Microwave loss mechanisms in Ba0.25Sr0.75TiO3 films

Trilayer Au(Pt)/Ba0.25Sr0.75TiO3/(Pt)Au thin film varactors are fabricated on high resistive Si substrate and characterized at dc, rf and microwave frequencies. In the frequency range of 10-45 GHz the varactors reveal relatively low losses, the loss tangent is less than 0.025 at 45 GHz. Due to the thick and highly conductive Pt/Au electrodes the metal losses are less than 10%. However, the loss tangent of the ferroelectric film is still 3-5 times higher than that in Ba0.27Sr0.73TiO3 single crystal. The analysis of the dc field dependent loss tangent and permittivity in a wide frequency range show that these additional losses are mainly due to the charged defects. Extrapolation of measured low frequency (1 MHz) loss tangents to the microwave region using the power law ω1/3 is in good agreement with the experiment. We assume that the oxygen vacancies within the grain boundaries of ferroelectric film act as charged defects and cause extrinsic microwave losses. The knowledge of the extrinsic loss mechanism and corresponding microstructure defects is useful in optimization of the varactor design, deposition, and/or annealing process and further improvement of the varactor performance.