Development of high performance lead-free Piezoelectric thin films

Abstract

Lead-based piezoelectric materials have been widely used in electrical and electronic devices. However, the usage of toxic lead causes severe human health and environmental concerns. Utilizing lead-based materials for commercial applications will be prohibited in many countries, once the lead-free alternatives become available. In this project, attempts have been made to develop high performance lead-free 0.94(Bi0.5Na0.5)TiO3-0.06BaTiO3 (BNBT)-based piezoelectric thin films by Pulsed Laser Deposition (PLD) and Laser Molecular Beam Epitaxy (MBE).

The growth of BNBT-based thin films has shown high sensitivity to the processing conditions, which lead to substantially different physical properties. In this work, thin films of 0.5 mol% Mn-doped BNBT were deposited on SrRuO3 coated SrTiO3 (001) substrates by PLD. The effects of oxygen partial pressure and substrate temperature on the properties of samples were studied. It is found that the film deposited at 200 mTorr and 700 °C rendered the highest remnant polarization of 33.0 μC/cm2, and the largest piezoelectric coefficient of 120.0 pm/V.

The physical properties of BNBT thin film can be enhanced through site engineering. By depositing x mol% Fe-doped BNBT thin films (x = 0, 0.5, 1.0, 1.5, 2.0) on SrRuO3 electroded SrTiO3 (001) substrates by PLD. The sample at the composition of x = 1.5 has shown better physical properties than the films with other Fe doping levels, indicating the effectiveness of Fe doping.

The efficacy of site engineering was further evaluated by comparing the performance of undoped, 0.5 mol% Mn-doped, 1.0 mol% Sm-doped, and 1.5 mol% Fe-doped BNBT thin films grown on La0.7Sr0.3MnO3 buffered SrTiO3 (001) substrates by Laser MBE. The Fe-doped BNBT thin film exhibited the highest remnant polarization of 37.0 μC/cm2, whereas the Mn-doped sample showed the largest piezoelectric coefficient of 112.5 pm/V.

The ferroelectric and piezoelectric properties of our BNBT-based thin films presented in this thesis were found to be comparable to those of the lead-based counterparts, and much better than some of the widely studied lead-free piezoelectric thin films up to date. Our results demonstrated the success in developing high performance lead-free BNBT-based thin films, and their potential applications in ferroelectric memory and piezoelectric micro-sensors and actuators applications, etc.