ELECTROMECHANICAL CHARACTERIZATION OF LEAD MAGNESIUM NIOBATE BASED THIN FILMS

Open Access
Author:
Shetty, Smitha
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
January 04, 2019
Committee Members:
  • Susan E Trolier-Mckinstry, Dissertation Advisor
  • Susan E Trolier-Mckinstry, Committee Chair
  • Clive A Randall, Committee Member
  • Venkatraman Gopalan, Committee Member
  • Chris Giebink, Outside Member
  • Thomas Shaw, Special Member
Keywords:
  • relaxor ferroelectricity
  • PMN
  • D33
  • single beam laser interferometer
Abstract:
Lead Magnesium Niobate (PMN) is the most studied relaxor ferroelectric compound with cation order intimately interacting with nanoscale polar heterogeneities. In undoped bulk PMN, the cation order can hardly be changed, while the polar order is quite responsive to external stimuli (electric field and temperature). Moreover, PMN-based oxides are of technological importance, displaying strong piezoelectric effects, high electrostrictive strains, and unusually high dielectric permittivity. The local compositional heterogeneity associated with short-range ordering of Mg and Nb in PbMg1/3Nb2/3O3 (PMN) is correlated with its characteristic relaxor ferroelectric behavior. Fully-ordered PMN has not been prepared as a bulk material. This work reports the growth and characterization of thin film PMN heterostructures with long-range cation order. These films were grown at temperatures below 1073 K by artificially reducing the degree of disorder via synthesis of heterostructures with alternate layers of Pb(Mg2/3Nb1/3)O3 and PbNbO3, as suggested by the random-site model. 100-nm-thick, phase-pure films were grown epitaxially on (111) SrTiO3 substrates using alternate target timed pulsed-laser deposition of Pb(Mg2/3Nb1/3)O3 and PbNbO3 targets with 20% excess Pb. Selected area electron diffraction confirmed the emergence of (1/2, 1/2, 1/2) superlattice spots with randomly distributed ordered domains as large as ~150 nm. These heterostructures exhibited a dielectric constant of 800, loss tangents of ~0.03 and 2 x remanent polarization of ~11 μC/cm2 at room temperature. Polarization-electric field hysteresis loops, Rayleigh data, and optical second-harmonic generation measurements are consistent with the development of ferroelectric domains below 140 K. Temperature-dependent permittivity measurements demonstrate reduced frequency dispersion compared to short range ordered PMN films. Although perfectly layered heterostructures could not be grown, the novelty of this work lies in achieving the highest cation order in PMN. Further, these engineered films exhibit a continuum between normal and relaxor ferroelectric behavior as a function of temperature. The dynamic nonlinear dielectric responses were also studied as a function of temperature and electric field for the PMN thin films with long- and short-range ordering. It was found that long-range ordering decreased the dispersion in the first and third harmonic displacement current relative to short-range ordered films. For both sets of films, a second harmonic component of the dielectric response was also detected. The phase angles of the higher harmonic responses were used to probe the continuum in ferroelectric- relaxor behavior in these films. At high temperatures (e.g. above the freezing temperature), the presence of nanopolar clusters is believed to cause the strong dispersion in the third harmonic response in both long range and short-range films. However, at lower temperatures, changes in the sign of χ3 suggest long-range ferroelectric ordering, such that the response of mobile interfaces (believed to be domain walls) dominate the response. Additionally, miniaturization of PMN based ferroelectric films for actuator applications necessitates quantification of piezoelectric properties with scaling. A single beam laser interferometer based on a modified Mirau detection scheme with a vertical resolution of ∼5 pm was developed for localized d33 measurements on patterned piezoelectric films. The tool provides high spatial resolution (∼2 μm), essential for understanding scaling and processing effects in piezoelectric materials. This approach enables quantitative information on d33, currently difficult in local measurement techniques such as piezoresponse force microscopy. The interferometer is built in a custom microscope and employs a phase lock-in technique in order to detect sub-Angstrom displacements. d33 measurements on single crystal 0.67PbMg0.33Nb0.67O3-0.33PbTiO3 and bulk PbZrTiO3-5A ceramics demonstrated agreement within <3% with measurements using a double beam laser interferometer. Substrate bending contributions to out-of-plane strain, observed in thin continuous PbZr0.52Ti0.48O3 films grown on Si substrates is reduced for electrode diameters smaller than 100 μm. Direct scanning across 5 μm and 10 μm features etched in 0.5 μm thick PbZr0.52Ti0.48O3 films doped with 1% Nb poled at either room temperature or 150 °C confirmed minimal substrate contributions to the effective d33,f. Furthermore, enhanced d33,f values were observed along the feature edges due to partial declamping from the substrate, thus validating the application of single beam interferometry on finely patterned electrodes.