Dielectric and tunable properties of Pb0.25BaxSr0.75−xTiO3 thin films fabricated by a modified sol-gel method

A modified sol-gel method was used to fabricate (Pb_0.25Ba_x Sr_0.75−x)TiO₃ (PBST) thin films with x = 0.05,0.1,0.15 and 0.2 on Pt/TiO₂/SiO₂/Si substrate. The structure, surface morphology, dielectric and tunable properties of PBST thin films were investigated as a function of barium content (x). X-ray diffraction and scanning electron microscopy analysis showed that we could get pure PBST perovskite phase and relative fine density thin films with smooth surface. It was found that the crystal lattice constant, grain size, room temperature dielectric constant, dielectric loss and tunability of Ba solutionizing PST thin films increased with the increase in Ba content. For (Pb_0.25Ba_0.2Sr_0.55)TiO₃ thin film, it had the highest dielectric constant of 1390 and the largest tunability of 80.6%. The figure of merit parameter reached a maximal value of 28.9 corresponding to the (Pb_0.25Ba_0.05 Sr_0.7)TiO₃ thin film, whose dielectric constant, dielectric loss and tunability measured at 1 MHz were 627, 0.024 and 69.4%, respectively.     

Dielectric dispersion and tunability of sol-gel derived Ba x Sr1−x TiO3 thin films

Ba_0.5Sr_0.5TiO₃, Ba_0.6Sr_0.4TiO₃, Ba_0.7Sr_0.3TiO₃ and Ba_0.8Sr_0.2TiO₃ thin films were fabricated by a modified sol-gel technique on Pt(111)/Ti/SiO₂/Si(100) substrates. All Ba _x Sr_1–x TiO₃ films crystallized in the perovskite structure with a crack free microstructure and clear grain boundaries. Highest relative permittivity and dielectric tunability was observed in the Ba_0.7Sr_0.3TiO₃ thin film. Ba_0.7Sr_0.3TiO₃ and Ba_0.8Sr_0.2TiO₃ compositions demonstrated ferroelectric hysteresis loops indicating the presence of ferroelectricity at room temperature. The paraelectric compositions of Ba_0.5Sr_0.5TiO₃ and Ba_0.6Sr_0.4TiO₃ showed significant tunability with negligible loss tangent. The tunability of Ba_0.5Sr_0.5TiO₃ thin film decreased with the increase of frequency from 100 kHz to 100 MHz. As the frequency increases, especially above 10 MHz, the relative permittivity decreases while the loss tangent increases. Since Ba_0.5Sr_0.5TiO₃ thin film is paraelectric at room temperature, relaxation due to ferroelectric domains cannot occur. Therefore this behaviour has originated from the contact resistance and finite sheet resistance of both the bottom and top electrodes. To analyse the thin film capacitor, the parallel plate capacitor structure can be modeled based on an equivalent circuit, which contain electrode and contact resistance.     

Microstructure and electrical properties of Ba0.7Sr0.3(Ti1 − xZrx)O3 thin films prepared on copper foils with sol-gel method

Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0, 0.1, 0.2) (BSZT) thin films have been prepared on copper foils using sol-gel method. The films were annealed in an atmosphere with low oxygen pressure so that the substrate oxidation was avoided and the formation of the perovskite phase was allowed. The X-ray diffraction results show a stable polycrystalline perovskite phase, with the diffraction peaks of the BSZT films shifting toward the smaller 2θ with increasing Zr content. Scanning electron microscopy images show that the grain size of the BSZT thin films decreases with increasing Zr content. High resolution transmission electron microscopy shows the clear lattice and domain structure in the film. The dielectric peaks of the BSZT thin films broaden with increasing Zr content. Leakage current density of Ba_0.7Sr_0.3(Ti_1 − xZr_x)O₃ (x = 0.1) thin film is the lowest over the whole applied voltage.     

Calcium-doping for enhanced temperature stability of (Ba0.6Sr0.4)1−xCaxTiO3 thin film dielectric properties

Ternary (Ba_0.6Sr_0.4)_1−xCa_xTiO₃ (BSCT) (x = 0, 0.1, 0.2, 0.3 and 0.4) thin films with thickness of around 500 nm were prepared on Pt(111)/TiO₂/SiO₂/Si substrates by sol-gel methods. BSCT forms the complete solid solutions in a single cubic perovskite structure. The lattice constant, dielectric constant, tanδ and tunability of BSCT decrease, whereas the temperature stability of dielectric properties increases with increasing the Ca concentration. From 25 to 100 °C, the decrease of tunability is about 11% for BSCT with 40 at.% of Ca. BSCT thin films exhibit the comparable tunability, low loss and enhanced temperature stability.     

Enhanced dielectric and tunable properties of barium strontium titanate thin films through introducing Nd(Zn1/2Ti1/2)O3 and adjusting Ba/Sr

Ba_0.6Sr_0.4TiO₃ (BST) and 0.06Nd(Zn_1/2Ti_1/2)O₃–0.94Ba _x Sr_1?x TiO₃ (NZT–BST) thin films with x = 0.6, 0.7, 0.75, and 0.8 were fabricated on Pt/Ti/SiO₂/Si substrates by sol–gel method. The structures, surface morphology, dielectric, and ferroelectric properties, and thermal stability of BST and NZT–BST thin films were investigated as a function of NZT and Ba content. It was found that introducing NZT into BST decreased significantly dielectric loss, however, along with the tunability. On this basis, increasing Ba/Sr in NZT–BST thin films led to the simultaneous increase of dielectric constant and tunability of thin films. As a result, optimized dielectric and tunable properties were obtained for 0.06Nd(Zn_1/2Ti_1/2)O₃–0.94Ba_0.7Sr_0.3TiO₃ thin film with the highest FOM value of 43.22. It awakens us that, for reducing dielectric loss, introducing a certain amount of low permittivity oxides or non-ferroelectrics like NZT into weak ferroelectric perovskite tunable materials, not into paraelectric perovskite tunable materials, may obtain more excellent dielectric and tunable performances.     

Preparation and characterization of Ba1 − xSrxTiO3 thin films deposited on Pt/SiO2/Si by sol-gel method

BST thin films have been investigated as potential candidates for use in frequency agile microwave circuit devices. Stoichiometric (Ba_1 − xSr_x)TiO₃ (BST) thin films have been prepared on Pt/SiO₂/Si substrates using sol-gel method. The BST films were characterized by X-ray fluorescence (XRF) spectroscopy analysis, X-ray diffraction (XRD), scanning electron microscope (SEM) and electrical measurements. The relationships of processing parameters, microstructures, and dielectric properties are discussed. The results show that the films exhibit pure perovskite phase through rapid thermal anneal at 700 °C and their grain sizes are about 20-40 nm. The dielectric constants of BST5, BST10, BST15 and BST20 are 323, 355, 382 and 405, respectively, at 80 kHz.     

Preparation and characterization of La-doped Ba(1 − x)SrxTiO3 powders and thin films

Ba_(1 − x)Sr_xTiO₃ powders with different Ba/Sr ratios (x = 0.10, 0.25, 0.40, 0.55, 0.70) and La-doped Ba_0.9Sr_0.1TiO₃·yLa powders (y = 0.002, 0.004, 0.006, 0.008, 0.010) have been prepared by sol-gel technology using dehydrated barium-acetate, strontium-carbonate, lanthanum-nitrate, and titanium-isopropoxide as raw materials. The experimental results show that the dielectric properties of Ba_(1 − x)Sr_xTiO₃ powders depend on the Ba/Sr ratios. When the Sr fraction is 0.10, the dielectric constant is relatively higher and the dielectric loss is relatively lower, which are more than 2000 and less than 2.0 × 10^− 2 at 1000 Hz, respectively, the most important is that this kind of powder has better frequency stability. La-doping can increase the dielectric constant distinctly, but the dielectric loss can also be increased. Their dielectric properties at 1.0 × 10³ Hz are better than those at 1.0 × 10⁵ Hz. At 1.0 × 10³ Hz the dielectric constant is much higher, while the dielectric loss is much lower. The dielectric constant of different La-doping contents is nearly 3.5 × 10⁴ and the dielectric loss is less than 0.20 when La fraction is 0.008. The La-doped BST sample also has better frequency stability, especially at high frequency. La-doped BST thin films are successfully deposited on mild steel substrates by using plasma spray system with suspension precursors of Ba_0.90Sr_0.10TiO₃·0.8La powders. The XRD patterns of Ba_0.90Sr_0.10TiO₃ and Ba_0.90Sr_0.10TiO₃·0.8La powders are almost the same. No new peaks appear after La-doping, but the peaks move slightly to a larger degree, which indicates that the element La has entered the lattice of the Ba_0.90Sr_0.10TiO₃ and has made the constant of the crystal cell reduce. The XRD pattern of the thin films is just like that of the Ba_0.90Sr_0.10TiO₃·0.8La powders except a peak corresponding to Fe substrate. The SEM results show that the thin films have a uniform and smooth surface. The morphology of cross-section shows a columnar grain structure indicating smooth surface and uniform thickness of the film. The thickness of the film is about 15 um. The thin films obtained are expected to be prospective material for applications in tunable microwave devices.     

Self-buffered BaxSr1−xTiO3 films by sol-gel and RF magnetron sputtering method

Ba_xSr_1−xTiO₃ (BST) films are fabricated by sol-gel and RF (radio frequency) magnetron sputtering method. A buffer layer with columnar grains by sol-gel method is introduced to improve the dielectric anomaly in BST films. We find that the presence of buffer layer can increase the differential dielectric constant against temperature in sol-gel derived BST films while not so with sputtered films. We explain this by an ‘expanded layer thickness model’ and an unstable crystallized surface, respectively. The obtained (dε/ε) dT is up to 6% around 11 °C by the sol-gel method.     

Bottom electrodes dependence of microstructures and dielectric properties of compositionally graded (Ba1−xSrx)TiO3 thin films

Compositionally graded (Ba_1−xSr_x)TiO₃ (BST) thin films, with x decreasing from 0.3 to 0, were deposited on Pt/Ti/SiO₂/Si and Ru/SiO₂/Si substrates by radio frequency magnetron sputtering technology. The microstructure and dielectric properties of the graded BST thin films were investigated. It was found that the films on Ru electrode have better crystallization, and that RuO₂ is present between the Ru bottom electrode and the graded BST thin films by X-ray diffraction and SEM analysis. Dielectric measurement reveals that the graded BST thin films deposited on Ru bottom electrode have higher dielectric constant and tunability. The enhanced dielectric behavior is attributed to better crystallization as well as smaller space charge capacitance width and the formation of RuO₂ that is more compatible with the BST films. The graded BST films on Ru electrode show higher leakage current due to lower barrier height and rougher surface of bottom electrode.     

Morphologies and crystal structures of nano-sized Ba1−xSrxTiO3 primary particles prepared by flame spray pyrolysis

Nano-sized Ba_1−xSr_xTiO₃ (BST) powder was prepared by flame spray pyrolysis using “CA-assisted” spray solution. The effects of the mole ratios of Ba to Sr components on the mean sizes, morphologies, and crystal structures of the BST powder prepared by flame spray pyrolysis were investigated. The precursor powders obtained by flame spray pyrolysis had large size, fractured and hollow structures irrespective of the mole ratios of Ba to Sr components. The post-treated BST powders had slightly aggregated morphology of the primary particles with nanometer sizes. The slightly aggregated BST powders turned to nano-sized primary particles by a simple milling process. The milled BaTiO₃ particles post-treated at temperature of 1000 °C had spherical-like shape. On the other hand, the milled Ba_0.5Sr_0.5TiO₃ and SrTiO₃ particles had square shape. The mean sizes of the milled BaTiO₃, Ba_0.5Sr_0.5TiO₃ and SrTiO₃ particles were each 110, 32, and 48 nm. Phase pure BST powder was obtained at a post-treatment temperature of 1000 °C irrespective of the mole ratios of Ba to Sr components. The BaTiO₃ powder had tetragonal crystal structure. On the other hand, the BST except for the BaTiO₃ composition had cubic crystal structures at post-treatment temperature of 1000 °C. The mean crystallite sizes of the milled Ba_1−xSr_xTiO₃ primary particles were changed from 29 to 37 nm according to the mole ratios of Ba to Sr components.     

Large area Ba1 − xSrxTiO3 thin films for microwave applications deposited by pulsed laser ablation

Large area Ba_1 − xSr_xTiO₃ (BST) thin films with x = 0.4 or x = 0.5 were deposited on 75 mm diameter Si wafers in a pulsed laser deposition (PLD) chamber enabling full-wafer device fabrication using standard lithography. The deposition conditions were re-optimized for large PLD chambers to obtain uniform film thickness, grain size, crystal structure, orientation, and dielectric properties of BST films. X-ray diffraction and microstructural analyses on the BST films grown on Pt/Au/Ti electrodes deposited on SiO₂/Si wafers revealed films with (110) preferred orientation with a grain size < 100 nm. An area map of the thickness and crystal orientation of a BST film deposited on SiO₂/Si wafer also showed (110) preferred orientation with a film thickness variation < 6%. Large area BST films were found to have a high dielectric tunability of 76% at an electric field of 400 kV/cm and dielectric loss tangent below 0.03 at microwave frequencies up to 20 GHz and a commutation quality factor of ~ 4200.     

Crystallization and electrical properties of Ba<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>TiO<Subscript>3</Subscript> thin films on SrRuO<Subscript>3</Subscript>/Pt hybrid bottom electrode

Ba_0.7Sr_0.3TiO₃ (BST) thin films were deposited on Pt and SrRuO₃(SRO)/Pt hybrid bottom electrodes by radio frequency magnetron sputtering. X-ray analysis indicated that both films were polycrystalline. Dielectric measurements showed that the films on SRO/Pt hybrid bottom electrode had lower dielectric constant and loss than the films on single Pt and the dielectric properties were frequency-independent. The leakage current density of Ba_0.7Sr_0.3TiO₃ thin films on hybrid bottom electrode was also lower. Leakage mechanism investigations showed that the contact between the electrode-film interfaces of thin films on SRO/Pt hybrid bottom electrode was ohmic. Based on the results, the effects of SRO/Pt hybrid bottom electrode on the crystallization and electrical properties of BST thin films were discussed.     

Nanograins dependent dielectric constant, tunability, phase transition, impedance spectroscopy and leakage current of (Pb1 − xSrx)TiO3 thin films

Nanostructured (Pb_1 − xSr_x)TiO₃ (PST) (x = 0.1, 0.2 and 0.3) thin films have been prepared by chemical solution deposition process using spin coating technique. The solution as such was deposited on Pt/Ti/SiO₂/Si substrates and annealed at 650 °C/3h. Nanograins dependent dielectric properties of PST films show dielectric constant up to the higher frequency region, low losses, large tunability and phase transition at small temperature. The impedance data has been fitted by Cole-Cole model to study the effect of grain boundaries on the dielectric properties. The current-voltage characteristics have been measured to study leakage current in PST films and described by Poole-Frenkel emission model. It is suggested that the key carrier transport process in PST films is emission of electrons from a trap state near the metal-film interface into a continuum of states associated with each conductive dislocation. The activation energy value for carrier transport in PST films is obtained from temperature-dependent current-voltage characteristics.     

Dielectric properties of barium strontium titanate thick films prepared by electrophoretic deposition

Ba_xSr_1−xTiO₃ (x = 0.5, 0.6, 0.7) thick films were prepared by electrophoretic deposition (EPD) technique on platinum metallic foils using BaTiO₃ and SrTiO₃ nanoparticles with different molar proportion of 1:1, 3:2 and 7:3, respectively. An isostatic pressure method was used to increase density of the thick films before high temperature sintering. Microstructures of the deposited films were examined with XRD and SEM techniques. Porosity of the thick films decreased after the isostatic pressure process. The Ba_0.5Sr_0.5TiO₃ thick films of 10 μm, 15 μm and 20 μm showed a tunability of 28.8%, 33.3% and 33.9%, respectively, at room temperature and at a biasing field of 2 kV/mm. The dielectric constant was from 2138 to 3446 and dielectric loss was from 0.016 to 0.011 at zero bias field at 10 kHz. The temperature dependence of dielectric constant was also measured and the effect of porosity and thickness on the electrical performance of the thick films was discussed.     

Ferroic metal-oxide films grown by polymer assisted deposition

Polymer assisted deposition is a versatile technique to grow simple and complex metal-oxide thin films. In this paper we report the structural and electrical properties of ferroic materials, namely La_0.67M_0.33MnO₃ (M = Sr and Ca) and Ba_1 − xSr_xTiO₃ (x = 0.3, 0.5, and 0.7) prepared using this process. The films were prepared on single crystalline LaAlO₃ substrates. The films were highly c-axis oriented and epitaxial in nature. The ferromagnetic La_0.67Sr_0.33MnO₃ and La_0.67Ca_0.33MnO₃ films show intrinsic transport properties with maximum magnetoresistance values (at applied field of 5 T) of − 50% and − 88%, respectively. The highest dielectric constant (∼ 1010) and tunability (∼ 69%) of Ba_1 − xSr_xTiO₃ film occurs at x = 0.3 for films, which is at the phase boundary of tetragonal and cubic.     

Synthesis and characterization of Ba0.7Sr0.3TiO3, BaZr0.3Ti0.7O3 thin film heterostructures

The paper presents synthesis of Ba_0.7Sr_0.3TiO₃ (BST), Ba_0.7Sr_0.3TiO₃ (BZT) thin films and BZT/BST heterostructures using modified Pechini method. The La_0.7Sr_0.3MnO₃ has been used as a conducting bottom layer to form metal ferroelectric metal capacitor. The thin films are spin coated on SiO₂/n-Si (100) substrates. The thin films thus deposited are characterized for crystal structure, morphology, dielectric and complex impedance properties. The results show that BZT/BST heterostructures show reduced loss tangent tan δ and useful value of figure of merit γ in RF range of frequencies. The results on dielectric properties could be analyzed in terms of the Maxwell–Wagner Model and contribution due to superlattice effects.     

Microwave dielectric properties of Ba2 − xSrxLa3Ti3NbO15 ceramics

High dielectric constant and low loss ceramics in the system Ba_2 − xSr_xLa₃Ti₃NbO₁₅ (x = 0-1) have been prepared by conventional solid-state ceramic route. Ba_2 − xSr_xLa₃Ti₃NbO₁₅ solid solutions adopted A₅B₄O₁₅ cation-deficient hexagonal perovskite structure for all compositions. The materials were characterized at microwave frequencies. They show a linear variation of dielectric properties with the value of x. Their dielectric constant varies from 48.34 to 43.03, quality factor Q_u × f from 20,291 to 39,088 GHz and temperature variation of resonant frequency from 8 to 1.39 ppm/°C as the value of x increases. These low loss ceramics might be used for dielectric resonator (DR) applications.     

Preparation,microstructure and dielectric properties of Ba0.5Sr0.5TiO3 thin films grown on Pt/Ti/SiO2/Si substrates by pulsed laser deposition

Barium strontium titanate Ba_0.5Sr_0.5TiO₃ (BST) thin films have been deposited on Pt/Ti/SiO₂/Si substrates by a pulsed laser deposition method. The low-frequency dielectric responses of the BST films, grown at different substrate temperatures (T_s), were measured as functions of frequency in the frequency range from 1 kHz to 1 MHz. With increase of T_s, the grain size of BST thin films became larger and the crystallinity was greatly improved, and then the dielectric permittivity increased, while the dielectric dispersion rose drastically. The origin of the large dielectric relaxation is believed to result from the aggravation of oxygen diffusion at the BST/Pt interface for the BST thin films grown at comparatively higher temperatures. This concept could be further explained by considering the influence of post-annealing in oxygen ambient on the dielectric properties of BST thin films. Our results reveal that the dielectric properties are strongly dependent on the processing conditions and the microstructure of thin films.     

Different microstructure and dielectric properties of Ba1−xCaxTiO3 ceramics and pulsed-laser-ablated films

A comparative study of the microstructure and dielectric properties between Ba_1−xCa_xTiO₃ (BCT) ceramics and films were performed in the whole Ca concentration range of x = 0-1. The ceramics were prepared by conventional solid-state reaction technique and the films by the method of pulsed-laser deposition. X-ray diffraction (XRD) study of the BCT ceramics exhibited a pure tetragonal phase for x = 0-0.25, a tetragonal-orthorhombic diphase for x = 0.25-0.85 and a pure orthorhombic phase for x = 0.90-1.00. And the dielectric phase transition temperature from tetragonal to cubic was marginally affected by the Ca doping into BaTiO₃. However, BCT films deposited on Pt/Si/SiO₂/Si substrates showed a different microstructure and dielectric properties. Tetragonal-orthorhombic diphase was not found in the BCT films for x = 0.25-0.85, and a large decrease of the Curie point and diffuse phase transition were observed in the BCT films. Based on the compositional analysis, such phenomena were ascribed to the occupancy of some Ca²⁺ to the Ti⁴⁺ sites in the BCT films.     

Synthesis, characterization, and dielectric properties of Ba(Ti1−xSnx)O3 nanopowders and ceramics

We prepared Ba(Ti_1−xSn_x)O₃ powders and ceramics by means of the sol-gel process, with dibutyltin dilaurate as the Sn precursor. The samples were characterized by means of Fourier-transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy, and also determined the dielectric properties of the ceramics. The powders synthesized by means of the sol-gel process had a grain size on the nanometer scale, with the grains mainly composed of a cubic BaTiO₃ phase. Sn can disperse into BaTiO₃ more uniformly in the sol-gel technique using dibutyltin dilaurate as the Sn precursor. With increasing Sn concentration, the grain size of the Ba(Ti_1-xSn_x)O₃ ceramics increased and the maximum dielectric constant (?_max) first increased and then decreased. At a Sn concentration of 5 mol%, ?_max reached its maximum value (19,235).