Preparation and ferroelectric properties of barium-ion-doped strontium bismuth tantalate thin films

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Journal of Physics and Chemistry of Solids 69 (2008) 480–484

Preparation and ferroelectric properties of barium-ion-doped

strontium bismuth tantalate thin ﬁlms

Chung-Hsin Lu

, Da-Pong Chang

Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC

Abstract

(Sr0.5Ba0.5)xBiyTa2Oz (SBBT) ﬁlms were prepared on Pt/Ti/SiO2/Si substrates by a metal-organic decomposition method. At low

temperatures, the phase transformation of films from fluorite-type structure to SBBT structure was suppressed with increasing the amounts of bismuth ions. As temperature increased, the phase transformation was improved by excess bismuth ions. Adding excess bismuth ions also resulted in an increase in the grain size of the prepared films. The value of the remanent polarization of thin film was also increased by adding excess bismuth contents. It was found that the ferroelectric characteristics of these films were significantly affected by the composition in the films.

Keywords: A. Thin ﬁlms; B. Chemical synthesis; D. Microstructure

1. Introduction

Recently, the layered-structure perovskites have at-tracted considerable attention for their applications of low-voltage and high-speed nonvolatile random access memory (NvRAM). The chemical formula of these layered-structure compounds is (Bi2O2)2+(Ax1Bx

O3x1)2, where A and B indicates cations, and x

represents numbers of perovskite layers between two (Bi2O2)2+ layers. These materials were synthesized by

Aurivillus in 1949 and hence were named as Aurivillus compounds. Comparing with Pb(Zr,Ti)O3-based materials,

thin ﬁlms of the Aurivillus compounds with x ¼ 2, such as SrBi2Ta2O9, BaBi2Ta2O9, and SrBi2Nb2O9, exhibit several

advantages. The excellent properties of low leakage current, long retention time, fatigue-free, and stable imprint characteristics improve the applications in FeRAM

[1–11]. The processes for preparing SrBi2Ta2O9-based

thin ﬁlms on Si-based substrates have been widely examined.

Strontium bismuth tantalite (SrBi2Ta2O9, SBT) is one of

the promising materials of Aurivillus compounds with

excellent ferroelectric properties of long fatigue endurance and large remanent polarization (Pr)[12–16]. The previous

studies have revealed that the layered-structured perovs-kites exhibit the characteristics of low leakage current, low operating voltage, stable imprinted characteristics, and high polarization retention up to long switching cycles. Because of the slight difference of cation radii between Sr2+and Ba2+, the formation of solid solutions of these compounds is feasible. In present study, barium ions were doped into SrBi2Ta2O9 ﬁlms to form solid solutions.

(Sr0.5Ba0.5)BiyTa2Ozthin ﬁlms were prepared via a

metal-organic decomposition (MOD) method on Pt/Ti/SiO2/Si

substrates. Varying the stoichiometry of bismuth ions, the formation of the layered-structure perovskites, crystal-linity, and the ferroelectric properties were investigated in this study.

2. Experimental

Strontium 2-ethylhexanoate [Sr(C8H15O2)2], barium

2-ethylhexanoate [Ba(C8H15O2)2], bismuth 2-ethylhexanoate

[Bi(C8H15O2)2], and tantalate ethoxide [Ta(OC2H5)5] were

used as the starting materials. MOD was employed to fabricate (Sr0.5Ba0.5)xBiyTa2Oz (SBBT) thin ﬁlms. The

starting materials were mixed in toluene and then were

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spin-coated onto Pt/Ti/SiO2/Si substrates. The

as-depos-ited films were baked on the hot plate at 150 1C. After the solvent was removed, the films were pyrolyzed at 400 1C to burn out the remaining organics. The pyrolyzed films were annealed at 580–800 1C for 1 h with oxygen flow to enhance

the crystallinity. The crystallite phases were identiﬁed by X-ray diffraction (XRD) at room temperature using a MAC Science MXP3 XRD system with Cu Karadiation at

20 25 30 35 40 45 50

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Pt Pt

Fluorite type structure SBBT y=2.6 y=2.4 y=2.2 y=2.0 y=1.8 Intensity 2 θ (115)

Fig. 1. XRD patterns of (Sr0.5Ba0.5)BiyTa2Oz thin ﬁlms with different

y values annealed at 630 1C for 1 h.

20 25 30 35 40 45 50 (0010) (2010) (200) (115) y=2.6 y=2.4 y=2.2 y=2 y=1.8 Pt Pt SBBT Intensity 2 θ

Fig. 2. XRD patterns of (Sr0.5Ba0.5)BiyTa2Ozthin ﬁlms with different y

values annealed at 730 1C for 1 h.

20 25 30 35 40 45 50 SBBT Intensity Temperature ( °C) 800 730 680 630 580 2 θ (115) Pt (0010) Pt (2010) (200)

Fig. 3. XRD patterns of (Sr0.5Ba0.5)Bi2.4Ta2Oz thin ﬁlms heated at

(a) 580 1C, (b) 630 1C, (c) 680 1C, (d) 730 1C, and (e) 800 1C.

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40 kV and 30 mA. The surface morphologies and structure were examined by scanning electron microscopy (Hitachi model S-800 microscope, 20 kV) and atomic force micro-scopy using tapping mode with amplitude modulation (Nanoscope IIIa, Digital Instruments Company, Santaba-bara). The ferroelectric hysteresis measurements were conducted on ﬁlms in metal–ferroelectric–metal conﬁgura-tion using a standardized ferroelectricity test system in the virtual ground.

3. Results and discussion

3.1. Formation of (Sr0.5Ba0.5)BiyTa2Ozthin films

Fig. 1 shows the XRD patterns of (Sr0.5Ba0.5)BiyTa2Oz

thin films with different y values annealed at 630 1C. At y ¼ 1.8–2.2, two structures, layered-structure perovskites (SBBT structure) and fluorite-type structure, are observed. With an increase in y, only fluorite-type structure is

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obtained, and the SBBT structure disappears. When the ﬁlms are annealed at 630 1C, SBBT structure and ﬂuorite-type structure coexist regardless of variations in y. Fig. 2

illustrates the XRD patterns of (Sr0.5Ba0.5)BiyTa2Oz thin

films with different y values annealed at 730 1C. As the annealing temperature increases to 730 1C, the SBBT structure becomes the dominant phase for y ¼ 2–2.6. For y ¼ 1.8, a minor tungsten bronze type coexists with the perovskite structure. It is observed that there is a phase transformation from fluorite-type structure to SBBT structure with increasing temperature. In addition, it is also found that increasing the bismuth contents can enhance the crystallinity of the films.

Fig. 3illustrates the XRD patterns of the films at y ¼ 0.4 heated at various temperatures. At 580 1C, only the fluorite structure exists. After heating at 630 1C, the crystallinity of the fluorite structure increases. The layered structure starts to form from 680 1C; however, a small amount of fluorite phase still remains. After heating at 730 1C, pure layered structure is completely formed.

Fig. 4illustrates the phase diagram of SBBT. At 580 1C, the coexistence of SBBT structure and fluorite-type structure at y ¼ 1.8–2.2 indicates that the phase transfor-mation takes place with less bismuth-ion doped at low temperatures. The phase transformation of fluorite type to SBBT structure is suppressed when excess bismuth-ion is doped. When temperature arises, this transformation is facilitated by excess bismuth-ion. Therefore, the fluorite-type structure has transferred to SBBT structure comple-tely above 730 1C. However, the thin films decompose at high temperature, and tungsten bronze-type oxide is formed.

In order to examine the effects of bismuth contents on the preferred orientation of the prepared ﬁlms, the degree of orientation is deﬁned as follows:

Dð200Þ¼

½Ið200Þ=Ið115ÞFilm

½Ið200Þ=Ið115ÞPowder

1, (1)

where I200and I115are the diffraction intensity of the (2 0 0)

and (1 1 5) planes, respectively. It is found that when the value of y increases from 1.8 to 2.6, the degree of orientation in a-axis signiﬁcantly decreases. It reveals that adding excess bismuth-ion suppresses the preferred orien-tation in a-axis.

3.2. Microstructures and ferroelectric properties of (Sr0.5Ba0.5)BiyTa2Ozthin films

The scanning electron micrographs of (Sr0.5Ba0.5)Bi

y-Ta2Oz thin ﬁlms heated at 730 1C are shown in Fig. 5.

When y ¼ 1.8, the grain size of the films is around 90 nm. When y increases to 2.6, the grain size of the films also increases to around 200 nm. This indicates that increasing the bismuth contents results in an increase in the grain size of the films. The ferroelectric characteristics of the prepared films heated at 730 1C are analyzed by the hysteresis measurement. The obtained data are illustrated

in Fig. 6. When the value of y is small, the values of 2Pr

and 2Ecof thin ﬁlms are also small. Increasing the contents

of bismuth ions, the values of 2Pr and 2Ec gradually

increases. When y is equal to 2.4, the values of 2Prand 2Ec

of thin ﬁlms becomes 12.459 mC/cm2 and 28.44 kV/cm, respectively. After 1010switching cycles, these ﬁlms exhibit good anti-fatigue properties. The value of 2Pronly slightly

decreases 3.55%. The above results reveals that (Sr0.5Ba0.5)BiyTa2Oz with good ferroelectric ﬁlms are

successfully prepared in this study.

4. Conclusions

(Sr0.5Ba0.5)xBiyTa2Oz ﬁlms were synthesized on Pt/Ti/

SiO2/Si substrates by a metal-organic decomposition

method with different contents of strontium, barium, and bismuth ions. At low temperature, the phase transforma-tion of films from fluorite-type structure to SBBT structure was suppressed with increasing quantity of bismuth ions. As temperature increased, the phase transformation was improved by excess bismuth ions and decomposition was also avoided. Adding excess bismuth ions caused an increase in the grain size of the prepared films. The value

Fig. 6. Ferroelectric characteristics of (Sr0.5Ba0.5)xBi2.4Ta2Oz ﬁlms

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of the remanent polarization of thin film was also increased by adding excess bismuth contents. It was found that the ferroelectric characteristics of these films were substantially influenced by the bismuth contents in the films.

References

[1] K. Amanuma, T. Hase, Y. Miyasaka, Preparation and ferroelectric properties of SrBi2Ta2O9 thin ﬁlms, Appl. Phys. Lett. 66 (1995)

221–223.

[2] C.A. Pazaraujo, J.D. Cuchiaro, M.C. Scott, L.D. Mcmilan, Fatigue-free ferroelectric capacitors with platinum electrodes, Nature 374 (1995) 627–629.

[3] J.F. Scott, F.M. Ross, C.A. De Araujo Paz, M.C. Scott, M. Huffman, Structure and device characteristics of SrBi2Ta2O9-based

nonvolatile random-access memories, Mater. Res. Soc. Bull. 21 (1996) 33–39.

[4] S.B. Desu, D.P. Vijay, X. Zhang, B.P. He, Orientation growth of SrBi2Ta2O9 ferroelectric thin ﬁlms, Appl. Phys. Lett. 69 (1996)

1719–1721.

[5] T.J. Boyle, C.D. Buchheit, M.A. Rodriguez, H.N. Al-Shareef, B.A. Hernandez, B. Scott, J.W. Ziller, Formation of SrBi2Ta2O9. Part I:

Synthesis and characterization of a novel sol–gel solution for production of ferroelectric SrBi2Ta2O9thin ﬁlms, J. Mater. Res. 11

(1996) 2274–2281.

[6] P.C. Joshi, S.O. Ryu, X. Zhang, S.B. Desu, Properties of SrBi2

Ta2O9 ferroelectric thin ﬁlms prepared by a modiﬁed

metal-organic solution deposition technique, Appl. Phys. Lett. 70 (1997) 1080–1082.

[7] K. Kato, C. Zheng, J.M. Finder, S.K. Dey, K. Torii, Sol–gel route to ferroelectric layer-structured perovskite SrBi2Ta2O9and SrBi2Nb2O9

thin ﬁlms, J. Am. Ceram. Soc. 81 (1998) 1869–1875.

[8] C.H. Lu, J.T. Lee, Strontium bismuth tantalate layered ferroelectric ceramics: reaction kinetics and thermal stability, Ceram. Int. 24 (1998) 285–291.

[9] C.H. Lu, Y.C. Chen, Sintering and decomposition of ferroelectric layered perovskites: strontium bismuth tantalate ceramics, J. Eur. Ceram. Soc. 19 (1999) 2909–2915.

[10] C.H. Lu, C.Y. Wen, Phase formation and ferroelectric characteristics of nonfatigue barium bismuth tantalate thin ﬁlms, J. Appl. Phys. 86 (1999) 6335–6341.

[11] C.H. Lu, C.H. Wu, Preparation, sintering, and ferroelectric proper-ties of layer-structured strontium bismuth titanium oxide ceramics, J. Eur. Ceram. Soc. 22 (2002) 707–714.

[12] Y. Zhu, S.B. Desu, T. Li, S. Ramanathan, SrBi2Ta2O9 thin ﬁlms

made by liquid source metal-organic chemical vapor deposition, J. Mater. Res. 12 (1997) 783–792.

[13] P.Y. Chu, R.E. Jones Jr., P. Zurcher, D.J. Taylor, B. Jiang, S.L. Gillespie, Y.T. Lii, Characteristics of spin-on ferroelectric SrBi2Ta2O9

thin ﬁlm capacitors for FERAM application, J. Mater. Res. 11 (1996) 1065–1069.

[14] C.H. Lu, B.K. Fang, Secondary phase formation and microstructural development in the interaction between SrBi2Ta2O9ﬁlms and Pt/Ti/

SiO2/Si substrates, J. Mater. Res. 12 (1997) 2104–2110.

[15] B.H. Kim, D.K. Kang, Characteristics of ferroelectric Sr0.7Bi2.1

Ta2.0O9 thin ﬁlms grown by liquid delivery metal organic chemical

vapor deposition process, Thin Solid Films 492 (2005) 140–145. [16] A.B. Panda, A. Tarafdar, S. Sen, A. Pathak, P. Pramanik,

Preparation of nanocrystalline SrBi2Ta2O9 powders using