Epitaxial Ferroelectric/Superconductor Structures on Si-on-Sapphire for Tuneable Microwave Circuits

摘要

Heterostructures with a thin (300 nm) Ba_xSr_(1-x)TiO_3 (BSTO) or KtaO_3 (KTO) ferroelectric layer between two YBa2Cu3O_x (YBCO) superconducting films were laser deposited on Si-on-Sapphire substrates (SOS). To avoid chemical interaction between Si and YBCO, a double Y-ZrO_2/Ce0_2 (9.5% Y_2O_3) buffer layer was grown epitaxially on the (OOl)Si surface. Such a five layer structure may be considered as a basis of several voltage controlled, microwave integrated devices. The (001) planes of both BSTO and KTO were normal to the c-axis of the YBCO in the trilayers. The superconducting Tc's of the bottom and top YBCO layers were 88-90K. The dielectric permittivity, Er, and the high frequency losses of epitaxially grown BSTO (x=0, 0.25, 0.50, 0.75) and KTO layers were measured in the temperature interval 15-3OOK (f=100 kHz). The position of the maximum in Er(T) for BSTO (x=0, 0.25) was well correlated with the temperature variation of the Curie point. About the same sensitivity of the ε_r to an applied bias voltage was detected at T<100K for the KTO layer. There were no indications of an hysteresis in theε_r -V dependencies for BSTO (x =0) and KTO at 77K, The losses, tan8, in the BSTO and KTO layers at 50-100 K were in the range of 2·10~(-2)-7·10~(-3) (f=100kHz).