$Ba0.25Sr0.75TiO3$ thin-film varactors on $SrRuO3$ bottom electrode

$Ba0.25Sr0.75TiO3∕SrRuO3$ (BSTO/SRO) thin-film varactors were fabricated on (001)$LaAlO3$ substrates and characterized at rf and microwave frequencies of up to 10 GHz in the temperature range of 50–300 K. X-ray analysis reveals epitaxial growth of (001)-oriented BSTO films with extended out-of-plane lattice parameter $b=3.925Å$⁠. The tensile out-of-plane strain $(u33=0.3%)$ of BSTO films results in an out-of-plane component of the relative permittivity $ε33=490$⁠. The renormalized Curie-Weiss constant and Curie temperature are $C33=8.0×104K$ and $T33=140K$⁠, respectively. An interfacial layer presumably forms at the $Au∕Pt$ top electrode with a thickness-to-relative permittivity ratio of $di∕εi=0.3–0.4nm$ and reduces the apparent relative permittivity to $εa=250$⁠. The relative tunability of the varactor (at 10 V dc bias) increases from 40% (295 K) up to 80% (125 K) as the temperature decreases in the whole frequency range. The varactor loss tangent increases with increasing frequency and decreasing temperature (from $tanδ=0.01$ at 1 MHz and 295 K up to $tanδ=1.7$ at 10 GHz and 125 K, 0 V dc bias). The extrinsic loss of the BSTO film may be ascribed to charged defects associated with oxygen vacancies, at least in the temperature range of 150–300 K. This type of loss is mainly contributing to the total varactor loss balance in the frequency range of up to 500 MHz. At higher frequencies, the loss due to the SRO electrode series resistance starts to dominate. The commutation quality factor of the BSTO/SRO varactor is about 2000 at frequencies of up to 100 MHz and temperatures of less than 230 K, which is quite acceptable for practical applications.