Phase relation, dielectric and ferroelectric properties of lead-free ferroelectric (1-x)Na0.5Bi0.5TiO3-xBaTiO3 thin films obtained by PLD and RF-PLD

Sodium bismuth titanate (Na_1/2 Bi_1/2) TiO3, pure or in solid solution with BaTiO₃, (1-x)Na_0.5Bi_0.5TiO₃-xBaTiO₃, can be a promising lead-free ferroelectric material for replacing lead zirconate titanate. In bulk, at morphotropic phase boundary (x = 0.06) it has attractive properties such as remanent polarization of 38 muC cm^-2 and a coercive field of 73 kV cm^-1 at room temperature, with a relatively high Curie temperature (320degC). In this work, the morphological, structural and electrical properties of NBT-BT thin films (with x values from 0 to 0.08) prepared by pulsed laser deposition (PLD) and radiofrequency assisted pulsed laser deposition (RF-PLD) were investigated. A parametric study concerning the influence of substrate temperature, RF power, x value and the reactive gas pressure was carried out. The deposited layers were characterized by XRD, SEM, AFM, TEM and SE (Spectroscopic Ellipsometry). Dielectric and ferroelectric properties were investigated up to 110 MHz using an impedance analyzer and a RT 66A Ferroelectric Test System. We report structural, dielectric and ferroelectric properties of sodium bismuth titanate - barium titanate (NBT-BT), thin films grown on Pt(111)/Si and MgO (100) by pulsed laser deposition (PLD) and radiofrequency beam discharge assisted PLD (RF-PLD). Structural investigations show that XRD spectra of PLD-grown films correspond to single-phase pseudoperovskite and indicate coexistence of rhombohedral and tetragonal phases (morphotropic phase boundary, MPB), as for the target. These films are polycrystalline and randomly oriented. The spectra of RF-PLD grown films correspond to (001)-oriented single-phase pseudoperovskite with rhombohedral distortion, as if the MPB would be displaced towards a slightly higher barium titanate content. Since the other deposition parameters have been the same, it seems that RF-PLD growth induces a higher s- tress in the deposited films than normal PLD and favours the rhombohedral structure.