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.     

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.     

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

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

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

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

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

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

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

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

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

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

Structure, dielectric, ferroelectric, and optical properties of (1 − x)Ba(Zr0.2Ti0.8)O3 − x(Ba0.7Ca0.3)TiO3 thin films prepared by sol-gel method

We report high dielectric tunabilities of (1 − x)Ba(Zr_0.2Ti_0.8)O₃ − x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) (x = 0.15, 0.30, 0.40, 0.45, 0.50, and 0.55) thin films prepared by a sol-gel method. The films show a pure perovskite structure with random orientation. They have moderate dielectric constant ranging from 350 to 500 and low dielectric loss near 3.0% at 1 kHz with 0 V bias at room temperature. The dielectric tunability of the BZT-0.55BCT thin films is up to 65% at 400 kV/cm and 100 kHz. The films exhibit a high optical transmission in the range of 420 nm-1500 nm. Their optical band gap energies are about 3.90 eV.     

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

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

Annealing effect on dielectric and leakage current characteristics of Mn-doped Ba0.6Sr0.4TiO3 thin films as gate insulators for low voltage ZnO thin film transistor

We report on the dielectric properties and leakage current characteristics of 3 mol% Mn-doped Ba_0.6Sr_0.4TiO₃ (BST) thin films post-annealed up to 600 °C following room temperature deposition. The suitability of 3 mol% Mn-doped BST films as gate insulators for low voltage ZnO thin film transistors (TFTs) is investigated. The dielectric constant of 3 mol% Mn-doped BST films increased from 24 at in-situ deposition up to 260 at an annealing temperature of 600 °C due to increased crystallinity and the formation of perovskite phase. The measured leakage current density of 3 mol% Mn-doped BST films remained on the order of 5 × 10^− 9 to 10^− 8 A/cm² without further reduction as the annealing temperature increased, thereby demonstrating significant improvement in the leakage current characteristics of in-situ grown Mn-doped BST films as compared to that (5 × 10^− 4 A/cm² at 5 V) of pure BST films. All room temperature processed ZnO-TFTs using a 3 mol% Mn-doped BST gate insulator exhibited a field effect mobility of 1.0 cm²/Vs and low voltage device performance of less than 7 V.     

Interfacial characteristics and dielectric properties of Ba0.65Sr0.35TiO3 thin films

Ba_0.65Sr_0.35TiO₃ (BST) thin films were deposited on Pt/Ti/SiO₂/Si substrates by radio frequency magnetron sputtering technique. X-ray photoelectron spectroscopy (XPS) depth profiling data show that each element component of the BST film possesses a uniform distribution from the outermost surface to subsurface, but obvious Ti-rich is present to BST/Pt interface because Ti⁴⁺ cations are partially reduced to form amorphous oxides such as TiO_x (x < 2). Based on the measurement of XPS valence band spectrum, an energy band diagram in the vicinity of BST/Pt interface is proposed. Dielectric property measurements at 1 MHz reveal that dielectric constant and loss tangent are 323 and 0.0095 with no bias, while 260 and 0.0284 with direct current bias of 25 V; furthermore, tunability and figure of merit are calculated to be 19.51% and 20.54, respectively. The leakage current density through the BST film is about 8.96 × 10^− 7 A/cm² at 1.23 V and lower than 5.66 × 10^− 6 A/cm² at 2.05 V as well as breakdown strength is above 3.01 × 10⁵ V/cm.     

Improved dielectric properties of chemical solution derived Pb0.5Sr0.5TiO3 thin films by a layer-by-layer annealing method

Pb_0.5Sr_0.5TiO₃ thin films were prepared on Pt/TiO₂/SiO₂/Si and LaNiO₃ (LNO)/Si substrates by using chemical solution deposition technique, and a layer-by-layer annealing method was used in an attempt to improve the dielectric properties of the thin films. The structure, dielectric, and ferroelectric properties of the thin films were investigated. Improved dielectric properties of the thin films were clearly confirmed: the dielectric constant and dielectric loss for the films on Pt/TiO₂/SiO₂/Si substrates annealed at 650 °C were 1064 and 0.027, respectively, at 1 kHz, with a dielectric tunability of more than 50%; similarly, the films prepared on LNO/Si substrates, showed a high dielectric constant of 1280 and a low dielectric loss of 0.023, at 1 kHz. P-E hysteresis loop measurements indicated that the remanent polarization and coercive field for the films on Pt/TiO₂/SiO₂/Si substrates annealed at 650 °C were 15.7 μC/cm² and 51 kV/cm, respectively.     

Dependence of dc and ac conduction on the pre-sintering temperature in sol–gel derived Ba0.5Sr0.5TiO3 thin films

The effect of pre-sintering temperature on the activation energies of dc and ac conduction in sol–gel spin-coated Ba_0.5Sr_0.5TiO₃ (BST) thin films is reported in this paper. Layer by layer coating of BST sol on Pt/TiN/SiO₂/Si substrates was done with varying pre-sintering temperatures (drying temperature for individual layers). Dominant mechanism of dc conduction was identified as Schottky process whereas ac conduction was governed by trap controlled hopping process. The activation energy for dc conduction decreased from 0.45 to 0.38 eV with increase in the pre-sintering temperature from 400 to 600 °C; whereas, that for ac conduction increased from 0.17 to 0.21 eV. The results have been correlated with grain growth phenomena and reduction of oxygen vacancies in the films with increase in the pre-sintering temperature.     

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.     

Composition dependent preferential orientation, dielectric and ferroelectric properties of Pb(ZrxTi1 − x)O3 thin films derived by sol-gel process

Pb(Zr_xTi_1 − x)O₃ (x = 0.35, 0.40, 0.60, 0.65) thin films were prepared by sol-gel spin on technique. From the X-ray diffraction analysis, PZT films with Zr-rich compositions (x = 0.60 and 0.65) had (111) preferential orientation and the preferential orientation changed to (100) for Ti-rich compositions (x = 0.35 and 0.40). The dielectric measurements on the above compositions at room temperature showed that the dielectric constant values were higher in Zr-rich compositions compared to Ti-rich compositions. The ferroelectric behavior measured in terms of the remnant polarization (P_r) and coercive field (E_c) up to an applied field of 260 kV/cm depicted that the Zr-rich PZT films with (111) preferential orientation had higher P_r and lower E_c values compared to the Ti-rich PZT films with (100) preferential orientation can be understood from the domain switching mechanism.     

AC Conductivity and Dielectric Relaxation Behavior of Sol-gel BaxSr1-xTiO3 Thin Films

BaxSr1-xTiO3 sol-gel thin films with x=0.5, 0.7 and 0.8 have been fabricated as Al/BST/Pt capacitor. The AC conductivity and dielectric properties over a frequency rang of 10 Hz and 1 MHz have been studied in order to explore the ion dynamics and relaxation mechanisms in the films. The frequency dependent conductivity plots show three regions of conduction processes. Dielectric results show that ε at low frequencies increases as Sr content decreases, whereas at high frequencies, it shows opposite variation, which is attributed to the dipole dynamics. The electric modulus plots reveal the relaxation peaks which are not observed in the ε plots and the contribution of the grains, grain boundaries and electrode to the relaxation mechanisms.     

Microwave dielectric properties of La1−xBix(Mg0.5Sn0.5)O3 ceramics

The La_1−xBi_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. An apparent density of 6.50 g cm⁻³, a dielectric constant (?_r) of 20.2, a quality factor (Q × f) of 58,100 GHz and a temperature coefficient of resonant frequency (τ_f) of −84.2 ppm °C⁻¹ were obtained for La_0.97Bi_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

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.     

Effect of Nb substitution on the structural, dielectric and magnetic properties of multiferroic BiFe1−xNbxO3 ceramics

Polycrystalline BiFe_1−xNb_xO₃ ceramics have been synthesized by standard solid-state reaction method. The effect of Nb substitution on the dielectric, magnetic and magnetoelectric properties of the BiFeO₃ multiferroic perovskite was studied. X-ray diffraction pattern revealed that all the samples with x = 0.00-0.10 showed rhombohedral perovskite structure. We obtained single phase upto doping concentration of x = 0.05 and with further increase in Nb concentration, some impurity peaks appeared. An anomaly in the dielectric constant (?) and dielectric loss (tan (δ)) in the vicinity of the antiferromagnetic Néel temperature (T_N) was observed. Nb substitution reduced the antiferromagnetic Néel temperature (T_N) in BiFe_1−xNb_xO₃. Proper amount of Nb could decrease the dielectric loss. Magnetic hysteresis loops measured at 5 K/300 K and temperature dependent magnetization curves indicated ferromagnetism in Nb substituted BiFeO₃ ceramics. The room temperature magnetic moment was found to increase with increase in Nb concentration. The dependence of dielectric constant on the magnetic field is an evidence of magnetoelectric coupling in BiFe_1−xNb_xO₃ ceramics.     

Effect of process parameters and post deposition annealing on the optical, structural and microwave dielectric properties of RF magnetron sputtered (Ba0.5,Sr0.5)TiO3 thin films

Thin films of (Ba_0.5,Sr_0.5)TiO₃ (BST5) in the thickness range 400-800 nm have been deposited by RF magnetron sputtering on to quartz substrates at ambient temperature. All the properties investigated, i.e. structure, microstructure, optical and microwave dielectric, show a critical dependence on the processing and post processing parameters. The surface morphology as studied by atomic force microscopy reveals ultra fine grains in the case of as deposited films and coarse grain morphology on annealing. The as-deposited films are X-ray amorphous and exhibit refractive index in the range 1.9-2.04 with an optical absorption edge value between 3.8 and 4.2 eV and a maximum dielectric constant of 35 at 12 GHz. The dispersion in refractive index fits into the single effective oscillator model while the variation in the optical parameters with oxygen percentage in the sputtering gas can be explained on the basis of packing fraction changes. On annealing the films at 900 °C they crystallize in to the perovskite structure accompanied by a decrease in optical band gap, increase in refractive index and increase in the microwave dielectric constant. At 12 GHz the highest dielectric constant achieved in the annealed films is 175. It is demonstrated that with increasing oxygen-mixing percentage in the sputtering gas, the microwave dielectric loss decreases while the dielectric constant increases.