Voltage tunable Ba0.6Sr0.4TiO3 thin films and coplanar phase shifters

Not only the ferroelectric properties of Ba_0.6Sr_0.4TiO₃ thin films but also the design, fabrication and microwave properties of coplanar waveguide (CPW) phase shifter were investigated. When the applied voltage changes from zero to 20 V range at a frequency of 1 kHz, the dielectric constant, tunability, remanent polarization (2P_r) and coercive electric field (Ec) of the BST films are 1266, 29.5%, 2.29 μC/cm² and 22.27 kV/cm, respectively. The structure of CPW phase shifter was designed by ANSOFT High Frequency Structure Simulator (HFSS) and Agilent Advanced Design System (ADS). The designed phase shifter consists of 56 same sections. It is observed from the photograph of the fabricated device that the lines of electrodes are regular and the widths of bottom electrodes overlapped by top electrodes are about 5 μm. It was found that the phase shift gradually increases as the voltage increases from 0 V to 20 V, which may be due to the tunability of BST films varies with external dc field. About 168° phase shift was achieved at 28 GHz with a bias voltage of 20 V.     

High dielectric tunability of Ba0.6Sr0.4TiO3 thin film deposited by radio-frequency magnetron sputtering

In microwave tunable devices, one of the major challenges encountered is the simultaneous minimization of the material's dielectric loss and maximization of dielectric tunability. In this work, Ba_0.6Sr_0.4TiO₃ thin film with the thickness of 300 nm was deposited on Pt/SiO₂/Si substrates using radio-frequency magnetron sputtering technique, and its dielectric properties were investigated. Due to the high temperature annealing process at substrate temperature of 600 °C, well-crystallized Ba_0.6Sr_0.4TiO₃ film was deposited. The dielectric constant and dielectric loss of the film at 100 kHz are 300 and 0.033, respectively. Due to the good crystallinity of the Ba_0.6Sr_0.4TiO₃ films deposited by radio-frequency magnetron sputtering, high dielectric tunability up to 38.3% is achieved at a low voltage of 4.5 V.     

钛酸锶钡薄膜中微观结构缺陷及其形成机理

采用脉冲激光沉积方法,在(001)单晶铝酸镧(LaAlO3)衬底上成功制备出钛酸锶钡(Ba0.75Sr0.25TiO3)外延薄膜.利用高分辨电子显微学对其微观结构进行了详细研究,并探讨了其微观结构缺陷的形成机理.研究发现,在钛酸锶钡外延薄膜中存在失配位错和穿透位错,且存在直线型和锯齿型的两种反相畴界.失配位错是由于钛酸锶钡薄膜与LaAlO3单晶衬底之间存在较大的晶格失配形成的,可分解为不全位错;穿透位错可以分解为不全位错伴随有堆垛层错的形成.反相畴界的形成是由于衬底表面存在台阶,直线型反向畴界是由于形核点离台阶处较近产生的,而锯齿型反向畴界是由于形核点离台阶处较远而形成.研究结果可为其它钙钛矿型外延薄膜中微观结构缺陷的形成机理提供理论指导.     

Effect of bottom electrodes on dielectric properties of high frequency Ba0.5Sr0.5TiO3 parallel plate varactor

Three types of Ba_0.5Sr_0.5TiO₃ (BST) thin film parallel plate varactor with different bottom electrodes were fabricated. The bottom electrodes of three types of varactor were perovskite conducting oxide La_0.7Sr_0.3MnO₃ (LSMO), Pt and Au, respectively. Dielectric properties of the BST thin films were characterized in the frequency range from 10 MHz to 15 GHz. The microstructure of the BST thin films was investigated by X-ray diffraction and scanning electron microscope. The microstructural analysis shows that the BST thin films grown on LSMO and Pt bottom electrodes are polycrystalline textured with columnar grains. Dielectric measurement indicates that the BST thin film grown on LSMO bottom electrode has a maximum dielectric constant and a little higher loss tangent.     

Effect of MgWO4 content on properties of Ba0.5Sr0.5TiO3 composite ceramics for tunable microwave applications

xMgWO₄-(1 − x) Ba_0.5Sr_0.5TiO₃ (x = 0.0, 5.0, 15.0, 25.0 and 35.0 wt%) composite ceramics were prepared via solid state reaction processing. Their structural and dielectric properties were systematically characterized. A significant increase in grain size was observed with increasing MgWO₄ content, which was accompanied by obvious variations in dielectric properties of the composite ceramics. It is found that the permittivity peaks of the samples gradually shifted to low temperatures with increasing MgWO₄ content. At the same time, tunabilities of the composite ceramics decreased, but their Q values increased. The sample with 35 wt% MgWO₄ possesses a high tunability of 16.8% (∼10 kHz), a low permittivity of 65 and an appropriate Q value of 309 (∼4.303 GHz), which meet the requirements of high power and impedance matching, thus making it a promising candidate for applications as electrically tunable microwave devices.     

Dielectric and tunable characteristics of Ba0.4Sr0.6TiO3-BaWO4 composite ceramics for microwave applications

Phase compositions, microstructure and microwave dielectric properties, of BaWO₄ (BW)-Ba_0.4Sr_0.6TiO₃ (BST) composite ceramics, prepared by the traditional solid-state route, were systematically characterized. Meanwhile, mechanism of dielectric tunability of those materials was discussed. Dielectric properties of the BW-BST composites at a DC bias field near the phase transition temperature could be interpreted by using Johnson's phenomenological equation. The sample with x = 0.60 exhibited a tunability of 29.5%, a dielectric permittivity of 192 and a Q value of 231 (at 2.700 GHz), which make it a promising candidate for applications in electrically tunable microwave devices.     

Structural characterization of sputter-deposited Ba0.48Sr0.52TiO3/LaNiO3 artificial superlattice structure by X-ray reflectivity and diffraction

Ba_0.48Sr_0.52TiO₃/LaNiO₃ artificial superlattices with a symmetric sublayer structure were successfully fabricated on Nb-doped SrTiO₃ (001) single-crystalline substrate with rf magnetron sputtering. X-ray reflectivity and high-resolution diffraction measurements were employed to characterize the microstructure of these films. Formation of a superlattice structure was confirmed from the appearance of Bragg peaks separated by Kiessig fringes in X-ray reflectivity curves and a diffraction pattern. The fitted result from X-ray reflectivity curves shows that the densities of the Ba_0.48Sr_0.52TiO₃ and LaNiO₃ sublayers were slightly less than their bulk values. The appearance of discernible satellite peaks beside the main peak of the crystal truncation rod observed for deposited films clearly demonstrates that a well-defined superlattice structure can be formed with rf sputtering. The artificial Ba_0.48Sr_0.52TiO₃/LaNiO₃ superlattices exhibit a large dielectric constant and a small dielectric loss. The smaller is the stacking periodicity of superlattices, the larger is the dielectric constant and dielectric loss.     

Synthesis and dielectric characterization of Ba0.6Sr0.4TiO3 ferroelectric ceramics

Ferroelectric ceramics Ba_0.6Sr_0.4TiO₃ (BST 40) were prepared, by solid-state reaction in the temperature range 1210-1450 °C. Maximum values of the ceramic densities were around 94% of their theoretical value. X-ray diffraction techniques (XRD) and scanning electron spectroscopy (SEM) were used to analyze the structure and the surface morphology of ceramics. Rounded, well defined or abnormal granular growth was observed in the SEM images, vs. sintering conditions and purity of the raw materials. In all samples, BST 40 ceramic is the major phase, but there are also present small amounts of secondary phases, as revealed in XRD diffraction patterns. Permittivity and dielectric loss measurements were performed in the temperature range − 150 to + 150 °C, and 150 Hz-5 MHz frequency values. Permittivity values rising from 1200 to 12,500, with increasing sintering temperatures, were recorded. Narrow and well defined transition peaks were noticed at higher sintering temperatures. Curie temperature was around 2 °C, for samples with the mentioned composition. Permittivity and losses vs. frequency show different behavior whether BST ceramics are in polar or non-polar state and with the distance toward phase transition. Microwave measurements performed at room temperature have shown lower values of permittivity, compared with similar data at low frequency, and dielectric losses lower than 1% at 0.7 GHz. The sintering conditions (temperatures, sintering time, etc.) and purity of the raw materials lead to important changes of transition temperatures in the polymorphic diagram, which we have built—for the other Ba1−xSrxTiO3 compositions (x = 0.25-0.90) sintered at 1260 °C for 2 h.     

Epitaxial growth of LiNbO3 thin films in a microwave oven

Oriented LiNbO₃ thin films were prepared using a polymeric precursor solution deposited on (0001) sapphire substrate by spin coating and crystallized in a microwave oven. Crystallization of the films was carried out in a domestic microwave oven. The influence of this type of heat treatment on the film orientation was analyzed by X-ray diffraction and electron channeling patterns, which revealed epitaxial growth of films crystallized at 550 and 650 °C for 10 min. A microstructural study indicated that the films treated at temperatures below 600 °C were homogeneous and dense, and the optical properties confirmed the good quality of these films.     

Thickness-ratio-dependent dielectric properties of Bi1.5Zn1.0Nb1.5O7/Ba0.5Sr0.5TiO3 bilayered thin films

Bi_1.5Zn_1.0Nb_1.5O₇ (BZN)/Ba_0.5Sr_0.5TiO₃ (BST) thin films were prepared on Pt/Ti-coated sapphire substrates by radio frequency magnetron sputtering. The specific relationship between the dielectric properties and the thickness ratio of the BZN thickness to the BST thickness was investigated. The presence of BZN films effectively reduced the dielectric loss of the thin films. The thickness-ratio-dependent dielectric constant and dielectric loss behaviors were in good accordance with the simulation results based on the series connection theory. The optimum thickness ratio was determined to be around 0.5, exhibiting a maximum commutation quality factor of about 16,000. The built-in electric field at the region near the BZN–BST interface may be responsible for the asymmetric characteristic of the electric-field-dependent dielectric properties of the BZN/BST thin films.     

Optical rib waveguide based on epitaxial Ba0.7Sr0.3TiO3 thin film grown on MgO

Barium strontium titanate (Ba_0.7Sr_0.3TiO₃, abbreviated as BST) thin films were grown on MgO (001) single crystal substrates using pulsed laser deposition. A heteroepitaxial growth relationship of <001> _BST // <001> _MgO was confirmed by X-ray diffraction. The bidimensional structure of a rib waveguide was designed using the effective index method. A BST/MgO rib waveguide was fabricated using photolithography and reactive ion etching techniques. A single-mode near-field output pattern was observed using an end-fire coupling method at a wavelength of 1550 nm, which is consistent with our numerical calculation.     

Transmission photoacoustic spectroscopy of flash-evaporated CuIn0.75Ga0.25Se2 thin films

Photoacoustic spectroscopy (PAS) has proved to be an effective technique for the evaluation of inherent defect population in a wide range of materials for various applications. This paper demonstrates the use of this technique in transmission mode and hence evaluates the optical properties of flash evaporated CuIn_0.75Ga_0.25Se₂ (CIGS) thin films. Both the photoacoustic and transmission spectra were recorded at room temperature using high resolution near-infrared of the gas-microphone type PAS which revealed a very broad transmission region (about 300 meV) near the fundamental band edge in the as-grown CIGS thin films due to the presence of several shallow defect levels. The post-deposition heat treatment of the samples under selenium ambient followed by annealing under inert and forming gas ambient showed significant changes in the behavior of the PAS spectra particularly near the fundamental band edge. The absorption coefficient has been derived from these spectra to determine the band gap energy values and the activation energies for several defect related energy levels. Using the PAS, the energy band gap values were in the range of 1.197 to 1.202 eV. The optical transmission spectra were also recorded from the spectrophotometer. The transmission data was used to determine the energy band gap values which were calculated to be in the range of 1.159 to 1.194 eV. These values were found to be in good agreement to each other as well as to those reported in the literature.     

UV-visible spectra of perovskite iron-doped Ba0.72Sr0.28TiO3

The ultraviolet-visible spectra of the pure barium strontium titanate (Ba_0.72Sr_0.28TiO₃) and the Fe³⁺-doped Ba_0.72Sr_0.28TiO₃ were measured. The pure barium strontium titanate exhibited an ultraviolet (UV) absorption effect. The addition of Fe₂O₃ resulted in an increase of absorption wavelength. This red-shift effect was very strong even though the amount of Fe³⁺ was very small. The similar phenomenon was also observed in pure BaTiO₃ and Fe³⁺-doped BaTiO₃. The UV-absorption behavior was discussed in terms of Fe³⁺ replacing Ti⁴⁺ in the titanate with perovskite structure.     

Preparation and colossal magnetoresistance in a trilayer La0.67Sr0.33MnO3/La0.75MnO3/La0.67Sr0.33MnO3 device by dc magnetron sputtering

Epitaxial trilayer films of La_0.67Sr_0.33MnO₃ (LSMO)/La_0.75MnO₃ (L0.75MO)/La_0.67Sr_0.33MnO₃ (LSMO) have been prepared on (0 0 1) oriented LaAlO₃ substrates by dc magnetron sputtering. The structure and MR are studied. All as-deposited trilayer films exhibit a semiconductor to metal transition at temperature ranging from 116 to 185 K. The MR is also shown to be dependent on the thickness of the middle oxide layer. A maximum MR value of 32% (ΔR/R₀) has been obtained at 132 K under 0.4 T magnetic field for a LSMO (300 nm)/L0.75MO (70 nm)/LSMO (300 nm) trilayer film. The MR of trilayer film prefers to that of both LSMO and L0.75MO single layer films.     

Influence of rare-earth addition on microstructure and dielectric behavior of Ba0.6Sr0.4TiO3 ceramics

Ba_0.6Sr_0.4TiO₃ (BST) ceramics with 0.5 mol% various trivalent rare-earth additions prepared by a solid-state route are investigated. A strong correlation is observed between the microstructure, dielectric properties and rare-earth element dopant. The results display that comparing with the lattice constants of undoped and doped rare-earth BST, the structure transforms from cubic to tetragonal structure. In addition, the dopant improves the tetragonal distortion with the ionic radius of rare earth decreasing, and then deteriorates it with further decreasing. Large ions rare-earth additions effectively suppress the grain growth of BST. It is found that the temperature-permittivity characteristics for the BSTR (R, namely, rare earth) system could be controlled using various rare-earth elements. Especially, such as Sm, Eu, Gd dopants are effective to satisfy the tunable microwave devices application due to the decrease of permittivity and the improvement of dissipation factors of BST ceramic with the accompanying high-tunability.     

Percolative property and microwave dielectric characterization of Ba0.4Sr0.6TiO3-Mg2TiO4 diphasic ceramics

Ba_0.4Sr_0.6TiO₃-Mg₂TiO₄ composite ceramics were fabricated via conventional solid state reaction process. Dielectric properties of the composites were investigated systematically. It was shown that their dielectric constants decreased monotonously from 1197 to 60 with the contents of increasing Mg₂TiO₄ from 0 to 70 wt%. Tunability had a slight change from 24.0% to 20.5% as the volume fraction of Mg₂TiO₄ was increased from 0 to 62 vol%. However, it decreased substantially from 20.5% to 13.4% above the threshold from 62 vol% to 71 vol%. Such an obvious change in tunability was well explained by connectivity levels of the binary composites. Q values of the composite ceramics were more than 200 (tg δ < 0.005) at microwave frequencies of less than 10 GHz. Dielectric dispersion was observed at about 22 GHz using interdigital capacitor (IDC) stucture.     

The influence of annealing temperatures on the properties of Bi2VO5.5/LaNiO3/Si thin films

Bi₂VO_5.5 ferroelectric thin films were fabricated on LaNiO₃/Si(100) substrate via chemical solution deposition. Ferroelectric and dielectric properties of the thin films annealed at 500-700 °C were studied. The thin film annealed at 700 °C exhibited more favorable ferroelectric and dielectric properties than those annealed at lower temperatures. The values of remnant polarization 2P_r and coercive field E_c for the film annealed at 700 °C are 10.62 µC/cm² and 106.3 kV/cm, respectively. The leakage current of the film is about 1.92 × 10^− 8 A/cm² at 6 V. The possible mechanism of the dependence of electrical properties of the films on the annealing temperature was discussed.     

Structural engineering of Ba0.5Sr0.5TiO3 epitaxial films

Ba_0.5Sr_0.5TiO₃ single-layered and multilayered films were epitaxially grown on a (001) LaAlO₃ substrate using single target and dual target pulsed laser deposition, respectively. Compared to the single-layered films, the multilayered films exhibited broader phase transition and improved thermo-stability. The microstructure of these epitaxial films was investigated using high-resolution transmission electron microscopy in details. Misfit and threading dislocations were observed in the single-layered film, while the threading dislocations were dramatically decreased and no misfit dislocations were found in the multilayered film. It is suspected that the difference in dislocation densities is responsible for the different behaviors of the permittivity with temperature.     

Enhanced ferroelectric properties of multilayer SrBi2Ta2O9/SrBi2Nb2O9 thin films for NVRAM applications

Ferroelectric SrBi₂Ta₂O₉/SrBi₂Nb₂O₉ (SBT/SBN) multilayer thin films with various stacking periodicity were deposited on Pt/TiO₂/SiO₂/Si substrate by pulsed laser deposition technique. The X-ray diffraction patterns indicated that the perovskite phase was fully formed with polycrystalline structure in all the films. The Raman spectra showed the frequency of the O–Ta–O stretching mode for multilayer and single layer SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBNT) samples was 827–829 cm⁻¹, which was in between the stretching mode frequency in SBT (813 cm⁻¹) and SBN (834 cm⁻¹) thin films. The dielectric constant was increased from 300 (SBT) to 373 at 100 kHz in the double layer SBT/SBN sample with thickness of each layer being 200 nm. The remanent polarization (2P_r) for this film was obtained 41.7 μC/cm², which is much higher, compared to pure SBT film (19.2 μC/cm²). The coercive field of this double layer film (67 kV/cm) was found to be lower than SBN film (98 kV/cm).     

Effect of Na/K excess on the electrical properties of Na0.5Bi0.5TiO3-K0.5Bi0.5TiO3 thin films prepared by sol-gel processing

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (NKBT) thin films derived from different amounts of Na/K excess content were fabricated via an aqueous sol-gel method on a Pt(111)/Ti/SiO₂/Si substrate, and the effect of Na/K excess content on the microstructure and electrical properties of the NKBT thin films was investigated. A second phase appears when Na/K excess content is below 20 mol%. Appropriated Na/K excess can enhance the polarization and dielectric properties due to compensation of Na/K loss that occurred during heat treatment. The 20 mol% excess derived NKBT thin film exhibits the best ferroelectric and dielectric properties with a remnant polarization (Pr) of 13.6 μC/cm², and a coercive field (Ec) of 104.8 KV/cm, together with a dielectric constant of 406 and a dissipation factor of 0.064. Similar to the dielectric response change with Na/K excess content, the decreasing concentration of charged defects is the main reason resulting in the increase of the piezoelectric property. The film with a 20 mol% excess content exhibited an effective d₃₃^? of about 56 pm/V. Also, the NKBT with a 20 mol% excess content exhibits the lowest current density of 5.6 × 10^− 5 A/cm² at 10 V.