Lead-Free Ferroelectric (Na1/2Bi1/2)TiO3-BaTiO3 Thin Films in the Morphotropic Phase Boundary Composition: Solution Processing and Properties

Ferroelectric (Na1/2Bi1/2)TiO3-BaTiO3 (BNT-BT) single perovskite thin films have been prepared, for the first time, by chemical solution deposition onto Pt/TiO2/SiO2/(100) Si substrates. A solution process has been developed for the preparation of precursor solutions. The handling of all the chemical reagents, refluxes, and distillations was carried out in air. In addition, the resulting solutions are stable in air, presenting a low gelation and trend to the formation of precipitate. Solutions containing a 10 mol% of Na excess, and a 10 mol% excess of Na+10 mol% excess of Bi, were prepared. These solutions were spin-coated onto the substrates, obtaining amorphous layers that were crystallized in O-2 by rapid thermal processing. Multiple deposition and crystallization were carried out to grow crystalline films with a thickness between 125 and 140 nm. At temperatures below 600 degrees C, the perovskite phase coexists with a pyrochlore phase. At temperatures over 700 degrees C or soaking times over 60 s, an interdiffusion substrate/film interface is formed. Single perovskite BNT-BT thin films are obtained at 650 degrees C, with soaking times of only 6 s. There is an effect of the Na excess and Na+Bi excess on the film microstructure and the dielectric/ferroelectric properties. The films containing Na+Bi excess have a larger grain size, higher dielectric permittivities (at room temperature K'similar to 400), and higher saturation and polarization values (P-s similar to 23 mu C/cm(2) and P-r similar to 12 mu C/cm(2)). The results of this work prove the feasibility of preparing these leadfree compositions in thin film form, resulting in materials with promising functional properties when integrated with Si semiconductor substrates.