5⫻10 K. Compared with the parameters obtained in SrTiO3 ceramics 共T 1 ⫽84 K, T 0 ⫽25.1 K, and C⫽8⫻104 K兲,12 and single crystal 共T 1 ⫽84 K, T 0 ⫽35.5 K, and C⫽8⫻104 K兲,13 these values are reasonable. For dielectric loss tan ␦, as T⬍50 K, the tan ␦ increases quickly, corresponding to the increase in . This is also similar to the behavior observed in SrTiO3 ceramics or single crystals. Compared with the observation in SrTiO3 single crystals, an additional dielectric loss peak with frequency dispersion is observed around 150 K 共at 1 kHz兲 for the thin film, hereafter denoted as mode I. A similar peak has been also observed in other samples. The relaxation rate for the polarization derived from the temperature dependence of the imaginary part of the permittivity for the sample is plotted in the inset of Fig. 1共a兲. The data were fitted to the Arrhenius law 4
⫽ 0 exp关 U/k B T 兴 ,
共2兲
where 0 is the attempt frequency, U is the activation energy for relaxation, and T is the temperature. The fit parameters are U⫽0.25 eV and 0 ⫽⬃1⫻1011 Hz. By applying dc electric fields, the dielectric behavior is greatly modulated. Figure 2 shows the temperature dependence of the dielectric behavior in the electric-field range of 10–80 kV/cm at frequencies of 0.1, 1, and 10 kHz. It can be seen that a broad dielectric constant peak, denoted as peak A, is induced; this is similar to the peak that has been widely reported in the earlier literature in SrTiO3 single crystals.8,14–16 Peak A induced by the electric field in the
vs. T curve indicates the occurrence of an ‘‘electric-fieldinduced peak,’’ which was assigned to a ‘‘ferroelectric peak’’ by some authors.14–16 On the other hand, for mode I around 150 K, although the intensity decreases with the increasing electric field, the relaxation rate follows the Arrhenius law with almost same activation energy U and attempt frequency 0 , and the temperature of mode I remains almost the same. In order to explore further the nature of the dielectric behavior under dc electric fields, the dc electric field was extended to 350 kV/cm. As shown in the inset of Fig. 3, with increasing electric field, the intensity of mode I (tan ␦) is continually decreased, and finally disappears at ⬃350 kV/ cm. However the temperature (T m ) at which the tan ␦ peak occurs is independent of the electric field. This is different from any ‘‘ordered phase’’ or ‘‘induced ferroelectric peak,’’ whose temperature increases with increasing electric field,8,14–16 but is similar to the so-called ‘‘defect mode’’ 共or ‘‘impurity mode’’兲 observed in SrTiO3 doped with Bi.12 This indicates that mode I is due to some defect/impurity effects, rather than an intrinsic mechanism 共lattice vibration兲. It is well known that SrTiO3 is a typical soft mode incipient ferroelectric.13 In SrTiO3 samples, even for highquality SrTiO3 single crystals, low concentration unavoidable defects/impurities 共for example, oxygen vacancies兲, and/or strain due to the mismatch between the film and the substrate may be present, hence, some dielectric anomalies, especially in the ⬙ part, can be observed. This has been confirmed in many studies.12,17–19 In the present work, for the SrTiO3 thin film, the concentration of defect/impurity and strain is expected to be higher than that of single crystals. We suggest that mode I is similar to the defect mode 共or impurity mode兲 observed in single-crystal SrTiO3 共Ref. 8兲 and Bi:SrTiO3, 17 and could be attributed to reorientation and/or hopping of the dipoles formed by the defect/impurity interacting with the soft modes of the lattice of SrTiO3. For the reorientation of the dipoles, the polarization strength can be qualitatively described by the Debye– Langevin model.20 Adopting the modified Debye–Langevin model, the polarization step can be written as12 ⌬⫽nq 2 ␦ 2 /3k B T,
共3兲
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where n is the concentration of the dipoles, q is the number of electrons, and ␦ is the effective dipole moment taking into account of the coupling effect between the dipole and the lattice. As mentioned above, the relaxation rate versus temperature follows the Arrhenius law, and the T m is independent of the electric field, i.e., the infinite frequency 0 and the potential barrier remain the same with increasing dc electric field; however, the intensity of the peak is greatly suppressed with increasing electric field. This implies that application of the dc bias has no influence on the frequency of the reorientation of the dipoles and the potential barrier. However, the local environment of the dipole is modulated, in other words, the coupling between the dipoles 共formed by defects and/or impurities兲 and the lattice vibration–soft mode is modified by the application of the dc electric field, i.e., the effective dipole moment ␦ in Eq. 共3兲 is modulated. In fact, from the previous work of Fleury and Worlock,16 we know that the soft modes of SrTiO3 can be ‘‘harden’’ by increasing dc bias. In the present work, with increasing dc bias, the system also becomes more and more ‘‘stiff,’’ and the effective dipole moment ␦ decreases, which leads to the decrease in the intensity of the polarization step. Obviously, this phenomenon is expected to be observed in the so-called ‘‘softmode materials,’’ in which the system is soft, the matrix has high polarizability, and the polarization effect of the defects and/or impurities can be greatly amplified, and their polarization gives the high permittivity. Indeed, two phononassisted impurity dielectric modes were observed in Bi:SrTiO3, 12 whose activation energies of the dielectric relaxation are exactly the same as those of the phonons observed. In Li-doped KTaO3, 21 a dielectric relaxation peak was also observed with activation energies of the dielectric relaxation of 0.22 eV. At this moment, a detailed physical picture is not clear, and further studies need to be conducted. The dielectric behavior of SrTiO3 thin films deposited on SrTiO3 single-crystal substrates is reported in this letter. Compared to results of the SrTiO3 single crystals, an additional dielectric loss peak with frequency dispersion is observed around 150 K 共at 1 kHz兲. This peak is suppressed and finally disappears with increasing dc bias, however, the tem-
perature at which the loss peak occurs is independent of the electric field. This peak is attributed to the defect mode or impurity mode, which comes from the coupling effect between the dipoles of the defects/impurities and the soft mode of the host lattice of SrTiO3. This work was supported by a grant from DARPA under Contract No. DABT63-98-1-002. The authors would like to thank Dr. X. X. Xi for his helpful discussion. O. G. Vendik, Ferroelectrics 12, 85 共1976兲. O. G. Vendik, I. G. Mironenko, and L. T. Ter-Martirosyan, Microwaves RF 33, 67 共1994兲; O. G. Vendik, E. Kollberg, S. S. Gevorgian, A. B. Kozyrev, and O. I. Soldatenkov, Electron. Lett. 31, 654b 共1995兲. 3 A. B. Kozyrev, T. B. Samoilova, A. A. Golovkov, E. K. Hollmann, D. A. Kalinikos, V. E. Loginov, A. M. Prudan, O. I. Soldatenkov, D. Galt, C. H. Mueller, T. V. Rivkin, and G. A. Koepf, J. Appl. Phys. 84, 3326 共1998兲. 4 F. W. Van Keuls, R. R. Romanofsky, D. Y. Bohman, M. D. Winters, F. A. Miranda, C. H. Mueller, R. E. Treece, T. V. Rivkin, and D. Galt, Appl. Phys. Lett. 71, 3075 共1997兲. 5 R. E. Treece, J. B. Thompson, C. H. Mueller, T. Rivkin, and M. W. Cromar, IEEE Trans. Appl. Supercond. 7, 2363 共1997兲. 6 D. Galt, J. Price, J. A. Beall, and R. H. Ono, Appl. Phys. Lett. 63, 3078 共1993兲; D. Galt, J. Price, J. A. Beall, and T. E. Harvey, IEEE Trans. Appl. Supercond. 5, 2575 共1995兲. 7 D. Fuchs, C. W. Schneider, R. Schneider, and H. Rietschel, J. Appl. Phys. 85, 7362 共1999兲. 8 A. Chen, A. S. Bhalla, R. Guo, and L. E. Cross, Appl. Phys. Lett. 76, 1929 共2000兲. 9 H-c. Li, W. Si, A. D. West, and X. X. Xi, Appl. Phys. Lett. 73, 190 共1998兲; 73, 464 共1998兲. 10 X. X. Xi, A. M. Clark, J. H. Hao, and W. Si, Integr. Ferroelectrics 24, 239 共1999兲. 11 J. H. Barrett, Phys. Rev. 7, 7403 共1952兲. 12 A. Chen and Y. Zhi, Phys. Rev. B 61, 11363 共2000兲. 13 K. A. Mu¨ller and H. Burkhard, Phys. Rev. B 19, 3593 共1979兲. 14 C. Frenzel and E. Hegenbarth, Phys. Status Solidi A 23, 517 共1974兲. 15 H. Unoki and T. Sakudo, J. Phys. Soc. Jpn. 23, 546 共1967兲. 16 P. A. Fleury and J. M. Worlock, Phys. Rev. 174, 613 共1968兲; J. Hemberger, P. Lunkhemer, R. Viana, R. Bohmer, and A. Loidl, Phys. Rev. B 52, 13159 共1995兲. 17 A. Chen, Y. Zhi, and L. E. Cross, Phys. Rev. B 62, 228 共2000兲. 18 R. Mizaras and A. Loidl, Phys. Rev. B 56, 10726 共1997兲. 19 R. Viana, P. Lunkenheimer, J. Hemberger, R. Bo¨hmer, and A. Loidl, Phys. Rev. B 50, 601 共1994兲. 20 H. Fro¨hlish, Theory of Dielectrics 共Clarendon, Oxford, U.K., 1958兲. 21 J. J. van Der Klink, D. Rytz, F. Borsa, and U. T. Hochli, Phys. Rev. B 27, 89 共1983兲. 1 2
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