High Dielectric Tunability and Figure of Merit at Low Voltage in (001)-Oriented Epitaxial Tetragonal Pb0.52Zr0.48TiO3 Thin Films

Ferroelectric thin films with a high dielectric tunability (<i>η</i>) have great potential in electrically tunable applications, including microwave tunable devices such as phase shifters, filters, delay lines, etc. Using a modified Landau-Devonshire type thermodynamic potential, we show that the dielectric tunability <i>η</i> of a (001) tetragonal ferroelectric film can be analytically solved. After a survey of materials, a large <i>η</i> value above 60% was predicted to be achievable in a (001)-oriented tetragonal Pb(Zr_0.52Ti_0.48)O₃ (PZT) film. Experimentally, (001)-oriented PZT thin films were prepared on LaNiO₃-coated (100) SrTiO₃ substrates by using pulsed laser deposition (PLD). These films exhibited good dielectric tunability (<i>η</i> ~ 67.6%) measured at a small electric field <i>E</i> of ~250 kV/cm (corresponding to 5 volts for a 200 nm thick film). It only dropped down to ~54.2% when <i>E</i> was further reduced to 125 kV/cm (2.5 volts for 200 nm film). The measured dielectric tunability <i>η</i> as functions of the applied electric field <i>E</i> and measuring frequency <i>f</i> are discussed for a 500 nm thick PZT film, with the former well described by the theoretical <i>η</i>(<i>E</i>) curves and the latter showing a weak frequency dependence. These observations validate our integrated approach rooted in a theoretical understanding.