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ABSTRACT: The (Ba1−xSrx)TiO3 (BST) ferroelectric thin films exhibit outstanding dielectric properties, even at high frequencies (>1 GHz), and large, electric-field
dielectric tunability. This feature makes them suitable for developing a new class of tunable microwave devices. The dielectric
properties and dielectric tuning property of BST thin films are closely related to the film compositions, substrate types,
and post-deposition process. The successful implementation of BST films as high-frequency dielectrics in electrically tunable
microwave devices requires a detailed understanding of both their processing and material properties. This paper will review
the recent progress of BST thin films as active dielectrics for tunable microwave devices. The technical aspects of BST thin
films, such as processing methods, post-annealing process, film compositions, film stress, oxygen defects, and interfacial
structures between film and substrate, are briefly reviewed and discussed with specific samples from the recent literature.
The major issues requiring additional investigations to improve the dielectric properties of BST thin films for tunable microwave
applications are also discussed.
Journal of Electronic Materials 04/2012; 32(10):1125-1134. · 1.47 Impact Factor
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ABSTRACT: Phononic crystals, which are structures with periodic variations of density andor sound velocities, can exhibit phononic bandgaps where propagation of acoustic waves is forbidden. The ability of phononic crystals to manipulate sound, in a similar manner which photonic crystal control light, makes them particularly useful for applications such as acoustic filters or very efficient waveguides. Since the positions of the phononic bandgaps depend solely on the densities and the sound velocities of the constituent materials, it is expected that a modulation in the densities andor sound velocities of the constituent materials would result in a phononic bandgap shift. It is known that ferroelectric ceramic, such as barium strontium titanate (BST), will undergo a ferroelectric-to-paraelectric phase transition across the Curie temperature. During the phase transition, there is a large variation in the sound velocities of BST which result in a shift of the phononic bandgaps. In this work, we calculated the phononic bandstructures of BST-based structures by the plane-wave expansion method and showed that the phononic bandgaps can be shifted thermally.
The Journal of the Acoustical Society of America 06/2008; 123(5):3281. · 1.55 Impact Factor
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ABSTRACT: Compositionally graded (Ba1−xSrx)TiO3 (BST) thin films (with 0.0 ≤ x ≤ 0.25) were grown by pulsed laser deposition on the (100)MgO single-crystal substrates covered with a conductive La0.5Sr0.5CoO3 (LSCO) layer as a bottom electrode. Their epitaxial growth, dielectric response, and microstructure were characterized. The epitaxial relationships between the BST, LSCO, and MgO can be determined as [001]BST//[001]LSCO//[001]MgO and (100)BST//(100)LSCO//(100)MgO, from the x-ray diffraction (rocking curve, ϕ scans) and electron-diffraction patterns. Dielectric data showed that the room temperature values of the dielectric constant and dielectric loss of the graded BST films were 630 and 0.017 at 100 kHz, respectively. Cross-sectional transmission electron microscopy (TEM) images reveal that both the BST films and the LSCO bottom electrode grow with a columnar structure, and they have flat interfaces and overall uniform thickness across the entire specimen. Cross-sectional high-resolution TEM images reveal that at the LSCO/MgO(100) interface, an interfacial reaction is not seen, whereas edge-type interfacial dislocations with their extra half-planes residing in the LSCO side are observed with an average interval of 2.20 nm, close to the theoretical value of 2.15 nm. At/near the LSCO/BST interface, the graded BST films grow perfectly and coherently on the LSCO lattice because they have the same type of crystal structure and almost same lattice constants, and no interfacial dislocations are observed. Planar TEM images show that the graded films exhibit granular and/or polyhedral morphologies with an average grain size of 50 nm, and the aligned rectangular-shaped voids were also observed. High-resolution TEM images show that the length sizes of voids vary from 8 to 15 nm, and with width of 5 to 10 nm along the 〈001〉 direction in the (100) plane.
Journal of Materials Research. 02/2008; 23(03):737 - 744.
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ABSTRACT: The low-frequency internal friction (0.1 Hz<f<10 Hz) of (100−x%)Pb(Mg1/3Nb2/3)O3−x%PbTiO3 (x=0, 13, 23, 33) ceramics has been measured in the temperature range between 90 and 500 K. All of the internal friction peaks show a height that is inversely proportional to the measurement frequency, thus they can be attributed to first-order phase transitions. The phase transitions between rhombohedral, monoclinic, and tetragonal phases in ceramics with x=33 occur over a very broad temperature range from 150 to 400 K, indicating that the giant piezoelectric effect in this material may be attributed to a field-induced phase transition process.
Journal of the American Ceramic Society 07/2007; 90(10):3167 - 3170. · 2.27 Impact Factor
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ABSTRACT: The optical properties of barium strontium titanate (Ba0.7Sr0.3TiO3; BST) thin films are described. The BST thin films were epitaxially grown upon MgO (001) substrates by pulsed laser deposition. The crystallographic properties of the BST thin films were examined by x-ray diffraction. The BST thin films were highly optically transparent in the visible region. The optical waveguide properties were characterized by a prism coupling technique. An inverse-WBK method was employed to determine the refractive-index profile along the thickness of the BST films. Optical losses were measured by a moving fiber method, and the optical losses were found to be 0.93 dB/cm for the TE0 mode and 1.29 dB/cm for the TM0 mode at 1550 nm. Electro-optic (E-O) properties were measured by a phase-modulation detection method at 632.8 nm, and the BST films exhibited a predominantly quadratic E-O effect with a quadratic E-O coefficient of 6.64 x 10(-18) m2/V2.
Applied Optics 04/2006; 45(9):1972-8. · 1.41 Impact Factor
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ABSTRACT: The effects of Ca doping on the Curie temperature, structural, dielectric, and elastic properties of Ba[sub 0.4]Sr[sub 0.6−x]Ca[sub x]TiO₃ (0≤x≤0.3) has been studied. Powder x-ray diffraction revealed that the cubic lattice constant a decreases linearly with increasing x from 0 to 0.15, while showing an anomalous expansion between x=0.15 and x=0.18. This, together with the anomalies in the dielectric constants, Curie temperature T[sub C], and elastic constants observed for 0.15<x<0.18, suggests that a small amount of Ca ions substituted for Ti ions. Correlated with the evolution of a with x, T[sub C] increases linearly with increasing x from 0 to 0.15; while deviating from linear behavior for x>0.15, T[sub C] increases persistently up to x=0.25 and thereafter shows a decrease. These variations of T[sub C] with x have been interpreted in terms of Ca-doping-induced A-site cation size variance, a substitution of a small amount of Ca ions for Ti ions, and structural phase separation. Upon cooling the longitudinal elastic constant C[sub L] shows drastic softening near T[sub C], arising from the electrostrictive coupling between the polarization fluctuations and the elastic strains. Moreover, it was found that Ca doping induces the hardening of C[sub L] just below T[sub C], and the magnitude of the relative hardening of C[sub L] (i.e., ΔC[sub L]/C[sub L]) increases with increasing x, implying that the tetragonal ferroelectric phase was increasingly stabilized by Ca doping. Author name used in this publication: J. Wang Author name used in this publication: X. G. Tang Author name used in this publication: Y. Wang Author name used in this publication: H. L. W. Chan Author name used in this publication: C. L. Choy
Journal of Applied Physics 08/2005; 98(8):084108. · 2.17 Impact Factor
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ABSTRACT: The changes in the structure and optical properties of indium tin oxide (ITO) thin films that were subjected to water electrolysis using an alternating current power source were investigated. X-ray diffraction indicated that the film was reduced to metallic indium after the treatment. Scanning electron microscopy revealed that the film became porous after the treatment. These structural changes had led to a dramatic degradation in the optical properties of ITO. The electrochemical mechanism responsible for the changes was discussed.
Journal of the American Ceramic Society 03/2005; 88(4):1007 - 1009. · 2.27 Impact Factor
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Ferroelectrics. 01/2005; 324(1):11-19.
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ABSTRACT: (Bi1/2Na1/2)TiO3 with 0–6 mol% Ba(Cu1/2W1/2)O3 (BNT-BCW), a new member of the BNT-based group, has been prepared following the conventional mixed oxide route. The compacted bodies were sintered at 1130°C for 2 h to get dense ceramics. The addition of BCW into BNT ceramics facilitated the poling process because of a reduction in leakage current. 0.995BNT·0.005BCW ceramics exhibit a relatively high piezoelectric constant (d33= 80 × 10−12 C/N) and a relatively low dielectric loss (tan δ= 1.5%). Increased amount of BCW was found to increase the dielectric constant and loss of BNT-BCW ceramics and to suppress the grain growth. During sintering, some BCW diffuses into the lattice of BNT to form a solid solution and some remains on the grain boundaries.
Journal of the American Ceramic Society 12/2004; 86(10):1809 - 1811. · 2.27 Impact Factor
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ABSTRACT: To develop a high-performance pyroelectric infrared detector, large-size and high-quality 0.2 mol% Fe-doped Pb(Mg[sub ⅓]Nb[sub ⅔])O₃-0.38PbTiO₃ single crystals have been grown by a modified Bridgman technique with a seed crystal. At 50 Hz, the doped single crystal showed a lower dielectric constant of 310 compared with the pure one. A low dielectric loss of 0.0067 was also obtained by doping with small concentration of iron ions. Piezoelectric coefficient (d[sub 33]) of <001> oriented sample was ~260 pC/N. The pyroelectric properties and their temperature dependence were measured by the dynamic technique. At room temperature, the pyroelectric coefficient and calculated detectivity figure of merit F[sub D] are 568 µC/m² K and 53 µPa[sup -½], respectively. The values of current response figure of merit F[sub i] and voltage response figure of merit F[sub v] are 227.2 pm/V and 0.083 m²/C, respectively. These excellent pyroelectric properties as well as being able to produce large-size and high-quality single crystals make this kind of single crystal very promising for high-performance infrared detectors and other pyroelectric applications.
Applied Physics Letters 05/2004; 84(23):4711-4713. · 3.84 Impact Factor
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ABSTRACT: Lead magnesium niobate-lead titanate 0.67Pb (Mg[sub ⅓]Nb[sub ⅔])O₃-0.33PbTiO₃ (PMN-0.33PT, abbreviated as PMN-PT) single crystals were used to fabricate PMN-PT/epoxy 1-3 composites with different volume fractions of PMN-PT ranging from 0.4 to 0.8. The electromechanical properties of the 1-3 composites were determined by the resonance technique. Theoretical modeling of the 1-3 composites matched quite well with the measured material properties. It was demonstrated that the thickness electromechanical coupling coefficients of the composites could reach as high as 0.8. A 2.4 MHz plane ultrasonic transducer was fabricated using a PMN-PT/epoxy 1-3 composite with 0.37 volume fraction of PMN-PT. It shows a -6 dB bandwidth of ∼61% and an insertion loss of -14 dB. Author name appeared in this publication: Hasou Luo Author name appeared in this publication: Kei C. Cheng
IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 10/2003; · 1.69 Impact Factor
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ABSTRACT: Multilayered Pt /( Ba <sub>0.5</sub> Sr <sub>0.5</sub>) TiO <sub>3</sub>/ Pb ( Zr <sub>0.52</sub> Ti <sub>0.48</sub>) O <sub>3</sub>/( Ba <sub>0.5</sub> Sr <sub>0.5</sub>) TiO <sub>3</sub>/ Pt (BST/PZT/BST) thin films with different thicknesses of the BST layers were prepared by the pulsed-laser deposition method. The existence of a BST layer between the PZT and Pt electrode can greatly improve the fatigue properties of the PZT film. However, the heterostructure with thicker BST layers exhibits lower remnant polarization because of a lower electric field applied on the PZT layer. So, the thickness of BST layers should be decreased to decrease the working voltage of the multilayered film. A heterostructure with very thin BST layers (thickness ∼7.5 nm) has good ferroelectric properties, such as high remnant polarization and rare fatigue resistance after 10<sup>10</sup> switching cycles. A possible reason for the effect of BST is that the BST layer can absorb oxygen vacancies or other point defects from the PZT layer and greatly improve its fatigue properties. © 2003 American Institute of Physics.
Applied Physics Letters 07/2003; · 3.84 Impact Factor
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ABSTRACT: We report an observation of triple-like P–E hysteresis loops and an electric field-induced tetragonal–orthorhombic phase transition in the 0.62Pb(Mg1/3Nb2/3)O3–0.38PbTiO3 single crystal. The stability of the induced orthorhombic phase was studied as a function of temperature and electric field, which is applied along the 〈011〉 direction of the original tetragonal phase.
physica status solidi (a) 06/2003; 198(1):R1 - R3. · 1.21 Impact Factor
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ABSTRACT: Multilayered Pt/(Ba[sub 0.5]Sr[sub 0.5])TiO₃ /Pb(Zr[sub 0.52]Ti[sub 0.48])O₃/(Ba[sub 0.5]Sr[sub 0.5])TiO₃/Pt (BST/PZT/BST) thin films with different thicknesses of the BST layers were prepared by the pulsed-laser deposition method. The existence of a BST layer between the PZT and Pt electrode can greatly improve the fatigue properties of the PZT film. However, the heterostructure with thicker BST layers exhibits lower remnant polarization because of a lower electric field applied on the PZT layer. So, the thickness of BST layers should be decreased to decrease the working voltage of the multilayered film. A heterostructure with very thin BST layers (thickness ~7.5 nm) has good ferroelectric properties, such as high remnant polarization and rare fatigue resistance after 10[sup 10] switching cycles. A possible reason for the effect of BST is that the BST layer can absorb oxygen vacancies or other point defects from the PZT layer and greatly improve its fatigue properties.
Applied Physics Letters 03/2003; 82:4325. · 3.84 Impact Factor
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ABSTRACT: The dielectric properties of (Pb(Mg1/3Nb2/3)O3)0.67(PbTiO3)0.33 single crystals in the temperature range from room temperature to 250 °C have been studied. A very sharp change of dielectric permittivity due to the spontaneous relaxor–ferroelectric phase transition was observed. The most interesting result is that the dielectric permittivity in ferroelectric phase is strongly dependent on the cooling rate across the relaxor–ferroelectric phase transition. We assume that the cooling rate can influence the grain size of ferroelectric subdomains in ferroelectric phase and can therefore influence the dielectric properties. © 2002 American Institute of Physics.
Applied Physics Letters 12/2002; 81(24):4580-4582. · 3.84 Impact Factor
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ABSTRACT: The spectral characteristics of a new type of long-period grating (LPG) structure, step-changed LPG, are investigated theoretically and experimentally. It is shown that the spectral profile of such gratings can be tailored by adjusting the step parameters. A good agreement between simulated results and measured spectra is obtained. This type of LPG has potential applications in gain flattening of erbium-doped fiber amplifiers.
IEEE Photonics Technology Letters 06/2002; · 2.19 Impact Factor
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ABSTRACT: A new scheme is proposed for simultaneous strain and temperature measurement with a single-fiber Bragg grating. The grating sensor was written on the splice joint between two fibers and has two resonance peaks because of the different refractive index between the two fibers. By bonding one section of the grating onto the substrate of large thermal expansion coefficient, the two resonance peaks exhibit different strain and temperature response and can thus be used for strain/temperature discrimination. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 33: 200–202, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10275
Microwave and Optical Technology Letters 05/2002; 33(3):200 - 202. · 0.62 Impact Factor
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ABSTRACT: The dielectric properties, internal friction, and Young's modulus of (1-x%) Pb(Mg[sub ⅓]Nb[sub ⅔])O₃-x%PbTiO₃ (for x=13, 23, and 33) ceramics have been measured. A phase-transition-like internal friction peak associated with Young's modulus softening has been observed at temperature T[sub R-F], which can be attributed to the relaxor-to-ferroelectrics (R-F) phase transition. Therefore, the R-F phase transition can be explained in terms of the paraelectric-to-ferroelectric phase transition of paraelectric matrix in the materials.
Applied Physics Letters 03/2002; 81:2059. · 3.84 Impact Factor
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ABSTRACT: The dielectric properties of (Pb(Mg[sub ⅓]Nb[sub ⅔])O₃)0.67(PbTiO₃)0.33 single crystals in the temperature range from room temperature to 250 °C have been studied. A very sharp change of dielectric permittivity due to the spontaneous relaxor-ferroelectric phase transition was observed. The most interesting result is that the dielectric permittivity in ferroelectric phase is strongly dependent on the cooling rate across the relaxor-ferroelectric phase transition. We assume that the cooling rate can influence the grain size of ferroelectric subdomains in ferroelectric phase and can therefore influence the dielectric properties.
Applied Physics Letters 03/2002; 81:4580. · 3.84 Impact Factor
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Ferroelectrics. 01/2002; 271(1):69-74.
