K. Venkata Saravanan

Large three-photon absorption in Ba0.5Sr0.5TiO3 films studied using Z-scan technique

Large picosecond nonlinearities in Ba0.5Sr0.5TiO3 thin films, grown at different temperatures in situ on (100) MgO substrates using rf magnetron sputtering technique, were studied using the Z-scan technique. The nonlinear absorption mechanism, studied near 800 nm using ∼2 and 25 ps pulses, switched from reverse saturable absorption type in the films deposited at temperature 600 °C. The magnitude of the 3PA coefficient was estimated to be ∼10−21 cm3/W2. Two-photon absorption (2PA) was the dominant mechanism recorded with ∼6 ns pulses. The observed behavior is correlated with morphological and crystallographic texture of the films. The linear refractive index and optical band gap of the films have also been calculated and these show a strong dependence on the substrate temperature.

Effect of misfit strain and surface roughness on the tunable dielectric behavior of Ba0.5Sr0.5TiO3 thin films

The effect of substrate temperature on the crystallographic texture and surface morphology of Ba0.5Sr0.5TiO3 deposited by rf magnetron sputtering on (111) oriented Pt (150 nm)/Ti (10 nm)/SiO2 (300 nm)/Si (100) substrates is reported. The onset of crystallinity is observed above 500 °C, beyond which all films are oriented along the (111) plane up to a temperature of 800 °C. Over this temperature range, the misfit strain and root mean square roughness (rmsroughness) peak at 600 °C, decreasing beyond this peak value. The tunability of the dielectric constant at 100 kHz is highest (60% for a field strength of 300 kV/cm) at the lowest values of rmsroughness and misfit strain and decreases as these values increase. It is also shown that the morphology of the Pt underlayers is strongly affected by the processing conditions. This in turn influences the tunable behavior of the BST films as much as the substrate temperature during growth and the consequent microstructural variations.

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.

Effect of CeO2 and Nd2O3 on the Microstructure and Microwave Dielectric Properties of (Zr0.8,Sn0.2)TiO4 Ceramics

(Zr0.8,Sn0.2)TiO4 (ZST) ceramics were prepared by solid-state reaction method with 1 wt% ZnO and 0.5–1.5 wt% CeO2 or Nd2O3 as sintering aids. The effect of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of ZST ceramics were investigated. The dielectric constant (ϵr) and temperature coefficient of the resonance frequency (τf) were not significantly affected by the addition of these additives. The unloaded quality factors (Qu) were effectively promoted by CeO2 and Nd2O3 additions. ϵr values of 40 and 38.3, Qxfo values of 57,500 and 59,300 were obtained for the samples sintered with 1.5 and 0.5 wt% of CeO2 and Nd2O3 respectively. The improvement in Qxfo value is primarily attributed to the increase in uniform grain size and density.

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).

Tunable Interdigital Capacitors with Ba0.5Sr0.5TiO3 Thin Films on Low-k Substrates for Frequency Agile Microwave Devices

A tunable interdigitated capacitor is developed using Ba0.5Sr0.5TiO3 ferroelectic thin film and low-k fused silica substrate for applications in the microwave frequency range. The capacitance and quality factor of tunable IDC structure on BST5 film as a function of frequency up to 5 GHz are extracted using the conformal mapping method and partial capacitance technique. In this paper, the tunable capacitance properties of Au/Ba0.5Sr0.5TiO3 thin-film interdigital capacitors on low-k substrates are presented.