SrTiO3-based varactors for Ka-band application: Preparation and properties

SrTiO3(STO)-based varactors with YBa2Cu3O7¿x (YBCO) electrodes were pulsed laser deposited on MgO substrate by using intermediate oxygen relaxation technique, i.e. the STO film was deposited in sequence of four 100nm thick layers with intermediate cooling in oxygen down to 300 °C. The oxygen treatment as well as introducing of a homoepitaxial MgO layer in between the MgO substrate and STO film, reduced the residual strain in the STO film and improved microwave properties of the varactors. Typical parameters of the varactors measured at 77K in a frequency range: 25 - 50 GHz were: capacitance, C, 30-40 fF, tuning C(0V)/C(40V) of 1.2, Q-factor of 20-40 and leakage current less than 0.25 nA, when a DC bias of 40V was applied. These results show applicability of the fabricated varactors at Ka band and higher frequencies.