K. Sudheendran

Microwave dielectric and Raman scattering studies on bismuth zinc niobate thin films

Bismuth zinc niobate (Bi1.5Zn1.0Nb1.5O7) thin films were deposited on fused silica substrates by pulsed laser deposition. Its phase composition and microstructure were characterized by x-ray diffraction and atomic force microscopy. The as deposited films were amorphous and all of them were crystallized after a postdeposition annealing at 600 °C for 30 min in air. The crystallite size of the films was found to be varying between 31 and 60 nm with respect to the oxygen pressure in the deposition chamber. The films were found to be less oriented with the decreasing crystallite size. The films deposited under different oxygen pressures showed a microwave dielectric permittivity in between 98–124 and a loss tangent in between 0.005 and 0.007 over the X-band frequency range. From the Raman scattering data of the bulk and thin films, it was found that the Raman modes in thin films are relatively more intense and additional Raman modes were appearing in the films in the low frequency range, which reveal that the ...

Effect of the amorphous-to-crystalline transition in Ba0.5Sr0.5TiO3 thin films on optical and microwave dielectric properties

Thin films of Ba0.5Sr0.5TiO3 were deposited at ambient temperature on fused silica substrates by RF magnetron sputtering followed by post-deposition annealing at temperatures between 400 and 1000 °C. The post-deposition annealing resulted in an amorphous–crystalline phase transition in the films. The amorphous–crystalline transition is accompanied by an increase in the refractive index, a decrease in the optical band gap and an increase in the microwave dielectric constant. At 800 °C the films exhibit a refractive index of 2.21 at 550 nm, an optical band gap of 3.49 eV and a microwave dielectric constant of 265, all of which indicate the crystalline nature of the films. The films lose oxygen when annealed at high temperatures as evidenced by the optical and microwave dielectric behaviour. The observed behaviour is explained in terms of defect centres created in the form of oxygen vacancies due to the annealing process and the structural phase transition.

Crystallographic texture, morphology, optical, and microwave dielectric properties of dc magnetron sputtered nanostructured zirconia thin films

Nanocrystalline zirconia thin films have been deposited at ambient temperature by dc magnetron sputtering on glass and quartz substrates. The crystallite size as calculated from the x-ray diffraction patterns in the films varies between 10 and 25nm and is dependent on oxygen percentage in the sputtering gas. Interestingly, the presence of monoclinic and cubic phase is observed for the films deposited on glass at 40%, 60%, and 80% of oxygen in the sputtering gas, while those deposited on quartz showed only the monoclinic phase. Refractive index decreased with increase in percentage of oxygen in the sputter gas. Significantly, even at 100% oxygen in the sputtering gas, films of thickness of the order of 500nm have been grown starting from the metallic Zr target. The dielectric constants were measured using the extended cavity perturbation technique at X-band frequency (8–12GHz). The dielectric constant and loss tangent showed a very small decrease with increase in frequency but exhibited a stronger dependen...

Broadband Microwave Dielectric Properties of BST Thin Films on Quartz Substrates

Thin Films of (Ba 0.5 , Sr 0.5 ) TiO 3 have been deposited on quartz substrates by RF magnetron sputtering. The percentage of oxygen in sputtering atmosphere was varied between 25 to 100%. The as-deposited films were x-ray amorphous and crystallized on post deposition annealing at 900°C for 1 min. Microstructure as determined by Atomic Force Microscope (AFM) showed a strong dependence on oxygen mixing percentage (OMP) in the sputtering gas. The microwave dielectric properties of these thin films are determined up to 20 GHz using a calibration comparison technique with patterned coplanar waveguide (CPW) transmission lines deposited on the surface of the film. Two CPW test structures of 100-micron gap and 200 micron width are deposited simultaneously one on the test film and the other on the bare substrate by directly sputtering silver through a shadow mask. The scattering parameters of these two CPW transmission lines were measured using a Vector Network Analyzer and a probe station mounted with the GSG probes of 250-micron pitch. The dielectric constant of the BST films crystallized on quartz vary between 150–220 and the loss tangent is found to be varying from 0.018 to 0.021 depending upon the processing conditions. The relaxation currents present in the films due to the structural defects associated with the processing conditions may be responsible for the observed dielectric loss as well as the frequency dependent permittivity.

Tunable dielectric characteristics of (111)-oriented barium strontium titanate thin films deposited on platinized Si substrates

We report the fabrication and characterization of Ba0.5Sr0.5TiO3 (BST5) thin films on Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. The x-ray diffraction (XRD) studies revealed that the BST5 films are highly oriented along (111) direction. The misfit strain in the films was about −0.68%, which is compressive in nature. The Root Mean Square roughness (RMS roughness) measured using dynamic force microscope (DFM) was about 2.4 nm. The low frequency (40 Hz–1MHz) dielectric properties measured using a Metal-Insulator-Metal structure show no dispersion either in dielectric constant or tan δ. The dielectric constant, loss tangent, tunability and figure of merit measured at 1MHz were about 598, 0.008, 54% (at 200 kV/cm) and 66, respectively. Varactors of circular patch capacitor (CPC) structure were patterned on the BST5 layer using UV photolithography and metal lift off process. The microwave dielectric response of the parallel plate varactors, measured from 1.5–10 GHz showed a tunability of ∼30% (at 12 V dc bias) and a quality factor of 23.5 (at 0 V dc bias).

Influence of Post Deposition Annealing Process on the Optical and Microwave Dielectric Properties of Bi1.5Zn1.0Nb1.5O7 Thin Films

Thin films of Bi1.5Zn1.0Nb1.5O7 (c-BZN) were deposited at ambient temperature on fused silica substrates by pulsed laser deposition (PLD) followed by a post deposition annealing at a temperature between 200–800°C. The post deposition annealing resulted in the formation of crystalline c-BZN thin films from the as deposited amorphous thin films. For the films that are annealed at 500°C, diffraction peaks are found to be appearing. The grain size and surface roughness of the films are found to be increasing with the increase in annealing temperature. The band edge of the as deposited c-BZN thin films are about 3.6 eV. There is a gradual increase of the optical band gap of c-BZN thin films with the annealing temperature. The as-deposited films and the films annealed up to 300°C shows a lower value of dielectric permittivity and loss tangent. There is a sudden increase of dielectric permittivity and loss tangent for the films annealed at 400°C and above. The dielectric permittivity was decreasing slightly for the films annealed above 700°C.

Tunable materials and their microwave characterization

Tunable materials are those whose dielectric constant is dependent on a dc bias electric field. This property is a primary hallmark of ferroelectrics. Because of the loss issues, instead of in the ferroelectric state, this property is exploited in their paraelectric phase eventhough the tunability is slightly lower. Apart from these materials there are other oxides and biopolymers which are known to exhibit this property. This property can be exploited with small applied voltages if the material is in thin film form. A transmission line patterned on such a film can exhibit voltage dependent electrical length and this property can be manipulated to create a range of microwave devices like tunable filters, phase shifters, delay lines, attenuators etc. Such voltage dependent devices can easily be controlled by digital to analog converters (DACs) and hence by software. These devices find end applications in areas like phased array antenna, software defined radio and tunable metamaterials. The major issues in exploitation of the property exhibited by some materials are developing device quality thin films and extracting their microwave dielectric parameters like broadband dielectric constant, loss and tunability. New microwave characterization techniques are needed for thin films, taking in to account the fact that they are always deposited on a dielectric or conducting substrate and the thickness of the film is too small compared to the wavelength involved. Tunability measurement brings in additional issues like incorporating the dc field applying procedure in such a manner that it will not interfere with the microwave characteristics of the films or measuring instruments. In this paper we are demonstrating various techniques that can be used for the microwave characterization of tunable thin films. The microwave dielectric properties of thin films were characterized at different frequencies using different techniques by involving coplanar waveguide transmission lines (CPW) for broadband measurements and circular patch capacitor based measurement for measurement in the lower microwave bands.

Nanomechanical and Dielectric Properties of Sol-Gel Derived PZT Thin Films Annealed with Microwave Energy

Lead zirconium titanate Pb (Zr0.58Ti0.42) O3 (PZT) thin films were deposited on silicon substrates by sol-gel process. The structural, morphological, dielectric and nano mechanical properties of these films have been studied systematically. The thicknesses of the films were around 160 nm. During the deposition of each layer, the films were baked in a microwave oven which in turn resulted in a low temperature crystallization of these thin films compared to the conventional method. The crystallite sizes of the films were ranged between 25–35 nm depending up on the annealing temperature. The dielectric constant of the films decreased with increase in frequency whereas the loss has increased and ranged between (480–100) and (0.03–0.30), respectively. The DFM (Dynamic force microscopy) images of the films shows the spherical grains with uniform microstructure and an average grain size of 80–110 nm. The hardness of the films ranged between 2–5 GPA. The film annealed at 600°C was having a dielectric constant of 400 with hardness of 4.8 GPa which is suitable combination of properties for micromechanical systems.

Ab initio and dielectric studies of succinic acid and maleic acid in 1,4-dioxane

In a continuing effort to understand the hydrogen bond through the study of dielectric and computational conformal studies of dilute solutions, succinic acid and maleic acid are studied in solutions of 1,4-dioxane solvent. Dielectric studies give an account of the net dipole moment of the system under study, which is then compared with the values obtained from conformal analysis. The dielectric measurements were made at 303 K at a frequency of 9.83 GHz using a X-band microwave test bench in order to determine the relaxation times and the dipole moments. The static dielectric permittivity and the high frequency dielectric permittivity were measured using a LCR meter and an Abbes refractometer, respectively. The results are inspected in comparison with the dipole moment results of ab initio calculations of some of the conformers in gas phase and in liquid phase. Gaussian-03 software package with 6–31G(d) basis set optimisation was used for this purpose. Onsagers reaction field model is used to study the solvation of the dicarboxylic acids in 1,4-dioxane. The results are interpreted in terms of the intermolecular and intramolecular hydrogen bond interactions in the dilute systems.

Raman spectra and dielectric studies in Ti substituted Bi2 (Zn2/3Nb4/3)O7 pyrochlores

Bi2Zn2/3-x/3Nb4/3–2x/3TixO7 (m – BZNT) with x = 0 to 0.4 were synthesized in the conventional solid state route. The dielectric constants of these ceramics at microwave frequencies are found to be increasing from 70 to 114 when x increased from 0 to 0.4. Raman scattering studies were carried out to investigate the effect of Ti4+ substitution on the dielectric properties of Bi2 (Zn2/3Nb4/3)O7 (m-BZN) ceramics. The observed Raman peaks in the m-BZNT are slightly shifted towards lower frequency in comparison to that of m-BZN clearly indicated that the Ti4+ ions are indeed occupying substitution sites in the host m – BZN. The anomalous change in intensity and full with at half maxima (FWHM) of Raman modes at 749 and 847 cm−1 suggest change in oxygen octahedral closely related to dielectric properties of m-BZNT.