High-Temperature Superconducting/Ferroelectric Structures for Tunable Microwave Components

At the NASA Lewis Research Center, ferroelectric films, such as SrTiO₃ and Ba_xSr_1-xTiO₃, are being used in conjunction with YBa₂Cu₃O_7-d high-temperature superconducting (HTS) thin films to fabricate tunable microwave components, such as filters, varactors, and local oscillators. These structures capitalize on the variation of the dielectric constant of the ferroelectric film upon the application of a dc electric field as well as on the low microwave losses exhibited by the high-temperature superconducting films relative to their conventional conductor counterparts. (For example, the surface resistance for a YBa₂Cu₃O_7-d thin film at 10 GHz and 77 K is more than two orders of magnitude lower than that of copper at the same frequency and temperature.) SrTiO₃ and Ba_xSr_1-xTiO₃ films are used because their crystal structure and lattice parameters are similar to those of YBa₂Cu₃O_7-d, thus enabling the growth of highly textured YBa₂Cu₃O_7-d films with high critical current densities on the underlying ferroelectric film, or alternatively, of highly textured ferroelectric film on the underlying YBa₂Cu₃O_7-d film.

Our efforts have been concentrated so far in determining the deposition parameters required for optimal ferroelectric thin-film growth (i.e., maximum tunability and lowest loss) and in investigating different varactor configurations to determine which geometry is the most advantageous in terms of tunability, losses, and required bias for a given communication application. For example, we have observed that for optimized SrTiO₃ films in a parallel plate capacitor, tunabilities of up to 47 percent and dissipation losses (tan d) of 0.05 are attainable at 1 MHz , 80 K, and within the 0- to 5-V bias range. In contrast, for an interdigital configuration, tunabilities of up to 70 percent and tan d ranging from 0.015 to 0.001 (depending on bias) have been observed at 1 MHz and 77 K within the 0- to 100-V bias range.

Efforts are underway to use these results in developing tunable receiver front-end preselect filters as well as in low-phase noise, tunable local oscillators for K-band applications. These components represent a hitherto unavailable technology to meet the stringent performance requirements of foreseeable satellite and wireless communication systems (e.g., bandwidth, in-band insertion losses, out-of-band rejection, and noise, amongst others) in a more advantageous fashion than with currently available technology (e.g., dielectric-filled cavity and waveguide filters, and dielectric resonator oscillators). Prototypes of high-temperature superconducting/ferroelectric tunable components such as a low-phase noise K-band local oscillator, a preselect C-band filter, and a low-loss K-band phase shifter are under development at NASA Lewis.

Bibliography

Miranda, F.A., et al.: HTS/Ferroelectric Thin Films for Tunable Microwave Components. IEEE Trans. Appl. Supercond., vol. 5, no. 2, 1995, pp. 3191-3194.

Miranda, F.A., et al.: Electrical Response of Ferroelectric/Superconducting/Dielectric Ba_xSr_1-xTiO₃/ YBa₂Cu₃O_7-d LaAlO₃ Thin-Film Multilayer Structures. Supercond. Sci. Technol., vol. 8, 1995, pp. 755-763.

Miranda, F.A.; et al.: Effect of SrTiO₃ Deposition Temperature on the Dielectric Properties of SrTiO₃/YBa₂Cu₃O_7-d/LaAlO₃ Structures. Presented at the 8th International Symposium on Integrated Ferroelectronics, Tempe, Arizona, Mar. 17-20, 1996. To be published in Integrated Ferroelectronics, 1997.

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