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Fabrication of SiO2-doped Ba0.85Sr0.15TiO3 glass–ceramic films and the measurement of their pyroelectric coefficient
Materials Science and Engineering B99 (2003) 511 /515 www.elsevier.com/locate/mseb
Fabrication of SiO2-doped Ba0.85Sr0.15TiO3 glass ceramic films and the measurement of their pyroelectric coefficient /
Shaobo Liu *, Meidong Liu, Shenglin Jiang, Churong Li, Yike Zeng, Yanqiu Huang, Dongxiang Zhou Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China Received 14 June 2002; received in revised form 10 October 2002; accepted 21 October 2002
Abstract SiO2-doped Ba0.85Sr0.15TiO3 (SBST) glass /ceramic (g /c) films with perovskite structure have been prepared on Pt/Ti/SiO2/Si substrates by sol /gel technique. Differential thermal analysis (DTA), X-ray diffraction (XRD) and atomic force microscopy (AFM) are employed to analyze the synthesize process and microstructure of SBST g /c films. The ferroelectricity and crystallization behavior of SBST films are discussed. It is found that the starting synthesize temperature of SBST15 film is larger than that of pure barium strontium titanate (BST) film for about 60 8C. The grain sizes decrease and the ferroelectricity of SBST g /c films is degenerated, but their loss tangent and leakage current density decrease with increasing SiO2 contents. The temperature coefficient of dielectric (TCD) and the pyroelectric coefficient g of the films are measured. The results show that TCD and the pyroelectric coefficient g of SBST5 film at 20 /25 8C are, respectively, 4.6% 8C 1 and 8.1 /10 8 C cm 2 K 1, which is about 2/3 value of the pure BST films. BST g /c film with 5 mol% SiO2 dopant is hopeful to be the advanced candidate material for uncooled infrared focal plane arrays (UFPAs) applied at near room temperature. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol /gel; BST thin film; Glass /ceramic; Pyroelectric coefficient
1. Introduction The infrared thermal imaging system that has been extensively applied is one of the important detecting devices. Recently, the low cost and portable infrared focal plane arrays (FPAs) are urgently wanted. The traditional photons-type FPAs are mostly used at present; however, some disadvantages result in their limited application such as the cooled work ambient at 77 K and the consumptive devices. Thus, in the last decade uncooled infrared focal plane arrays (UFPAs) have gradually become a new access to achieve the peering array photography [1]. Though their detectivity is lower than that of photons-type FPAs for about 2/3 orders of magnitude, it can be solved through increasing the array scale and improving the signal readout methods. Currently, the sensitivity of UFPAs is less
* Corresponding author. Tel.: /86-027-87542693. E-mail address: [email protected] (S. Liu).
than 0.04 K, to some extent which is superior to the capability of the scanning imaging system in military use. Due to its outstanding pyroelectric coefficient, high dielectric constant, low loss tangent and high dielectric breakdown strength, ferroelectric barium strontium titanate (BST) has recently been identified as a promising candidate material for sensitive elements for UFPAs [2]. By adjusting the Ba/Sr ratio to 73/27, the Curie point (Tc) of bulk BST is close to room temperature at which it takes on the maximum pyroelectric coefficient. A UFPA system using such elements will achieve the optimal sensitivity when operated at near ambient temperature. But the fabrication process for bulk BST is not easily compatible with the semiconductive ultralarge-scale-integration (ULSI) techniques [3,4], which make bulk BST unsuitable for UFPA-sensitive elements. BST thin films with higher responsivity can be prepared by a more compatible sol /gel technique to substitute bulk BST. However, the diffuseness of the ferro-to-para phase transition leads to difficulty in obtaining optimal
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pyroelectricity in thin films, especially those derived from sol /gel techniques. BST films with the same Tc / 25 8C but different Ba/Sr ratios have been derived from different methods such as metal organic chemical vapor deposition (MOCVD), sol /gel method and r.f. magnetron sputtering. In earlier research, Tc of Ba0.85Sr0.15TiO3 thin film has been found to be close to 25 8C, but the leakage current density affected the electrical stability and reliability [5,6]. In this paper, SiO2 glass phase doped to BST thin films is expected to improve the film resistivity.
2. Experimental details Barium acetate (Ba(C2H3O2)2), strontium acetate (Sr(C2H3O2)2), ethyl silicate ((C2H5)4SiO4) and tetrabutyl titanate (Ti(OC4H9)4) were used as starting materials. Glacial acetic acid (CH3COOH) was used as catalyst. Ethylene glycol monoethyl ether (C2H5OC2H4OH) was used as solvent. N -N -dimethyl formamide (C3H7NO) was used to stabilize the solution. All the above reagents were at analytic purity. After stoichiometrically dissolved, successively mixed and stirred, four SiO2-doped Ba0.85Sr0.15TiO3 (SBST) clear sols with dopant content as 0, 5, 10, and 15 mol% were, respectively prepared, which were subsequently abbreviated as BST, SBST5, SBST10 and SBST15. Then sols were left in the atmosphere to hydrolyze and condense. The stock coating precursor solutions were achieved, all of which have the same concentration of 0.33 M, a pH value of 3.3 /3.5 and a viscosity of (9.1 /10.2) /10 6 m2 s 1. The film fabrication process is not described in detail in this paper, which has been adequately reported elsewhere [7 /9]. The sols were spun onto the Pt/Ti/SiO2/ Si substrates at 4000 rpm for 30 s to form wet films. The wet films were treated at 150 8C for 5 min to remove H2O, then pyrolyzed at 500 8C for 5 min to form inorganic thin films. The above process should be repeated to reach the desired thickness. Lastly, the inorganic thin films were annealed above 750 8C for 10 min to produce ferroelectric thin films. All processes were carried out in a rapid thermal annealer (RTA) with a O2-rich ambient to reduce the oxygen ion vacancies. For electrical measurement, Au top electrode with a diameter of 1.5 mm was d.c.-sputtered onto the film surface. The other electrode was the Pt/Ti/SiO2/Si substrate. Fig. 1 shows the schematic cross-sectional view of SBST films test structure. Using a TAS-100 thermal analyzer and at a heating rate of 10 8C min 1 till 850 8C, the differential thermal analysis (DTA) characteristics of the SBST wet gels were analyzed. The morphologies of thin films were observed via atomic force microscopy (AFM). Using a rotatingtarget X-ray diffractometer, Ni-filtered Cu Ka as radiation source, X-ray diffraction (XRD) patterns
Fig. 1. Schematic cross-sectional view of SBST films test structure.
were obtained to determine the crystallinity of SBST films. The current versus time (I /T ) characteristic curves of SBST films were measured by a modified Sawyer-Tower circuit. Using an HP4192A impedance analyzer, the film temperature coefficient of dielectric (TCD) was measured. By a charge pumping method, the film pyroelectric coefficient was measured.
3. Results and discussion 3.1. Differential thermal analysis The DTA characteristics of BST, SBST5 and SBST15 wet gels were shown in Fig. 2. In SBST5 and SBST15 gel curves, most parts of the two curves are essentially alike, except for the starting synthesize temperature. The endothermic peak at 804.2 8C is associated with the solid solution temperature between BST and SiO2. The endothermic peak at 88.5 8C in BST curve mostly results from the rapid volatilization of the water and organic C4H10O, the latter is hydrolysate of Ti(OC4H9)4. The exothermic peaks at 255.5 and 359 8C are related to the pyrolysis of organic compound (to produce CO2 and H2O). The exothermic peak at 478.5 8C results from the formation of Ti /O /Ti /O / /[TiO2]. The sharp
Fig. 2. Characteristics of the BST, SBST5 and SBST15 wet gels annealed at 700 8C.
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3.2. XRD analysis
Fig. 3. XRD patterns of SBST films on the Pt/Ti/SiO2/Si: (a) BST, (b) SBST5, (c) SBST10 and (d) SBST15 films.
Fig. 3 shows the XRD patterns of BST, SBST5, SBST10 and SBST15 films on the Pt/Ti/SiO2/Si substrate annealed at 700 8C. It can be seen from Fig. 3 that SBST15 film annealed at 700 8C exhibits amorphous characteristic. But the pure perovskite phase structure appears and the peaks become strong and sharp with the decreasing SiO2 contents. It means that pure BST grains are easier to grow than SBST. Their 2u degrees of diffraction peak are identical to those of pure BST ceramic powder. Thus, thin films annealed at 700 8C are polycrystalline with a random orientation. And it is confirmed that SiO2 phase cannot be doped into BST inner grain, it may be distributed into the grain boundary and becomes the glass phase.
3.3. AFM analysis exothermic peaks at 648.8, 692.7 and 711.2 8C are associated with the synthesize temperature of BST, BST5 and SBST15, respectively. The starting synthesize temperature of SBST15 film is larger than that of pure BST film for about 60 8C. But it is notably decreased by contrast to the sintering temperature of traditional ceramic process.
Because there are large amounts of organic solvent, organics and water in the gels. The films will experience substantial shrinkage and internal stress as a result of removal of these materials during the heat treatment. The films may generate pinholes, cracks or even peeling from the substrate. Our experiments have shown that these defects can be prevented by suitable pyrolysis
Fig. 4. Stereoscopic and surface morphologies of BST, SBST5 and SBST10 films by AFM: (a, d) BST, (b, e) SBST5, and (c, f) SBST10 films.
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Fig. 5. I /T characteristic curve of BST (a), SBST5 (b) and SBST10 (c) films at 25 8C. Table 1 TCD and pyroelectric coefficient g of BST films at 20 /25 8C Materials
BST film SBST5 film SBST10 film SBST15 film
Parameter TCD (% 8C 1)
g ( /10 8 C cm 2 K 1)
15 /20 8C
20 /25 8C
15 /20 8C
20 /25 8C
5.8 3.2 1.9 0.4
6.9 4.6 2.3 0.8
9.3 6.5 2.8 0.3
12 8.1 4.3 0.7
cycles using rapid thermal processing. Fig. 4 shows stereoscopic (a /c) and surface (d /f) morphologies of BST, SBST5 and SBST10 thin films by AFM. The morphologies suggest that films are mostly crack-free within the viewing area and the grains are dense and homogeneously distributed. The grain sizes range from 100 to 150, 50 to 100 and 30 to 60 nm, respectively, for BST, SBST5 and SBST10 thin films. The grain sizes decrease with increasing SiO2 contents. The same concentration of BST, SBST5, SBST10 sols lead to their equal thickness of 300 nm. 3.4. I /T characteristic curve The typical results of I /T characteristic curve of BST, SBST5 and SBST10 films at 25 8C are shown in Fig. 5. It shows that the phase shift exists between the input and output signals and the shift degree decrease with increasing SiO2 contents. The peaks in curves (a) and (b) are sharper than those in curve (c), which means BST and SBST5 films possess stronger ferroelectricity than SBST10 film. For the peak in I /T characteristic curve is associated with the spontaneous polarization (Ps) strength at the coercive electric field (Ec). However, the nonlinear characteristic of curve (a) is less than that of curve (b). According to the principle of a modified Sawyer-Tower circuit, it means that the loss tangent and the leakage current density of BST film are larger than that of SBST5. From curve (b), the SBST5 resistivity is close to the order of magnitude of corresponding bulk material. It can be explained from the more grain boundaries and the larger Schottky barrier height in SBST film since it has smaller grain size. But the grain
size in SBST10 film is too small, which is less than the demarcation size of 70 nm, to make the ferroelectricity appear. 3.5. TCD and pyroelectric coefficient Using an HP4192A impedance analyzer, the film TCD was measured. By a charge pumping method, the film pyroelectric coefficient was measured, as seen in Table 1. The results show TCD and the pyroelectric coefficient g of SBST5 g/c film at 20/25 8C are about 2/3 value of the pure BST films. Based on the whole analysis of resistivity and pyroelectricity, BST g/c film with 5 mol% SiO2 dopant is hopeful to be the advanced candidate material for UFPA system applied at near room temperature.
4. Conclusions By sol/gel technique, SBST g /c films with perovskite structure have been prepared on Pt/Ti/SiO2/Si substrates. The starting synthesize temperature of SBST15 film is larger than that of pure BST film for about 60 8C. The grain sizes decrease and the ferroelectricity of SBST g /c films is degenerated, but their loss tangent and leakage current density decrease with increasing SiO2 contents. SiO2 phase exists in the grain boundary and becomes glass phase. The TCD and the pyroelectric coefficient g of SBST5 film at 20 /25 8C are 4.6% 8C 1 and 8.1 /10 8 C cm 2 K 1, respectively, which is about 2/3 value of the pure BST films. SBST5 g /c film is hopeful to be the advanced candidate material for UFPAs applied at near room temperature.
Acknowledgements This research was supported by the National Advanced Materials Committee of China (NAMCC).
References [1] C. Huang, Appl. Opt. 20 (1999) 10. [2] K.C. Liddiard, SPIE 2746 (1996) 72.
S. Liu et al. / Materials Science and Engineering B99 (2003) 511 /515 [3] J.F. Scott, F.M. Ross, M.C. Scott, M. Huffman, Annu. Rev. Mater. Sci. 28 (1998) 79. [4] D. Dimos, H.C. Mueller, Annu. Rev. Mater. Sci. 28 (1998) 397. [5] S. Chen, M.D. Liu, C.R. Li, Thin Solid Films 375 (2000) 288. [6] S.B. Liu, M.D. Liu, Y.K. Zeng, Y.Q. Huang, Mater. Sci. Eng. B 90 (2002) 149.
515
[7] M.D. Liu, S.B. Liu, Y.K. Zeng, C.R. Li, Piezoelectrics Acoustoopt. 23 (2001) 41. [8] S.B. Liu, M.D. Liu, Y.K. Zeng, C.R. Li, Mater. Sci. Eng. C 22 (1) (2002) 73. [9] M.D. Liu, P.Y. Wang, C.R. Li, Sens. Actuators A 49 (1995) 191.
Fabrication of SiO2-doped Ba0.85Sr0.15TiO3 glass ceramic films and the measurement of their pyroelectric coefficient /
Shaobo Liu *, Meidong Liu, Shenglin Jiang, Churong Li, Yike Zeng, Yanqiu Huang, Dongxiang Zhou Department of Electronic Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China Received 14 June 2002; received in revised form 10 October 2002; accepted 21 October 2002
Abstract SiO2-doped Ba0.85Sr0.15TiO3 (SBST) glass /ceramic (g /c) films with perovskite structure have been prepared on Pt/Ti/SiO2/Si substrates by sol /gel technique. Differential thermal analysis (DTA), X-ray diffraction (XRD) and atomic force microscopy (AFM) are employed to analyze the synthesize process and microstructure of SBST g /c films. The ferroelectricity and crystallization behavior of SBST films are discussed. It is found that the starting synthesize temperature of SBST15 film is larger than that of pure barium strontium titanate (BST) film for about 60 8C. The grain sizes decrease and the ferroelectricity of SBST g /c films is degenerated, but their loss tangent and leakage current density decrease with increasing SiO2 contents. The temperature coefficient of dielectric (TCD) and the pyroelectric coefficient g of the films are measured. The results show that TCD and the pyroelectric coefficient g of SBST5 film at 20 /25 8C are, respectively, 4.6% 8C 1 and 8.1 /10 8 C cm 2 K 1, which is about 2/3 value of the pure BST films. BST g /c film with 5 mol% SiO2 dopant is hopeful to be the advanced candidate material for uncooled infrared focal plane arrays (UFPAs) applied at near room temperature. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol /gel; BST thin film; Glass /ceramic; Pyroelectric coefficient
1. Introduction The infrared thermal imaging system that has been extensively applied is one of the important detecting devices. Recently, the low cost and portable infrared focal plane arrays (FPAs) are urgently wanted. The traditional photons-type FPAs are mostly used at present; however, some disadvantages result in their limited application such as the cooled work ambient at 77 K and the consumptive devices. Thus, in the last decade uncooled infrared focal plane arrays (UFPAs) have gradually become a new access to achieve the peering array photography [1]. Though their detectivity is lower than that of photons-type FPAs for about 2/3 orders of magnitude, it can be solved through increasing the array scale and improving the signal readout methods. Currently, the sensitivity of UFPAs is less
* Corresponding author. Tel.: /86-027-87542693. E-mail address: [email protected] (S. Liu).
than 0.04 K, to some extent which is superior to the capability of the scanning imaging system in military use. Due to its outstanding pyroelectric coefficient, high dielectric constant, low loss tangent and high dielectric breakdown strength, ferroelectric barium strontium titanate (BST) has recently been identified as a promising candidate material for sensitive elements for UFPAs [2]. By adjusting the Ba/Sr ratio to 73/27, the Curie point (Tc) of bulk BST is close to room temperature at which it takes on the maximum pyroelectric coefficient. A UFPA system using such elements will achieve the optimal sensitivity when operated at near ambient temperature. But the fabrication process for bulk BST is not easily compatible with the semiconductive ultralarge-scale-integration (ULSI) techniques [3,4], which make bulk BST unsuitable for UFPA-sensitive elements. BST thin films with higher responsivity can be prepared by a more compatible sol /gel technique to substitute bulk BST. However, the diffuseness of the ferro-to-para phase transition leads to difficulty in obtaining optimal
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 5 1 6 - 0
512
S. Liu et al. / Materials Science and Engineering B99 (2003) 511 /515
pyroelectricity in thin films, especially those derived from sol /gel techniques. BST films with the same Tc / 25 8C but different Ba/Sr ratios have been derived from different methods such as metal organic chemical vapor deposition (MOCVD), sol /gel method and r.f. magnetron sputtering. In earlier research, Tc of Ba0.85Sr0.15TiO3 thin film has been found to be close to 25 8C, but the leakage current density affected the electrical stability and reliability [5,6]. In this paper, SiO2 glass phase doped to BST thin films is expected to improve the film resistivity.
2. Experimental details Barium acetate (Ba(C2H3O2)2), strontium acetate (Sr(C2H3O2)2), ethyl silicate ((C2H5)4SiO4) and tetrabutyl titanate (Ti(OC4H9)4) were used as starting materials. Glacial acetic acid (CH3COOH) was used as catalyst. Ethylene glycol monoethyl ether (C2H5OC2H4OH) was used as solvent. N -N -dimethyl formamide (C3H7NO) was used to stabilize the solution. All the above reagents were at analytic purity. After stoichiometrically dissolved, successively mixed and stirred, four SiO2-doped Ba0.85Sr0.15TiO3 (SBST) clear sols with dopant content as 0, 5, 10, and 15 mol% were, respectively prepared, which were subsequently abbreviated as BST, SBST5, SBST10 and SBST15. Then sols were left in the atmosphere to hydrolyze and condense. The stock coating precursor solutions were achieved, all of which have the same concentration of 0.33 M, a pH value of 3.3 /3.5 and a viscosity of (9.1 /10.2) /10 6 m2 s 1. The film fabrication process is not described in detail in this paper, which has been adequately reported elsewhere [7 /9]. The sols were spun onto the Pt/Ti/SiO2/ Si substrates at 4000 rpm for 30 s to form wet films. The wet films were treated at 150 8C for 5 min to remove H2O, then pyrolyzed at 500 8C for 5 min to form inorganic thin films. The above process should be repeated to reach the desired thickness. Lastly, the inorganic thin films were annealed above 750 8C for 10 min to produce ferroelectric thin films. All processes were carried out in a rapid thermal annealer (RTA) with a O2-rich ambient to reduce the oxygen ion vacancies. For electrical measurement, Au top electrode with a diameter of 1.5 mm was d.c.-sputtered onto the film surface. The other electrode was the Pt/Ti/SiO2/Si substrate. Fig. 1 shows the schematic cross-sectional view of SBST films test structure. Using a TAS-100 thermal analyzer and at a heating rate of 10 8C min 1 till 850 8C, the differential thermal analysis (DTA) characteristics of the SBST wet gels were analyzed. The morphologies of thin films were observed via atomic force microscopy (AFM). Using a rotatingtarget X-ray diffractometer, Ni-filtered Cu Ka as radiation source, X-ray diffraction (XRD) patterns
Fig. 1. Schematic cross-sectional view of SBST films test structure.
were obtained to determine the crystallinity of SBST films. The current versus time (I /T ) characteristic curves of SBST films were measured by a modified Sawyer-Tower circuit. Using an HP4192A impedance analyzer, the film temperature coefficient of dielectric (TCD) was measured. By a charge pumping method, the film pyroelectric coefficient was measured.
3. Results and discussion 3.1. Differential thermal analysis The DTA characteristics of BST, SBST5 and SBST15 wet gels were shown in Fig. 2. In SBST5 and SBST15 gel curves, most parts of the two curves are essentially alike, except for the starting synthesize temperature. The endothermic peak at 804.2 8C is associated with the solid solution temperature between BST and SiO2. The endothermic peak at 88.5 8C in BST curve mostly results from the rapid volatilization of the water and organic C4H10O, the latter is hydrolysate of Ti(OC4H9)4. The exothermic peaks at 255.5 and 359 8C are related to the pyrolysis of organic compound (to produce CO2 and H2O). The exothermic peak at 478.5 8C results from the formation of Ti /O /Ti /O / /[TiO2]. The sharp
Fig. 2. Characteristics of the BST, SBST5 and SBST15 wet gels annealed at 700 8C.
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3.2. XRD analysis
Fig. 3. XRD patterns of SBST films on the Pt/Ti/SiO2/Si: (a) BST, (b) SBST5, (c) SBST10 and (d) SBST15 films.
Fig. 3 shows the XRD patterns of BST, SBST5, SBST10 and SBST15 films on the Pt/Ti/SiO2/Si substrate annealed at 700 8C. It can be seen from Fig. 3 that SBST15 film annealed at 700 8C exhibits amorphous characteristic. But the pure perovskite phase structure appears and the peaks become strong and sharp with the decreasing SiO2 contents. It means that pure BST grains are easier to grow than SBST. Their 2u degrees of diffraction peak are identical to those of pure BST ceramic powder. Thus, thin films annealed at 700 8C are polycrystalline with a random orientation. And it is confirmed that SiO2 phase cannot be doped into BST inner grain, it may be distributed into the grain boundary and becomes the glass phase.
3.3. AFM analysis exothermic peaks at 648.8, 692.7 and 711.2 8C are associated with the synthesize temperature of BST, BST5 and SBST15, respectively. The starting synthesize temperature of SBST15 film is larger than that of pure BST film for about 60 8C. But it is notably decreased by contrast to the sintering temperature of traditional ceramic process.
Because there are large amounts of organic solvent, organics and water in the gels. The films will experience substantial shrinkage and internal stress as a result of removal of these materials during the heat treatment. The films may generate pinholes, cracks or even peeling from the substrate. Our experiments have shown that these defects can be prevented by suitable pyrolysis
Fig. 4. Stereoscopic and surface morphologies of BST, SBST5 and SBST10 films by AFM: (a, d) BST, (b, e) SBST5, and (c, f) SBST10 films.
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Fig. 5. I /T characteristic curve of BST (a), SBST5 (b) and SBST10 (c) films at 25 8C. Table 1 TCD and pyroelectric coefficient g of BST films at 20 /25 8C Materials
BST film SBST5 film SBST10 film SBST15 film
Parameter TCD (% 8C 1)
g ( /10 8 C cm 2 K 1)
15 /20 8C
20 /25 8C
15 /20 8C
20 /25 8C
5.8 3.2 1.9 0.4
6.9 4.6 2.3 0.8
9.3 6.5 2.8 0.3
12 8.1 4.3 0.7
cycles using rapid thermal processing. Fig. 4 shows stereoscopic (a /c) and surface (d /f) morphologies of BST, SBST5 and SBST10 thin films by AFM. The morphologies suggest that films are mostly crack-free within the viewing area and the grains are dense and homogeneously distributed. The grain sizes range from 100 to 150, 50 to 100 and 30 to 60 nm, respectively, for BST, SBST5 and SBST10 thin films. The grain sizes decrease with increasing SiO2 contents. The same concentration of BST, SBST5, SBST10 sols lead to their equal thickness of 300 nm. 3.4. I /T characteristic curve The typical results of I /T characteristic curve of BST, SBST5 and SBST10 films at 25 8C are shown in Fig. 5. It shows that the phase shift exists between the input and output signals and the shift degree decrease with increasing SiO2 contents. The peaks in curves (a) and (b) are sharper than those in curve (c), which means BST and SBST5 films possess stronger ferroelectricity than SBST10 film. For the peak in I /T characteristic curve is associated with the spontaneous polarization (Ps) strength at the coercive electric field (Ec). However, the nonlinear characteristic of curve (a) is less than that of curve (b). According to the principle of a modified Sawyer-Tower circuit, it means that the loss tangent and the leakage current density of BST film are larger than that of SBST5. From curve (b), the SBST5 resistivity is close to the order of magnitude of corresponding bulk material. It can be explained from the more grain boundaries and the larger Schottky barrier height in SBST film since it has smaller grain size. But the grain
size in SBST10 film is too small, which is less than the demarcation size of 70 nm, to make the ferroelectricity appear. 3.5. TCD and pyroelectric coefficient Using an HP4192A impedance analyzer, the film TCD was measured. By a charge pumping method, the film pyroelectric coefficient was measured, as seen in Table 1. The results show TCD and the pyroelectric coefficient g of SBST5 g/c film at 20/25 8C are about 2/3 value of the pure BST films. Based on the whole analysis of resistivity and pyroelectricity, BST g/c film with 5 mol% SiO2 dopant is hopeful to be the advanced candidate material for UFPA system applied at near room temperature.
4. Conclusions By sol/gel technique, SBST g /c films with perovskite structure have been prepared on Pt/Ti/SiO2/Si substrates. The starting synthesize temperature of SBST15 film is larger than that of pure BST film for about 60 8C. The grain sizes decrease and the ferroelectricity of SBST g /c films is degenerated, but their loss tangent and leakage current density decrease with increasing SiO2 contents. SiO2 phase exists in the grain boundary and becomes glass phase. The TCD and the pyroelectric coefficient g of SBST5 film at 20 /25 8C are 4.6% 8C 1 and 8.1 /10 8 C cm 2 K 1, respectively, which is about 2/3 value of the pure BST films. SBST5 g /c film is hopeful to be the advanced candidate material for UFPAs applied at near room temperature.
Acknowledgements This research was supported by the National Advanced Materials Committee of China (NAMCC).
References [1] C. Huang, Appl. Opt. 20 (1999) 10. [2] K.C. Liddiard, SPIE 2746 (1996) 72.
S. Liu et al. / Materials Science and Engineering B99 (2003) 511 /515 [3] J.F. Scott, F.M. Ross, M.C. Scott, M. Huffman, Annu. Rev. Mater. Sci. 28 (1998) 79. [4] D. Dimos, H.C. Mueller, Annu. Rev. Mater. Sci. 28 (1998) 397. [5] S. Chen, M.D. Liu, C.R. Li, Thin Solid Films 375 (2000) 288. [6] S.B. Liu, M.D. Liu, Y.K. Zeng, Y.Q. Huang, Mater. Sci. Eng. B 90 (2002) 149.
515
[7] M.D. Liu, S.B. Liu, Y.K. Zeng, C.R. Li, Piezoelectrics Acoustoopt. 23 (2001) 41. [8] S.B. Liu, M.D. Liu, Y.K. Zeng, C.R. Li, Mater. Sci. Eng. C 22 (1) (2002) 73. [9] M.D. Liu, P.Y. Wang, C.R. Li, Sens. Actuators A 49 (1995) 191.