Dielectric Tunability Research Articles

Doping effect on nanoscopic and macroscopic electrical properties of Barium Zirconate Titanate thin films

Lead-free Barium Zirconate Titanate thin films synthesized by sol-gel process were deposited on platinized silicon substrates. Atomic and piezoresponse force microscopies analysis have revealed low values for the surface roughness and local piezoelectric loops, respectively. Nanoscopic piezoelectric and ferroelectric properties decrease with zirconium content increasing. Dielectric measurements were made up to 1 MHz at room temperature. The highest dielectric permittivity and tunability are obtained for 10% of zirconium. Dielectric loss tangent decreases with frequency, zirconium content and electric field increasing. Thin films with 20% of zirconium could be particularly interesting for electronic applications.

Impact of electrode materials on microstructure, leakage current and dielectric tunable properties of lead-free BSZT thin films

Lead-free ferroelectric sol-gel thin films derived from barium strontium titanate (Ba0.85Sr0.15Zr0.1Ti0.9O3, BSZT) were deposited onto two types of electrode: (i) noble metallic Pt- and (ii) conductive oxides LaNiO3- and SrRuO3-coated silicon substrates. The present studies demonstrate that electrode materials impact significantly on morphology, crystallographic orientation, lattice mismatch-induced microstrains and electrical properties of BSZT thin films. Highly (100)-textured BSZT thin films with the highest crystallinity and lowest microstrain were found on conductive oxide electrodes, whereas Pt electrodes showed polycrystalline. Capacitors with oxide LaNiO3 electrode display predominated Schottky emission with a rather low leakage current density ~4.68 × 10−8 A/cm2, while Pt electrode capacitors indicate space-charge limited conduction. Dielectric characterizations in the frequency range of 103–5 × 106 Hz and the dc electric field up to ±800 kV/cm show that the permittivity, loss tangent, and tunability of BSZT films also strongly depend on the electrode materials used. The optimal dielectric tunability with a large figure of merit (FOM ≈ 22.53) and low dielectric loss (tanδ ≈ 2.9%), was found to be 66% for BSZT thin films on the conductive LaNiO3 electrode, suggesting that the as-deposited ferroelectric films are promising candidates for applications in tunable microwave elements and related electronic devices.

Achieving a high dielectric tunability in strain-engineered tetragonal K0.5Na0.5NbO3 films

Using a modified Landau-Devonshire type thermodynamic potential, we show that dielectric tunability η of a tetragonal ferroelectric film can be analytically solved. At a given electric field E, η is a function of the remnant polarization (P_0^f) and the small-field relative dielectric permittivity (chi _0^f), which are commonly measured material properties. After a survey of materials, a large η~80% is predicted to be achievable in a (001)-oriented tetragonal (K0.5,Na0.5)NbO3 film. This strain-stabilized tetragonal phase is verified by density functional theory (DFT) calculations. (K0.5,Na0.5)NbO3 films based on this design were successfully prepared via a sputtering deposition process on SrRuO3-buffered (100)SrTiO3 substrates. The resulted epitaxial films showed a sizable P_0^f (~0.21C m−2) and a large chi _0^f (~830–860), as well as a large η close to the theoretical value. The measured dielectric tunabilities as functions of E are well described by the theoretical η(E) curves, validating our integrated approach rooted in a theoretical understanding.

Formulation and Characterization of Sinterless Barium Strontium Titanate (BST) Dielectric Nanoparticle Ink for Printed RF and Microwave Applications

Here, we report a previously unreported low-temperature curable barium strontium titanate (BaXSr1−XTiO3) or BST dielectric nanoparticle ink which shows a high dielectric tunability for printed electronics/additive manufacturing applications. The newly formulated BST ink is optimized to print in aerosol jet printers and can be cured at 150°C, which will allow the fabrication of tunable radio-frequency (RF) and microwave (MW) devices on a wide range of flexible substrates. Characterization of high-frequency dielectric properties showed a high dielectric tunability (~ 15% at 10 GHz with 10 V/µm) and a high dielectric constant (~ 16 at 10 GHz). The linear-reversible tunability, which is very important for tunable devices, was confirmed by the tunability testing at 10 GHz. Characterization of temperature-dependent dielectric properties found < 10% variations of the dielectric constant at 10 GHz from −50°C to 125°C for this BST ink. Detailed information on BST nanoparticle characterization, ink formulation and characterization of dielectric properties is discussed.

Bismuth layer‐structured ferroelectrics with non‐sheet‐like polyhedral microstructures

AbstractBismuth layer‐structured ferroelectrics (BLSFs) are promising candidates for a variety of applications because of their high Curie temperature and environmentally benign nature as compared to their lead contained counterparts. Featured by the mica‐like grain growth habit and spontaneous polarization along crystallographic a‐b plane, it is difficult to obtain excellent ferroelectric properties for most of the BLSFs fabricated by the conventional ceramic route. Herein, a new approach is reported to obtain Bi2WO6 and Bi4Ti3O12 BLSF ceramics with non‐sheet‐like polyhedral microstructures. The intriguing ferroelectric hysteresis behavior, dielectric tunability, and domain structures are successfully observed. The Bi2WO6 ceramic with a unique single‐domain structure in each grain shows a typical ferroelectric polarization‐electric field hysteresis loop with a remnant polarization of ~17.8 μC cm−2 and a coercive field of ~43 kV cm−1. These values have never been obtained in Bi2WO6 ceramics with sheet‐like microstructure. The Bi4Ti3O12 ceramic with irregular‐shaped and curved tiny domains exhibits an antiferroelectric‐like behavior, which is significantly different from the reported Bi4Ti3O12 counterparts that have plate‐like anisotropic grains. The present study would stimulate microstructure design and property modulation for other layer‐structured materials.

A-site substitution induced diffuse phase transition for high tunability in Ba1-xCaxTi0.85Sn0.15O3 thin films with a low bias electric field

We fabricate BaTi0.85Sn0.15O3 thin films with various Ca doping contents on Pt(100)/Ti/SiO2/Si substrates by a sol-gel and spin-coating method to investigate the effects on dielectric tunable properties. The results show that the greater tunability under a low bias electric field can be achieved compared with undoped BaTi0.85Sn0.15O3 thin films. In particular, the 0.1 mol. % Ca-doped BaTi0.85Sn0.15O3 thin films achieve the highest tunability of 50.9% and the highest FOM value of 23.1 at 16 kV/mm, surpassing most values reported previously for the dielectric tunability of BTS thin films. The use of a low bias electric field avoids the need for a high voltage, thus reducing safety hazards. These outstanding properties are attributed to the broadening of the Curie peak near room temperature, which indicates the existence of diffuse ferroelectric phase transition behavior, a larger grain size, and the suppression of electron hopping between Sn2+ and Sn4+. The results show the great potential of Ca-doped BaTi0.85Sn0.15O3 thin films for dielectric tunable applications.

Microstructure and bidirectional dielectric tunability behaviour of Nd3+-doped KSr2Nb5O15 lead-free ceramics

The luminescence modulation behaviour under the in-situ electric field of rare-earth doped KSr2Nb5O15 ceramics opened a new door for the development of dielectric materials. Where the understanding the effect of rare-earth doping on the electric properties of host, especially at the similar doping concentration with luminescence researches (low concentrations) is very important for the exploration of mechanism of electric-luminescent coupling effect. In this work, Nd3+-doped KSr2Nb5O15 (KSN-xNd) ceramics were synthesized, and the electric properties were investigated systematically. Our results suggest that the Nd3+ doping slightly increased the phase transition temperatures and improved the piezoelectric response of KSr2Nb5O15. Most importantly, a bidirectional dielectric tunability is revealed in KSr2Nb5O15. The dielectric permittivity can be adjusted by the DC electric bias, with tunability ranging from −12.3% to 21.9%. The related mechanism and relationship between the bidirectional dielectric tunability and ferroelectricity are revealed by temperature-dependent dielectric and ferroelectric characterization. The researches of electric properties and bidirectional dielectric tunability of KSN-based ceramics paved the way to further exploration of electric-luminescent coupling mechanism.

Phase Manipulating toward Molybdenum Disulfide for Optimizing Electromagnetic Wave Absorbing in Gigahertz

AbstractMolybdenum disulfide (MoS2) has been proved to be a potential electromagnetic wave (EMW) absorber. However, the limited EMW attenuation mechanisms and conductivity have always been recognized as the major challenges impeding their further developments. In this study, a new dielectric tuning strategy giving rise to high EMW attenuation performance by manipulating phase content (with 0, 24, 50, and 100 wt% 1T phase) toward MoS2 is demonstrated. The greatly introduced 2H/1T interfaces facilitate the dipole distribution dynamics, and the metal‐semiconductor mixed phase enhances the electron transfer ability. Benefiting from the structural merits, the MoS2 with 50 wt% 1T absorber delivers the maximum reflection loss of −45.5 dB and effective absorbing bandwidth of ≈3.89 GHz, corresponding to nearly ten times higher than that of pure 2H counterpart. Moreover, the Computer Simulation Technology (CST) simulation and Lorentz transmission electron microscope are performed to visualize the structural advantages of MoS2 absorbers with mixed 2H/1T phases. By manipulating the phase compositions, this study provides a deep understanding and opens an avenue in developing efficient and high performance transition metal dichalcogenides (e.g., WS2, MoSe2, and WSe2) absorbers.

Anisotropic in-plane dielectric and ferroelectric properties of tensile-strained BaTiO3 films with three different crystallographic orientations

Ferroelectric properties of films can be tailored by strain engineering, but a wider space for property engineering can be opened by including crystal anisotropy. Here, we demonstrate a huge anisotropy in the dielectric and ferroelectric properties of BaTiO3 films. Epitaxial BaTiO3 films deposited on (100), (110), and (111) SrTiO3 substrates were fabricated by chemical solution deposition. The films were tensile-strained due to thermal strain confirmed by the enhanced Curie temperature. A massive anisotropy in the dielectric constant, dielectric tunability, and ferroelectric hysteresis loops was observed depending on the in-plane direction probed and the orientation of the films. The anisotropy was low for (111) BaTiO3, while the anisotropy was particularly strong for (110) BaTiO3, reflecting the low in-plane rotational symmetry. The anisotropy also manifested at the level of the ferroelectric domain patterns in the films, providing a microscopic explanation for the macroscopic response. This study demonstrates that the properties of ferroelectric films can be tailored not only by strain but also by crystal orientation. This is particularly interesting for multilayer stacks where the strain state is defined by the boundary conditions. We propose that other materials can be engineered in a similar manner by utilizing crystal anisotropy.

The other model antiferroelectric: PbHfO3 thin films from ALD precursors

Antiferroelectric PbHfO3 is grown from atomic layer deposition precursors lead bis(dimethylaminomethylpropanolate) and tetrakis dimethylamino hafnium with H2O and O3 oxidizers in thicknesses from 20 nm to 200 nm at a substrate temperature of 250 °C. X-ray analysis shows an as-grown crystalline PbO phase that diffuses into an amorphous HfO2 matrix upon annealing to form a randomly oriented, orthorhombic PbHfO3 thin film. Electrical characterization reveals characteristic double hysteresis loops with maximum polarizations of around 30 µC/cm2 and transition fields of 350 kV/cm–500 kV/cm depending on the thickness. Temperature-dependent permittivity and polarization testing show a phase transition at 185 °C, most probably to the paraelectric phase, but give no clear evidence for the intermediate phase known from bulk PbHfO3. The energy storage density for the films reaches 16 J/cm3 at 2 MV/cm. A dielectric tunability of 221% is available within 1 V for the thinnest film. These results highlight the unique spectrum of properties available for thin film perovskite antiferroelectrics.

Lead-free ferroelectric barium titanate -based thin film for tunable microwave device application

Lead-free ferroelectric barium titanate-based (Ba0.85Sr0.15Zr0.1Ti0.9O3, BSZT) thin films have been successfully deposited on Pt- coated silicon substrates by a spin-coating solgel method. Microstructure and dielectric respones of the Pt/BSZT/Pt thin film capacitors were investigated in detail. X-ray diffraction analysis results indicated that BSZT thin films were well crystallized in tetragonal perovskite structure with a random crystal orientation at rather low temperature of 650 °C. Dielectric constant and dielectric loss of the thin film capacitors were performed at various bias voltage ranges and frequency from 1kHz up to 5MHz. Hysteresis C-V curves with two maxima confirmed the ferroelectric nature in the film. The BSZT capacitors provide excellent dielectric tunability ∼ 42- 69% and high figure-of-merit (FOM) ∼ 4-17 at different bias voltages.

A facile control of epitaxial-like barium strontium titanate thin films with various Ba/Sr ratios synthesized by a hydrothermal-galvanic couple method

Barium strontium titanate (BaxSr1-xTiO3; BST) thin films exhibiting superior dielectric tunability have many technological applications. BST thin films with various Ba/Sr ratios were facilely produced on TiN-coated Si substrates by a hydrothermal-galvanic couple (HT-GC) method in mixed Ba(CH3COO)2 and Sr(CH3COO)2 alkaline solutions. A cubic single phase of BST films with the perovskite structure was obtained. Epitaxial-like growth in that strongly preferred (111) BST films grew over (111)-oriented TiN underlayer was observed for all the obtained films. When increasing the mole fraction of Ba2+ in the reaction solutions, the (111) diffraction peak of BST films shifted toward lower Bragg's angles, indicating a transition from Sr-rich to Ba-rich BST. From the X-ray photoelectron spectroscopy and electron probe for microanalysis, BST films with a full range of Ba/Sr ratios were obtained. The Sr ions were more readily incorporated into the films than Ba ions, which is associated with the lower formation energy of SrTiO3. Field-emission scanning electron microscopy revealed a spherical-particulate microstructure of the BST films. The formation mechanisms of the films by such a method were also discussed. The obtained BST films exhibited a promising dielectric tunability that could reach as high as 92%.

Flexible capacitive sensor based on 2D-titanium dioxide nanosheets/bacterial cellulose composite film

In this paper, titanium dioxide nanosheets (Ti0.91O2 NSs) were incorporated into bacterial cellulose (BC) film for dielectric property tuning while maintaining the flexibility of the resulting composite paper. By taking advantage of the improved dielectric constant, the nanosheets/BC composites were employed as capacitive sensors. The fabricated devices showed the highest sensing performance of ∼2.44 × 10−3 kPa−1 from 0 to 30 N when incorporating as little as 3 vol% of Ti0.91O2 NSs (or ∼2 wt% Ti). Stable operation and high robustness of the sensor were demonstrated, where simple human motions could be efficiently monitored. This study provided a route for preparing flexible and low-cost BC composite paper for capacitive sensor. The strategy for enhancing the dielectric properties as well as sensing performances of the BC demonstrated herein will be essential for the future development of biocompatible, low-cost, and eco-friendly wearable electronics.

Ti-doping induced antiferroelectric to ferroelectric phase transition and electrical properties in Sm-PbZrO3 thin films

The antiferroelectric (Pb0.985Sm0.01) (Zr1-xTix)O3 (Ti-PSZO) thin films were synthesized on Pt(111)/Ti/SiO2/Si substrates using a chemical solution deposition method. The films were crystallized in the perovskite phase with a preferential orientation along (111) direction. With Ti doping in PSZO, a gradual transformation from antiferroelectric to ferroelectric phase transition was noticed at room temperature owing to the Ti doping induced lattice distortion. The phase transition has been confirmed through the P - E hysteresis loops, X-ray diffraction (peak shifting), capacitance-voltage measurements, and Raman scattering analysis. The thin film with Ti = 0.15 doping displayed a ferroelectric behavior with high dielectric constant and large dielectric tunability of about 62%. Also, Ti doping altered the Curie temperature (Tc) and enhanced the order of dielectric diffuseness. It is believed that Ti-doping in PSZO is an effective way to induce an antiferroelectric - ferroelectric phase transition and to tailor the electrical characteristics of PSZO thin films.

Effects of Mn and Y co-doping on the energy storage properties of Ba0.67Sr0.33TiO3 films

Enhanced energy storage properties were observed in Ba0.67−xYxSr0.33Ti0.993Mn0.007O3 (BSTM-xY, x = 0, 0.003, 0.006 and 0.012) films prepared by radio frequency magnetron sputtering technique. All film samples present polycrystalline with perovskite structures, and compact microstructures with homogeneous grain size distribution were observed. The enhanced energy storage properties are attributed to the “slimmer” hysteresis loops and increased maximum polarization as the introduction of Y3+ ion. The energy storage density (W) and recoverable energy storage density (Wre) increase with increasing Y3+ contents. A large Wre ~ 17.8 J/cm3 accompanied by stable energy efficiency (η ~ 77%) for BSTM-0.012Y film was observed. The dielectric properties, tunability and leakage current density are enhanced by optimizing Y3+ doping contents.

Enhanced recoverable energy storage density of barium strontium titanate-based thin films with compositionally graded structure

The compositionally graded structure thin films have attracted much attention due to its high dielectric constant and tunability, which show a great potential in energy storage field. Here, Ba1−xSrxTiO3 (BST)-based compositional thin films with graded structure are deposited onto Pt/Ti/SiO2/Si substrate by sol–gel technique. The formation of such a novel-graded structure is favorable for obtaining a relatively giant recoverable energy storage density (Wrec) of 33.7 J cm−3 and efficiency (η) of 54.7%, respectively, under the breakdown strength (Eb) of 4150.5 kV cm−1. It indicates that the introduction of interface is effective to improve resistance of penetration under high applied electric field. Additionally, Ba/Sr ratio is also another key factor affecting the microstructure and electrical performance of the BST-based thin film. Excessive Sr2+ doping content may bring in phase change, decreasing reversible polarization according to Rayleigh law. The formation of compositionally graded thin film structure to obtain high energy storage performance will lay far-reaching impact on the sustainable energy and promote the development of BST-based dielectric capacitors.

A wide-frequency range dielectric tuning of BaTiO3 by embedding metal nanocrystals.

Fe nanocrystals (NCs) were embedded into the epitaxial BaTiO3 (BTO) matrix. According to optimized growth processes, a novel nanocomposite system was constructed, which consisted a well epitaxial BTO layer and three-dimensional Fe NCs. Based on this, the different dielectric response in the regions of low temperature-high frequency and low frequency-high temperature were revealed by the contribution of hopping and interfacial polarizations, respectively. With the increased amount of Fe NCs, the obvious enhancement in the low-frequency conductivity, middle frequency capacitance, and high-frequency inductive effect was found. The embedded metal NCs play an important role in tuning the dielectric behaviors and AC conductivity of oxide dielectrics. This significant rectification effect in wide-frequency ranges opens up a new direction for designing embedded nano-capacitors.

Effects of Nb and Mg doping on the dielectric and electromechanical properties of PbZrO[formula omitted] thin films

Antiferroelectric PbZrO3-based thin films are extensively studied for energy storage applications. Indeed, extensive work has been performed on chemical doping to increase dielectric breakdown or (antipolar to polar) AP→P phase transition field, in order to achieve higher energy storage density. However, reduction of the AP→P phase transition field is worth investigating for alternative applications, e.g., high tunability filters or low power actuators. This paper discusses the effects of Nb and Mg co-doping on the functional properties of PbZrO3 thin films (∼200 nm in thickness) processed by chemical solution deposition on platinized Si substrates. Overall, Nb and Mg co-doping stabilizes the ferroelectric phase, significantly reduces the AP→P phase transition field from 461 to 173 kV/cm and increases the low-field dielectric permittivity. At an optimal 2.5% Nb, 1.25% Mg doping concentration, the PbZrO3 films show the highest dielectric tunability of 88.8%, while maintaining a very high strain of 0.74%, and low AP→P phase transition field of 232 kV/cm.

Microstructure and dielectric properties of compositionally graded Ba1-xSrxTiO3 multilayer tunable capacitors

Li2O/B2O3-added Ba1-xSrxTiO3 (B1-xSxT) ceramics, where 0.2 ≤ x ≤ 0.35, were well densified at 920 °C with pure perovskite structure. The dielectric constant, tunability, and figure of merit (FOM) of B1-xSxT ceramics increased with x because of the decreasing Curie temperature (TC). The specimen with x = 0.35, whose TC was close to room temperature, exhibited a large tunability of 27.4 % and FOM of 110 at 10 kV/cm. A compositionally graded multilayer (CGML), which was sintered at 920 °C, was fabricated using B1-xSxT thick films to produce a temperature-stable tunable capacitor, and it evinced a dense microstructure and a continuous interface between the B1-xSxT thick film and the Ag electrode. This CGML capacitor showed a large tunability (51 %) and FOM (150) at 20 kV/cm. It also exhibited stable tunability (17–28 % at 10 kV/cm) at temperatures between 30–90 °C. Therefore, the B1-xSxT CGML capacitor is a suitable candidate for temperature-stable tunable capacitors.

Dielectric Tunability Properties in (110)-Oriented Epitaxial 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 Thin Films Prepared by PLD Method.

Epitaxial 0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3 (BZT-BCT) thin films with single-crystal perovskite structure have been grown by pulsed laser deposition (PLD) on the (110) SrRuO3/SrTiO3 substrates. Temperature-dependent dielectric measurements show obvious characteristics of a diffused phase transition. Typical P-E hysteresis loops with a distinct ferroelectric imprint phenomenon are observed in these BZT-BCT thin films with a remnant polarization of 2.0 μC/cm2 and coercive field of 187 kV/cm. Small leakage currents (<1 × 10−6 A/cm2) are obtained in these thin films under an electrical field of 240 MV/m. These BZT-BCT thin films have shown large dielectric tunability values ranging from 75.8% to 85.7%, under a wide temperature range from 200 K to 330 K and a frequency range between 100 Hz and 100 kHz, which shows their good temperature and frequency stability. Such excellent dielectric tunability properties in these (110)-oriented BZT-BCT thin films promise their great potentials in practical phase shifter applications.