The Effects of Annealing Atmosphere on the Electrical Properties of MgNb2O6/ITO Heterostructures

The effects of annealing atmosphere (N₂, air and O₂) on the electrical properties of sol–gel‐derived MgNb₂O₆/ITO heterostructures are discussed in this work. All samples exhibited the amorphous phase and were highly transparent. The percentage of Nb⁴⁺ content increased when the films were annealed in the oxygen‐deficient conditions, which could lead to semiconducting films. In addition, the results show that the electrical properties of sol–gel‐derived MgNb₂O₆ thin films could be tuned based on the annealing atmosphere. Moreover, the conduction mechanisms of MgNb₂O₆/ITO heterostructures are also discussed in this study. The results show that MgNb₂O₆ thin films have potential for use in multifunctional optoelectronic applications, due to their flexible electrical properties and good transparency.     

Sol‐gel derived TiNb2O7 dielectric thin films for transparent electronic applications

To reduce power consumption of transparent oxide‐semiconductor thin film transistors, a gate dielectric material with high dielectric constant and low leakage current density is favorable. According to previous study, the bulk TiNb₂O₇ with outstanding dielectric properties may have an interest in its thin‐film form. The optical, chemical states and surface morphology of sol‐gel derived TiNb₂O₇ (TNO) thin films are investigated the effect of postannealing temperature lower than 500°C, which is crucial to the glass transition temperature. All films possess a transmittance near 80% in the visible region. The existence of non‐lattice oxygen in the TNO film is proposed. The peak area ratio of non‐lattice oxygen plays an important role in the control of leakage current density of MIM capacitors. Also, the capacitance density and dissipation factor were affected by the indium tin oxide (ITO) sheet resistance at high frequencies. The sample after postannealing at 300°C and electrode‐annealing at 150°C possesses a high dielectric constant (>30 at 1 MHz) and a low leakage current density (<1 × 10^?6 A/cm² at 1 V), which makes it a very promising gate dielectric material for transparent oxide‐semiconductor thin film transistors.     

Fabrication of Sr0.5Ba0.5Nb2O6 Nanocrystallite‐Precipitated Transparent Phosphate Glass‐Ceramics

Transparent (Sr_0.5Ba_0.5)Nb₂O₆ (SBN50) nanocrystallite‐precipitated phosphate glass‐ceramics were prepared by a conventional glass‐ceramic process. x(SrO–BaO–2Nb₂O₅) ? (100–4x)P₂O₅ (xSBNP) glasses with a refractive index of 1.9–2.0 exhibited high water resistance owing to the presence of Q⁰ and Q¹ phosphate units. Both bulk and surface crystallization of the SBN50 phase were observed in 20SBNP and 21SBNP glass‐ceramics. Although the nominal content of SBN50 crystals in the 21SBNP glass was larger than that in the 20SBNP glass, the latter exhibited better crystallinity of SBN50 and a higher number density of precipitated SBN nanocrystallites. By tuning the two‐step heat‐treatment and the chemical composition, transparent SBN50‐precipitated glass‐ceramics were successfully obtained. Given that no remarkable increase of the relative dielectric constants was observed after crystallization of the SBN50 nanocrystallites, it is postulated that the relative dielectric constant of the bulk is mainly governed by the amorphous phosphate region, and that the contribution of precipitation of the SBN50 nanocrystallites to the dielectric constant is not very significant in this system.     

Synthesis and Characterization of Eu3+‐Doped Transparent Glass–ceramics Containing Nanocrystalline SrIINbIVO3

The glass–ceramics containing a rarely achievable nanocrystalline Sr^IINb^IVO₃ phase in the 53.75SiO₂–18.25K₂O–9Bi₂O₃–9SrO–9Nb₂O₅–0.5CeO₂–0.5Eu₂O₃ (mol%) glass system were prepared by the melt‐quench technique followed by a two‐stage controlled heat treatment. The unusual oxidation state of Nb in Sr^IINb^IVO₃ crystal is 4+ and upon heat treatment of the samples at lower temperature of 500°C for several hours, the glass composition and chemical environment around Nb ions played a key role for the formation of Sr^IINb^IVO₃ in the glass–ceramics. The microstructure of the glass–ceramics was studied using TEM and FESEM. The TEM images advocate 10–40 nm crystallite size of Sr^IINb^IVO₃. FTIR study confirms that all the samples consist of SiO₄, BiO₃, BiO₆, and NbO₆ structural units. The refractive index at different wavelengths was found to vary in the range 1.7105–1.7905 and increase with increase in heat‐treatment time. The luminescence spectra of Eu³⁺‐doped glass and glass–ceramics were recorded at 465 nm excitation wavelength and the luminescence intensity is found to be increased with heat‐treatment time due to increase in crystallinity. The high intensity ratio of ⁵D₀→⁷F₂ to ⁵D₀→⁷F₁ indicates that the Eu³⁺‐doped nanocrystalline Sr^IINb^IVO₃ glass–ceramics are promising candidate materials as red‐light source.     

Low‐Temperature Sintering and Piezoelectric Properties of CuO‐Added KNbO3 Ceramics

Dielectric and piezoelectric properties of CuO‐added KNbO₃ (KN) ceramics were investigated. The CuO reacted with the Nb₂O₅, formed a CuO–Nb₂O₅‐related liquid phase during the sintering, and assisted the densification of the KN ceramics at low temperatures. Moreover, some of the Cu²⁺ ions replaced the Nb⁵⁺ ions in the matrix and behaved as a hardener. The dielectric and piezoelectric properties of the KN ceramics were considerably influenced by the relative density. The 1.0 mol% CuO‐added KN ceramic sintered at 960°C for 1.0 h, which showed a maximum relative density, exhibited a high phase angle of 86.9°, P_r of 14.8 μC/cm², and E_c of 1.8 kV/mm. This specimen also exhibited good dielectric and piezoelectric properties: ε^T₃₃/ε_o of 364, d₃₃ of 122 pC/N, k_p of 0.29, and Q_m of 611.     

Green Up‐Conversion Photoluminescence and Dielectric Relaxation of Ho3+/Yb3+‐Codoped Bi2Ti2O7 Pyrochlore Thin Films

Ho³⁺/Yb³⁺‐codoped Bi₂Ti₂O₇ pyrochlore thin films were prepared by a chemical solution deposition method, and their visible up‐conversion (UC) photoluminescence and dielectric relaxation were studied. Ho and Yb can be doped into Bi₂Ti₂O₇ lattice and single pyrochlore phase is maintained. Intense visible UC photoluminescence can be observed under the excitation of a 980‐nm diode laser. Two UC emission bands centered at 551 nm and 665 nm in the spectra can be assigned to ⁵F₄, ⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transitions of Ho³⁺ ions, respectively. The dependence of their UC emission intensity on pumping power indicates that both the green and red emissions of the thin films are two‐photon process. In addition, a Stokes near‐infrared emission centered at 1200 nm can be detected, which is due to ⁵I₆→⁵I₈ transition of Ho³⁺ ions. The thin films prepared on indium tin oxide–coated glass substrates exhibit a relatively high dielectric constant and a low dielectric loss as well as a good bias voltage stability. The dielectric relaxation of the thin films was also analyzed based on the temperature‐ and frequency‐dependent dielectric properties. This study suggests that Ho³⁺/Yb³⁺‐codoped Bi₂Ti₂O₇ thin films are promising materials for developing multifunctional optoelectronic thin film devices.     

Wavelength Tailorability of Broadband Near‐Infrared Luminescence in Cr4+‐Activated Transparent Glass‐Ceramics

Four Cr⁴⁺‐activated transparent glass‐ceramics containing different species of silicate nano‐crystals (Zn₂SiO₄, Mg₂SiO₄, Li₂ZnSiO₄, and Li₂MgSiO₄) were successfully prepared. Absorption spectra, photoluminescence spectra, lifetime decay curves, and quantum yield of these transparent glass‐ceramics were measured. According to the crystal field strength of Cr⁴⁺‐incorporated tetrahedral sites, the broadband near‐infrared (NIR) luminescence of Cr⁴⁺ can be tailored from 1130 to 1350 nm and the lifetime of Cr⁴⁺ luminescence can be prolonged from 6 to 100 μs. Quantum yield in the transparent glass‐ceramics containing Li₂ZnSiO₄ nano‐crystals reached at 17%, which is the highest value of NIR luminescence in transition‐metal‐activated glass materials.     

Insulating characteristics of zinc niobium borate glass‐ceramics

Zinc borate glasses with different concentrations of Nb₂O₅ were prepared and later were heat treated for prolonged times. Prepared samples were characterized by XRD, SEM, DSC, IR and optical transmission spectroscopy techniques. Later, dielectric properties viz., dielectric constant, loss tangent, electric modulii, electrical impedance and a.c. conductivity over wide ranges of frequency and temperature, were investigated as a function of Nb₂O₅ concentration. Finally, the dielectric breakdown strength was measured in air medium at ambient temperature. The results of characterization techniques viz., XRD, SEM and DSC indicated that multiple crystal grains (with sizes varying from 0.1 to 1 μm) are dispersed in the residual glass phase. The concentration of crystal grains found to increase with increase in Nb₂O₅ content. The XRD studies have further revealed that the bulk samples are composed of columbite ZnNb₂O₆ crystal phases. Using generalized gradient approximation (GGA) quantitative information on these crystal phases viz., the lattice parameters, optical band gap and band structure were evaluated. The analysis of results of IR spectral studies have indicated that there is an increasing degree of polymerization of glass network with increase in Nb₂O₅ content due to the increased connectivity between various structural groups in the glass network. The optical absorption spectra indicated an increase in optical transmittance of the bulk samples with increase in Nb₂O₅ content. The dielectric parameters are observed to decrease, whereas the dielectric breakdown strength is observed to increase to a large extent due to the crystallization of the glass with the Nb₂O₅. The increase is attributed to the formation of ZnNb₂O₆ crystalline phases that contain intertwined ZnO₆ and NbO₆ structural units. As a whole, zinc borate glasses exhibited a significant increase in the electrical insulating strength due to the crystallization with Nb₂O₅ as the crystallizing agent. Further, the value of dielectric constant is also found to be the optimal with no dispersion with frequency up to 450 K. Overall, the studied glass‐ceramics meet the necessary physical conditions to be used as insulating layers in the display panels and hence may be considered for such applications.     

Electro-optical performances of nanostructured SrTiOx films: The effect of plasma power,Ar/O2 ratio and annealing

The present work evaluates the effects of plasma power and oxygen mixing ratios (OMRs) on structural, morphological, optical, and electrical properties of strontium titanate SrTiO_x (STO) thin films. STO thin films were grown by magnetron sputtering, and later thermal annealing at 700°C for 1 h was applied to improve film properties. X-ray diffraction analysis indicated that as-deposited films have amorphous microstructure independent of deposition conditions. The films deposited at higher OMR values and later annealed also showed amorphous structure while the films deposited at lower OMR value and annealed have nanocrystallinity. In addition, all as-deposited films were highly transparent (~80%–85%) in the visible spectrum and exhibited well-defined main absorption edge, while the annealing improved transparency (90%) within the same spectrum. The calculated direct and indirect optical band gaps for films were in the range of 3.60-4.30 eV as a function of deposition conditions. The refractive index of the films increased with OMRs and the postdeposition annealing. The frequency dependent capacitance measurements at 100 kHz were performed to obtain film dielectric constant values. High dielectric constant values reaching up to 100 were obtained. All STO samples exhibited more than 2.5 μC/cm² charge storage capacity and low dielectric loss (less than 0.07 at 100 kHz). The leakage current density was relatively low (3 × 10⁻⁸Acm⁻² at +0.8 V) indicating that STO films are promising for future dynamic random access memory applications.     

Aqueous Solution–Gel Preparation and Dielectric Properties of Ba(Mg1/3Nb2/3)O3 Thin Films with Long‐Range Order

In the present research, the Ba(Mg_1/3Nb_2/3)O₃ (BMN) thin films with high 1:2 long‐range order (LRO) were prepared by an entire aqueous solution–gel route and a heat treatment at low temperature. The BMN precursor solutions used for spin‐coating consist of tri‐metal ions citrate complexes, which are formed through the combination of three single citrate complexes with NH₄⁺ acting as crosslinks. The metal‐coordinated bonds of the citrate complexes can be easily broken by heat treatment, which can result in the crystallization of BMN films at low annealing temperature. The crystalline structure and LRO degree of BMN films were also investigated. It is shown that the LRO degrees increase with the increase in annealing temperature. The dielectric constant increases and the dielectric loss decreases with the annealing temperature up to 750°C due to the improvement of the film densification and the LRO degree. The further increase in annealing temperature slightly decreases the dielectric constant and increases the dielectric loss due to the appearance of large aggregates. It is shown that the charge carriers mainly contribute to the dielectric response below 100 KHz, while the dipolar response starts to take effect for the frequency higher than 100 KHz.     

Dielectric performance of pyrochlore-type Bi2MgNb2-xTaxO9 ceramics: The effects of tantalum doping

The solid solutions based on the pyrochlore-type system Bi₂MgNb_2-xTa_xO₉ were formed in the compositional range х = 0–2.0 (Bi_1·6Mg_0·8Nb_1.6-tTa_tO_7.2, t = 0–1.6). The Rietveld method was used to refine the structure for Bi₂MgNb_2-xTa_xO₉ (x = 0, 1.0, 2.0). The increasing tantalum content led to the slight decrease in the cubic unit cell parameters from 10.56934 (4) Å for x = 0 and 10.54607 (3) Å for x = 2 (sp.gr. Fd-3m:2). At the same time, tantalum additions suppressed grain growth in the pyrochlore ceramics during sintering and made it possible to obtain materials with an average grain size of 1–2 μm (Bi_1·6Mg_0·8Ta_1·6O_7.2). The increase in the Ta⁵⁺ concentration led to the decrease in the dielectric permeability from 104 (Bi_1·6Mg_0·8Nb_1·6O_7.2) to 20 (Bi_1·6Mg_0·8Ta_1·6O_7.2) at room temperature, while the dielectric loss tangent remained lower than 0.002, which is due to the small grain size and the high porosity of the samples. An increase in temperature has practically no effect on the values of the dielectric permittivity in the entire frequency range. The samples have weak through conductivity. The activation energies of electrical conductivity varied in the range of 0.84–1.00 eV, and the less tantalum, the lower the activation energy. The electrical properties of the samples at 200 Hz to 1 MHz are described by the simplest parallel scheme.     

Effects of element chemical states and grain orientation growth of ITO targets on photoelectric properties of the film

A study was carried out to compare element chemical states and grain orientation growth between two ITO targets, which were respectively sintered at 1560 °C (target A#) and 1600 °C (target B#). The lower sintering temperature can be beneficial to increase mass content ratio of In₂O₃ to SnO₂, reduce the production of Sn²⁺ ions and the component of O-In as well as increase oxygen holes, and can also promote grain orientation growth of In₂O₃ and In₄Sn₃O₁₂ phase. Three groups of ITO films were deposited at 230 °C using these targets to investigate the effects of sputtering parameters on the photoelectric properties of ITO films. Under different sputtering pressures, the sheet resistance for target A# has higher sensitivity to low O₂ flow, while target B# displays higher sensitivity to high O₂ flow. In the case of sputtering pressure of 0.5 Pa, ITO films for target A# displays the highest visible transmittance. In addition, annealing process could decrease the sheet resistance and improve the transmittance of ITO films because of its effect on the crystallinity and surface morphology of ITO films.     

Microwave dielectric properties of bismuth zinc niobate thin films deposited on alumina by pulsed laser deposition

Bi₂Zn_2/3Nb_4/3O₇ thin films were prepared on Al₂O₃ substrates by pulsed laser deposition. The phase compositions and microstructures were characterized by X-ray diffraction and atomic force microscopy. The as-deposited films were all amorphous in nature. All films were crystallized after the post annealing at the temperature range of 700–900 °C for 30 min in air. The texture characteristics change with annealing temperature. A split post dielectric resonator method was used to measure the microwave dielectric performance at the resonant frequencies of 10, 15 and 19 GHz. For the films annealed at 900 °C, the preferential orientation is similar to the monoclinic BZN bulk. The microwave dielectric constants at 10, 15 and 19 GHz are 69.4, 58.9 and 47.9, respectively, which are closer to these of the monoclinic BZN bulk.     

Low-temperature firing and microwave dielectric properties of MgNb2-xVx/2O6-1.25x ceramics

MgNb_2-xV_x/2O_6-1.25x (0.1≤x≤0.6) ceramics with orthorhombic columbite structures were prepared at low-temperature by a solid-phase process. The phase component, microscopic morphology, low-temperature sintering mechanism and microwave dielectric performance of MgNb_2-xV_x/2O_6-1.25x ceramics were comprehensively investigated. Low-temperature sintering densification of dielectric ceramics was achieved via the nonstoichiometric substitution of vanadium (V) at the Nb-site. In contrast to pure MgNb₂O₆ ceramics, the sintering temperature of MgNb_2-xV_x/2O_6-1.25x (x = 0.2) ceramics was reduced by nearly 300 °C owing to the liquid-phase assisted sintering mechanism. The liquid phase arises from the autogenous low-melting-point phase. Meanwhile, MgNb_2-xV_x/2O_6-1.25x (x = 0.2) samples with nonstoichiometric substitution could achieve a more than 900% improvement in the Q × f value, compared with stoichiometrically MgNb_2-xV_xO₆ (x = 0.1, 0.2) ceramics. Finally, MgNb_2-xV_x/2O_6-1.25x dielectric ceramics possess outstanding microwave dielectric properties: ε_r = 20.5, Q × f = 91000, and τ_f = -65 ppm/°C when sintered at 1030 °C for x = 0.2, which provides an alternative material for LTCC technology and an effective approach for low-temperature sintering of Nb-based microwave dielectric ceramics.     

Phase formation and dielectric properties of Ln2(Ln′0.5Nb0.5)2O7 (Ln = rare earth element)

Weberites and pyrochlores (A₂B₂O₇), both fluorite-related superstructures, are attractive dielectric ceramics due to their ability to accommodate diverse cations, thus allowing their properties to be tailored. This study focuses on the fundamental understanding of the structure–dielectric property relationships in fluorite-related oxides. Specifically, Ln₃NbO₇ and Ln₂(Ln′_0.5Nb_0.5)₂O₇ (where the ionic radius of Ln′ is smaller than that of Ln) compounds are investigated. It has been previously shown that weberite-type Ln₃NbO₇ exhibits a composition dependent dielectric relaxation above room temperature. It is here shown that a dielectric relaxation also occurs in La₂(Ln′_0.5Nb_0.5)₂O₇ (Ln′ = Yb³⁺, Er³⁺, and Dy³⁺) compounds near or below ?158 °C. The temperature, at which the maximum permittivity occurs, is different for different compositions (?132 °C for La₂(Yb_0.5Nb_0.5)₂O₇, ?197 °C for La₂(Er_0.5Nb_0.5)₂O₇, and ?187 °C for La₂(Dy_0.5Nb_0.5)₂O₇ at 1 MHz) and is correlated with the distortion of the NbO₆ octahedra. The room temperature dielectric permittivity of all three compounds was measured to be between 40 and 50 at 1 MHz.     

Preparation and electrical properties of Bi2Zn2/3Nb4/3O7 thin films deposited at room temperature for embedded capacitor applications

Bi₂Zn_2/3Nb_4/3O₇ thin films were deposited at room temperature on Pt/Ti/SiO₂/Si(1 0 0) and polymer-based copper clad laminate (CCL) substrates by pulsed laser deposition. Bi₂Zn_2/3Nb_4/3O₇ thin films were deposited in situ with no intentional heating under an oxygen pressure of 4 Pa and then post-annealed at 150 °C for 20 min. It was found that the films are still amorphous in nature, which was confirmed by the XRD analysis. It has been shown that the surface roughness of the substrates has a significant influence on the electrical properties of the dielectric films, especially on the leakage current. Bi₂Zn_2/3Nb_4/3O₇ thin films deposited on Pt/Ti/SiO₂/Si(1 0 0) substrates exhibit superior dielectric characteristics. The dielectric constant and loss tangent are 59.8 and 0.008 at 10 kHz, respectively. Leakage current density is 2.5 × 10^?7 A/cm² at an applied electric field of 400 kV/cm. Bi₂Zn_2/3Nb_4/3O₇ thin films deposited on CCL substrates exhibit the dielectric constant of 60 and loss tangent of 0.018, respectively. Leakage current density is less than 1 × 10^?6 A/cm² at 200 kV/cm.     

Eu2+‐Doped Yellow‐Emitting CsBaB3O6 Glass‐Ceramic Prepared by Melt Quenching and Re‐Crystallization Method

Eu³⁺‐doped cesium barium borate glass with the composition of Cs₂O·2BaO·3B₂O₃ was prepared by the conventional melt quenching method. The glass‐ceramic sample was obtained from the re‐crystallization of the as‐made glass to change the amorphous glass into a crystalline host. This reduces the Eu³⁺ in glass to Eu²⁺ ions resulting in a yellow‐emitting phosphor of Eu²⁺‐activated CsBaB₃O₆. The samples were investigated by the XRD patterns and SEM micrograph, the optical absorption, the photoluminescence spectra, and decay curves. The as‐made glass has only Eu³⁺ centers. Under the excitation of blue or near‐UV light, Eu²⁺‐doped CsBaB₃O₆ presents yellow‐emitting color from the allowed inter‐configurational 4f–5d transition in the Eu²⁺ ions. The maximum absolute luminescence quantum efficiencies of Eu²⁺‐doped CsBaB₃O₆ phosphor was measured to be 47% excited at 430 nm light at 300 K. By taking into account the efficient excitation in blue wavelength region, this new phosphor could be a potential yellow‐emitting phosphor for an application in white light‐emitting diodes fabricated with blue chips.     

A Novel Glass‐Ceramic with Ultra‐Low Sintering Temperature for LTCC Application

In this paper, the phase compositions and the dielectric properties of 3ZnO–2B₂O₃ glass‐ceramic prepared by solid‐state method were investigated. The X‐ray diffraction patterns show that all sintered samples consist of Zn₃B₂O₆ and α‐Zn(BO₂)₂. The dielectric properties changed significantly with the sintering temperature. After sintering at 650°C for 30 min, the glass‐ceramic exhibits optimum dielectric properties: a dielectric constant of 7.5 and a dielectric loss of 0.6 × 10^?3 at 10 MHz. The chemical compatibility with Ag electrode under the co‐fired process illustrates a potential application in low temperature co‐fired ceramic field for the glass‐ceramic.     

Ultra‐Low Temperature Sintering and Dielectric Properties of SiO2‐Filled Glass Composites

Ultra low temperature co‐fired ceramics system based on zinc borate 3ZnO–2B₂O₃ (3Z2B) glass matrix and SiO₂ filler was investigated with regard to the phase composition, the microstructure and the dielectric properties as functions of the filler content and sintering temperature. The softening temperature of 554°C and the crystallization temperature of around 650°C for the glass were confirmed by Differential Thermal Analysis result. The X‐ray diffraction results show that all SiO₂‐filled samples were made up of SiO₂, α‐Zn(BO₂)₂, Zn₃B₂O₆ phases. And there was no chemical reaction between SiO₂ and the glass during densification. And then the dielectric constant decreased with the increasing content of SiO₂. At the level of 15 wt% SiO₂ addition, the composites can be densified at a sintering temperature of 650°C for 30 min, and showed the optimal dielectric properties at 1 MHz with the dielectric constant of 6.1 and the dielectric loss of 1.3 × 10^?3, which demonstrates a good potential for use in LTCC technology.     

Resistive Switching Behaviors of Sol‐Gel‐Derived MgNb2O6 Thin Films on ITO/glass Substrate

In this study, transparent amorphous MgNb₂O₆ (MNO) films were fabricated via the sol‐gel method to form an Al/MNO/indium tin oxide/glass structure. The resistive switching (RS) behavior of the devices was investigated. From the DC voltage sweep test, the air‐annealed MNO samples exhibited stable and reproducible bipolar resistive switching (BRS) behavior; however, the samples annealed in an O₂‐rich environment showed no RS property. These results suggest that the RS behavior of the MNO memory devices is highly related to the oxygen vacancy concentration and distribution within the MNO films. In addition, forming‐free unipolar resistive switching (URS) behavior was observed when the MNO films were annealed under an N₂H₂ atmosphere. In order to determine the origin of the BRS and URS behaviors, cross‐sectional high‐resolution transmission electron microscopy images of the MNO samples were acquired. The RS behavior of the MNO films can be ascribed to the release and recombination of electrons and oxygen vacancies.