Microwave response of BiFeO3 films in parallel-plate capacitors
Paper in proceedings, 2011
BiFeO3 (BFO) is extensively considered for its multiferroic and multifunctional properties. Non-volatile memory and sensors are only some of the applications to mention. Its relatively high piezoelectric constant makes BFO attractive for applications in thin film bulk acoustic wave resonators (FBARs). Low permittivity is beneficial for high power applications, and the high Curie temperature ensures low temperature dependence of its parameters. In this work the dielectric and piezoelectric properties BFO films in parallel-plate configuration are studied at microwave frequencies in a view of their applications in tunable FBARs. The 150 nm thick BFO films are grown by pulsed laser deposition on platinum bottom electrodes. Fused silica is used as the substrates. X-ray diffraction analysis indicates that the BFO films are strongly (111)pseudocubic oriented as a result of growth facilitated by the Pt(111) texture. The dielectric response of the BFO films is measured at 1 MHz and in the frequency range 1-30 GHz under different dc electric fields. The dc bias is changed from zero (non-poled state) up to 33 V/µm and then reversed down to zero. The measured break down field, ca. 500 kV/cm, is sufficiently high to ignore any effects associated with leakage current. The 1 MHz permittivity and loss tangent correspond to those measured in the microwave range. Their frequency dependences (only permittivity is shown in Fig. 1) reveal no remarkable relaxation phenomena which may be considered as evidence that non-180 domain wall processes are strongly limited as it should be in (111) oriented BFO films. The permittivity is higher than that reported for BFO ceramics (ca. 30), most likely, due to compressive out-of-plane strain caused by large difference in thermal expansion coefficients of the BFO film and silica substrate. Additionally, we assume that vibrations of the domain walls may contribute to the total permittivity since irreversible polarization behaviour is detected in the whole frequency range (Fig. 1). As an example of the irreversible polarization response, Fig. 2 shows the permittivity and loss tangent at 10 GHz versus electric field varying in the arrow directions starting from a non-poled state. Fig. 3 shows frequency dependences of the loss tangent of a BFO film at different dc electric fields. Increase of loss tangent with frequency in the whole range confirms negligible contribution of the leakage current. The dependences reveal resonant peaks due to both intrinsic (field independent) and field induced (at 3.2 GHz and 5.5 GHz) piezoeffects. The resonances arise at different frequencies due to reflections of acoustic waves from different interfaces of the multilayer test structure. The rather intensive field induced resonance at ca. 3.3 GHz indicates that electrostrictive coefficient of the BFO films is high enough for its application in the tunable FBARs.