All-oxide thin-film varactors with SrMoO3-bottom electrodes and Mn/Ni-doped BST for sub-6 GHz applications

This paper presents a comprehensive study and performance analysis of all-oxide thin-film Au/Pt/Ba0.5Sr0.5TiO3 /SrMoO3 varactors with 100 nm Ba0.5Sr0.5TiO3 (BST) tunable dielectric layers doped with 1%, 3% and 5% of Mn and Ni grown epitaxially on a 600 nm SrMoO3 bottom electrode. Performance parameters such as tunability, quality factor, figure-of-merit, leakage current and acoustic resonances of the varactors are investigated. The investigation covers frequencies up to 6 GHz and a broad temperature range from −40◦ to 80◦ , using biasing field strengths of up to ± 100 V∕μm, in order to evaluate the feasibility and the performances of BST-doped all-oxide varactors for sub-6 GHz applications. Results confirm the anticipated advantages of Mn and Ni doping: a substantial reduction in leakage current along with an increase in quality factor, however, at the cost of reduced tunability. The observed acoustic resonances are shifted towards higher frequencies as compared to those for varactors with undoped BST, which implies in a larger frequency operating range for doped all-oxide varactors. In addition, the amplitudes of the acoustic resonances decline with doping. Moreover, they can be effectively suppressed for thinner BST layers, which has been proved exemplarily for a varactor with a 25 nm Mn-doped BST layer and a tunability of 55 %, requiring an ultimately low tuning voltage of only 2.9 V.