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Dielectric properties of Ž Ba 0.5 Sr0.5 . TiO 3 thin films Feng Yana,b,U , Peng Bao a , Zhigang Zhang a , Jinsong Zhua , Yening Wang a , Helen L.W. Chanb , Chung-Loong Choy b a

National laboratory of Solid State Microstructures, Department of Physics, Nanjing Uni¨ ersity, Nanjing 210093, PR China b Department of Applied Physics, The Hong Kong Polytechnic Uni¨ ersity, Hong Kong, PR China Received 16 June 1999; received in revised form 29 November 1999; accepted 29 November 1999

Abstract The dielectric properties of ŽBa 0.5 Sr0.5 .TiO 3 ŽBST. thin films with high electrical resistivity were investigated. BST films are deposited on PtrTiO 2rSiO 2rSi substrates by a metal-organic deposition ŽMOD. method. The dielectric permittivity and ac conductivity of the films are measured in the frequency range 102᎐105 Hz. The dielectric permittivity r decreases slightly with frequency f, following the relationship r s aq bf ny1 Ž a, b and n are constants, n - 1.. The ac conductivity Ž f . increases with frequency as Ž f . ; f n. These results indicate that the phonon-assisted jumps of electrons between localized states play an important role in the dielectric properties of BST thin films. The dielectric permittivity and ac conductivity of the BST thin films increase with grain size, and decrease with increasing temperature. A preliminary explanation is given. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Dielectric properties; Conductivity; Barium; Strontium; Titanium; Heat treatment

1. Introduction There has been significant interest and effort in developing ferroelectric thin films for various electronic devices, such as dynamic and ferroelectric random access memories ŽDRAM and FRAM.. Considerable attention has been focused on the use of ŽBa 1y x Sr x .TiO 3 ŽBST. as DRAM capacitors w1᎐5x due to its high dielectric permittivity and low loss. To successfully use BST films for technological applications, the electrical properties need to be better understood. It was reported that, after the application of a dc voltage to a BST film, in addition to an essentially instantaneous polarization, there is a current in the film with a power-law time dependence of approximately tyn , where n - 1 w2,4,6,7x. Because of its tyn

behavior, this current dominates at short times, but the true leakage current will dominate at long times w2,8᎐10x. The true leakage current is interface-limited w8᎐10x and the Schottky emission equation has been widely used to describe the conduction behavior. But the mechanism of the time dependent current, the so called Curie-von Schweidler relaxation current w2,4,6,7,11x, has not been carefully studied. Through a Fourier transform, the relaxation current can be transformed into a frequency dispersion of the dielectric permittivity, which has been reported in several papers w4,6,7x. Since the leakage is very important in technological applications, we study the dielectric behavior of Ba 0.5 Sr0.5TiO 3 thin films in order to gain a deeper understanding of the Curie-von Schweidler relaxation. 2. Experiments

U

Corresponding author. E-mail address: [email protected] ŽF. Yan..

The Ba 0.5 Sr0.5TiO 3 thin films were prepared on PtrTirSiO 2rSi substrates by a metal-organic deposi-

0040-6090r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 0 . 0 1 2 3 0 - X

F. Yan et al. r Thin Solid Films 375 (2000) 184᎐187

185

Table 1 The approximate diameter of grains of BST thin films annealed in different temperatures Annealing temperature Ž⬚C. Approximate diameter of grains Žnm.

600 30

650 50

700 100

750 200

studies revealed that the crystalline grain diameter increased with annealing temperature. Dielectric measurements were performed on a set of BST thin films with various grain sizes. The film thickness was approximately 120 nm. Pt top electrodes with area of 0.01 mm2 and thickness of 100 nm were deposited by magnetron sputtering on each film. The capacitance and ac conductance were measured as functions of frequency from 100 Hz to 100 kHz using an impedance analyzer ŽHP 4194A.. The measurement temperature was varied from 12 to 80⬚C. 3. Results and discussion Fig. 1a,b shows the dielectric permittivity Ž r . and ac conductivity Ž f . of BST thin films annealed at various temperatures. It is seen that the dielectric permittivity increases with increasing annealing temperature, i.e. with increasing grain size. The ac conductivity also increases with increasing grain size and the origin will be discussed later. The dielectric permittivity and ac conductivity can be fitted to the following frequency Ž f . dependence as indicated by the solid lines in Fig. 1:

Fig. 1. Frequency dependence of Ža. dielectric permittivity and Žb. ac conductivity of BST thin films annealed at different temperatures measured at 18⬚C. Symbols are experimental data. Solid lines are fitted curves.

tion ŽMOD. method described by Tahan et al. w12x. A solution of 0.4 molrl concentration was prepared by dissolving appropriate ratios of barium and strontium acetates in glacial acetic acid. Titanate IV isopropoxide was dissolved in ethylene glycol and then added to the above solution. The clear yellowish BST solution was spin coated onto the substrates and the coated films were pyrolyzed at 400⬚C for 5 min. This procedure was performed several times to achieve the desired film thickness. Then the films were annealed at different temperatures ranging from 550 to 750⬚C in oxygen ambient. The samples annealed at different temperatures were investigated by X-ray diffraction. The BST films annealed above 600⬚C exhibited a crystalline structure. As shown in Table 1, scanning electron microscopy ŽSEM.

r s aq bf ny 1

Ž1.

s cf n

Ž2.

where the power n Ž n - 1. is the dispersion parameter and a, b and c are temperature-dependent parameters. The values of the parameters are shown in Table 2. For the samples annealed at temperature above 600⬚C, the dispersion parameter n is almost constant. Fig. 2a,b shows the frequency dependence of r and for BST thin films annealed at 750⬚C measured at different temperatures. The dielectric permittivity decreases with increasing temperature, but it does not follow the Curie᎐Weiss law. The ac conductivity also Table 2 The parameter of Eqs. Ž1. and Ž2. fitting the dielectric constant and ac conductance of BST thin films annealed in different temperature Annealing temperature Ž⬚C.

a

b

c

n

600 650 700 750

157.6 67.9 98.8 205.0

228.7 428.9 499.3 438.4

2.09= 10y9 2.36= 10y9 3.02= 10y9 3.45= 10y9

0.9018 0.9334 0.9295 0.9301

F. Yan et al. r Thin Solid Films 375 (2000) 184᎐187

186

Table 3 The parameter of Eqs. Ž1. and Ž2. fitting the dielectric constant and ac conductance of BST thin film measured in different temperature Measurement temperature Ž⬚C.

a

b

c

n

12 32 50 78

119.1 170.5 209.4 207.4

563.0 520.2 529.2 489.5

3.00= 10y9 4.14= 10y9 6.23= 10y9 6.89= 10y9

0.9311 0.9095 0.8669 0.8464

decreases with increasing temperature. The r and data can be fitted to Eqs. Ž1. and Ž2., respectively, as indicated by the solid lines. The resulting parameters are given in Table 3. The dispersion parameter n decreases with increasing temperature, which is consistent with the result reported by Zafar et al. w4x. According to the theory of Austin and Mott w13x, the ac conductivity in the high temperature regime Žabsolute temperature T ) ⌰r2. due to the hopping of localized electrons is: Ž f . A Ž e 2r␣ 5 . ND NATy1 f ln Ž rf . y WrkT 4

4

Ž3.

where ⌰ is the Debye temperature, ␣ is a constant, e is the electron charge, ND is the density of the localized electrons, NA is the density of acceptors, W is the electron hopping energy and is the frequency of phonons, which is in the order of 1013 Hz at room temperature. At low frequency Ž f < ., Eq. Ž3. reduces to Eq. Ž2., which was shown above to be consistent with our data. It is therefore reasonable to suggest that the ac conductivity of BST film arises from the hopping of localized electrons. However, Eq. Ž3. cannot explain the decrease of the dispersion parameter n with increasing temperature as shown in Table 3. Using the Kramers᎐Kronig relation w14x, Eq. Ž1. can be derived from Eq. Ž2.. Besides the contribution of the localized electrons, the motion of ions also contributes to the dielectric permittivity. However, the ions relax at high frequency, thus contributing with an approximately constant term in the frequency range of our measurement. Therefore, the parameter a which is the frequency-independent part of the observed permittivity consists of the contribution of both localized electrons and ions. Eq. Ž3. shows that as the electron becomes more localized Ž ␣ becomes larger. the conductivity becomes lower. Therefore, if we assume that smaller grain size leads to a higher degree of localization, then the decrease of conductivity with decreasing grain size, as shown in Fig. 1b, can be readily understood. The time-dependent Curie-von Schweidler relaxation current is related to the frequency-dependent conductivity by a Fourier transform w6,7x, so it also arises from the hopping of localized electrons. In order to lower the relaxation current, the density of the localized electrons ND should be reduced. It is expected that localized electrons originate from oxygen vacancies, impurities and non-stoichiometric composition. We have annealed a BST sample in Ar ambient at 400⬚C for 1 h in order to increase the number of oxygen vacancies. The consequent increase in the ac conductivity probably reflects the increase in the number of localized electrons associated with the increase of oxygen vacancies. 4. Conclusion The experimental results indicate that the ac conductivity of BST thin films is largely due to the hopping of localized electrons. This implies that the Curie-von Schweidler relaxation current Žand possibly the leakage current . may be reduced by finding ways to decrease the density of localized electrons in the films.

Fig. 2. Frequency dependence of Ža. dielectric permittivity and Žb. ac conductivity of BST thin film annealed at 750⬚C measured at different temperatures. Symbols are experimental data. Solid lines are fitted curves.

Acknowledgements The authors thank Prof. H.M. Shen, Mr Z. Yang, Dr X.B. Chen and Dr X.L. Liang for their help in the

F. Yan et al. r Thin Solid Films 375 (2000) 184᎐187

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w7x T. Horikawa, T. Makita, T. Kuroiwa, N. Mikami, Jpn. J. Appl. Phys. 34 Ž1995. 5478. w8x S. Zafar, R.E. Jones, B. Jiang, B. White, V. Kaushik, S. Gillespie, Appl. Phys. Lett. 73 Ž1998. 3533. w9x S. Maruno, T. Kuroiwa, N. Mikami, K. Sato, S. Ohmura, M. Kaida, T. Yasue, T. Koshikawa, Appl. Phys. Lett. 73 Ž1998. 954. w10x M. Copel, J.D. Baniecki, P.R. Duncombe, D. Kotecki, R. Laibowitz, D.A. Neumayer, T.M. Shaw, Appl. Phys. Lett. 73 Ž1997. 1832. w11x A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectrics Press, London, 1983. w12x D.M. Tahan, A. Safari, L.C. Klein, J. Am. Ceram. Soc. 79 Ž1996. 1593. w13x I.G. Austin, N.F. Mott, Adv. Phys. 18 Ž1969. 41. w14x M. Pollak, T.H. Geballe, Phys. Rev. 122 Ž1961. 1742.