Dielectric properties of SrTiO3/LaNiO3 composite structure prepared by “sol-gel” method

In this work, Pt/SrTiO₃(STO)/LaNiO₃(LNO) and Pt/STO/Pt structures were fabricated on Si substrates using sol–gel method. Dielectric tunability characteristics of these two structures are investigated over temperature range of 300–80 K. As temperature decreases, relative dielectric constant and tunability of STO films in both structures increase linearly, while loss tangent (tan?) decreases. STO prepared on LNO exhibits better crystallinity and dielectric tunability than that prepared on Pt. Furthermore, LNO top pole array with 0.2 mm diameter was prepared on STO/LNO using photosensitive sol–gel method, resulting in LNO/STO/LNO structure with symmetric electrode. Dielectric tunability tests reveal that, compared with Pt/STO/LNO structure, LNO/STO/LNO structure exhibits similar tunability, lower tan?, higher figure-of-merit (FOM) values, and better tunability symmetry. Relative dielectric constant, tunability (350 kV/cm), and FOM are approximately 391, 60%, and 142.6 at 80 K, respectively. Further analysis shows that because of Schottky barrier at Pt/STO interface, the FOM and tunability symmetry of Pt/STO/LNO structure are worse than those of LNO/STO/LNO structure. These results demonstrate that LNO electrode performs better than conventional inert metal electrodes.     

Low Dielectric Loss and Good Dielectric Thermal Stability of xNd(Zn1/2Ti1/2)O3–(1−x)Ba0.6Sr0.4TiO3 Thin Films Fabricated by Sol–Gel Method

xNd(Zn_1/2Ti_1/2)O₃–(1?x)Ba_0.6Sr_0.4TiO₃ (xNZT–BST) thin films were fabricated on Pt/Ti/SiO₂/Si substrates by sol–gel method with x = 0, 3%, 6%, and 10%. The structures, surface morphology, dielectric and ferroelectric properties, and thermal stability of xNZT–BST thin films were investigated as a function of NZT content. It was observed that the introduction of NZT into BST decreased grain size, dielectric constant, ferroelectricity, tunability, and significantly improved dielectric loss and dielectric thermal stability. The corresponding reasons were discussed. The 10%NZT–BST thin film exhibited the least dielectric loss of 0.005 and the lowest temperature coefficient of permittivity (TCP) of 3.2 × 10^?3/°C. In addition, the figure of merit (FOM) of xNZT–BST (x = 3%, 6%, and 10%) films was higher than that of pure BST film. Our results showed that the introduction of appropriate NZT into BST could modify the dielectric quality of BST thin films with good thermal stability. Especially for the 3%NZT–BST thin film, it showed the highest FOM of 33.58 for its appropriate tunability of 32.87% and low dielectric loss of 0.0098.     

Dielectric enhancement of BaSrTi1.1O3/BaSrTi1.05O3/BaSrTiO3 multilayer thin films prepared by RF magnetron sputtering

BaSrTi_1.1O₃/BaSrTi_1.05O₃/BaSrTiO₃ multilayer (BSTM) thin films and BaSrTiO₃ (BST) thin films were deposited on LaNiO₃ (LNO)/SiO₂/Si substrates by radio frequency (RF) magnetron sputtering at substrate temperature of 400 °C, respectively. X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM) investigations revealed that all the films have uniform and crack-free surface morphology with a perovskite structure. The dielectric constant of the BSTM thin films was increased and dielectric loss was decreased compared with those of uniform BST thin films. The dielectric constant of 420, dielectric loss of 0.017, and dielectric tunability of 38% were achieved for the BSTM thin films.     

Preparation of LaNiO3/SrTiO3/LaNiO3 capacitor structure through sol–gel process

In this paper, SrTiO₃/LaNiO₃ (STO/LNO) bilayer films were prepared on lanthanum aluminate (LAO) substrates by use of the sol–gel technique. An array of LNO electrodes with diameters of 200 μm was prepared on the surface of STO/LNO bilayer films. Therefore, LaNiO₃/SrTiO₃/LaNiO₃ (LNO/STO/LNO), a capacitor structure with symmetrical top and bottom electrodes, was obtained. The XRD analysis showed that the obtained capacitor structure has a biaxial texture. The dielectric test suggested that the relative dielectric constant of the LNO/STO/LNO structure is symmetric, has a high tunability, and has a low dielectric dissipation factor (tan δ) in response to varying electric field bias. As the temperature decreased, the relative dielectric constant of the STO film increased, the tunability increased, and the tan δ decreased. At test conditions of 80 K and 100 kH, the tunability and the figure of merit (FOM) reached 56% and 107, respectively.     

Achieving high breakdown strength and figure of merit of Ba0.6Sr0.4TiO3 films through coating a Y3Fe5O12 layer

Low tunability and figure of merit significantly limited the application of Ba_0.6Sr_0.4TiO₃ (BST) ferroelectric film, which originates from the low electric breakdown strength and high dielectric loss of BST layer. Garnet structured Y₃Fe₅O₁₂ (YIG) exhibits the merits of good microwave dielectric property and a much high resistivity, which are helpful for enhancing the breakdown strength and suppressing the dielectric loss. In this work, Y₃Fe₅O₁₂/Ba_0.6Sr_0.4TiO₃ (YIG/BST) composite films were fabricated via chemical solution deposition method. The composite films exhibited a low dielectric loss (0.006) and an almost frequency independent dielectric constant in a frequency range from 10 kHz to 1 MHz. The electric breakdown strength was significantly enhanced from less than 400 kV/cm to around 800 kV/cm through coating a YIG layer, causing an excellent tunability of 72.84% and an ultra-high figure of merit (FOM=118) at 800 kV/cm in YIG/BST film. It is physically clarified that the conduction loss plays an important role in BST film while the intrinsic loss is the dominate factor for the YIG/BST composite films.     

Low-temperature fabrication of Ba1−xSrxTiO3 thin films with good dielectric properties on platinized silicon substrates

Ba_0.7Sr_0.3TiO₃ (BST) thin films 500 nm in thickness were prepared on technologically desirable Pt/TiO₂/SiO₂/Si(1 0 0) substrates by ion beam sputtering (IBS) and post-deposition annealing method. The effect of annealing temperature on the structural and dielectric properties of BST thin films was systematically investigated. A sharp transition in their tunable dielectric behaviours was observed in good agreement with the evolution of crystal structure from amorphous to crystalline phase. It was demonstrated that the perovskite phase could crystallize in BST films at a very low temperature, around 450 °C. The lowering of perovskite crystallization temperature in the BST films was explained in terms of the high energetic process nature of IBS technique. A high dielectric tunability of 42% at E (electric field intensity) = 500 kV/cm and a low loss tangent of 0.013 at zero bias were both obtained in the 450 °C-annealed film, thereby resulting in the highest figure-of-merit factor among all the different temperature annealed films. Moreover, the 450 °C-annealed film showed superior leakage current characteristics with a low leakage current density of about 10^?4 A/cm² at E = 800 kV/cm.     

Giant dielectric phenomenon of Ba0.5Sr0.5TiO3/CaCu3Ti4O12 multilayers due to interfacial polarization for capacitor applications

Ba_0.5Sr_0.5TiO₃/CaCu₃Ti₄O₁₂ (BST/CCTO) multilayers with different stack sequences were deposited on LaNiO₃(LNO)/SiO₂/Si substrates by a sol-gel process. The dielectric properties of BST/CCTO multilayers are significantly affected by the deposition sequence, the layer thickness and the impurities, effects that are interpreted using the Maxwell-Wagner interfacial polarization model. Impurities are generated by elemental interdiffusion at the interfaces of BST/CCTO, and less at the interfaces of BST/LNO and CCTO/LNO. The dielectric permittivity of the CCTO/BST/LNO/SiO₂/Si sample reaches 352,200 at 10 kHz, and stabilizes above 20,000 in the range of 100 kHz to 1 MHz. This work demonstrates an effective approach to enhance dielectric properties for film capacitor applications by constructing multilayers with specific deposition sequences and layer thicknesses.     

Dielectric properties of Fe-doped Ba0.65Sr0.35TiO3 thin films fabricated by the sol–gel method

Fe-doped Ba_0.65Sr_0.35TiO₃ (BST) thin films have been fabricated on Pt/Ti/SiO₂/Si substrate using the sol–gel method. The structural and surface morphology, dielectric, and leakage current properties of undoped and 1 mol% and 2 mol% Fe-doped BST thin films have been studied in detail. The results demonstrate that the Fe-doped BST films exhibit improved dielectric loss, tunability, and leakage current characteristics as compared to the undoped BST thin films. The improved figure of merit (FOM) of Fe-doped BST thin film suggests a strong potential for utilization in microwave tunable devices.     

Preparation and characterization of crystalline Ba0.5Sr0.5TiO3 thin films on FTO transparent electrodes

To achieve an economic advantage, nanocrystalline Ba_0.5Sr_0.5TiO₃ (BST) thin films were prepared on FTO-coated glass substrates using an aqueous sol–gel process. The crystal structures and chemical states of the obtained thin films were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD patterns confirmed the perovskite structure of the BST samples, while XPS analysis demonstrated that Ba element was in its single oxidation state in the film with annealing temperature of 600 °C. In contrast, Ba was present in a mixed oxidation state upon increasing the annealing temperature. It was found that the BST thin film prepared at a relatively low annealing temperature of 600 °C exhibited optimal crystallinity and superior dielectric properties, along with excellent optical transparency. More specifically, this film exhibited a dielectric tunability of 44.81% (1.33 MV/cm), a figure of merit of 56.01, and an average transmittance of 87.2% in the visible region.     

Dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates

The dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates have been investigated. The BST films were screen printed and sintered at temperatures below 1300 °C. At temperatures below the Curie point the BST films exhibit tunability in the range 15–35% under a DC bias field of 2 kV/mm. The dielectric loss is critically dependent on film thickness with lower losses (<10⁻²) for the thicker films (>100 μm). A relaxation process appears to take place for the BST films in the MHz to GHz frequency regime. The variation of permittivity with bias field exhibits hysteretic behaviour in both the ferroelectric and paraelectric regions. This is believed to arise due to the non-uniform composition and existence of micro/nano-polar phases in the films.     

Ferroelectric Phase Transition,Interfaces Quality,and Stress Evolution of TiOx/BaxSr1−xTiO3 Structures

Ba(Sr,Ti)O₃ material presents a remarkable property that lies in the possibility to change the permittivity by applying a dc electric field, i.e., BST is a tunable material. That makes BST a very interesting material for the development of reconfigurable devices in microelectronics. In this study, we focus our work on Ba(Sr,Ti)O₃ with Ba/Sr = 30/70, the films are deposited by radio‐frequency magnetron sputtering on Al₂O₃ (0001). A buffer layer of TiO_x is used to control the film orientation. The influence of this buffer layer on the dielectric properties, the interfaces quality with respect to the film thickness, and the temperature is analyzed. An increase of 30% of the relative permittivity was measured and a tunability of 50% was attained at 300 KV/cm. The dielectric measurements on BST/TiO_x as a function of the temperature show a shift of the Curie temperature (T_c = ?40°C) in comparison to BST without TiO_x layer (T_c = ?80°C). We demonstrate that the Curie temperature does not correspond to the maximum permittivity. The important stress measured on the films (930 MPa) could explain this behavior.     

Ultraviolet pulsed laser crystallization of Ba0.8Sr0.2TiO3 films on LaNiO3-coated silicon substrates

In this work, Ba_0.8Sr_0.2TiO₃ (BST) films on LaNiO₃-buffered SiO₂/Si (LNO/SiO₂/Si) substrates were crystallized by pulsed laser irradiation. Solution-derived amorphous barium–strontium–titanate precursor layers were crystallized with a KrF excimer laser in oxygen ambient at fluences ranging from 50 to 75 mJ cm⁻². With the substrate temperature set to 500 °C, the number of pulses and film thickness were varied until high-quality crystallinity could be achieved. It was found that films with a thickness of 40 nm are fully crystallized with a uniaxial {00l} orientation which is predetermined by the LaNiO₃ orientation. On the other hand, for 160 nm thick films, crystallization was observed after 12,000 pulses in the 70 nm close to the surface, while the rest of the film remained amorphous. The large temperature difference between the film surface and interface due to the low thermal conductivity of the amorphous BST is suggested as the origin of this behavior. Films thicker than 80 nm cracked on crystallization due to the stress caused by the different thermal expansion coefficients of film and substrate, as well as the large temperature variations within the BST film.     

Investigation on Tunable Performance of BMN/BST Multilayer and BMN–BST Composite Thin Films

The BMN/BST multilayer and BMN–BST composite thin films have been fabricated by pulsed laser deposition on Au/TiO_x/SiO₂/Si substrate. The multilayer thin films comprising one, two, and four periodic compositional Bi_1.5MgNb_1.5O₇/Ba_0.6Sr_0.4TiO₃ (BMN/BST) layers (PCBLs) have been elaborated with the final same thickness. The four PCBLs show the largest dielectric constant of ~168 and tunability of 40.6% at a maximum applied bias field of 0.67 MV/cm and the lowest loss tangent of ~0.006, whereas the figure of merit (FOM) is 72. The BMN–BST composite thin films exhibit medium dielectric constant of ~238, low loss tangent of ~0.0053, and superior tunable dielectric properties at room temperature. Calculations of tunability and FOM display a maximum value of 49.3% at 670 kV/cm and ~88, respectively.     

Impact of microwave sintering on dielectric properties of screen printed Ba0.6Sr0.4TiO3 thick films

The influence of 30 GHz microwave sintering compared to conventional sintering has been investigated on polycrystalline Ba_0.6Sr_0.4TiO₃ (BST60) thick films with respect to an application as tunable dielectrics. The BST thick films were prepared as metal–insulator–metal (MIM) capacitors on alumina substrates. The average grain size (440 nm) and the porosity (approx. 30%) of the sintered films are only little affected by the sintering method. However, permittivity, dielectric loss and tunability have been influenced substantially. The dielectric improvement by microwave sintering is interpreted in terms of an increased crystal quality (ξ_S) and/or a decrease of defect concentrations. It is assumed that microwave sintering preferably heats up parts of the film where an increased defect density exists and therefore causes a selective heating process. This may heal up charged defects, inhomogeneities, and structural defects.     

Thickness effects on microwave properties of (Ba,Sr)TiO3 films for frequency agile technologies

We fabricated (Ba_0.6Sr_0.4)TiO₃ (BST) thin films of various thicknesses on sapphire (−1 1 2 0) substrates using metal-organic decomposition method. These films showed grain growth from 160 to 650 nm with an increase in the thickness from 90 to 1050 nm. At microwave frequencies, the measured capacitances of the planar capacitors decreased with the film thickness because the electro-magnetic field propagates across high permittivity BST films to the low permittivity sapphire substrate. However, we found that the BST-thin film permittivity remained large up to 90 nm thick, based on electro-magnetic field analysis using the finite element method. On the other hand, the BST thin film tunability decreased with the film thickness.     

Effects of vacancy defects caused by non-stoichiometric ratio on dielectric properties of ABO3 perovskite (Ba0.5Sr0.5)xTiO3 ceramics

Non-stoichiometric Ba_0.5Sr_0.5TiO₃ (BST50) ceramics with varying A/B ratios, namely (Ba + Sr)/Ti, were prepared by a conventional solid-state reaction approach. The effects of vacancy defects caused by varying the A/B ratio on the structure and dielectric properties of BST50 ceramics were systematically investigated. A remarkable change in grain size was found when the A/B ratio was increased, which led to apparent variations in the dielectric properties of the BST50 ceramics. The Curie temperature (T_c) and dielectric permittivity peak (ε_max) increased first and then decreased with increasing A/B ratio, and reached the maximum at A/B = 1. Simultaneously, the dielectric diffusion parameter of BST50 ceramics was studied by the Lorenz-type formula. All samples exhibited diffusion phase transition behavior, and T_c was frequency independence. When A/B < 1, the Q value remained at a high level; in contrast, when A/B > 1, the Q value was significantly reduced. For this BST50 system, high tunability of 24.95% (at 30 kV/cm), low dielectric loss of 0.0017 (at 10 kHz), and high figure of merit (FOM) of 147 were achieved simultaneously at A/B = 1.01.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Processing and dielectric characterization of Ba0.3Sr0.7TiO3 thin films on alumina substrates

Ba_0.3Sr_0.7TiO₃ (BST) thin films were prepared from the sols based on alkaline earth acetates and titanium propoxide in 2-methoxyethanol–acetic acid solvents and deposited on polished alumina substrates by spin coating. The perovskite phase crystallizes upon heating at/above 700 °C. By increasing the annealing temperature from 700 to 900 °C the grain size increases from 40 to 80 nm, due to the increased driving force for crystallization. The annealing time has got only a minor influence on grain size as a consequence of constrained conditions of the film. The dielectric permittivity and tunability (ɛ_0 V/ɛ_200 V) of BST films, measured at 1 MHz, strongly depend on the grain size, exhibiting the values of 345 and 1.47, and 722 and 1.93 for the films with 40 and 80 nm-sized grains, respectively.     

Preparation of La1?x Sr x CoO3 Oxide Electrode Material and its Influence on the Structure and Dielectric Performance of the Supported Ba1?x Sr x TiO3 Thin Films

Using the sol–gel method, La_1−x Sr _x CoO₃ (LSCO) electrode films were first fabricated on the Si (100) substrates, followed by the growth of Ba_1−x Sr _x TiO₃ (BST) thin films on the LSCO electrode film. The crystal structure and surface morphology of these films were characterized by XRD and SEM. The effects of Sr-doping and annealing temperature on the structure and electric resistivity of the LSCO films and the dielectric properties of the BST films were studied. Results show that the La_0.5Sr_0.5CoO₃ electrode annealed at 750 °C has the lowest electric resistivity, 1.1 × 10⁻³Ω cm. The relative permittivity of the La_0.5Sr_0.5CoO₃-supported BST films first increases and then decreases with Sr-doping. The relative permittivity of the BST film decreases while the dielectric loss increases with frequency. Among the studied BST films, Ba_0.5Sr_0.5TiO₃ has the largest relative permittivity and the smallest dielectric loss (95 and 0.1, respectively) when the frequency is 1 kHz.     

Effects of annealing temperature on the microstructure,ferroelectric and dielectric properties of W-doped Na0.5Bi0.5TiO3 thin films

The effects of annealing temperature on the structure, morphology, ferroelectric and dielectric properties of Na_0.5Bi_0.5Ti_0.99W_0.01O_3+δ (NBTW) thin films are reported in detail. The films are deposited on indium tin oxide/glass substrates by a sol-gel method and the annealing temperature adopted is in the range of 560–620 °C. All the films can be well crystallized into phase-pure perovskite structures and show smooth surfaces without any cracks. Particularly, the NBTW thin film annealed at 600 °C exhibits a relatively large remanent polarization (P_r) of 20 μC/cm² measured at 750 kV/cm. Additionally, it shows a high dielectric constant of 608 and a low dielectric loss of 0.094 as well as a large dielectric tunability of 62%, making NBTW thin film ideal in the room-temperature tunable device applications.