- Email: [email protected]
Ba0.5Sr0.5TiO3 bilayered thin films
Materials Science and Engineering B 178 (2013) 911–916
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Thickness-ratio-dependent dielectric properties of Bi1.5 Zn1.0 Nb1.5 O7 /Ba0.5 Sr0.5 TiO3 bilayered thin films Ruguan Li, Shuwen Jiang ∗ , Libin Gao, Yanrong Li State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
a r t i c l e
i n f o
Article history: Received 14 January 2013 Received in revised form 1 April 2013 Accepted 23 April 2013 Available online 10 May 2013 Keywords: Dielectric properties Tunable dielectric Thin films Bismuth zinc niobate (BZN) Barium strontium titanate (BST)
a b s t r a c t Bi1.5 Zn1.0 Nb1.5 O7 (BZN)/Ba0.5 Sr0.5 TiO3 (BST) thin films were prepared on Pt/Ti-coated sapphire substrates by radio frequency magnetron sputtering. The specific relationship between the dielectric properties and the thickness ratio of the BZN thickness to the BST thickness was investigated. The presence of BZN films effectively reduced the dielectric loss of the thin films. The thickness-ratio-dependent dielectric constant and dielectric loss behaviors were in good accordance with the simulation results based on the series connection theory. The optimum thickness ratio was determined to be around 0.5, exhibiting a maximum commutation quality factor of about 16,000. The built-in electric field at the region near the BZN–BST interface may be responsible for the asymmetric characteristic of the electric-field-dependent dielectric properties of the BZN/BST thin films. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Barium strontium titanate, Ba1−x Srx TiO3 , is a ferroelectric film widely studied as a suitable material in tunable microwave devices because of its strong electric-field-dependent permittivity [1–3]. However, in general the loss tangent of these films is relatively high, e.g. tan ı > 0.01 for Ba0.5 Sr0.5 TiO3 (BST) thin films at room temperature [4,5]. It is a major drawback for their applications. Recently, cubic pyrochlore Bi1.5 Zn1.0 Nb1.5 O7 (BZN) thin films have gained a great deal of attention in tunable applications because of their very low dielectric loss and a certain dielectric tunability. However, the tunability of these films is relatively low and it required very high electric fields to achieve a high tunability [6–8]. In other words, the tunability of BZN is much smaller than BST at the same electric fields. An interesting issue to address is how to realize composite thin films that utilize the optimum combination of the high tunability of BST and the low loss of BZN. BST–BZN composite thin films by using BZN as a dopant [9,10] and BST/BZN heterolayered thin films [11–13] with good dielectric properties have been reported. Among these approaches, the BZN/BST heterolayered thin films reported by Fu et al. [13] were one of the best tunable materials achieved to date. We have reported capacitors employing BZN/BST bilayered films for radio frequency (RF) applications; the capacitors exhibit a high tunability and a high Q-factor [14].
∗ Corresponding author. Tel.: +86 28 83200866; fax: +86 28 83202569. E-mail address: [email protected] (S. Jiang). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.04.010
The fact that BZN/BST bilayered thin films are composed of two layers of different materials means that the different thickness ratios of the two layers would result in different dielectric properties. However, only a few reports on the thickness-ratio-dependent dielectric properties are available. Furthermore, considering that the thickness ratio affects the tunability and dielectric loss, an optimal thickness ratio must exit for the best performance of the material. This study investigates the specific relationship between the thickness ratio and dielectric properties, including permittivity, tunability, and dielectric loss. In addition, the optimum thickness ratio was determined. Ferroelectric thin films have been deposited on different substrates, including single crystal substrates and silicon wafers. The lattices of single crystal substrates present a close match to BST, whereas the common Si wafers do not provide lattice matching to BST [5]. The low resistivity of Si makes the realization of low loss microwave transmission lines challenging, which is another problem of the use of Si in tunable applications. The resistivity of sapphire is higher than that of silicon and HR silicon, and the sapphire are relatively low cost compared to other single crystal substrates, such as MgO or LaAlO3 . Therefore, sapphire was used as the substrate in this study. The Curie temperature of BST material is known to depend dramatically on the composition ratio between Ba and Sr in Ba1−x Srx TiO3 . BST has similarly sized maximum tunability near the Curie temperature, and the tunability and the loss tangent of BST decrease away from the Curie temperature. The choice of the composition depends on the operating temperature and the specific requirements of the particular microwave application [5]. Among
912
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
Table 1 Details of the deposition duration, thickness, and thickness ratio of the BZN/BST films. Sample number
1 2 3 4 5
BST
BZN
Thickness ratio of BZN vs. BST
Duration (min)
Thickness (nm)
Duration (min)
Thickness (nm)
180 180 180 180 180
160 160 160 160 160
0 30 50 70 90
0 70 110 140 160
the compositions, BST with a composition of Ba0.5 Sr0.5 TiO3 has a Curie temperature close to room temperature and a relatively large dielectric tunability. In the BZN material system, there are two main phases depending on composition: cubic Bi1.5 Zn1.0 Nb1.5 O7 and pseudo-orthorhombic Bi2 (Zn1/3 Nb2/3 )2 O7 . It has been reported that BZN thin films with a composition of Bi1.5 Zn1.0 Nb1.5 O7 showed dielectric tunable properties, but thin films with a composition of Bi2 (Zn1/3 Nb2/3 )2 O7 were nearly tunable [6]. Consequently, the particular BST (Ba0.5 Sr0.5 TiO3 ) and BZN (Bi1.5 Zn1.0 Nb1.5 O7 ) composition are chosen in this study. 2. Experimental The BST thin films were deposited on Pt/Ti-coated sapphire substrates by RF magnetron sputtering. A series of BZN thin films with different thicknesses were sputtered on the BST thin films. BST was deposited by sputtering from a ceramic Ba0.5 Sr0.5 TiO3 target with an RF power of 200 W, a substrate temperature of 800 ◦ C, and an Ar/O2 (30% O2 ) gas mixture at a total pressure of 4 Pa. BZN was deposited with a power of 150 W from a ceramic Bi1.5 Zn1.0 Nb1.5 O7 target at 750 ◦ C in a gas pressure of 4 Pa (85% Ar and 15% O2 ). The thickness of the thin films was controlled by allowing different growth time and was measured by conducting a cross-sectional scanning electron microscopy (SEM). Details of the deposition duration, thickness, and thickness ratio of the films are tabulated in Table 1. For the electrical measurements, Pt/Ti (∼200 nm/30 nm) top electrodes were deposited on the thin films by performing an electron-beam evaporation and patterned by conducting a lift-off process. The thin Ti adhesion layer was used to enhance the adhesion between the films and the platinum electrodes. An annealing in 4 Pa of oxygen ambient at 600 ◦ C for 30 min was performed to improve the interfacial contact between the films and the electrodes. The phase composition and crystallization of the BST thin films and BZN/BST bilayered films were characterized by X-ray diffraction (XRD). The dielectric properties of the thin films were measured at 100 kHz using an HP4294A Precision Impedance Analyzer at room temperature.
0 0.44 0.69 0.88 1
where dBZN is the BZN thickness and dBST is the BST thickness. Fig. 2 shows the applied-electric-field-dependent dielectric constant (extracted from measured capacitances) and the loss tangent of the thin films with different thickness ratios. The maximum dielectric constant of the BST thin films (i.e., x = 0) was ∼250, as shown in Fig. 2(a). The maximum dielectric constant of the thin films decreased with increased thickness ratio. Fig. 2(b) shows that the presence of the BZN films effectively reduced the dielectric loss of the thin films, for example, the dielectric loss dropped from 1.17% at a thickness ratio of 0 to 0.45% at a thickness ratio of 1. The dielectric loss reduction is attributed to the low loss of the BZN films. An asymmetric characteristic of the electric-field-dependent dielectric properties of the thin films was observed and will be discussed later. The following discussions present the specific relationship between the dielectric properties and the thickness ratio of the BZN/BST thin films. The measured capacitance Cm can be evaluated from the series connection of the BST capacitance, CBST , and the BZN capacitance, CBZN , as follows [16]: 1 1 1 = + Cm CBST CBZN
(2)
Given the same area, the average dielectric constant, εav , of the bilayered thin films can be expressed as: dBST dBZN dtotal = + εav εBST εBZN
(3)
where dtotal is the total thickness of the BZN/BST thin films and εBST and εBZN are the permittivity of the BST and BZN thin films, respectively. Using this expression with Eq. (1) provides a
3. Results and discussion Fig. 1 shows the XRD patterns of the BST thin films and the BZN/BST bilayered films. The results showed that the BST thin films were cubic pervoskite polycrystalline structure, and the BZN thin films were cubic pyrochlore polycrystalline structure. No impurity phases can be detected in the BZN/BST bilayered films, which mean that no reaction between BST and BZN layer occurred during the deposition process. The SEM cross-section image in the inset of Fig. 1 shows that the BZN/BST bilayered thin films have distinct interfaces. The key parameter used in this study was the thickness ratio x, which is defined as x=
dBZN dBST
(1)
Fig. 1. XRD patterns of BST thin films and BZN/BST bilayered films with a thickness ratio of 0.44. Inset is the SEM cross-sectional image for the BZN/BST with a thickness ratio of 0.44.
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
913
Fig. 3. Simulation and experimental results of the thickness-ratio-dependent dielectric constant of the BZN/BST thin films. The simulation is based on the series connection theory.
n=
Fig. 2. (a) Dielectric constant and (b) loss tangent of BZN/BST thin films as function of applied electric field at 100 kHz under forward-biased electric fields. The thickness ratio is defined as the ratio of the BZN thickness to the BST thickness.
relationship between the average dielectric constant εav and the thickness ratio x εav =
(1 + x) · εBST · εBZN εBZN + x · εBST
(4)
When x→ ∞, the εav → εBZN , whereas when x → 0, the average dielectric constant approaches εBST . Fig. 3 presents the comparison between the experimental and simulation results from Eq. (4). The experimental results of the thin films were the maximum dielectric constant extracted from Fig. 2(a). In the simulation results, εBST was 250 and εBZN was 140. This BZN dielectric constant value is close to the value for the BZN deposited on Si substrates in our laboratory [17]. Fig. 3 shows that the experimental results are in good accordance with the simulation results, indicating that the relationship between the dielectric constant and the thickness ratio comply with the theory, wherein the dielectric constant decreases as the thickness ratio increases. A further thickness ratio increase of over 0.6 causes the dielectric constant to decrease slowly and gradually approach the value for the BZN thin films. The most concerned property of ferroelectric thin films for tunable applications is the dependence of the permittivity of this thin film on the applied electric field. This characteristic is commonly described by the relative tunability nr , as expressed by Eq. (5), or the tunability n, as defined by Eq. (6): nr =
εmax − εmin × 100% εmax
(5)
εmax εmin
(6)
where εmax and εmin are the maximum permittivity and the minimum permittivity under the applied fields, respectively. The relative tunability of the films under the ±1 MV/cm electric fields as a function of the thickness ratio is plotted in Fig. 4. Given the asymmetric characteristic of the electric-field-dependent dielectric constant of the films, the minimum dielectric constant was replaced by the average value of the dielectric constant under the +1 and −1 MV/cm electric fields. Fig. 4 shows that the relative tunability of the films decreases almost linearly as the thickness ratio increases. Considering that the dielectric tunability of BZN is much smaller than BST, the tunable characteristic of the BZN/BST bilayered thin films is believed to be dominated by the BST. When electric fields are applied in series along the BZN and BST films, the BZN layers consume part of the applied fields. The available electric fields applied to the BST layers decrease with increasing thickness ratio; thus, the dielectric tunability decreases as the thickness ratio increases. Fortunately, the dielectric tunability of the BZN/BST bilayered films was prevented from being abruptly
Fig. 4. Thickness-ratio-dependent relative tunability of the BZN/BST thin films. The straight line represents linear fit to the experimental data points.
914
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
Fig. 5. Simulation and experimental loss tangent of BZN/BST thin films as a function of the thickness ratio. The simulation is based on the series connection theory.
degraded because the BZN permittivity was close to the value for BST and a certain tunability was also present in BZN. The main advantage of inserting BZN between the BST and the top electrodes is the reduction in the dielectric loss of the thin films. Fig. 2(b) shows that the presence of BZN films effectively reduces the dielectric loss of the thin films. The following presents the specific relationship between the dielectric loss of the films and the thickness ratio. BZN/BST bilayered thin films are composed of two layers, thus the power consumption of the bilayered films should be the sum of the consumption of the two layers, as follows: U2 U2 U2 = BST + BZN R RBST RBZN
(7)
where U is the total voltage applied in BZN/BST bilayered films, UBST and UBZN are the voltages applied in the BST and BZN layers, respectively. R represents the resistance of BZN/BST bilayered films, RBST and RBZN represent the resistance of BST and BZN layers, respectively. With dielectric loss, Eq. (7) can be written as 2 2 U 2 · ωCm tan ı = UBST · ωCBST tan ıBST + UBZN · ωCBZN tan ıBZN
(8)
where ω is the angular frequency and tan ıBST and tan ıBZN are the loss tangents of the BST and BZN thin films, respectively. In addition, Gauss’s law provides: D1 = D2 → εBST · EBST = εBZN · EBZN
(9)
Using Eq. (8) with Eqs. (1), (2), and (9) provides a relationship between the loss tangent of the BZN/BST bilayered thin films (tan ı) and the thickness ratio as follows: tan ı =
εBZN . tan ıBST + x · εBST · tan ıBZN εBZN + x · εBST
(10)
where tan ı → tan ıBZN in the limit of x→ ∞, and tan ı → tan ıBST when x → 0. The simulation results and the measured maximum loss tangent of the thin films as a function of the thickness ratio are plotted in Fig. 5. In the simulation, tan ıBST and tan ıBZN is 1.17% and 0.1%, respectively. Fig. 5 shows that the experimental results are in good accordance with the simulation results. Apparently, the dielectric loss of the films decreases as the thickness ratio increases, approaching the loss of the BZN films. Both the dielectric tunability and dielectric loss of the thin films decreased with increasing thickness ratio. Obtaining the material for the best device performance is based on the optimal tradeoff between the tunability and dielectric loss. The figure of merit
Fig. 6. Commutation quality factor (k) of the BZN/BST thin films as a function of thickness ratio. The higher value of K indicates a more excellent performance of the material. Thus the optimum thickness ratio was determined to be around 0.5, exhibiting a maximum commutation quality factor of about 16,000.
(FOM), defined as the ratio of the relative tunability to the loss tangent, is commonly used to reflect the trade-off between tunability and dielectric loss. However, the simple form of FOM does not have a well-defined physical meaning [5]. The commutation quality factor (K), first proposed by Vendik et al. to characterize a two-state, one-port switchable network [18], can be used instead of FOM to evaluate the optimal trade-off between the tunability and loss of a material better. K is given by [18]: K=
(n − 1)2 n · tan ımax . tan ımin
(11)
where n is the tunability as defined in Eq. (6) and tan ımax and tan ımin are the loss tangents of the materials under zero and nonzero applied electric field, respectively. The FOM is more helpful to the device engineer, and K is more useful in comparing materials [5]. Therefore, the theory was also applied to evaluate the ferroelectric material properties [18,19]. The higher value of K indicates a more excellent performance of the material. The commutation quality factor of the thin films as a function of the thickness ratio is shown in Fig. 6. K is about 10,000 at a thickness ratio of 0. Initially, K increases as the thickness ratio increases. The maximum K value of about 16,000 was obtained at a thickness ratio of around 0.5. A further thickness ratio increase of over 0.6 reduces K, decreasing to about 10,000 at a thickness ratio of around 1. The optimized thickness ratio for the thin films has an important role for the best performance of the material. Consequently, the thickness ratio of about 0.5 is recommended for the BZN/BST bilayered thin films. The dielectric properties of BZN films are almost thickness independence in this study. However, the permittivity of BST films in this study showed obvious difference when film thickness larger than 300 nm or smaller than 100 nm. It is known that BST shows thickness-dependent properties [19]. Therefore, the optimum thickness ratio of 0.5 should not be applied to bilayered films that BST thickness over the range of 100–300 nm. For the case of BST film thickness larger than 300 nm or smaller than 100 nm, the analysis of their dielectric properties is identical to the analysis in this study and the corresponding optimum thickness ratio would be obtained. The asymmetry in the dielectric behaviors of the thin films is observed in Fig. 2. This asymmetry may be related to the builtin electric field at the region near the interface between the BZN and BST. As reported in our previous work [15], the Fermi energy
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
915
Fig. 7. Sketch map of the paralleled capacitance structure. (a) positive bias voltages are applied to the top electrodes, (b) negative bias voltages are applied to the top electrodes. E and Ei are the electric field applied to the thin films and built-in electric field at the BZN–BST interface, respectively. T-E and B-E denote the top electrodes and bottom electrodes, respectively.
level of BZN is about 0.4 eV deeper in the band gap than that of the BST. After joining the BST and BZN thin films, the electrons near the BZN–BST interface flowed from the BST layer to the BZN layer, leaving positively charged ions in the BST region. The region near the interface lost its neutrality and became charged. As a result, a built-in electric field (Ei ) was established at the charged region near the interface. The built-in electric field opposes the diffusion process for electrons, thus this field points from the BST layer to BZN layer. This process is continued until a thermal equilibrium is reached, as indicated by a constant Fermi energy level. When positive bias voltages are applied to the top electrodes, as shown in Fig. 7(a), the built-in field is opposite to the direction of the applied electric fields. In this condition, the built-in field reduces the effective electric fields applied to the thin films. Conversely, the built-in field is in the same direction as the applied electric field when negative bias voltages are applied to the top electrodes, as shown in Fig. 7(b). In this condition, the built-in field enhances the effective electric field applied to the thin films. This behavior results in a higher dielectric constant and loss tangent under positive biases compared with that under equivalent negative biases, as observed in Fig. 2. Consequently, the presence of the built-in electric field at the region near the BZN–BST interface shifts the peaks of the electric-field-dependent property curves to higher electric fields, possibly causing the asymmetric characteristic. The above analysis is based on the dielectric properties of the thin films under forward-biased electric fields. Fig. 8 shows the electric-field-dependent dielectric constant of the films under backward-biased fields. The analysis of their dielectric properties is identical to the analysis above, and similar results were achieved. Comparing Figs. 2(a) and 8, very little difference in the shape of the
electric-field-dependent dielectric constant curves are observed. From another perspective, the built-in electric field at the region near the BZN–BST interface should be responsible for the asymmetry in the dielectric behaviors of the thin films. The corresponding electric fields of the peaks of the electric-field-dependent dielectric constant curves and the dielectric constant in Fig. 8 are slightly different from those in Fig. 2(a). The dielectric properties under forward and backward biases should have no differences if the films have no defects. The concentration of the movable ions or charges accumulated near the interfaces and oxygen vacancies formed during deposition may be responsible for the observed difference. As mentioned above, the built-in electric field at the region near the BZN–BST interface should be responsible for the asymmetric behavior of the electric-field-dependent permittivity of the BZN/BST thin films. It is considered that the build-in electric field may result from the asymmetric structure of the BZN/BST bilayered films. Since the build-in field may also result from defects such as oxygen vacancies or bismuth vacancies in BZN films during film deposition, it is necessary to check the origin of the built-in field. Two sets of samples have been prepared: T-E/BST/BZN/BE/Sapphire and T-E/BZN/BST/BZN/B-E/Sapphire. Fig. 9 shows the electric field dependent dielectric constant of the two samples under forward and backward biases. The asymmetric behavior of the BST/BZN bilayered films showed reverse characteristics from the BZN/BST films. The corresponding electric fields of maximum permittivity of BST/BZN films occurred at negative biases, and it showed a higher dielectric constant under negative biases compared with that under equivalent positive biases. Besides, the asymmetric behavior of bias field dependent permittivity was
Fig. 8. Electric-field-dependent dielectric constant of BZN/BST thin films under backward-biased electric fields.
Fig. 9. Electric-field-dependent dielectric constant of BST/BZN bilayered films and BZN/BST/BZN sandwich films under forward and backward biases.
916
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
much improved for the BZN/BST/BZN sandwich films with symmetry structure. These results indicate that the build-in electric field results from the asymmetric structure of the BZN/BST bilayered films. As shown in Fig. 9, there is a little difference of the fielddependent-permittivity between forward and backward biases. This may be caused by the defects accumulated near the interfaces. 4. Conclusions The thickness-ratio-dependent dielectric behaviors of BZN/BST thin films were investigated. The relative dielectric tunability of the thin films decreased almost linearly as the thickness ratio increased. The thickness-ratio-dependent dielectric constant and dielectric loss behaviors were in good accordance with the simulation results based on the series connection theory. The optimum thickness ratio was determined around 0.5, exhibiting a maximum commutation quality factor of about 16,000. The built-in electric field at the region near the BZN–BST interface may be responsible for the asymmetric characteristic of the electric-field-dependent dielectric properties of the BZN/BST thin films. Acknowledgement The authors gratefully appreciate the support from the National Natural Science Foundation of China Grant Nos. 50972024 and 60871049.
References [1] B. Acikel, T.R. Raylor, P.J. Hansen, J.S. Speck, R.A. York, IEEE Microwave and Wireless Components Letters 12 (2002) 237. [2] A. Tombak, J. Maria, F.T. Ayguavives, Z. Jin, G.T. Stauf, A.I. Kingon, A. Mortazawi, IEEE Transactions on Microwave Theory and Techniques 51 (2003) 462. [3] S. Courrèges, Y. Li, Z. Zhao, K. Choi, A. Hunt, S. Horst, J.D. Cressler, J. Papapolymerou, IEEE Microwave and Wireless Components Letters 19 (2009) 356. [4] A. Kumar, S.G. Manavalan, V. Gurumurthy, S. Jeedigunta, T. Weller, Materials Science and Engineering B 139 (2007) 177. [5] P. Bao, T.J. Jackson, X. Wang, M.J. Lancaster, Journal of Physics D: Applied Physics 41 (2008) 063001. [6] W. Ren, S. Trolier-McKinstry, C.A. Randall, T.R. Shrout, Journal of Applied Physics 89 (2001) 767. [7] J. Lu, S. Stemmer, Applied Physics Letters 83 (2003) 2411. [8] X. Zhang, W. Ren, Materials Science and Engineering B 177 (2012) 975. [9] L. Yan, L.B. Kong, L.F. Chen, K.B. Chong, C.Y. Tan, C.K. Ong, Applied Physics Letters 85 (2004) 3522. [10] H. Tian, Y. Wang, D. Wang, J. Miao, J. Qi, H.L.W. Chan, C.L. Choy, Applied Physics Letters 89 (2006) 142905. [11] S. Wang, M. Guo, X. Sun, T. Liu, M. Li, X. Zhao, Applied Physics Letters 89 (2006) 212907. [12] W. Fu, L. Cao, S. Wang, Z. Sun, B. Cheng, Q. Wang, H. Wang, Applied Physics Letters 89 (2006) 132908. [13] W. Fu, H. Wang, L. Cao, Y. Zhou, Applied Physics Letters 92 (2008) 182910. [14] R. Li, S. Jiang, L. Gao, L. Wang, Y. Li, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 58 (2011) 1140. [15] R. Li, S. Jiang, L. Gao, Y. Li, Journal of Applied Physics 112 (2012) 074113. [16] I.D. Kim, H.L. Tuller, H.S. Kim, J.S. Park, Applied Physics Letters 85 (2004) 4705. [17] S. Jiang, Y. Li, Japanese Journal of Applied Physics 48 (2009) 121402. [18] I. Vendik, O. Vendik, E. Kollberg, IEEE Transactions on Microwave Theory and Techniques 48 (2000) 802. [19] A.K. Tagantsev, V.O. Sherman, K.F. Astafiev, J. Venkatesh, N. Setter, Journal of Electroceramics 11 (2003) 5.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Thickness-ratio-dependent dielectric properties of Bi1.5 Zn1.0 Nb1.5 O7 /Ba0.5 Sr0.5 TiO3 bilayered thin films Ruguan Li, Shuwen Jiang ∗ , Libin Gao, Yanrong Li State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China
a r t i c l e
i n f o
Article history: Received 14 January 2013 Received in revised form 1 April 2013 Accepted 23 April 2013 Available online 10 May 2013 Keywords: Dielectric properties Tunable dielectric Thin films Bismuth zinc niobate (BZN) Barium strontium titanate (BST)
a b s t r a c t Bi1.5 Zn1.0 Nb1.5 O7 (BZN)/Ba0.5 Sr0.5 TiO3 (BST) thin films were prepared on Pt/Ti-coated sapphire substrates by radio frequency magnetron sputtering. The specific relationship between the dielectric properties and the thickness ratio of the BZN thickness to the BST thickness was investigated. The presence of BZN films effectively reduced the dielectric loss of the thin films. The thickness-ratio-dependent dielectric constant and dielectric loss behaviors were in good accordance with the simulation results based on the series connection theory. The optimum thickness ratio was determined to be around 0.5, exhibiting a maximum commutation quality factor of about 16,000. The built-in electric field at the region near the BZN–BST interface may be responsible for the asymmetric characteristic of the electric-field-dependent dielectric properties of the BZN/BST thin films. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Barium strontium titanate, Ba1−x Srx TiO3 , is a ferroelectric film widely studied as a suitable material in tunable microwave devices because of its strong electric-field-dependent permittivity [1–3]. However, in general the loss tangent of these films is relatively high, e.g. tan ı > 0.01 for Ba0.5 Sr0.5 TiO3 (BST) thin films at room temperature [4,5]. It is a major drawback for their applications. Recently, cubic pyrochlore Bi1.5 Zn1.0 Nb1.5 O7 (BZN) thin films have gained a great deal of attention in tunable applications because of their very low dielectric loss and a certain dielectric tunability. However, the tunability of these films is relatively low and it required very high electric fields to achieve a high tunability [6–8]. In other words, the tunability of BZN is much smaller than BST at the same electric fields. An interesting issue to address is how to realize composite thin films that utilize the optimum combination of the high tunability of BST and the low loss of BZN. BST–BZN composite thin films by using BZN as a dopant [9,10] and BST/BZN heterolayered thin films [11–13] with good dielectric properties have been reported. Among these approaches, the BZN/BST heterolayered thin films reported by Fu et al. [13] were one of the best tunable materials achieved to date. We have reported capacitors employing BZN/BST bilayered films for radio frequency (RF) applications; the capacitors exhibit a high tunability and a high Q-factor [14].
∗ Corresponding author. Tel.: +86 28 83200866; fax: +86 28 83202569. E-mail address: [email protected] (S. Jiang). 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.04.010
The fact that BZN/BST bilayered thin films are composed of two layers of different materials means that the different thickness ratios of the two layers would result in different dielectric properties. However, only a few reports on the thickness-ratio-dependent dielectric properties are available. Furthermore, considering that the thickness ratio affects the tunability and dielectric loss, an optimal thickness ratio must exit for the best performance of the material. This study investigates the specific relationship between the thickness ratio and dielectric properties, including permittivity, tunability, and dielectric loss. In addition, the optimum thickness ratio was determined. Ferroelectric thin films have been deposited on different substrates, including single crystal substrates and silicon wafers. The lattices of single crystal substrates present a close match to BST, whereas the common Si wafers do not provide lattice matching to BST [5]. The low resistivity of Si makes the realization of low loss microwave transmission lines challenging, which is another problem of the use of Si in tunable applications. The resistivity of sapphire is higher than that of silicon and HR silicon, and the sapphire are relatively low cost compared to other single crystal substrates, such as MgO or LaAlO3 . Therefore, sapphire was used as the substrate in this study. The Curie temperature of BST material is known to depend dramatically on the composition ratio between Ba and Sr in Ba1−x Srx TiO3 . BST has similarly sized maximum tunability near the Curie temperature, and the tunability and the loss tangent of BST decrease away from the Curie temperature. The choice of the composition depends on the operating temperature and the specific requirements of the particular microwave application [5]. Among
912
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
Table 1 Details of the deposition duration, thickness, and thickness ratio of the BZN/BST films. Sample number
1 2 3 4 5
BST
BZN
Thickness ratio of BZN vs. BST
Duration (min)
Thickness (nm)
Duration (min)
Thickness (nm)
180 180 180 180 180
160 160 160 160 160
0 30 50 70 90
0 70 110 140 160
the compositions, BST with a composition of Ba0.5 Sr0.5 TiO3 has a Curie temperature close to room temperature and a relatively large dielectric tunability. In the BZN material system, there are two main phases depending on composition: cubic Bi1.5 Zn1.0 Nb1.5 O7 and pseudo-orthorhombic Bi2 (Zn1/3 Nb2/3 )2 O7 . It has been reported that BZN thin films with a composition of Bi1.5 Zn1.0 Nb1.5 O7 showed dielectric tunable properties, but thin films with a composition of Bi2 (Zn1/3 Nb2/3 )2 O7 were nearly tunable [6]. Consequently, the particular BST (Ba0.5 Sr0.5 TiO3 ) and BZN (Bi1.5 Zn1.0 Nb1.5 O7 ) composition are chosen in this study. 2. Experimental The BST thin films were deposited on Pt/Ti-coated sapphire substrates by RF magnetron sputtering. A series of BZN thin films with different thicknesses were sputtered on the BST thin films. BST was deposited by sputtering from a ceramic Ba0.5 Sr0.5 TiO3 target with an RF power of 200 W, a substrate temperature of 800 ◦ C, and an Ar/O2 (30% O2 ) gas mixture at a total pressure of 4 Pa. BZN was deposited with a power of 150 W from a ceramic Bi1.5 Zn1.0 Nb1.5 O7 target at 750 ◦ C in a gas pressure of 4 Pa (85% Ar and 15% O2 ). The thickness of the thin films was controlled by allowing different growth time and was measured by conducting a cross-sectional scanning electron microscopy (SEM). Details of the deposition duration, thickness, and thickness ratio of the films are tabulated in Table 1. For the electrical measurements, Pt/Ti (∼200 nm/30 nm) top electrodes were deposited on the thin films by performing an electron-beam evaporation and patterned by conducting a lift-off process. The thin Ti adhesion layer was used to enhance the adhesion between the films and the platinum electrodes. An annealing in 4 Pa of oxygen ambient at 600 ◦ C for 30 min was performed to improve the interfacial contact between the films and the electrodes. The phase composition and crystallization of the BST thin films and BZN/BST bilayered films were characterized by X-ray diffraction (XRD). The dielectric properties of the thin films were measured at 100 kHz using an HP4294A Precision Impedance Analyzer at room temperature.
0 0.44 0.69 0.88 1
where dBZN is the BZN thickness and dBST is the BST thickness. Fig. 2 shows the applied-electric-field-dependent dielectric constant (extracted from measured capacitances) and the loss tangent of the thin films with different thickness ratios. The maximum dielectric constant of the BST thin films (i.e., x = 0) was ∼250, as shown in Fig. 2(a). The maximum dielectric constant of the thin films decreased with increased thickness ratio. Fig. 2(b) shows that the presence of the BZN films effectively reduced the dielectric loss of the thin films, for example, the dielectric loss dropped from 1.17% at a thickness ratio of 0 to 0.45% at a thickness ratio of 1. The dielectric loss reduction is attributed to the low loss of the BZN films. An asymmetric characteristic of the electric-field-dependent dielectric properties of the thin films was observed and will be discussed later. The following discussions present the specific relationship between the dielectric properties and the thickness ratio of the BZN/BST thin films. The measured capacitance Cm can be evaluated from the series connection of the BST capacitance, CBST , and the BZN capacitance, CBZN , as follows [16]: 1 1 1 = + Cm CBST CBZN
(2)
Given the same area, the average dielectric constant, εav , of the bilayered thin films can be expressed as: dBST dBZN dtotal = + εav εBST εBZN
(3)
where dtotal is the total thickness of the BZN/BST thin films and εBST and εBZN are the permittivity of the BST and BZN thin films, respectively. Using this expression with Eq. (1) provides a
3. Results and discussion Fig. 1 shows the XRD patterns of the BST thin films and the BZN/BST bilayered films. The results showed that the BST thin films were cubic pervoskite polycrystalline structure, and the BZN thin films were cubic pyrochlore polycrystalline structure. No impurity phases can be detected in the BZN/BST bilayered films, which mean that no reaction between BST and BZN layer occurred during the deposition process. The SEM cross-section image in the inset of Fig. 1 shows that the BZN/BST bilayered thin films have distinct interfaces. The key parameter used in this study was the thickness ratio x, which is defined as x=
dBZN dBST
(1)
Fig. 1. XRD patterns of BST thin films and BZN/BST bilayered films with a thickness ratio of 0.44. Inset is the SEM cross-sectional image for the BZN/BST with a thickness ratio of 0.44.
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
913
Fig. 3. Simulation and experimental results of the thickness-ratio-dependent dielectric constant of the BZN/BST thin films. The simulation is based on the series connection theory.
n=
Fig. 2. (a) Dielectric constant and (b) loss tangent of BZN/BST thin films as function of applied electric field at 100 kHz under forward-biased electric fields. The thickness ratio is defined as the ratio of the BZN thickness to the BST thickness.
relationship between the average dielectric constant εav and the thickness ratio x εav =
(1 + x) · εBST · εBZN εBZN + x · εBST
(4)
When x→ ∞, the εav → εBZN , whereas when x → 0, the average dielectric constant approaches εBST . Fig. 3 presents the comparison between the experimental and simulation results from Eq. (4). The experimental results of the thin films were the maximum dielectric constant extracted from Fig. 2(a). In the simulation results, εBST was 250 and εBZN was 140. This BZN dielectric constant value is close to the value for the BZN deposited on Si substrates in our laboratory [17]. Fig. 3 shows that the experimental results are in good accordance with the simulation results, indicating that the relationship between the dielectric constant and the thickness ratio comply with the theory, wherein the dielectric constant decreases as the thickness ratio increases. A further thickness ratio increase of over 0.6 causes the dielectric constant to decrease slowly and gradually approach the value for the BZN thin films. The most concerned property of ferroelectric thin films for tunable applications is the dependence of the permittivity of this thin film on the applied electric field. This characteristic is commonly described by the relative tunability nr , as expressed by Eq. (5), or the tunability n, as defined by Eq. (6): nr =
εmax − εmin × 100% εmax
(5)
εmax εmin
(6)
where εmax and εmin are the maximum permittivity and the minimum permittivity under the applied fields, respectively. The relative tunability of the films under the ±1 MV/cm electric fields as a function of the thickness ratio is plotted in Fig. 4. Given the asymmetric characteristic of the electric-field-dependent dielectric constant of the films, the minimum dielectric constant was replaced by the average value of the dielectric constant under the +1 and −1 MV/cm electric fields. Fig. 4 shows that the relative tunability of the films decreases almost linearly as the thickness ratio increases. Considering that the dielectric tunability of BZN is much smaller than BST, the tunable characteristic of the BZN/BST bilayered thin films is believed to be dominated by the BST. When electric fields are applied in series along the BZN and BST films, the BZN layers consume part of the applied fields. The available electric fields applied to the BST layers decrease with increasing thickness ratio; thus, the dielectric tunability decreases as the thickness ratio increases. Fortunately, the dielectric tunability of the BZN/BST bilayered films was prevented from being abruptly
Fig. 4. Thickness-ratio-dependent relative tunability of the BZN/BST thin films. The straight line represents linear fit to the experimental data points.
914
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
Fig. 5. Simulation and experimental loss tangent of BZN/BST thin films as a function of the thickness ratio. The simulation is based on the series connection theory.
degraded because the BZN permittivity was close to the value for BST and a certain tunability was also present in BZN. The main advantage of inserting BZN between the BST and the top electrodes is the reduction in the dielectric loss of the thin films. Fig. 2(b) shows that the presence of BZN films effectively reduces the dielectric loss of the thin films. The following presents the specific relationship between the dielectric loss of the films and the thickness ratio. BZN/BST bilayered thin films are composed of two layers, thus the power consumption of the bilayered films should be the sum of the consumption of the two layers, as follows: U2 U2 U2 = BST + BZN R RBST RBZN
(7)
where U is the total voltage applied in BZN/BST bilayered films, UBST and UBZN are the voltages applied in the BST and BZN layers, respectively. R represents the resistance of BZN/BST bilayered films, RBST and RBZN represent the resistance of BST and BZN layers, respectively. With dielectric loss, Eq. (7) can be written as 2 2 U 2 · ωCm tan ı = UBST · ωCBST tan ıBST + UBZN · ωCBZN tan ıBZN
(8)
where ω is the angular frequency and tan ıBST and tan ıBZN are the loss tangents of the BST and BZN thin films, respectively. In addition, Gauss’s law provides: D1 = D2 → εBST · EBST = εBZN · EBZN
(9)
Using Eq. (8) with Eqs. (1), (2), and (9) provides a relationship between the loss tangent of the BZN/BST bilayered thin films (tan ı) and the thickness ratio as follows: tan ı =
εBZN . tan ıBST + x · εBST · tan ıBZN εBZN + x · εBST
(10)
where tan ı → tan ıBZN in the limit of x→ ∞, and tan ı → tan ıBST when x → 0. The simulation results and the measured maximum loss tangent of the thin films as a function of the thickness ratio are plotted in Fig. 5. In the simulation, tan ıBST and tan ıBZN is 1.17% and 0.1%, respectively. Fig. 5 shows that the experimental results are in good accordance with the simulation results. Apparently, the dielectric loss of the films decreases as the thickness ratio increases, approaching the loss of the BZN films. Both the dielectric tunability and dielectric loss of the thin films decreased with increasing thickness ratio. Obtaining the material for the best device performance is based on the optimal tradeoff between the tunability and dielectric loss. The figure of merit
Fig. 6. Commutation quality factor (k) of the BZN/BST thin films as a function of thickness ratio. The higher value of K indicates a more excellent performance of the material. Thus the optimum thickness ratio was determined to be around 0.5, exhibiting a maximum commutation quality factor of about 16,000.
(FOM), defined as the ratio of the relative tunability to the loss tangent, is commonly used to reflect the trade-off between tunability and dielectric loss. However, the simple form of FOM does not have a well-defined physical meaning [5]. The commutation quality factor (K), first proposed by Vendik et al. to characterize a two-state, one-port switchable network [18], can be used instead of FOM to evaluate the optimal trade-off between the tunability and loss of a material better. K is given by [18]: K=
(n − 1)2 n · tan ımax . tan ımin
(11)
where n is the tunability as defined in Eq. (6) and tan ımax and tan ımin are the loss tangents of the materials under zero and nonzero applied electric field, respectively. The FOM is more helpful to the device engineer, and K is more useful in comparing materials [5]. Therefore, the theory was also applied to evaluate the ferroelectric material properties [18,19]. The higher value of K indicates a more excellent performance of the material. The commutation quality factor of the thin films as a function of the thickness ratio is shown in Fig. 6. K is about 10,000 at a thickness ratio of 0. Initially, K increases as the thickness ratio increases. The maximum K value of about 16,000 was obtained at a thickness ratio of around 0.5. A further thickness ratio increase of over 0.6 reduces K, decreasing to about 10,000 at a thickness ratio of around 1. The optimized thickness ratio for the thin films has an important role for the best performance of the material. Consequently, the thickness ratio of about 0.5 is recommended for the BZN/BST bilayered thin films. The dielectric properties of BZN films are almost thickness independence in this study. However, the permittivity of BST films in this study showed obvious difference when film thickness larger than 300 nm or smaller than 100 nm. It is known that BST shows thickness-dependent properties [19]. Therefore, the optimum thickness ratio of 0.5 should not be applied to bilayered films that BST thickness over the range of 100–300 nm. For the case of BST film thickness larger than 300 nm or smaller than 100 nm, the analysis of their dielectric properties is identical to the analysis in this study and the corresponding optimum thickness ratio would be obtained. The asymmetry in the dielectric behaviors of the thin films is observed in Fig. 2. This asymmetry may be related to the builtin electric field at the region near the interface between the BZN and BST. As reported in our previous work [15], the Fermi energy
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
915
Fig. 7. Sketch map of the paralleled capacitance structure. (a) positive bias voltages are applied to the top electrodes, (b) negative bias voltages are applied to the top electrodes. E and Ei are the electric field applied to the thin films and built-in electric field at the BZN–BST interface, respectively. T-E and B-E denote the top electrodes and bottom electrodes, respectively.
level of BZN is about 0.4 eV deeper in the band gap than that of the BST. After joining the BST and BZN thin films, the electrons near the BZN–BST interface flowed from the BST layer to the BZN layer, leaving positively charged ions in the BST region. The region near the interface lost its neutrality and became charged. As a result, a built-in electric field (Ei ) was established at the charged region near the interface. The built-in electric field opposes the diffusion process for electrons, thus this field points from the BST layer to BZN layer. This process is continued until a thermal equilibrium is reached, as indicated by a constant Fermi energy level. When positive bias voltages are applied to the top electrodes, as shown in Fig. 7(a), the built-in field is opposite to the direction of the applied electric fields. In this condition, the built-in field reduces the effective electric fields applied to the thin films. Conversely, the built-in field is in the same direction as the applied electric field when negative bias voltages are applied to the top electrodes, as shown in Fig. 7(b). In this condition, the built-in field enhances the effective electric field applied to the thin films. This behavior results in a higher dielectric constant and loss tangent under positive biases compared with that under equivalent negative biases, as observed in Fig. 2. Consequently, the presence of the built-in electric field at the region near the BZN–BST interface shifts the peaks of the electric-field-dependent property curves to higher electric fields, possibly causing the asymmetric characteristic. The above analysis is based on the dielectric properties of the thin films under forward-biased electric fields. Fig. 8 shows the electric-field-dependent dielectric constant of the films under backward-biased fields. The analysis of their dielectric properties is identical to the analysis above, and similar results were achieved. Comparing Figs. 2(a) and 8, very little difference in the shape of the
electric-field-dependent dielectric constant curves are observed. From another perspective, the built-in electric field at the region near the BZN–BST interface should be responsible for the asymmetry in the dielectric behaviors of the thin films. The corresponding electric fields of the peaks of the electric-field-dependent dielectric constant curves and the dielectric constant in Fig. 8 are slightly different from those in Fig. 2(a). The dielectric properties under forward and backward biases should have no differences if the films have no defects. The concentration of the movable ions or charges accumulated near the interfaces and oxygen vacancies formed during deposition may be responsible for the observed difference. As mentioned above, the built-in electric field at the region near the BZN–BST interface should be responsible for the asymmetric behavior of the electric-field-dependent permittivity of the BZN/BST thin films. It is considered that the build-in electric field may result from the asymmetric structure of the BZN/BST bilayered films. Since the build-in field may also result from defects such as oxygen vacancies or bismuth vacancies in BZN films during film deposition, it is necessary to check the origin of the built-in field. Two sets of samples have been prepared: T-E/BST/BZN/BE/Sapphire and T-E/BZN/BST/BZN/B-E/Sapphire. Fig. 9 shows the electric field dependent dielectric constant of the two samples under forward and backward biases. The asymmetric behavior of the BST/BZN bilayered films showed reverse characteristics from the BZN/BST films. The corresponding electric fields of maximum permittivity of BST/BZN films occurred at negative biases, and it showed a higher dielectric constant under negative biases compared with that under equivalent positive biases. Besides, the asymmetric behavior of bias field dependent permittivity was
Fig. 8. Electric-field-dependent dielectric constant of BZN/BST thin films under backward-biased electric fields.
Fig. 9. Electric-field-dependent dielectric constant of BST/BZN bilayered films and BZN/BST/BZN sandwich films under forward and backward biases.
916
R. Li et al. / Materials Science and Engineering B 178 (2013) 911–916
much improved for the BZN/BST/BZN sandwich films with symmetry structure. These results indicate that the build-in electric field results from the asymmetric structure of the BZN/BST bilayered films. As shown in Fig. 9, there is a little difference of the fielddependent-permittivity between forward and backward biases. This may be caused by the defects accumulated near the interfaces. 4. Conclusions The thickness-ratio-dependent dielectric behaviors of BZN/BST thin films were investigated. The relative dielectric tunability of the thin films decreased almost linearly as the thickness ratio increased. The thickness-ratio-dependent dielectric constant and dielectric loss behaviors were in good accordance with the simulation results based on the series connection theory. The optimum thickness ratio was determined around 0.5, exhibiting a maximum commutation quality factor of about 16,000. The built-in electric field at the region near the BZN–BST interface may be responsible for the asymmetric characteristic of the electric-field-dependent dielectric properties of the BZN/BST thin films. Acknowledgement The authors gratefully appreciate the support from the National Natural Science Foundation of China Grant Nos. 50972024 and 60871049.
References [1] B. Acikel, T.R. Raylor, P.J. Hansen, J.S. Speck, R.A. York, IEEE Microwave and Wireless Components Letters 12 (2002) 237. [2] A. Tombak, J. Maria, F.T. Ayguavives, Z. Jin, G.T. Stauf, A.I. Kingon, A. Mortazawi, IEEE Transactions on Microwave Theory and Techniques 51 (2003) 462. [3] S. Courrèges, Y. Li, Z. Zhao, K. Choi, A. Hunt, S. Horst, J.D. Cressler, J. Papapolymerou, IEEE Microwave and Wireless Components Letters 19 (2009) 356. [4] A. Kumar, S.G. Manavalan, V. Gurumurthy, S. Jeedigunta, T. Weller, Materials Science and Engineering B 139 (2007) 177. [5] P. Bao, T.J. Jackson, X. Wang, M.J. Lancaster, Journal of Physics D: Applied Physics 41 (2008) 063001. [6] W. Ren, S. Trolier-McKinstry, C.A. Randall, T.R. Shrout, Journal of Applied Physics 89 (2001) 767. [7] J. Lu, S. Stemmer, Applied Physics Letters 83 (2003) 2411. [8] X. Zhang, W. Ren, Materials Science and Engineering B 177 (2012) 975. [9] L. Yan, L.B. Kong, L.F. Chen, K.B. Chong, C.Y. Tan, C.K. Ong, Applied Physics Letters 85 (2004) 3522. [10] H. Tian, Y. Wang, D. Wang, J. Miao, J. Qi, H.L.W. Chan, C.L. Choy, Applied Physics Letters 89 (2006) 142905. [11] S. Wang, M. Guo, X. Sun, T. Liu, M. Li, X. Zhao, Applied Physics Letters 89 (2006) 212907. [12] W. Fu, L. Cao, S. Wang, Z. Sun, B. Cheng, Q. Wang, H. Wang, Applied Physics Letters 89 (2006) 132908. [13] W. Fu, H. Wang, L. Cao, Y. Zhou, Applied Physics Letters 92 (2008) 182910. [14] R. Li, S. Jiang, L. Gao, L. Wang, Y. Li, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 58 (2011) 1140. [15] R. Li, S. Jiang, L. Gao, Y. Li, Journal of Applied Physics 112 (2012) 074113. [16] I.D. Kim, H.L. Tuller, H.S. Kim, J.S. Park, Applied Physics Letters 85 (2004) 4705. [17] S. Jiang, Y. Li, Japanese Journal of Applied Physics 48 (2009) 121402. [18] I. Vendik, O. Vendik, E. Kollberg, IEEE Transactions on Microwave Theory and Techniques 48 (2000) 802. [19] A.K. Tagantsev, V.O. Sherman, K.F. Astafiev, J. Venkatesh, N. Setter, Journal of Electroceramics 11 (2003) 5.