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The effects of anisotropic dielectric properties of ferroelectric Ba1-xSrxTiO3 (BST) films on the characteristics of phase shifter have been studied in microwave regions at room temperature. Ferroelectric BST films with (001) and (011) orientation were epitaxially grown on (001) and (011) MgO substrates, respectively, by pulsed laser deposition method. The structures of BST films were investigated using x-ray diffraction measurement. The microwave properties of orientation engineered BST films were investigated using coplanar waveguide transmission lines that were fabricated on BST films using a thick metal layer by photolithography and etching process. The measured differential phase shift and insertion loss (S21) for (011) BST films are larger than those for (001) BST films. Dielectric constants of the ferroelectric BST films are calculated from the measured S21 using a modified conformal-mapping model.
The ferroelectric (Ba0.6Sr0.4)TiO3 (BST) films were prepared on (001) MgO single crystals by pulsed laser deposition. Coplanar waveguide (CPW) type phase shifters controlled by external dc bias field were fabricated on BST films using a 2 μm thick metal layer to reduce metal loss. Microwave properties of the CPW phase shifter were measured using a HP 8510C vector network analyzer from 0.1 – 20 GHz. The fabricated CPW phase shifters (8 mm long) exhibited differential phase angle of 100 ° at 10 GHz with a dc bias field of less than 80 kV/cm between center and ground conductors. Furthermore, a stable differential phase angle (102 ± 3.5 o) was observed from another CPW while changing the power of incident microwave from -10 to +30 dBm. Gap size dependent dielectric constant of the BST film was observed and a simple correction method was suggested in the paper. These results demonstrate the possible application of ferroelectric tunable devices on a high power tunable wireless telecommunication.
Ferroelectric BaTiO 3 thin films with perovskite structure were grown by sol-gel spin-on processing onto (111)Pt/Ti/SiO2/Si substrates. In order to investigate the effects of space charge in BaTiO3 thin films, we measured the relative dielectric constant and the ac conductivity of the films as a function of frequency, ac oscillation amplitude and temperature. Dielectric constant and dielectric loss were 147 and 0.03 at 100 kHz, respectively. Also, BaTiO3 thin films exhibited marked dielectric relaxation above the Curie temperature and in the low frequency region below 100 Hz. This low frequency dielectric relaxation is attributed to the ionized space charge carriers such as oxygen vacancies and defects in BaTiO3 film and the interfaical polarization. The thermal activation energy for the relaxation process of the ionized space charge carriers was 0.72 eV.
The modifications of the laser ablated target surface morphology and the particulates on YBa2Cu3O7−x (YBCO) thin films have been investigated. As the number of laser pulse per site increases, the ablated target surface changes to conical structures. After the texturing of the target surface, the ablated surface morphology changed identically, independent of the initial surface roughness of the polished targets. The YBCO thin films deposited using the stationary target have an almost particulate-free surface. However, in the case of the YBCO films deposited using the rotating target, there is a considerable increase in the density of particulates on the film surface.
We have investigated the effect of substrate temperatures ranging from 680 °C to 800 °C on the orientation of pulsed laser deposited YBa2Cu3O7−x.(YBCO) thin films on (100) LaAlO3 substrates. X-ray diffraction studies indicate that there is a progressive change in the dominant orientation of the films from c-axis oriented (c-axis perpendicular to the substrate surface) to a-axis oriented (a-axis perpendicular to the substrate surface) grown as the substrate temperature is lowered. Two YBCO bilayers, which are the a-axis oriented YBCO film on the top of c-axis oriented YBCO film (a/c) bilayer and c-axis oriented YBCO film on the top of a-axis oriented YBCO film (c/a) bilayer, have been grown by in situ two step pulsed laser deposition and were characterized by scanning electron microscopy (SEM), Rutherford backscattering (RBS) spectrum, x-ray diffraction (XRD), and Tc measurements. Through SEM and XRD studies, double layer structures of the a/c and c/a bilayers were confirmed indirectly.
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