Development of high-performance lead-free piezoelectric thin films and nanostructures for microelectronic devices

Abstract

In this thesis, high-performance lead-free piezoelectric epitaxial thin films and nanostructures based on (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 (BZT-xBCT) and (Bi0.5Na0.5)TiO3-BaTiO3 (BNBT) were prepared using pulsed laser deposition (PLD) and laser molecular beam epitaxy (LMBE). Firstly, by investigating the effects of deposition temperature and oxygen partial pressure on the physical properties of BZT-0.5BCT thin films, the optimal deposition temperature and oxygen partial pressure were determined to be 850 ˚C and 200 mTorr, respectively. A series of BZT-xBCT homogenous thin films were then prepared under the optimized conditions to study the compositional dependence of structural and electrical properties. A morphology phase boundary (MPB)-like behaviour was observed at the composition of x=0.5 with a high remanent polarization Pr of 17.8 μC/cm2 and a large piezoelectric coefficient d33 of 100 ± 5 pm/V. The compositional gradient BZT-xBCT multilayers were fabricated. A higher remanent polarization and a larger piezoelectric coefficient were attained in compositionally downgraded (DG) thin films compared to the compositionally upgraded (UG) one, owing to the retained in-plane compressive strain and large built-in electric field, respectively. The origin of the internal electric field was also confirmed to be the flexoelectric effect. The morphology engineering was extended to BCTm/BZTm (m presents the number of unit cells for BCT and BZT in one period) symmetric superlattices prepared by LMBE. The growth rate was precisely controlled with the aid of reflection high-energy electron diffraction (RHEED). A built-in electric field was observed in all superlattices and its strength was periodicity dependent, i.e. the longer the periodicity, the larger the built-in electric field. An excellent piezoelectric coefficient was obtained in BCT3/BZT3 superlattice with d33+ of 150 ± 5 pm/V. Finally, Mn doped BNBT (MnBNBT) thin film was fabricated with a high Pr of 33.0 μC/cm2 and a large d33 of 120.0 ± 20 pm/V. By changing the deposition temperature, the defect level was finely tuned and minimized in the MnBNBT thin film deposited at 700˚C, giving rise to the excellent ferroelectric and piezoelectric properties. In summary, the high-performance lead-free piezoelectric thin films and nanostructures fabricated in this work presented their promising prospect on substituting lead-based materials as well as potential on applications of piezoelectric microelectronic devices.