This review paper presents a discussion on dielectric substrate materials suitable for the preparation of YSa2C@-, thin-film based microwave integrated circuits. The requirements on the properties of the substrate materials are specified. They cover the properties crucial both for the preparation of high-quality YBa2Cu&-, films and for the design of microwave elements. The former includes mainly the lattice match, the match of thermal expansivities, chemical stability, and absence of twinning. The latter includes the relative dielectric permittivity (e) and the related tolerances, the microwave loss tangent, and the substrate area required for the accommodation of a microwave circuit.The properties of the currently available substrates suitable for YSM film epitaxy are discussed in view of these reauirements. The main attention is paid to the microwave properties. Current achievements and potential difficulties of the clystal growth technology in the preparation of the substrates are taken into account as well.
The voltage-dependent dielectric constant (ε) of SrTiO3 (STO) thin films is the basis for developing cryogenic capacitors for tunable microwave applications. In this study, the effect of microwave signal level on nonlinear response at 1.7–1.9 GHz was examined by measuring the level of the third order intermodulation distortion (IMD) signal relative to the input signal level. Small signal dielectric properties such as capacitance, tuning, and loss (tan δ) were also measured at 1 MHz, 3 GHz, and 10 GHz, at temperatures from 4.2 to 300 K. Planar capacitors were comprised of highly (100)-oriented, 1 μm thick STO films deposited via magnetron sputtering onto CeO2-buffered (11_02)-oriented sapphire substrates, with 10 μm gaps between the electrodes. Deviations from the anticipated cubic dependence of the third order IMD product on incident power, for incident power ranges from −10 to 22 dBm, were attributed to conductivity nonlinearity. At incident power levels of 22 dBm and with no dc bias applied to the capacitor, the level of the third order IMD product was 21 dB below the fundamental signal level. Application of a 107 V/m dc electric field bias across the capacitor suppressed the third order IMD by an additional 10 dB. The nonlinear properties of thin film STO capacitors as a function of microwave voltage were determined by comparing the experimental and theoretical dependencies of the IMD products.
The preparation of high-temperature superconducting YBa2Cu3O7-x thin films on both sides of the SrTiO3 substrate by DC magnetron sputtering is reported. The films on both sides of the substrate exhibited well-oriented crystalline structure with the c axis normal to the substrate surface, Tc=80-91 K and Rs less than 0.05 Omega at 77 K and 60 GHz. The properties of the resultant SrTiO3 capacitor with YBCO electrodes were examined at frequency of 1 GHz. The following parameters were obtained: relative dielectric permittivity of 4*103 at 77 K and dielectric loss tangents of 3*10-4 at 77 K. No hysteresis in the dependence of the capacitance on the DC bias voltage was observed.
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