Jpn.J. Appl. Phys.Vol.4l (2002)pp. 6814{819Part l, No. 11B, November 2002
@zNZ The Japan Society of Applied Physics

Preparationand Propertiesof Bia-"LayTi3Op FerroelectricThin Films
Using Excimer UV Irradiation
WataruSereuorol andShin-ichiHtnnuol
TakashiHeynssr, DaichiTocRwn,Mio YerraADAl,
Department of Materials Science,Shonan Institute of Technology, I -I -25 Tsujido-Nishil<nigan, Fuiisawa, Kanaguwa 251'851 l, Japanl Departrnent of Applied Chemistry, Graduate School of Engineering, Nagoya (Jniversity, Furo-cho, Chlkum.ku, Nagaya ,164'8603,Japan(Received May 29,20fl2; acceptedfor publication July 18, 2002)
Low-temperature processing of Bi+-,La,Ti3O12(BLT) thin films was investigated by chemical solution dcporition using anexcimer UV irradiation, and their ferroelectric properties, crystallinity and microstructure werÇ chuactorlrêd, BLJTthin films
were prepared on Ft(200 nm)ÆiOz(SOnm)/SiO2/Si substratesby a spin-coating technique from alkoxidc prceurflor solutions.
The excimer UV irradiation onto as-deposited BLT thin films was highly effective in removing organic $pêclcËof the gelfilms, leading to the decreaseof the crystallization temperature and an increase of the crystallinity. The UV.pmcenred BLT
films started to crystallize at550'C and showed a high crystallinity and a high (117) prefened orientation for 600oC.unnealed
films. BLT thin films prepared at 600'C showed a homogeneous and dense microstructure with grain sizer of 20tl-300 nm.
The excimer UV irradiation onto as-crystallized BLI thin films was also effective in improving the ferroelectric propeniesof the thin films. Bir.rslao.zsTigOrzthin films preparedat 600'C using excimer UV inadiation showed a well-saturatEdP*Ethin films exhibited good fatigue
hysteresisloop with a P. of 9.8 pClcmz and E" of 78 kV/cm. Moreover, the 8i3.35La6.75Ti3O12
enduranceup to 1010switching cycles. [DOI: 10.1143/JJAP.4l.6814)
KEYWORDS: Bic-xLaxTisOrzthin film, chemical solution deposition,excimer UV irradiation,low-temperaturecrystalllzatlon,ferroelectricproperties

1.

an excimer UV irradiation, and investigatedtheir ferroelectricproperties, crystallinity and microstructures.

Introduction

Bi+TirOrz (BIT) is an attractive ferroelectric material forseveralapplications such as nonvolatile memories becauseof
its large remanentpolarization (P.), small coercivefield (8"),
and high Curie temperature.Compared with SrBizTazOeand
its related materials,BIT thin films are known to crystallize
at lower temperatureswith a high crystallinity. However,BIT
contains unstable Bi ions which are easily evaporatedduring the heating process.This volatility of Bi ions affects theferroelectricand fatigue characteristics.Bi3+ ions in the BIT
structurecan be substitutedby ions such as La3*, Nd3+ andV5+ for the improvement of its properties'!-3)Among them,Bia-,La,TlOrz (BLT) has been intensively investigatedasa promising ferroelectric material for nonvolatile random'
accessmemory becauseof its fatigue-free property and lowvoltage operation, compared to those of Pb(Zr,Ti)O3 eZT).
BLT thin films are typically depositedby a spin-coating
techniqueusing a chemical solution processand crystallized
by furnace annealing or rapid thermal annealing (RIA) at650-750"C.
Regarding the preparation of BLT thin films by the sol-gel
method, it has been repofted4'5)that processing factors suchas the control of Bi-excesscomposition as well as annealing
temperatureand time strongly affect the crystallinity, the orientation, the crystalline structure and ferroelectric propertiesof BLT films. Moreoveç the high-temperature annealing processis not suitable for high-density memory devices, because
the silicon semiconductor is often seriously damaged during
the annealingprocess.Recently, in order to lower the processing temperature,a nertvprocess for film preparation using anexcimer laser or Hg UV lamp accompanying a photolysis reaction of organic speciesand crystallization has been activelyinvestigated.6-8)
In this work, we focused on the low-temperature processing of BLT thin films by chemical solution deposition using

2.

ExperimentalProcedures

Figure 1 shows the procedure for the preparation ofa BUI precursor solution by chemical solution deposition. Bi(OtCsHrrh, La(OiC:Hz)l and Ti(OiClHz)+ wereused as starting materials. All procedures were conducted in a dry Nz atmosphere.The desired amounts ofBi(OtCsH11)3,La(OtCrHrh and Ti(O'CrHt)o correspondLao,;sTilOr2 compositions
ing to Bil.zsLao.rsTilOrz and 8i3,35
were dissolved in absolute 2-methoxyethanol,and subsequently ecetylâcctonew&s addedto the solution as a stabilizing agcnt,The solution wûs refluxedfor l8h and condensed
to approximatcly 0.1 mol//. The homogeneity and stability

Bi(OtCsH,,)3, La(OiC3H7):,Ti(OiC3FI7)4

2-Methoxyethanol
Acetylacetone
C for l8h(0.1M)

IndryN,BLT Coating Solution
Fig. l. Procedure for preparation of BLT coating solution by chemical solution deposition.

6814

of lhc eoating solution was significantly improved by thelddltion of acetylacetone.BLT thin films were prepared on
Pt(200nm)lTioz (50 nm)/SiOz/Si substratesby a spin-coating
techniquc from alkoxide precursor solutions of BLT. Asdepoxltædprecursorfilms were dried at 150"C for 5 min, andthen were inadiated using an Xe excimer UV lamp (172nm)
0t 300*C for 30min in Oz atmosphere. The excimer UVpnnersed films were calcined at 500"C for 10min in an Oz
attnosphere,and then were annealedat 550-700'C for 30 min
In an Oz atmosphere by RIA. The film thickness of BLT
wan adjustedto approximately 200nm by repeating the coating/calciningcyclesfour times.
Optical properties of BLT precursor solution irradiatedusing a UV lamp (Ushio Electrics, UXM-501MD: 210580 nm) were evaluatedusing a UV-V spectrometer.The crystallinity and crystalline phaseof BLT thin films were examined by X-ray diffraction (XRD). The surface morphologyof the thin films was observed using an atomic force microscope (AFM) and a field-emission scanningelectron microscope (FE-SEM). The chemical speciesin the excimer UVprocessedthin films were analyzedby a Fourier transform infrared spectrometer(FI-IR).
Prior to electrical measurements.circular Pt electrodes of
0.20 mm diameter were deposited by rf sputtering onto thesurfaceof the films, which was followed by annealing at crystallization temperaturesfor 5 min. The ferroelectric properties of the films were also evaluatedby means of a ferroelectric test system(TFA-ANALYZER 2000, AixACCT. Inc.) at
100H2.The appliedvoltagewas l-15 V.
Results and Discussion

3.

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Vol.4l(2W2)Pt.l, No.llBIpn.J,Appl.Phys.

3.1 Optical properties of BLT precursor solutions
In order to investigatethe changeof BLT precursor solution
after UV irradiation, optical properties of the BLT precursor
solution were evaluated using a UV-V spectrometer.Figure
2 shows the UV-V spectra of BLT precursor solutions irradiated using a UV lamp at room temperaturefor 3G-60minin N2. At the stageof UV-nonirradiation, the BLT precursor
solution showeda high transmittanceover a wide wavelength

region, as shown in Fig. 2(a). On the other hand, in the UVirradiation time of 30 to 60 min. the transmittance decreased
from approximately 65 to 40Vo at 500nm, as shown in Fig.
2(b) and (c). This may be due to the dissociationof organic
species,followed by the generation of organic radical and reduced Ti ion in the BLT precursor solution by the photolysis
reaction. Hunt and Wintere) have reported the photoinduced
reduction of Ti(IV) to Ti(ID alkoxides by Sunlight. Therefore, Ti(IV) in the BLT precursor is reduced to Ti(III) by UVlight. Due to the changein optical property of the BLT precursor solutions, UV irradiation is found to influence the chemical bonding between metal ion and organic ligands. Hence, inthis work, excimer UV light (l72nm) with a high energywas
employed to remove the organic species smoothly from theprecursor films during the fabrication processof BLT films.
FT-IR spectra of excimer UV-irradiated BLT precursor
thin films
Figure 3 shows FFIR spectra of as-depositedBLT thinfilms irradiated using an excimer UV lamp at 300'C for
30 min in 02 atmosphere.The absorbancepeak assigned tostretching vibrations of C-H groups was observedat approximately 2900cm-l for the samplescalcinedat 300'C without
irradiation. However, at the excimer irradiation temperatureof 300'C in 02 atmosphere,the organic speciesin precursor
gel films were completely removed during the inadiation timeof 30min. This may be attributableto the easierdecomposition of organic groups by an oxidation reaction in 02 atmosphereaswell asthe bond cleavageby the photolysisreaction.

3.2

3.3 Crystallization of BLT thinfilms
The crystallinity of BLT films was examined by changing
the composition of the films. BLI thin films with Bi-excess,exhibited a higher crystallinity than thoseBi3.35Las.75Ti3O12,
aswith a stoichiometriccompositionof Bi3.25Las.75Ti3O12,
shown in Fig. 4.
thin
Figure 5 shows XRD patterns of Bi3.35Lae.75Ti3O12
films without and with excimer UV inadiation at 300"C for
30min in an Oz atmosphere,followed by RIA at 550 and600'C for 30min. As-depositedthin films without excimer
UV-inadiation crystallized at 550'C and exhibited randomorientation with a weak (00/) diffraction peak, as shown inFig. 5(a). On the other hand, excimer UV-processedthin filmso(h.!
fiÉ-ot-r

OFHv)
ti-o-HIIs

Nonirradiation at 300"Cfor 30min in O,V^ c-H'()()

rt1lrradiation at 300oCfor 30min in O,

a.a.

j\lt

_J_a \ v

i*\ta

atl-'.
(ttaa laa
I

7/f.
t a a

ttarlttflfarar..
aatt

400

500

600

700

800

Wavelength(nm)Fig, 2. UV-V spectraof BLT precursor solutions with acacH (a) before UVinudintion and after inadiation for 15 min (b) and for 30min (c).

4000c=st..

a.arrlrtarrfl

3000 2000
1000
Wave number/ cm-r

400

Fig. 3. FT-IR spectra of as-depositedBLT gel thin films without and withexcimer UV inadiation at 300"C for 30min in 02 atmosphere.

6816

Jpn.J. Appl.Phys.Vol.41 (2N2) Pt. l, No. l lB

T. Hnyesur era/.

t--Ê
(t)

(h€,ot-rhh

(t)

(t)
(.t()C)

2030

4050

oof-er^3:f,I6030

2g / deg.

40

20 / deg.

Fig. 4. XRD pattemsof Bi3.25La6.tsTbOn(a) and Bi3.35l-as
75Ti:Orz (b)thin films preparedat 700'C using excimer UV irradiation.

crystallized at 550'C, and the films crystallized at 600'C
showed a high crystallinity and a high (ll7) preferred orientation, as shown in Fig. 5(b). The excimer UV-inadiationin 02 atmosphere was very effective for the increase of thecrystallinity and the promotion of (ll7) preferred orientationof BLT films. This may be attributable to the promotion of
the decomposition of residual organic species with the formation of 03 along with a photolysis reaction. Furthermore,
the calcinationat 500'C in an 02 atmospherewas effective inincreasing the crystallinity.
3.4 Surfacemorphology of Bij.j5la.6.75Ti
jO 12thin films
Figure 6 shows AFM images of the surfaces of
Bi3.35Las.75Ti3O12
thin films preparedat 550 and 600oC.The
thin films prepared at 550'C using the excimer UV irradiation showed a smooth surface microstructureconsisting offine grains of approximately 100nm in size, which grew tolargergrainsof 10f200 nm by annealingat 600'C, comparedwith those without the excimer UV irradiation. The excimer
UV inadiation was found to be useful for improving the surface morphology of BLT films.
3.5 Ferroelectric properties of BLT thinfilms
Figure 7 shows P*E hysteresis loops of 700"Cannealed BLT thin films with an excess composition of
B\35Lan.75Ti3O12
and with a stoichiometric composition of
Bl.25Lq.7sTi: O rz. 8i3.35Las.75Ti3
O 12 thin films showed improved ferroelectric properties with a remanent polarization,Pr, of l5.41tClcm2 and a coercive electric field, 8", of
64kV/cm, compared with those of a stoichiometric composition of Bi3.25
L ax.15T\O e.
Figure 8 shows P. and E" as a function of applied voltage for B\35Laa.75Ti3O12
thin films preparedat 550'C using
excimer UV inadiation. The thin films prepared at 550'C using excimer UV irradiation showed a saturationtendencyover
5 V. A well-saturated P-E hysteresisloop was obtained at an
applied voltage of 15V for the thin film preparedat 550'C,
which showed P, of 4.4 p,C/cm2 and E" of 72kY/cm.

a€r(t)og s 933
l0?0304A5060

2 0/ d e g .
Fig. 5. XRD puttcrnsof Bir.rsLa6.75Ti3O12
thin films using (a) excimer
UV nonirrudistion,und (b) irradiation at 300oC in 02 atmosphere.

Figure 9 shows P-E hysteresis loops at an applied
voltage of 5 V for Bi3.35lao.zsTi:O12thin films preparedat 550-700oC without and with excimer UV irradiation.
Bi3.35las.7sTigOrzthin films prepared at 550 and 600"C
without excimer UV irradiation exhibited P, of 1.0 and9.2p,Clcm2, ffid E. of 38 and 75kV/cm, respectively,asshown in Fig. 9(a). On the other hand, 8i3.35La6.75Ti3O1
thin films prepared at 550'C using excimer UV inadiation
showed P, of 2.4 pCtcm2 and E" of 44.4kYlcm. Furthermore, well-saturated P-E hysteresis curves were obtainedfor Bi3.35lao.zsTigOr2
thin films preparedat 600'C using excimer UV inadiation. They showed P. of 9.8 1,tC/cmzand E"
of 78 kV/cm, as shown in Fig. 9(b), which are sufficient for
ferroelectric memory device applications. The excimer UV
irradiation onto as-crystallized BLT thin films was also foundIpn.J. Appl,Phys.Vol.4l (2002)Pt. l. No. I lB

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Fig. 6. AFM imagesof the surfaceof 500 and 600"C-annealedBi3.35Las.75Ti3O12
thin films: 550'C without excimer UV inadiation (a)and with excimer UV inadiation (b), 600'C without excimer UV inadiation (c) and with excimer UV irradiation (d).

30

to be effective in improving the ferroelectric properties of thethin films.
Figure 10 showsthe fatigue property of Bi3.35Las.75Ti3O12
thin films prepared at 600'C using excimer UV irradiation
before and after the switching of 1010cycles at a frequencyof 1 MHz. Similar hysteresisloops were observedbefore andafter the switching, indicating good fatigue endurance.

ôlF!

()

U15\)

oH

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. rrtl.aConclusions$N.F{f.fGI

FlIt-15-200

0200

400

ElectricField/kY/cm
Fig, 7. P-E hysteresisloops of Bi3.25La6;6TizOy- (a) and 8i3.35l-a6.75Ti3O12G) thin films preparedat 700'C.

BLT thin films were synthesizedby chemical solution deposition using excimer UV irradiation, and their microstructure, crystal phase and ferroelectric properties were investigated. Our results are sufilmarized as follows:
l. Ferroelectric BLT thin films were successfully synthesized on Pt/TiO2/SiO2/Si substratesusing metal-organic
precursor solutions. Homogeneous and stable BLT precursor solutions were prepared by controlling the reaction of starting metal alkoxides in 2-methoxyethanol
with a key additive of acetylacetone.

6818

Jpn.J. Appl.Phys.Vol.41(2002)Pt. 1,No. 118

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)
101502(')
100?
;
50 ri-+15éF20Ol.

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0200400

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N.É-10L
ÂË . -20700'c650'CC-l-30- 1000

O-500

s00

1000

U600"c15550'C

Electric Field / kV/cm
Fig. 8. P. and E" as a function of applied voltage for 8i3.35l-a6.75T\Oethin films prepared at 550'C using excimer UV irradiation: (a) Saturation
property, (b) P-E hysteresis loop at applied volrage of 15 V.

.NF-15hri30-240N()

U+Pr

15

00200

400

ElectricField /kY/cm
Fig. 9. P-Ë hystcresisloops of 8i3.35l-a6.75T\Op thin films preparedat
the temperature of 550-700"C by KIA without (a) and with (b) excimer
UV irradiation.

102 104 106 108 1010 I

T2Nli-15-Pr^-30
Switching Cycles
Fig. 10. Fatigue property of Bi3.35la6.tsTitOn thin films prepared at600"C using excimer UV irradiation before and after l0l0 switching cycles.

3. The use of an excimer UV lamp further resulted in theeasy formation of single-phase BLT thin films with ahigh (ll7) preferred orientation and with a homogeneous microstructure consisting of fine grains.
4. The 600'C-annealed BLT thin films subjected to excimer UV inadiation showed P, of 9.81tClcmz and E"
of 78 kV/cm. Synthesized BLT films with appropriate
ferroelectric properties rue expected for use in the ferroelectric layers of FeRAM devices.
Acknowledgement

2. The excimer UV inadiation onto as-depositedfilms at300'C inO2 atmospherewas very effective in removing
the residual organic groups in the gel films and in lowering the crystallizationtemperatureof BLT to 550"C.

This work was supported by a Grant-in-Aid for Scientific
Research on Priority Area (B), No. 12134203 from
the Ministry of Education, Culture, Sports, Science andTechnology,Japan.

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