Study of barium strontium titanate (BST) thin films and BST/P (VDF-TrFE) 0-3 composites

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Study of barium strontium titanate (BST) thin films and BST/P (VDF-TrFE) 0-3 composites

 

Author: Adikary, S. U.
Title: Study of barium strontium titanate (BST) thin films and BST/P (VDF-TrFE) 0-3 composites
Degree: Ph.D.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Ferroelectric thin films
Ferroelectric devices
Department: Dept. of Applied Physics
Pages: xxii, 224 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1692719
URI: http://theses.lib.polyu.edu.hk/handle/200/4756
Abstract: Fabrication, characterization and modification of barium strontium titanate (BST) thin films and BST/ poly(vinylidene fluoride-trifluoroethylene) {P(VDF-TrFE)} composites were studied. BST thin films possess high dielectric permittivity, low dielectric loss and field dependent dielectric permittivity which can be used for DRAM capacitor and tunable high frequency applications. BST/P(VDF-TrFE) composites with high dielectric permittivity and high breakdown strength are ideal for capacitor and energy storage applications. Fabrication and characterization of BaxSr1-xTiO3 thin films of compositions x = 0.5, 0.6, 0.7, 0.8, 0.9 and 1 deposited on Pt/Ti/SiO2/Si substrates were studied. A modified sol-gel technique based on the low concentration precursor solution and layer by layer annealing was used for the thin film fabrication. Thin films were fabricated by spin coating on clean Pt/Ti/SiO2/Si substrates. A clear improvement of grain growth and crystallinity in BST thin films were achieved by the use of modified sol-gel method. The crystalline structure, microstructure, dielectric and ferroelectric properties of thin films of different compositions were analysed. Highest relative permittivity and dielectric tunability was observed in the Ba0.7Sr0.3TiO3 thin film. The behaviours of dielectric permittivity and tunability as a function of composition were analysed using Devonshire phenomenological theory. Only four compositions, namely Ba0.7Sr0.3TiO3 Ba0.8Sr0.2TiO3, Ba0.9Sr0.1TiO3 and BaTiO3 showed polarization-electric field (P-E) hysteresis loops at room temperature. BST compositions, which are paraelectric at room temperature, were studied for high frequency dielectric relaxation and tunability. Parallel plate thin film capacitor structure with Au/Cr top electrodes and Pt/Ti/SiO2/Si substrates were subjected to high frequency measurements. The extrinsic effects originated due to the contact resistance and finite sheet resistance of electrodes were modelled using an equivalent circuit. Effective dielectric permittivity of Ba0.5Sr0.5TiO3 and Ba0.6Sr0.4TiO3 bulk ceramics at microwave frequencies were determined by the microstrip ring resonator method. Modifications of the structure and properties of BST thin films were investigated by fabricating thin films with different compositional gradients. Rutherford backscattering spectroscopy (RBS) was used to characterize the compositional gradient of the thin films fabricated on Si and Pt/Ti/SiO2/Si substrates. The graded structure can be considered as a stack of BaxSr1-xTiO3 capacitors connected in series. Therefore, the effects of Curie transition peaks on dielectric properties are removed. Graded thin films with ferroelectric compositions showed good ferroelectric properties with higher remanent polarization values. Ba0.65Sr0.35TiO3 /P(VDF-TrFE) 70/30 0-3 composites with different volume fractions of ceramic powder o were fabricated and characterized. Ba0.65Sr0.35TiO3 powder was fabricated by the sol-gel technique. Composites were fabricated by dispersing fine Ba0.65Sr0.35TiO3 powder in a copolymer matrix and composites with ceramic powder volume fractions o of 0.1, 0.2, 0.3, 0.4 and 0.5 were fabricated. Dielectric properties as a function of o were analyzed using the Bruggeman model and the experimental data agreed quite well with the theoretical calculation. The relative permittivity of P(VDF-TrFE) 70/30 copolymer at room temperature is ~11 and the permittivity increased to ~80 in the composite with o= 0.5. The composites were then irradiated by 3MeV protons at an ambient temperature in a vacuum. Different proton dosages, namely, 40 Mrad, 60 Mrad, 80 Mrad and 100 Mrad were used. Structure and property modifications due to proton irradiation were studied using XRD, DSC, TSDC, dielectric and ferroelectric measurements. The irradiated composites exhibit a broad peak in the permittivity whose peak location shifts upward in temperature with increasing frequency. The values of the parameters obtained by fitting the peak temperature to the Vogel-Fulcher rule imply that the composite has transformed to a relaxor ferroelectric material after irradiation. The highest relative permittivity of ~160 (at 1kHz) was observed at room temperature for composite with o = 0.5, irradiated with a dosage of 80 Mrad. P-E hysteresis loops of composites are strongly depended on the volume fraction of ceramic powder and the effects of irradiation is less apparent in composites with higher volume fraction of ceramic powder.

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