Konstantin Astafiev
Ferroelectric thin films are widely studied nowadays as potential replacements for semiconductors in modern tunable microwave devices such as tunable filters, phase-shifters, frequency mixers, power dividers, etc. Recent progress in the deposition of complex oxide films shows promise of significant improvements in the quality of ferroelectric thin films from the point of view of dielectric loss and electrical tunability. However, there still exist certain problems which limit a wide penetration of ferroelectric thin films into the market. The most crucial issue which needs further investigation is the difference in polarization response between ferroelectric thin films and bulk materials. In comparison to bulk materials, thin films show lower dielectric constants, and consequently a smaller electrical tunability, and much higher dielectric losses at microwave frequencies. Another important question which has rarely been investigated is the problem of the reliability of tunable devices based on ferroelectric films. Therefore, a systematic, experimental investigation to understand the physics behind these problems is a task of primary importance. The present work is devoted to the detailed theoretical and experimental investigation of the polarization response in SrTiO3 and KTaO3 thin films at radio and microwave frequencies over a wide range of temperatures and applied bias fields. Specifically, the following has been performed: The polarization response of SrTiO3 and KTaO3 ferroelectric thin films has been investigated at radio frequencies over a wide temperature range using a planar capacitor structure. The sensitivity of the dielectric properties to the deposition technique, conditions of films growth and type of the substrate used, and hence to the microstructure of the films, is clearly demonstrated. While SrTiO3 is paraelectric in bulk form, the appearance of a ferroelectric phase in SrTiO3 thin films on LaAlO3 substrate has been experimentally documented, with a transition temperature lying in the range of 90-120K. It is theoretically shown in terms of Landau theory, that the behavior of the SrTiO3 thin films can not be attributed to the effect of the film/substrate mechanical coupling. The ferroelectricity of SrTiO3 thin films on LaAlO3 substrates has been investigated in detail. The hysteresis loops have been monitored in a wide temperature range using thin film planar capacitors, the driving field being predominantly in the plane of the film. The microwave dielectric loss in perovskite ferroelectric materials has been theoretically investigated. A model of the quasi-Debye loss mechanism in SrTiO3 and Ba0.6Sr0.4TiO3 materials at microwave frequencies has been developed. Experimental investigations of the dielectric loss at microwave frequencies in SrTiO3 and KTaO3 thin films have been performed using a half-wave length microstrip resonator with a resonant frequency of around 8GHz. The field and temperature dependences of the loss tangent for the SrTiO3 and KTaO3 thin-film based planar capacitors have been measured. The impact of electrical stressing on the microwave performance of SrTiO3 films deposited onto different substrates (MgO and LaAlO3) has been investigated. The most important characteristics of tunable microwave components, including tunability and loss tangent, have been monitored. The influence of the addition of a low-loss linear dielectric material to a tunable ferroelectric material has been investigated in terms of electrostatic considerations. Calculations of the dielectric loss and dielectric non-linearity of ferroelectric-dielectric composites have been performed using three different models. On the basis of the results obtained, a figure of merit for the composite material has been evaluated.
EPFL2004
