Stability and formation of pyrochlore phase in doped Sr0.8Bi2.3Ta2O9 thin films

(1)

Stability and formation of pyrochlore phase in

doped Sr

0:8

Bi

2:3

Ta

2

O

9

thin ﬁlms

San-Yuan Chen

a,*

,Bang-Chiang Lan

a

,Chang-Sheng Taso

a

,Shinn-Yih Lee

b

a

Department of Materials Science and Engineering, National Chiao-Tung University, 1001 Ta-hsueh Road, Hsinchu 300, Taiwan, ROC

b

Department of Ceramic and Materials Science, National Lien-Ho Institute of Technology, 1 Lien-Kung, Kung-Ching Li, Miao Li 360, Taiwan, ROC

Received 11 March 2002; received in revised form 15 November 2002

Abstract

Ferroelectric thin ﬁlms of bismuth-containing layered perovskite Sr0:8Bi2:3Ta2xMxO9(SBTM),where M is V,Ti,W,

and Zr,have been prepared on Pt/Ti/SiO2/Si substrates using the metal-organic decomposition method. The eﬀect of the

incorporated B-site cations on pyrochlore phase formation and microstructure evolution of SBTM films was investi-gated. The pyrochlore phase formation has been identified due to out-diffusion of titanium from underneath platinum layer to participate in the reaction with the films. Furthermore,the formation of pyrochlore phase in the SBTM films has been observed strongly dependent on the characteristics of incorporated M cation. The substitution of both W and V for Ta leads to the formation of pyrochlore phase at lower annealing temperature (750–800°C). On the other hand, the addition of Zr can retard the formation of pyrochlore phase from 850 to 900°C. A model based on the binding energy of octahedral structure is used to elucidate the formation and stability of the pyrochlore phase present in the SBT film.

1. Introduction

Excellent fatigue resistance in SrBi2Ta2O9

(SBT) ﬁlms has made this material attractive for nonvolatile memory applications [1,2]. The SBT compound is part of the family of Aurivillius compounds with a general formula of (Bi2O2)2þ

-(Am1BmO3mþ1)2,consisting of m-perovskite units

sandwiched between bismuth oxide layers. Here A

is the 12-fold coordinated cation in the perovskite structures,B is the octahedral site,and the bis-muth forms the rock-salt type interlayer (Bi2O2)2þ

between the perovskite blocks (Am1BmO3mþ1)2

[3,4]. As the ﬁlms were ﬁred at high temperature,

especially above 850°C,a secondary phase having

a pyrochlore structure was formed. Lu and Fang reported that the formation of the pyrochlore phase was primarily correlated with the interaction between the films and the titanium species that diffused outward from the titanium layer on sub-strates [5]. Rodriguez et al. studied the effect of Bi content on perovskite formation of SBT films and *_{Corresponding author. Tel.: +886-3 573 1818; fax: +886-3}

572 5490.

E-mail address:[email protected](S.-Y. Chen).

(2)

reported that the pyrochlore phase is easily

de-veloped above 775 °C. With the decrease of Bi

content,the formation tendency of pyrochlore phase was stronger. The estimated stoichiometry of bismuth-deﬁcient pyrochlore was Sr0:2(Sr0:5

-Bi0:7)Ta2O6:75 [6]. Furthermore,once the

pyroch-lore phase forms,it will be deleterious to the formation of the SBT ferroelectric phase. There-fore,the pyrochlore phase is undesirable as the SBT films are used in ferroelectric memories. Thus, it is interesting to find ways to avoid formation of this pyrochlore phase. Excess Bi is usually required to compensate for Bi loss due to the high volatility of Bi during processing [7] and Bi diffusion into the bottom electrode during annealing [8]. However, the pyrochlore phase formation could be pre-vented or retarded by doping some elements be-cause the pyrochlore structure was constructed via the 6-fold coordinated cation to the main frame-work of the unit cell in terms of binding energy, with additional cation incorporated into the framework of octahedral [9]. Therefore,the sta-bility and formation of the pyrochlore phase will be influenced by the characteristics of 6-fold co-ordinated cation.

For realizing the contribution of dopants on the formation and stability of the pyrochlore phase, the substitutional replacements of various dopants with diﬀerent binding energy for Ta5þ_{sites (B-site)}

were used in this work. The temperature

depen-dence of the phase formation in the Sr0:8

-Bi2:3Ta2xMxO9 (SBTM) ﬁlms on Pt/Ti/SiO2/Si

substrates will be ﬁrst investigated. The composi-tional distribution in the depth direction was ex-amined to study the possible stoichiometry of pyrochlore structure in the SBT-based materials. Furthermore,the eﬀect of various dopants on the formation of pyrochlore phase will be further studied and discussed.

2. Experiment

The starting materials for the metal-organic decomposition (MOD) process were bismuth 2-ethylhexanoate [Bi(CH3(CH2)3CH(C2H5)COO)3],

strontium 2-ethylhexanoate [Sr(CH3(CH2)3

CH-(C2H5)COO)2],lead 2-ethylhexanoate [Pb(CH3

-(CH2)3CH(C2H5)COO)2],titanium isopropoxide

Ti(OC3H7)4,zirconium n-propoxide,Zr(OC3H7)4,

vanadium isopropoxide V(OC3H7)5,tungsten

eth-oxide W(OC2H5)6 and tantalum ethoxide

[Ta(OC2H5)5] with 2-ethylhexanoic acid as the

solvent. The metal-organic precursors were mixed to form solutions with compositions of Sr0:8Bi2:3

-(Ta2xMx)O9,where M is Zr,Ti,V and W. Prior

to ﬁlm deposition,the substrate (Pt/Ti/SiO2/Si)

was cleaned in acetone and alcohol ultrasonic

baths,and then blown dry with N2 gas. The

so-lutions were spin-coated onto the substrate at a speed of 3000 rpm. After each coating,the as-deposited ﬁlm was dried on a hot plate at a

tem-perature of about 350 °C to remove the solvent

before application of the next coating. After the process was repeated 3 times,the as-deposited ﬁlms were annealed at 700–900°C in air for 30 min by directly placing the coated substrate into a preheated tube furnace.

The crystal structures of the films were analyzed by using X-ray diffraction (XRD) with Cu Ka ra-diation and a Ni filter. The chemical composition of the films was determined using inductively coupled plasma (ICP) mass spectroscopy. Three samples were measured by ICP analysis. The Sr/Bi/

Ta/Ti ratio normalized in Ta¼ 2 for Sr0:8

-Bi2:3Ta2xMxO9 ﬁlms annealed at 850 °C/1 h was

calculated with error deviation of 5%. The

sec-ondary-ion mass spectroscopy (SIMS) of the films was performed with a surface analysis system. The film thickness was measured by surface profilo-metry. Field-emission scanning electron micro-scopy was performed to investigate the surface morphology of the films.

3. Results

3.1. Phase formation of Sr0:8Bi2:3Ta2O9 ﬁlms

XRD studies of Sr0:8Bi2:3Ta2O9 ﬁlms as a

function of annealing temperature indicate that

the ﬁlms annealed at 600 °C are amorphous. At

650–700 °C,a broad diﬀraction peak appears at

around 2h¼ 28:5,which implies that the ﬁlm is

not fully crystallized. As the annealing

(3)

XRD pattern becomes sharper and the full width at half-maximum decreases indicating better crys-tallinity and an increase in grain size. However,

when the ﬁlms were annealed at 850

°C,be-sides the presence of the SBT perovskite,a small amount of pyrochlore phase was found to generate as shown in the XRD patterns of Fig. 1 (labeled with x). Furthermore,upon increasing annealing temperatures,the amount of SBT phase decreases with the corresponding increase in pyrochlore phase.

3.2. Formation of pyrochlore phase in doped-SBT ﬁlms

The peak at 29.7° shown in XRD patterns was not only found in the pure SBT but also in oxide-doped SBT systems. Vanadium (V) has the same valence as tantalum (Ta) but a smaller ionic radius than Ta. It was found that the (1 1 5) reﬂection in Fig. 2 for 800°C – annealed Sr0:8Bi2:3(Ta2xVx)O9

(SBTV) ﬁlms is slightly shifted to the lower dif-fraction angle with increasing vanadium content, indicating that the V ions could be dissolved into

TaO6 octahedral structure to substitute for Ta

ions. Nevertheless,further excess vanadium (x) beyond 0.2,the peak intensity of (1 1 5) rapidly decreases along with the accelerating formation of the pyrochlore phase that implies the layered per-ovskite structure becomes unstable. When the va-nadium content was added above 0.4,complete

secondary phase with no trace of perovskite has

been developed in the SBTV ﬁlms at 850 °C. In

contrast,although the pyrochlore phase can be

detected for the undoped SBT ﬁlms ðx ¼ 0Þ,the

ﬁlm is primarily composed of perovskite phase.

For conﬁrmation,the Pt/SiO2/Si (without Ti)

substrate was used and no pyrochlore phase was

detected for the SBT ﬁlms annealed at 850°C for

1 h. In contrast,as the SBT ﬁlms were deposited on Pt/Ti/SiO2/Si (with Ti) substrate,well-crystallized

pyrochlore phase has been completely developed at the same annealing condition. Furthermore,the surface morphology (Fig. 3) of SBT film on the former (without Ti) substrate is much different from that on the latter (with Ti) substrate. How-ever,it was doubted that the Ti out-diffused from the substrate directly either reacts with the atoms of SBT or first forms TiO2phase prior to reacting

with SBT structure to form the pyrochlore phase. For realizing the eﬀect of Ti on the formation of pyrochlore phase,Sr0:8Bi2:3Ta2xTixO9 (SBTT)

ﬁlms were heated at various temperatures. The obtained XRD patterns shown in Fig. 4 illustrate

that,at 850°C,even though the pyrochlore phase

can be identified,the peak intensity is much weaker compared to that of undoped SBT films. 3.3. SIMS analysis of Sr0:8Bi2:3Ta2xMxO9 films

The SIMS analysis in Fig. 5 illustrates that the titanium has diﬀused into the SBT ﬁlm during

Fig. 1. XRD patterns of Sr0:8Bi2:3Ta2O9ﬁlms annealed at

dif-ferent temperatures for 0.5 h.

Fig. 2. XRD patterns of Sr0:8Bi2:3Ta2xVxO9ﬁlms annealed at

(4)

annealing at 800 °C. As the vanadium was added to substitute for tantalum ion,the SIMS analysis of Sr0:8Bi2:3(Ta1:8V0:2)O9 ﬁlms in Fig. 6(b) shows

that no signiﬁcant amount of strontium and tan-talum was detected in the platinum layer at 800°C as compared to that of undoped Sr0:8Bi2:3Ta2O9

ﬁlms in Fig. 5(a). However,at 850°C (Fig. 6(b)),it was found that a small quantity of Sr with little Ta

appears in the platinum layer for Sr0:8Bi2:3

-(Ta1:8V0:2)O9 ﬁlms. Furthermore,the relative

con-tent of Ti in the Sr0:8Bi2:3(Ta1:8V0:2)O9 ﬁlms is

higher than that in SBT films. On the other hand, as shown in Fig. 7,with the addition of incorpo-rated Zr cation,the diffusion of Ti into the films is retarded. Therefore,a lower ratio of Ti/Ta can be obtained for the Zr-added SBT films compared to

Fig. 3. SEM plan views of Sr0:8Bi2:3Ta2O9 ﬁlms deposited

on (a) Pt/Ti/SiO2/Si and (b) Pt/SiO2/Si substrates annealed at

850°C for 1 h.

Fig. 4. XRD patterns of Sr0:8Bi2:3Ta2xTixO9ﬁlms annealed at

800 and 850°C for 0.5 h.

Fig. 5. Secondary ion mass spectroscopy of Sr0:8Bi2:3Ta2O9

ﬁlms deposited on Pt/Ti/SiO2/Si substrate and annealed at

(5)

the ﬁlms added with the V and W (not shown here).

4. Discussion

4.1. Pyrochlore phase development of

Sr0:8Bi2:3Ta2xMxO9 ﬁlms

As well known,SBT films are usually required to sinter at 750–800°C for obtaining good ferro-electric properties. However,depending on the cation ratio of the SBT compounds,a mixture of at least three phases: a pyrochlore structure,per-ovskite SBT and Sr–Bi-oxide compound was ob-served and reported for the film stoichiometry of 1.0:2.0:2.0 fired at 775°C [6]. On the other hand,

when the ﬁlms were annealed above 850

°C,be-sides the presence of the SBT perovskite,pyroch-lore phase was generated as shown in the XRD patterns of Fig. 1 (labeled with x). This phenom-enon implies that SBT becomes unstable and tends to be converted into the pyrochlore phase. Similar diﬀraction peaks were also observed by Desu [10] and Lu and Fang [5]. In general,the pyrochlore structure displays an aﬃnity for the 6-fold coor-dinated cation to establish the main framework of the unit cell in terms of binding energy. In other words,with additional cation incorporated into

the framework of octahedra (TaO6),the formation

and stability of cation-defect structure was re-markably inﬂuenced [9].

Some reports mention that the pyrochlore phase is usually observed at higher temperature annealing and the phase formation results from the interaction between SBT film and substrate [5– 7]. As evidenced from SIMS analysis in Fig. 5,the titanium has diffused into the SBT film during

annealing at 800°C. Therefore,it can be assumed

that the phase formation was strongly dependent on the diﬀusion of Ti. Abe et al. reported that the titanium on Pt/Ti/SiO2/Si substrates did

out-dif-fuse from the intervening layer into the platinum layer at elevated temperatures,and some titanium atoms reached the outside surface of the platinum

layer and were oxidized to become TiO2 [11].

Furthermore,a rough and granular morphology was usually observed on the platinum layer due to

Fig. 6. Secondary ion mass spectroscopy of Sr0:8Bi2:3Ta1:8

-V0:2O9ﬁlms deposited on Pt/Ti/SiO2/Si substrate and annealed

at (a) 800°C and (b) 850 °C for 0.5 h.

Fig. 7. Secondary ion mass spectroscopy of Sr0:8Bi2:3Ta1:8

-Zr0:2O9ﬁlms deposited on Pt/Ti/SiO2/Si substrate and annealed

(6)

the formation of TiO2 [12]. Based on the above

discussion,the Ti in the pyrochlore phase appar-ently results from the outward diﬀusion under the Pt electrode. Therefore,the pyrochlore phase could be considered due to the direct reaction

be-tween TiO2 and SBT to form the compound as

follows:

SrBi2Ta2O9þ xTiO2) SrBi2Ta2TixO9þ2x: ð1Þ

In addition,the XRD patterns in Fig. 2 illustrate that when excess vanadiumðx > 0:4Þ was added to substitute for the Ta in the SBT ﬁlms,complete secondary phase with no trace of perovskite has

been developed in the SBTV ﬁlms at 850 °C

compared to the undoped SBT ﬁlmsðx ¼ 0Þ. This

ﬁnding reveals that the addition of vanadium

causes the TaO6 perovskite structure to be more

unstable and promotes the formation of pyroch-lore phase that is probably due to the smaller ionic radius of V ion compared to that of Ta ion. However,in this work,with the Bi content ﬁxed,

i.e.,Bi 2:3 (enough for evaporation loss),and

changing the V content,it was found that the py-rochlore phase is easily developed in the SBTV than pure SBT system. This phenomenon dem-onstrates that the formation of pyrochlore phase is not only dependent on Bi content but also strongly dependent on the stability of sub-perovskite structure.

In addition,it was found (not shown here) that as the Ta ion in the SBT ﬁlms was replaced by the zirconium (Zr) ion to form Sr0:8Bi2:3(Ta1:8Zr0:2)O9

(SBTZ),the perovskite phase becomes more stable and the formation of pyrochlore phase was

ham-pered in the SBTZ ﬁlm at 850°C. However,as the

tungsten (W) ion was used to substitute for Ta ion in the SBT ﬁlms,an enhanced transformation of perovskite into pyrochlore phase was observed.

Namely,at 800°C,a strong peak of the pyrochlore

phase has been identiﬁed in the XRD patterns of Sr0:8Bi2:3(Ta1:8W0:2)O9 ﬁlms.

4.2. Estimated stoichiometry of pyrochlore phase Since the pyrochlore phase has been fully de-veloped in the Sr0:8Bi2:3(Ta1:8V0:2)O9at 850°C,we

can infer that the pyrochlore phase should contain smaller concentrations in Sr,Bi,larger content in

Ti and almost equal to Ta content than the pe-rovskit phase. Therefore,the stoichiometric com-position of pyrochlore phase can be postulated to be approximately to Sr0:8xBi2:3yTa2TizOm.

The chemical compositions of Sr0:8Bi2:3Ta2O9

ﬁlms after annealed at 800°C for 0.5 h were

ana-lyzed by ICP,showing that the molar ratio of Sr, Ta,and Bi in the film is very close to the compo-sitions in the precursor solutions except for a partial loss of Bi and an extra detectable Ti com-pared to those in precursor solutions. This might indicate that the bismuth indeed diffused into the platinum layer,and the Ti out-diffused from the substrate into the SBT films. As the SBT film was

further annealed at 850°C for 1 h,Table 1 shows

that the bismuth content was much reduced and a great quantity of titanium has been diﬀused into the SBT ﬁlm. The Sr/Bi/Ta/Ti ratio normalized in

Ta¼ 2 for the 850 °C-annealed ﬁlm is

approxi-mately to 0.71/2.03/2.0/1.67. For comparison,the ICP analysis was made for the SBTV and SBTZ

with x¼ 0:2 ﬁlms annealed at 850 °C for 1 h. For

the SBTV ﬁlms where the pyrochlore phase has been completely formed,the Sr/Bi/Ta/Ti/V ratio

normalized in Ta¼ 2 was calculated to be

ap-proximately 0.73/2.21/2.0/1.89/0.14. According to Eq. (1),the possible stoichiometry for the py-rochlore phase was estimated and expressed as SrBi2Ta2Ti2xMyO13 compound. Since the

ex-change of Sr/Bi is possible [13,14] as well as both Ti and M cations are assumed to occupy at B-sites and the possible stoichiometry of the pyrochlore phase is very close to SrBi2Ta2Ti2O13 compound

and the crystal structure is depicted in Fig. 8 that

Table 1

Sr/Bi/Ta/Ti/M ratio normalized in Ta¼ 2 for Sr0:8Bi2:3

-Ta2xMxO9ﬁlms annealed at 850°C/1 h (where M is V or Zr)

Precursor solution Annealed SBTM ﬁlms Sr/Bi/Ta Sr/Bi/Ta/Ti 0.8/2.3/2.0 0.71/2.03/2.0/1.67 Sr/Bi/Ta/V Sr/Bi/Ta/Ti/V 0.8/2.3/1.8/0.2 0.73/2.21/2.0/1.89/0.14 Sr/Bi/Ta/Zr Sr/Bi/Ta/Ti/Zr 0.8/2.3/1.8/0.2 0.81/2.32/2.0/0.79/0.21 The calculated data are averaged from three samples with the error deviation of5%.

(7)

has a Fd3m symmetry [15]. In this structure, (Ta,Ti), O(1) and (Bi,Sr) were assigned to sites 16c, 48f and 16d,respectively. The other O(2) is located in 8b. On the other hand,for the SBTZ with

x¼ 0:2 ﬁlm (no trace of pyrochlore phase

ob-served),the Sr/Bi/Ta/Ti/Zr ratio is approximately to 0.81/2.32/2.0/0.79/0.21 that is signiﬁcantly de-viated form the stoichiometric composition of the pyrochlore phase. Therefore,the pyrochlore phase formation was inhibited because the less Ti con-tent existed in the SBTZ ﬁlms.

4.3. Stabilization of pyrochlore phase

In general,the pyrochlore structure displays an aﬃnity for the 6-fold coordinated cation to es-tablish the main framework of the unit cell in terms of binding energy,with additional cation incorporated into the framework of octahedra. From above discussion,it was implied that the formation extent of pyrochlore phase is strongly correlated with the addition of incorporated cation (M element). In other words,in addition to sin-tering conditions (temperature and time),the sta-bilization of perovskite structure will be inﬂuenced by the addition of M element. Titanium,vana-dium,tungsten,and zirconium were selected in this

work to study the eﬀect of M substitution for Ta on the stability of pyrochlore phase. If we neglect the Bi2O3 layer in the bismuth-based ferroelectric

layered structure,the stabilization of TaO6

octa-hedral structure was considered from the view-point of having the substitution of M atom for Ta. According to PaulingÕs rule,when a cation with higher valency (Z) and smaller ion radius (r) was placed in the center of polyhedron such as octa-hedral structure,the octaocta-hedral tends to become unstable since the cation substitution probably induces lattice strain and distortion of TaO6

oc-tahedral structure. Therefore,the higher the Z=r value,the unstable the octahedral structure is. A reconstruction of octahedral structure was easier preceded and converted into a pyrochlore phase at a lower annealing temperature. The Z=r ratio for the Ta substitution by W,V,Ta,and Zr is 1.223:1.051:1:0.77 in the above-mentioned doped-SBT systems that are consistent with our experi-mental results if based on the same stoichiometric composition of Sr0:8Bi2:3(Ta1:8M0:2)O9. That is,the

tendency to form the pyrochlore phase for SBT with V added film is higher than that with Zr added. The fact that the addition of Zr can retard the formation of pyrochlore phase can be further evidenced from the SIMS analysis as shown in Fig. 7. A lower ratio of Ti/Ta can be obtained for the Zr-added SBT films compared to the films added with the V and W (no shown here). Therefore,a model based on the binding energy of octahedral structure can be used to explain the formation and stability of the pyrochlore phase present in the SBT film.

5. Conclusions

(1) The out-diﬀusion of Ti from the underlayer Pt during annealing process plays a very im-portant role in the formation of pyrochlore phase.

(2) The estimated stoichiometry of the pyrochlore phase was approximately to SrBi2Ta2Ti2y

-MxO13 compound based on ICP analysis.

(3) A pyrochlore phase was always observed at high temperature annealing irrespective of M-doped SBT compositions.

Fig. 8. Crystal structure of SrBi2Ta2Ti2O13 pyrochlore phase

(8)

(4) The formation temperature of the pyrochlore phase in M-doped SBT ﬁlms was dependent on the characteristics and added amount of M cation.

(5) The relative stability of the perovskite to py-rochlore structures in the SBTM ﬁlms can be

accounted for by the binding energy of TaO6

octahedral structure.

Acknowledgement

The authors gratefully acknowledge the Na-tional Science Council of the Republic of China for its ﬁnancial support through contract no. NSC-90-2215-E-009-061.

References

[1] C.A. Paz de Araujo,J.D. Cuchiaro,M.C. Scott,L.D. McMillan,J.F. Scott,Nature 374 (1995) 627.

[2] O. Auciello,J.F. Scott,R. Ramesh,Phys. Today 51 (1998) 22.

[3] G.A. Smolenskii,V.A. Isupov,A.I. Agranovskaya,Sov. Phys.–Solid States 1 (1959) 149.

[4] G.A. Smolenskii,V.A. Isupov,A.I. Agranovskaya,Sov. Phys.–Solid States 3 (1961) 651.

[5] C.-H. Lu,B.-K. Fang,J. Mater. Res 12 (1997) 2104. [6] M.A. Rodriguez,T.J. Boyle,B.A. Hernandez,C.D.

Buch-heit,M.O. Eatough,J. Mater. Res 11 (1996) 2282. [7] T. Hayashi,H. Takahashi,T. Hara,Jpn. J. Appl. Phys. 35

(1996) 4952.

[8] P.Y. Chu,R.E. Jones,P. Zurcher,D.J. Taylor,B. Jiang, S.J. Gillespie,Y.T. Li,J. Mater. Res 11 (1996) 1065. [9] J. Pannetier,J. Phys. Chem. Solids 34 (1973) 583. [10] S.B. Desu,D.P. Vijay,Mater. Sci. Eng. B 32 (1995) 75. [11] K. Abe,H. Tomita,H. Toyoda,M. Imai,Y. Yokote,Jpn.

J. Appl. Phys. 30 (1991) 2152.

[12] K. Sreenivas,I. Reaney,T. Maeder,N. Setter,C. Jagadish, R.G. Elliman,J. Appl. Phys. 75 (1994) 232.

[13] C. Voisard,D. Damjanovic,N. Setter,J. Eur. Ceram. Soc. 19 (1999) 1251.

[14] D. Mercurio,J.C. Champarnaud-Mesjard,B. Frit,P. Conﬂant,J.C. Boivin,T. Vogt,J. Solid State Chem. 112 (1994) 1.

[15] C.-H. Lu,B.-K. Fang,C.-Y. Wen,Jpn. J. Appl. Phys. 39 (2000) 5573.