Ba0.6Sr0.4TiO3 Films Research Articles

Ultra-tunable dielectric capacitors enhanced by coupling ferroelectric field effect and semiconductor field effect

Tunable ferroelectric film capacitors play an important role in tunable microwave devices and filter systems due to their high dielectric constant, low loss, and high dielectric tunability. However, there is a trade-off between low loss and high tunability, which limits further enhancement of dielectric performance. Here, we propose an ultra-tunable capacitor by designing a Ba0.7Sr0.3TiO3 (BST)–semiconductor heterostructure. In the tunable capacitor, the BST film is fabricated directly on p-type silicon substrates by magnetron sputtering, and a heterostructure layer is constructed. The coupling effects between the semiconductor depletion layer capacitance and the BST capacitance produce higher capacitance tunability than a traditional sandwich BST capacitor. Based on the coupling effects, a metal–ferroelectric–semiconductor–ferroelectric–metal capacitor is developed, which enables the capacitor to operate under both negative and positive biases, which has an n value (n=Cmax/Cmin) of 90 with 40 V bias voltage and a maximum Q of 1000. The results offer a potential approach to designing high-performance tunable capacitors on silicon with BST film that could build tunable filters for information processing in communication systems.

Influence of oxygen pressure during deposition on the microwave dielectric tunability of Ba0.5Sr0.5TiO3 thin films in PLD process

Thin films of Ba0.5Sr0.5TiO3 (BSTO) were pulsed laser deposited on platinized silicon substrates with varying oxygen pressure from 5 × 10−1 mbar to 5 × 10−6 mbar. A strong correlation between the oxygen partial pressure during the PLD process and the structural and microwave dielectric properties of the PLD-grown BSTO thin films is observed. The structural properties of the PLD-grown BSTO films were analyzed by XRD, Raman spectroscopy, FTIR spectroscopy, and XPS studies, and it is observed that oxygen vacancies are formed in low oxygen pressure deposited films. Dielectric studies at microwave frequencies show that high oxygen pressure deposited BSTO films show good microwave dielectric properties and crystallinity. At 1 GHz frequency, the BSTO film grown at 5 × 10−2 mbar oxygen pressure exhibits a dielectric constant ∼470 and dielectric loss ∼0.15. The oxygen vacancies change the nature of metal-oxide bonding (Ti-O bond), affect the polarizabilities, and, as a result, reduce the dielectric constant. A maximum dielectric tunability of 71 % is observed in microwave frequencies for the BSTO films grown at 5 × 10−2 mbar oxygen pressure.

Enhanced electromagnetic tunability of barium strontium titanate films via coating Pr modified yttrium iron garnet layer

It is hard to simultaneously realize the high dielectric tunability and low loss for the single Ba0.6Sr0.4TiO3 (BST) dielectric film made through chemical method. Garnet structured Y2.80Pr0.20Fe5O12 (YPrIG) exhibits the merits of high saturation magnetization and a much high resistivity, which are helpful for realizing magnetoelectric double tuning and suppressing the dielectric loss of BST film. In this work, Y2.80Pr0.20Fe5O12/Ba0.6Sr0.4TiO3 (YPrIG/BST) composite films were fabricated by sol-gel method. The composite films exhibited a low dielectric loss (0.0087), and got a magnetic tunability of 11.73% at 10 kOe and 1.44 MHz. Compared with BST film, the tunability of YPrIG/BST film was enhanced from 56.25% to 73.24% under the bias electric field of 800 kV/cm. Additionally, YPrIG/BST films exhibited an electromagnetic double adjustability. The electromagnetic tunability of YPrIG/BST composite films was as high as 80.40% at 1.44 MHz, under the DC bias electric field of 800 kV/cm and magnetic field of 10 kOe. This phenomenon can be explained in terms of the superposition effect of electric field and magnetic field.

Wide-range tunable Ba0.7Sr0.3TiO3 films capacitors on LiNbO3 substrates

Ba0.7Sr0.3TiO3 (BST) films on Y-cut LiNbO3 substrates are investigated to realize wide-range tunable BST capacitors. The performances of the BST films are improved by the optimal magnetron sputtering conditions. It is found that the permittivity tunability of the BST capacitor reaches 300% under an applied voltage of 5 V. Moreover, the film has a high dielectric constant 2700 under an applied voltage of 0 V, and the Q value of the BST capacitor exceeds 3000. The results demonstrate that the BST film on the LiNbO3 substrate has potential as a wide-range tunable capacitor for tunable surface acoustic wave filters.

Dielectric property and tunability of multilayer BST-BTO thin film in the terahertz range

The dielectric property and tunability of Ba0.6Sr0.4TiO3 (BST) and BaTiO3 (BTO) multilayer thin film were investigated by using terahertz time-domain spectroscopy (THz-TDS) at room temperature. The film samples used in the study had a same total thickness but different numbers of periodic layer N (a single BTO layer plus a single BST layer) and different layer thickness ratios x (x for total BTO layer thickness to total film thickness; 1-x for total BST layer thickness to total film thickness). We found that the permittivity of the samples declined with the increase of N, which can also be modulated by external optical pumping. The dielectric loss of the sample with N = 1 was much higher than that of other samples, which could be attributed to the interfacial polarization between BST and BTO thin films. In short, the dielectric property of layered BST-BTO composite structure can be appreciably changed by selecting the number of periodical layers and the corresponding layer thickness. These results indicate that the multilayer thin film structure is possible for the applications of tunable photonic devices in the terahertz range.

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 Ba0.6Sr0.4TiO3 (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/Ba0.6Sr0.4TiO3 (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.

High tunability and low loss via establishing an internal electric field in LiFe5O8/Ba0.6Sr0.4TiO3 composite films using chemical solution deposition method

It is hard to simultaneously realize the high dielectric tunability and low loss for the single Ba0.6Sr0.4TiO3 (BST) ferroelectric film or its composites with the linear dielectrics. In this work, LiFe5O8/Ba0.6Sr0.4TiO3 (LFO/BST) composite films were fabricated via chemical solution deposition method, where the internal electric field could be established inside the film owing to the lithium ions’ de-intercalating effect of LiFe5O8 (LFO) layer under the bias field. Thus, we successfully achieved a high dielectric tunability and low loss in the composite films. Owing to the coupling effect in composite films, the dielectric constant remained unchanged at high frequency, while the dielectric loss was reduced to 0.008. An excellent tunability of 71.30% under the bias electric field of −340 kV/cm was obtained in LFO/BST film, which was 41.58% higher than that of BST film. Additionally, the response characteristic of the dielectric constant to the bias field exhibited an asymmetric butterfly curve for LFO/BST composite films. The dielectric tunability in LFO/BST composite films was 19.30% and 71.30% at 1 MHz, under the DC bias field of +340 kV/cm and −340 kV/cm, respectively. These phenomena can be explained in terms of the internal electric field inside the film under the action of the bias field.

Application Of Ba0.5Sr0.5TiO3 (Bst) Film Doped With RuO2 (0%, 2%, 4% and 6%) On A Rice-Stalk Cutting Robot Model Based On A Line Follower With Hc-05 Bluetooth Control

Ba0.5Sr0.5TiO3 (BST) film doped with RuO2 with varying concentrations (0%, 2%, 4%, and 6%) on a p-type (100) silicon substrate has been successfully grown using the Chemical Solution Deposition (CSD) method and spin coating at a rotational speed of 3000 rpm for 30 seconds. The film on the substrate was then heated at 850 oC for 15 hours. BST film+RuO2 variations were characterized for their sensitivity as a light sensor. BST film+RuO2 variations were tested with a varied light source and reflective surface colors. Thin films have a range at the visible light wavelength, so LED lights were used as the light source in the present study. The light source selected was the blue LED because it had a high contrast in differentiating between dark and light-colored surfaces. Various electronic circuits assisted sensitivity characterization with the aim to produce a sensor that is highly sensitive to light. The response of BST film+ RuO2 variations differed according to the RuO2 doping concentration. BST film doped with RuO2 6% had a very good response to changes in light, so this film was applied as the line detector sensor in the line follower-based rice-stalk cutter model with an HC-05 Bluetooth control. Before being integrated with the microcontroller, the output voltage of the BST film was strengthened using an op-amp circuit so that the microcontroller could read the output voltage of the BST film.

Tunable Microwave Device Fabrication on Low‐Temperature Crystallized Ba0.5Sr0.5TiO3 Thin Films by an Alternating Deposition and Laser Annealing Process

AbstractThis article demonstrates a method to fabricate crystalline Ba0.5Sr0.5TiO3 (BST) thin films at a lower temperature of ≈300 °C using an excimer laser by an alternating depositing and annealing process. Firstly, a BST thin film with a thickness of ≈120 nm is deposited at 300 °C (laser energy of ≈2 J cm−2) and subsequently laser annealed (66 mJ cm−2) at 300 °C. This process is repeated five times to obtain thicker device quality films. XRD patterns, TEM, Raman, and UV–Vis–NIR spectroscopy confirm that phase formation of BST thin films has taken place. The band edge value for BST thin films is observed to decrease systematically from 4.65 eV (amorphous, 300 °C) to 3.56 eV (5‐layers, crystalline, 300 °C). A varactor device with a Circular Patch Capacitor (CPC) structure is patterned on the five‐stage laser annealed BST thin films processed at 300 °C, which shows a microwave tunability of 34% at 1 GHz compared to conventionally deposited BST films (prepared at 700 °C). This study provides a way for fabricating ferroelectric thin film based tunable devices at low temperatures, making the process compatible with low melting substrates in flexible electronics and other situations where there is a temperature constraint.

Matching conflicting oxidation conditions and strain accommodation in perovskite epitaxial thin-film ferroelectric varactors

Perovskite oxide materials of the general chemical formula ABO3 are a rich playground for epitaxial stacks of different functional layers for novel device applications. In the example of a tunable metal–insulator–metal ferroelectric varactor (tunable capacitor) made from the highest conducting perovskite SrMoO3 as an electrode and the tunable dielectric Ba0.5Sr0.5TiO3 (BST), we show how the extremely conflicting oxidation potentials can be conciliated in a fully functional heterostructure. Controlling the growth kinetics by the substrate temperature, oxygen pressure, and oxidation time, the formation of the non-conducting Mo6+ states can be effectively suppressed and the BST cation stoichiometry can be tuned. The cumulative impact of the cation nonstoichiometry, epitaxial strain, and oxygen deficiency in the BST films leads to the expansion of their c-axis lattice parameter via the formation of point defects. The dielectric permittivity of 440, the high tunability of 3.5, and the quality factor of 50 are achieved for the varactors at the frequency of 1 GHz. It turns out that the varactor performance is anti-correlated to the tetragonal lattice distortion of BST, which itself is interrelated to the oxidation conditions. The mechanism of the leakage current through oxygen deficient BST layers of the varactors is analyzed within the combined scenarios of the space-charge limited current and Poole–Frenkel field-assisted emission from traps. The achieved high capacitance per unit area of 0.04 pF/μm2 and moderate leakage currents of 0.025 μA/pF make these varactors suitable for applications in microwave microelectronic devices.

Study on the Flexoelectricity of BST Film Multilayer Structure

Single layer and bilayer Y3+-doped Ba0.65Sr0.35TiO3 (BST) thin-film cantilever beam, of which the single BST film thickness is 500 nm, was prepared on a composite silicon substrate. The flexoelectric coefficients of both types of BSTs were measured using a low frequency vibration method. The flexoelectric coefficient of the bilayer BST film multilayer structure is 1.66 μC/m, which doubles the effective flexoelectric coefficient comparing to the single layer BST. In addition, we discussed the effective piezoelectric coefficient of the BST film multilayer structure, which can reach as much as 2.08×10-10C/N due to the scaling effect.

Application of Ba0.5Sr0.5TiO3 (Bst) Film Doped with 0%, 2%, 4% and 6% Concentrations of RuO2 as an Arduino Nano-Based Bad Breath Sensor

Ba0.5Sr0.5TiO3 (BST) film doped with variations in RuO2 concentration (0%, 2%, 4%, and 6%) has been successfully grown on a type-p silicon substrate (100) using the chemical solution deposition (CSD) method and spin-coating at a speed of 3000 rpm for 30 s. The film on the substrate was then heated at 850 °C for 15 h. The sensitivity of BST film + RuO2 variations as a gas sensor were characterized. The sensitivity characterization was assisted by various electronic circuitry with the purpose of producing a sensor that is very sensitive to gas. The responses from the BST film + RuO2 variation were varied, depending on the concentration of the RuO2 dope. BST film doped with 6% RuO2 had a very good response to halitosis gases; therefore, this film was applied as the Arduino-Nano-based bad-breath detecting sensor. Before it was integrated with the microcontroller, the voltage output of the BST film was amplified using an op-amp circuit to make the voltage output from the BST film readable to the microcontroller. The changes in the voltage response were then shown on the prototype display. If the voltage output was ≤12.9 mV, the display would read “bad breath”. If the voltage output >42.1 mV, the display would read “fragrant”. If 12.9 mV < voltage output ≤ 42.1 mV, the display would read “normal”.

Investigation of Ba0.6Sr0.4TiO3 thick films by means of a novel THz-TDS approach

Ba0.6Sr0.4TiO3 (BST) ferroelectric thick films were grown on MgO(0 0 1) and Al2O3(0 0 0 1) single-crystal substrates by using a pulsed laser deposition method. Structural, morphological, optical, and terahertz characterization of the BST films were performed by X-ray Diffraction, Atomic Force Microscopy, Spectroscopic Ellipsometry (SE), and Terahertz Time-Domain Spectroscopy (THz-TDS). Single-phase samples with strong preferred (1 1 1) orientation and surface roughness lower than 1.5% of their thicknesses have been obtained for both types of substrates. SE was employed to extract the thickness and optical properties by using a 3-layer optical model (substrate/thin film/roughness). The inferred refractive index @630 nm is around 2.05, while the optical interference is visible until 3.3 eV. The THz-TDS measurements in transmission set-up were carried out one after the other on substrates before and after the BST film deposition. The standard THz-TDS analysis of double-layer samples proved difficult to complete in the cases in which a thin or thick film is deposited on a much thicker substrate of known dielectric properties. However, we have been able to extract the complex dielectric permittivity in the THz domain for BST samples with thicknesses of few microns, by developing a specific procedure of data analysis.

Effect of post-annealing on the chemical state and crystalline structure of PLD Ba0.5Sr0.5TiO3 films analyzed by combined synchrotron X-ray diffraction and X-ray photoelectron spectroscopy

A Ba0.5Sr0.5TiO3 (BSTO) thin film was grown on a MgO (100) substrate by pulsed laser deposition. The interrelation between the results derived from X-ray photoelectron spectroscopy (XPS) and 2D-reciprocal space maps (2D RSMs) recorded in high resolution diffraction produce complementary information about the chemical state and about the crystalline structure. Both methods confirm the presence of two BSTO1 and BSTO2 phases in the as-deposited and in the post-annealed states, having two chemical environments, two out-of-plane lattice parameters c1 and c2 and two tetragonal distortion ratios D1 and D2. Using the advantages of a combined study, the BSTO1 phase corresponds to the perovskite phase while the BSTO2 phase belongs to a BSTO phase having high tetragonal distortion with respect to the perovskite BSTO1 phase.The post-annealing has induced a transformation from BSTO1 phase to the BSTO2 phase in the surface layer and in the bulk of the BSTO film in addition to the appearance of a further crystalline phase corresponding to a third chemical state. This represents an important and a novel finding in the understanding of the influence of the post-thermal treatment on the structure and the chemical states of the BSTO film which could be accordingly monitored by XRD and XPS. In fact, upon post-annealing, a proportion of BSTO1 perovskite phase decreases while a strained BSTO2 phase increases and a newly formed BaTiO3-like phase appears.

Microstructure and optical properties of Ba0.6Sr0.4TiO3 thin films prepared by pulsed laser deposition

Growth aspects of barium strontium titanate, Ba0.6Sr0.4TiO3 (BST) thin films grown on Si (100) and quartz substrates by pulsed laser deposition technique were investigated. The main objective of the work was to study the effect of substrate temperature and oxygen partial pressure on the microstructure and optical properties of the BST films. X-ray diffraction results revealed that the thin films were amorphous when the substrate temperature was up to 673 K and the crystallinity was found to be improved at the substrate temperature of 873 K. There was an increase in the lattice parameter value from 3.97 to 4.01 Å with an increase in the oxygen partial pressure. Scanning electron microscopy revealed smooth and uniform surfaces, while atomic force microscopy revealed that the particle size increased with decreasing oxygen partial pressure. UV-visible spectroscopy showed that the band gap of the BST films deposited on quartz substrate decreased from 4.41 to 3.73 eV as deposition temperature increased from 303 K to 873 K.

Influence of Y and Mn alternately doped order on the microstructures and dielectric properties of Ba0.6Sr0.4TiO3 films

ABSTRACTThe influences of Y and Mn alternately doped order on the microstructures and dielectric properties of the Ba0.6Sr0.4TiO3 (BST) films were reported in this paper. The Y and Mn alternately doped BST films were designed as YBST/MnBST/… and MnBST/YBST…multilayer films orderly expressed as (Y/Mn)M and (Mn/Y)M for short, and prepared on Pt/Ti/SiO2/Si wafers by an improved sol-gel method, where Y and Mn represent yttrium doped BST layer and manganese doped BST layer, and M is cycle unit, respectively. The microstructures of the alternately doped BST films were observed by SEM and the capacitance-voltage curves at 100 kHz or 1 MHz were measured by a HP4284A LCR meter and the dielectric properties in the range of 1GHz were measured by an E4991A impedance analyzer. Compared to Y or Mn doped multilayer BST film, (Y/Mn)M and (Mn/Y)M show higher dielectric tunability and lower dielectric loss with higher dielectric constant. Moreover, (Y/Mn)M show better dielectric properties than (Mn/Y)M because (Y/Mn)M show granular microstructures independent of M, while the (Mn/Y)M show granular microstructures when M is 2 and add to a surface layer of columnar microstructures on the granular microstructures when M is 4. The related mechanisms were obtained in terms of the XRD phase structures, the cross-sectional SEM microstructures and the AFM morphologies.

Experimental study of effect of strain on electrocaloric effect in (001)-epitaxial (Ba,Sr)TiO3 thin films

The effect of strain on the electrocaloric effect in (001)-epitaxial (Ba,Sr)TiO3 (BST) thin films was experimentally investigated via indirect estimation by measuring the electric field dependence of polarization at various temperatures. Compressively strained (Ba0.3Sr0.7)TiO3 films were deposited on (001)cSrRuO3/(001)SrTiO3 and (001)cSrRuO3/(001)(La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT) by RF magnetron sputtering. The BST film on (001)cSrRuO3/(001)SrTiO3 showed a higher ferroelectric-to-paraelectric phase transition temperature than that on (001)cSrRuO3/(001)LSAT owing to the larger compressive strain. It was revealed that the electrocaloric effect is maximized at around the ferroelectric-to-paraelectric phase transition temperature modified by the compressive strain in the films.

High energy photon induced Fermi-level shift of Ba0.5Sr0.5TiO3 thin films

We have studied surface chemical states of Ba0.5Sr0.5TiO3 (BST) thin films as a function of high energy photon doses. BST thin films were deposited on Si substrates by Sputtering technique and post irradiations were carried out with high energy 60Co gamma radiation. Core-level and Fermi-level spectra were measured by X-ray photoelectron spectroscopy. The gamma-ray irradiated BST films showed a higher binding energy shift of Ba, Sr, and Ti core level with increasing gamma doses due to shift in Fermi level. The Fermi level is shifted towards conduction band by ~1.1eV for 200kGy gamma irradiated BST with respect to pristine. An increase in full width at half maximum of X-ray diffraction peak and surface roughness were also observed with increasing gamma doses. Higher leakage current and decrease in capacitance with gamma doses are further evidence of higher carrier concentration which is consistent with the shift in Fermi level.

Giant Enhancement of Polarization and Strong Improvement of Retention in Epitaxial Ba0.6Sr0.4TiO3‐Based Nanocomposites

In Ba0.6Sr0.4TiO3 (BSTO)‐based epitaxial nanocomposite films increased P r values are demonstrated by up to a factor of 3 compared to standard BSTO films. A strongly reduced temperature coefficient of polarization retention is also obtained, i.e., 0.07% °C−1 compared to 0.24% °C−1. Piezopoling with only marginal leakage current is also achieved up to 200 °C, the highest temperature studied. The origin of the improved performance is the incorporation of Sm2O3 nanopillars in the films which acted as stiff vertical nanoscaffolds, inducing a strong tetragonal distortion in the BSTO (up to 1.033(7) in terms of the out‐of‐plane/in‐plane lattice dimensions). The films have comparable performance to industry‐standard Pb(Zr,Ti)O3 films, at the same time as being Pb‐free.

Elasticity study of textured barium strontium titanate thin films by X-ray diffraction and laser acoustic waves

Ba0.65Sr0.35TiO3 (BST) thin films of 300 nm were deposited on Pt(111)/TiO2/SiO2/Si(001) substrates by radio frequency magnetron sputtering. Two thin films with different (111) and (001) fiber textures were prepared. X-ray diffraction was applied to measure texture. The raw pole figure data were further processed using the MTEX quantitative texture analysis software for plotting pole figures and calculating elastic constants and Young’s modulus from the orientation distribution function (ODF) for each type of textured fiber. The calculated elastic constants were used in the theoretical studies of surface acoustics waves (SAW) propagating in two types of multilayered BST systems. Theoretical dispersion curves were plotted by the application of the ordinary differential equation (ODE) and the stiffness matrix methods (SMM). A laser acoustic waves (LAW) technique was applied to generate surface acoustic waves (SAW) propagating in the BST films, and from a recursive process, the effective Young’s modulus are determined for the two samples. These methods are used to extract and compare elastic properties of two types of BST films, and quantify the influence of texture on the direction-dependent Young’s modulus.