Effect of microstructure on the dielectric properties of compositionally graded (Ba,Sr)TiO3 films

The demand for better performing radio frequency (RF) and microwave devices has led to research on lower dielectric loss and more temperature-stable dielectric materials. In this work, we report on an approach to decrease the temperature coefficient of capacitance (TCC) of (Ba,Sr)TiO3, which is desirable for improved stability of phase shifter elements, voltage-controlled oscillators, and other tunable devices. We have grown and characterized a series of single-composition and compositionally-graded films with both parallel plate and interdigitated (IDC) electrodes. Graded films with larger Ba2+ concentrations in the initially-grown layers, as well those films crystallized at lower temperatures, displayed the lowest TCC. An asymmetrically graded BaTiO3∕Ba0.50Sr0.50TiO3∕SrTiO3 film on polycrystalline alumina crystallized at 750 °C yielded the lowest observed TCC of −150 ppm∕°C, a dielectric constant of 240, 20% tuning of permittivity (0−40 VDC, 80 kV/cm), and losses below 1%, measured at 100 kHz. The microstructures of graded films were examined and found to play an important role in determining not only the TCC of graded films but also film dielectric constants. In particular, it was found that films with BaTiO3-rich compositions displayed overall finer grain structures that appear to suppress the ferroelectric behavior. The effect of electrode configuration on the TCC, dielectric constant, and dielectric losses in compositionally-graded films is also discussed.