(A) study on fabrication and characterization of electrically tunable microwave devices using dielectrics as a tuning material

Abstract

In this thesis, several critical issues in electrically tunable microwave devices employing (Ba,Sr)TiO3 films as a frequency-tuning capacitor have been addressed. The high-quality epitaxial (Ba,Sr)TiO3 and YBa2Cu3Oy (YBCO) thin films for tunable microwave devices have been successfully fabricated by pulsed laser deposition. Although (Ba,Sr)TiO3 films highly a-axis oriented normal to the substrate are obtained at substrate temperature of above ~600 ºC, the films deposited at 600~700 ºC show a large number of surface features resulted from the insufficient mobility due to low substrate temperature. Therefore (Ba,Sr)TiO3 films with good surface morphology can be obtained by increasing the substrate temperatures beyond 700 ºC. In addition, the in-plane alignment for the BaxSr1-xTiO3 (x=0.4, 0.5 and 0.6) films is achieved whenever the substrate temperature is ³660 ºC, the FWHM (Dw) values of the (200) peak increase from 0.06º to 0.35º with increasing the substrate temperature up to 780 ºC. From the viewpoints of the surface morphology and crystallinity, the optimum substrate temperature for the (Ba,Sr)TiO3 films appears to be 700~800 ºC. On the other hand, YBCO thin films showing the best superconducting characteristics are obtained at substrate temperature of 760~800 ºC under oxygen pressure (PO2) of 400~500 mTorr when MgO, LaAlO3 and CeO2-buffered Al2O3 single crystalline substrates are used. The YBCO thin films made under optimized parameters show good electrical properties enough to be used in the tunable microwave devices. The microwave dielectric properties of (Ba,Sr)TiO3 capacitors have been measured using the two kinds of tunable microwave resonator, i.e., a YBCO-based 5~6 GHz resonator for BaxSr1-xTiO3 (x = 0 or 0.1) capacitor and an Au-based 2~3 GHz resonator for BaxSr1-xTiO3 (x = 0.4, 0.5, and 0.6). The YBCO-based tunable microstrip resonators employing a dielectric BaxSr1-xTiO3 (x = 0 or 0.1) films have been fabricated. Variations of the capacitance a...