PHASE-FIELD SIMULATION OF PHASE TRANSITIONS, DOMAIN STABILITIES AND STRUCTURES IN FERROELECTRIC THIN FILMS

Open Access
Author:
Sheng, Guang
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 14, 2011
Committee Members:
  • Long Chen, Dissertation Advisor
  • Long Qing Chen, Committee Chair
  • Zi Kui Liu, Committee Chair
  • Venkatraman Gopalan, Committee Member
  • Qiang Du, Committee Member
  • Qi Li, Committee Member
Keywords:
  • thin film
  • ferroelectric
  • phase-field
  • domain stability
Abstract:
In this study, we use the phase-field model to study the phase transitions, domain stabilities, domain structures and domain evolutions in a series of representative ferroelectric thin films, including PbTiO3, BaTiO3, SrTiO3, and BiFeO3. The effect of temperature, mechanical and electrostatic boundary conditions as well as the size effect on the above ferroelectric properties were systematically investigated. We modify the previous phase-field model to study the domain stability and evolution in ferroelectric thin films under anisotropic strains. The effect of anisotropic strains on the phase transitions and domains structures of PbTiO3 and BaTiO3 thin films is studied using both thermodynamic calculations and phase-field simulations. The misfit strain - misfit strain domain stability diagrams, i.e. the graphical representations of stable ferroelectric phases and domain structures as a function of strains, are predicted at several representative temperatures. The similarity and significant differences between the diagrams from thermodynamic calculations assuming single domains and from phase-field simulations are analyzed. Typical domain structures as a result of anisotropic misfit strains are presented. A modified thermodynamic potential for strontium titanate bulk crystal is developed from interpreting experimental data with phase-field simulations. This modified potential is then applied to study the transition temperatures and domain stabilities in SrTiO3 thin films grown on different substrates with different strain states. The results will be compared with experimental measurements on polarizations in SrTiO3 thin films grown on DyScO3 and GdScO3 substrates. Phase-field model is also employed to study the domain stabilities and phase transitions in BiFeO3 thin films. It will be shown that the morphotropic phase boundary, or rhombohedral to tetragonal phase boundary predicted by phase-field method has very good agreement with experimental observations in BiFeO3 films deposited on LaAlO3 and YAlO3 substrates. Domain switching near the morphotropic boundary is studied. We also analyze the strain dependence on the coercive field and remnant polarization for BiFeO3 thin films grown on PMN-PT with tensile strains. A three dimensional phase-field model is applied to model the thickness effect on the domain stability in ferroelectric thin films, by introducing the strain relaxation mechanism and critical thickness for dislocation formation using both Matthews-Blakeslee (MB) and People- Bean (PB) models. PbTiO3 thin films are chosen as the example to construct the strain – thickness domain stability diagram. The relative domain fractions as a function of film thickness are also simulated and compared with experiment results in PbTiO3 thin films grown on SrTiO3 and KTaO3 substrates.