Pb(CH3COO)2 and 2-methoxyethanol with continuously stirring. After that, lanthanum nitrate ... the viscosity of the sols increased by PVP. The cross(x) in Fig.1 ...
Mater. Res. Soc. Symp. Proc. Vol. 928 © 2006 Materials Research Society
0928-GG14-19
Effect of Polyvinylpyrrolidone Molecular Weight on the Critical Thickness, Crystallization, Densification and Properties of PLZT Films Z.H. Du1, and J. Ma2 1 School of Materials science and engineering, Nanyang Technological Univerisity, Ceramics lab,N4.1-B3-03,Nanyang Avenue, singapore, singapore, 639798, Singapore 2 School of Materials science and engineering, Nanyang Technological Univerisity, Ceramics lab,N4.1-B3-03,Nanyang Avenue, Singapore, Singapore, 639798, Singapore
ABSTRACT PLZT 9.5/65/35 ceramic films were prepared from the sol-gel solutions containing the stress-relaxing agent of polyvinylpyrrolidone(PVP) with the molecular weight of 40000, 360000 and 630000 (abbreviated as PVP40, PVP360 and PVP630) respectively. It is found that PVP40 can increase the critical thickness of the PLZT films to 0.22µm(compared with the critical thickness of 0.077µm in case of without PVP) and PVP360 and PVP630 can increase the critical thickness up to 0.49µm and 0.93µm individually. With the molecular weight of PVP increasing, the perovskite formation in the PLZT films was improved but the morphology of the films tended to be nano-porous. The films modified by PVP360 can be densified by optimizing the heat treatment conditions. When pyrolyzed at 600°C and then annealed at 700°C, the one-coating films with the thickness of 0.5µm were dense and exhibited a very slim hysteresis loop with the remnant polarization of 5.96x10-4µC/cm2 and coercive field of 21.51kV/cm, and a transmittance of around 90% and above in the wavelength of 450nm~900nm. KEYWORDS: polyvinylpyrrolidone, PLZT, critical thickness, crystallization, densification INTRODUCTION Pb0.905La0.095(Zr0.65,Ti0.35)0.976O3 (abbreviated as PLZT(9.5/65/35)) ceramic films above 1µm thick are very promising materials to be applied in advanced electro-optical (EO) and waveguide-based devices1,2,3. Among various film deposition techniques, sol-gel processing is an attractive method due to its flexibility in composition controlling and cost-effectivity. However, crack formation in the gel films during heat treatment is one of the major challenges for sol-gel processing of such thick films. The critical thickness i.e. the maximum thickness achieved without crack formation by one coating is often 99.5%, Aldrich Chemicals Company), zirconium propoxide(70% solution in 1-propanol, Aldrich), titanium isopropoxide(97%, Aldrich), lanthanum nitrate(Merck KGaA, Germany) and three kinds of PVP(with average molecular weight of 40000, 360000, and 630000 respectively, Fluka Chemie GmbH, Gernamy) were chose as starting materials. The molar composition of the starting solution was PbCH3COO)2/Zr(n-OC3H7)4/Ti(iOC3H7)4 /La(NO3)3/PVP/AcAcH/CH3OC2H4OH=1.01/0.6345/0.3417/0.09/1.5/0.5/30, where the mole ratio of PVP was defined according to its monomer. The solution was prepared using 10% excess PbO for compensating lead loss in the following heat treatment. First, zirconium propoxide and titanium isopropoxide were dropped into a solution of acetylacetone(>99%, Merck KGaA, Germany) and 2-methoxyethanol(99.3%, Sigma-Aldrich chemie GmbH, Germany) respectively and then mixed to form a clear solution. The solution was added to the mixture of Pb(CH3COO)2 and 2-methoxyethanol with continuously stirring. After that, lanthanum nitrate was added. Once it dissolved, the whole solution was divided into four parts and the three types of PVP were dropped in respectively. The PVP modified sols will be refluxed at 80°C for 2 hours and consequently aging for 72hours before using. Preparation and characterization of the PLZT films The one-layer green PLZT films were spin coated on glass slides from the PVP modified sols. The films were heated at 300°C for 10mins and then heated up to 550°C at 40°C/min and dwelled for 10mins. The film thickness was measured by Surface Profiler (Alpha-Step 500, Tencor, USA). The cracks in the films were observed at a 500x optical microscopy. The crystallization behavior of the PLZT films affected by PVP molecular weight was studied by Rigaku ultima type X-Ray Diffractometer (XRD) with the incident angle of 1°. The films were spin coated on ITO coated aluminia-silicate glass at different spin speed to get the similar thickness of 0.2µm. All the samples were pyrolyzed at 300°C and 550°C for 10mins and then heated up to the 600°C or 650°C at 40°C/min and dwelled for 30mins respectively. The densification of the PVP360 modified PLZT films were studied by changing the heating rate(5°C/min, 20°C/min, 40°C/min) and the pyrolysis temperature(300°C, 450°C, 500°C,550°C, 600°C) and all the films were finally annealed at 700°C for 30mins. The studied films are prepared by one coating with thickness of around 0.5µm. The morphology of the annealed samples was observed using a field emission scanning electronic microscope (FESEM, JSM-6340F, JEOL, Tokyo, Japan). The ferroelectric and dielectric properties were studied by the standard Radiant RT6000HVS type ferroelectric tester and HP 4194A Impedance/Gain Phase Analyzer individually. The transmittance of samples was measured by the UV-Vis spectrometer (UV-2501PC, Shimadzu, Kyoto, Japan).
RESULTS AND DISCUSSION Effect of PVP molecular weight on the critical thickness of the films
Thickness(nm)
Fig.1 shows the one-coating thickness of PVP modified and no PVP modified PLZT films. It is indicated that adding PVP can significantly increase the one-coating thickness of PLZT films compared with that of the films without PVP360 modification. This is attributed to the viscosity of the sols increased by PVP. The cross(x) in Fig.1 denotes the thickness where the films began to crack. The critical thickness of the films should lie between the cross-marked point and the previous point close to the cross-marked point. Thus the films modified by PVP40 can increase the critical thickness up to 1000 without PVP ~0.22µm(compared with the critical 900 PVP40000 PVP360000 thickness of 0.077µm in case of without 800 PVP630000 PVP) and PVP360 can increase the critical x with cracks 700 X thickness to ~0.49µm. For the films 600 modified by PVP630, cracks were not 500 found even if the spin speed decreased to 400 300rpm and the films peeled off when the 300 X spin speed was 100rpm. So the critical 200 thickness of the PVP630 modified PLZT 100 X was around the thickness of the films coated 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 Spin Speed(rpm) at 300rpm, i.e. ~0.93µm. So larger the Fig.1 One-coating thickness of PLZT films as function molecular weight of PVP, higher the critical of the molecular weight of PVP thickness of the PLZT films to be obtained.
.
Effect of PVP molecular weight on the crystallization and morphology of the films Fig.2 shows the effect of PVP molecular weight on the crystallization of PLZT films. It is found that when all the films were annealed at 600°C, the films modified by PVP consisted of pyroclore phase and perovskite phase, while for the films without PVP modification, perovskite
*
0 - Py * - IT O - Pe
.
.
PV P 630
0
0
0
0 - Py * - IT O - Pe
PVP630
*
. .
Intensity
0
.
(b)
P VP 360
*
Intensity
0
(a)
0
P VP 40
P V P3 60
0
0
P V P4 0
*
. no PVP
no P VP
20
25
30
35
2θ
40
45
50
20
25
30
35
40
45
50
2Θ
Fig.2 XRD of the PLZT films as function of PVP molecular weight ((a) annealed at 600°C; (b) at 650°C)
phase didn’t formed and there was only a very strong pyroclore peak in the XRD pattern. The perovskite phase began to form in the films without PVP modification until the annealing temperature increased to 650°C. At this annealing temperature, the perovskite phase has already
dominated in the PVP modified PLZT films and with the PVP molecular weight increasing, the peak of pyroclore phase became weaker, indicating less pyroclore existed in the films. So it can be concluded that adding PVP can improve the perovskite formation and the larger-molecularweight PVP enhanced the formation more significantly. However, the larger-molecular-weight PVP has some negative effect in the morphology of the PLZT films, as shown in Fig.3. It is indicated that low-molecular-weight PVP i.e. PVP40 induced dense and smooth PLZT films, which is shown in Fig.3(a), while the large-molecularweight PVP caused the PLZT films to be rough and nano-porous. From Fig.3(b), the pores in the PLZT films modified PVP360 are about 20nm in average diameter and the PLZT films modified by PVP630 comprised the pores with average size of 23nm, slightly larger than that in the films modified by PVP360. The morphology of the films modified by PVP630 became islandlike and the island area was especially porous, as shown in Fig.3(c). The porous structure induced by adding the PVP360 and PVP630 deteriorated the electrical properties of the PLZT films and the transmittance was only around 70%(not shown here). It was therefore necessary to study the optimal heat treatment conditions to densify the PLZT films. (a)
(b)
1µm
1µm
(c)
1µm Fig.3 Morphology of the PLZT films modified by ((a)PVP40, (b)PVP360,(c)PVP630)
Densification of the PVP360 modified PLZT films Since the films modified by PVP360 have comparatively smooth and dense morphology than that modified by PVP630, the densification of the films as function of heating rate and pyrolysis temperature was studied and all the studied samples are one-coating and with the thickness of around 0.5µm(close to the critical thickness). The results were shown in Fig.4. Fig.4(a)(b)(c) demonstrates that the morphology of the films were significantly affected by the heating rate and faster heating rate can improve the densification of the films. When the films were heated up from room temperature to 700°C by 5°C/min and dwelled for 30mins, the morphology of the films was rather porous and the pore size was about 67nm. The pore size
decreased to about 33nm when the heating rate increased to 20°C/min and when the heating rate was 40°C/min, the pore size was only about 25nm. It should be noted that if the films were directly heated up to 700°C without the pyrolysis step, the morphology can’t be fully dense by only varying the heating rate. So the pyrolysis temperature effect on the morphology of the films was also investigated and the heating rate was fixed at 40°C/min. The results are shown in Fig.4(d)(e)(f)(g)(h). Fig.4(d) shows the morphology of the PLZT films pyrolyzed at 300°C for 10mins and finally annealed at 700°C for 30mins. The films have similar porous morphology to that of the films heated by 20°C/min(shown in Fig.4(b)). However, when the pyrolysis temperature increased to above 450°C, the densification of the films was significantly enhanced and only some pores with the size below 10nm were found among the grains of the films, as shown in Fig.4(e), (f), (g) and (h)). And for the films pyrolyzed at 600°C, the grain size is not uniform and there was some very small grains distributed among the large grains. Thus the pyrolysis at the temperature above 450°C helps to improve the densification of the PVP360 modified PLZT films. (b)
(a)
1µm
(c)
1µm (e)
(d)
1µm (g)
1µm (f)
1µm
1µm
(h) Fig.4 Morphology of the PVP360 modified PLZT films: (a)700°C/5°C/min,(b)700°C/20°C/min (c)700°C/40°C/min,(d)300°C/700°C, (e)450°C/700°C,(f)500°C/700°C (g)550°C/700°C, (h)600°C/700°C) 1µm
1µm
Electric and optical properties of the PVP360 modified PLZT films Since the films pyrolyzed at above 450°C and finally annealed at 700°C were comparatively dense, the films pyrolyzed at 600°C were chose to further study the electrical and optical properties. The dielectric properties of the films were measured under 1V and 10kHz and the films have a dielectric loss as low as 0.05. Fig.5 shows the hysteresis loops for the films
under the applied electric field of 200kV/cm. It is indicated that the films exhibited a very slim and saturated hysteresis loop with the remnant polarization of 5.96x10-4µC/cm2and the coercive field of 21.51kV/cm. The transmittance spectra of the films were shown in Fig.6. Fig.6(a) indicated that the transmittance of the sample (substrate+PLZT films) were very close to that of the ITO/glass substrate in the wavelength of 450nm~900nm, meaning the PLZT films were extremely transparent. The transmittance of the PLZT films(Tf) can be given by the following 1 relation9: Tf =
1+
1 1 − Tm TS
where Tm is the measured transmittance of the sample(substrate+film) and Ts is the transmittance of the substrate(i.e. ITO/glass in our case). According to this equation, the transmittance of the PLZT films was calculated and shown in Fig.6(b). It is demonstrated that the transmittance of the films can reach around 90% and above in the wavelength of 450nm~900nm. 100 2
P( µC/cm )
90
100
ITO/glass
90
80
0.004
80
70 0.002
-250
-200
-150
-100
50
100
150
200
E(kV/cm)
250
60
PLZT/ITO/Glass
50
T%
T%
0.000 -50 0
70
60
40
30
30
-0.002
20 -0.004
50
40
20
10
(a)
0
(b)
10 0
300
400
500
600
wavelength(nm)
700
800
900
300
400
500
600
700
800
900
Wavelength(nm)
Fig.5 Hysteresis loops for the PLZT Fig.6 Transmittance spectra of the PVP360 modified PLZT films pyrolyzed films pyrolyzed at 600°C. at 600°C((a)measured, (b)calculated).
CONCLUSIONS Our results show that PVP40 can increase the critical thickness of the PLZT films to 0.22µm and PVP360 and PVP630 can increase the critical thickness up to 0.49µm and 0.93µm respectively. With the molecular weight of PVP increasing, the perovskite formation in the PLZT films was improved but the morphology of the films tended to be nano-porous. The films modified by PVP360 can be densified by optimizing the heat treatment conditions. When pyrolyzed at 600°C and then annealed at 700°C, the one-coating films with the thickness of 0.5µm were dense and exhibited a very slim hysteresis loop with the remnant polarization of 5.96x10-4µC/cm2 and coercive field of 21.51kV/cm, and a transmittance of around 90% and above in the wavelength of 450nm~900nm. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
K. Nashimoto, H. Moriyama, S. Nakamura, et al, Opt. Fiber Commun.Conf., 4, PD10-1-PD10-3(2001) Alexei L. Glebov, Michael G. Lee, Lidu Huang, et al., IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 11, 422-430(2005) G. H. Jin, Y. K. Zou, V. Fuflyigin, et al., J. Lightwave Technology,18, 807-812(2000) H. KOZUKA et al, J. Sol-Gel Sci.Tech. 26, 681–686(2003) H. KOZUKA, M. Kajimura, et al, J. Sol-Gel Sci.Tech. 19,205-209 (2000) H. Kozuka, S. Takenaka, H. Tokita,et al, J. Euro.Ceram.Soc., 24, 1585-1588(2004) H. Kozuka, S. Takenaka, J. Am. Ceram. Soc., 85, 2696–702 (2002) S. Takenaka, H. Kozuka, Appl.Phys.Lett., 79, 3485-3487(2001) E Dogheche, B.Jaber, and D.Remiens, Appl.Opt.,37, pp.4245-4248(1998)
