Transmission Electron Microscopy Investigation Of Defects And Domains In Epitaxial Films Of Aurivillius And Ruddlesden-Popper Phases

Zurbuchen, Mark Alan

Transmission Electron Microscopy Investigation Of Defects And Domains In Epitaxial Films Of Aurivillius And Ruddlesden-Popper Phases

Open Access

Author:: Zurbuchen, Mark Alan
Graduate Program:: Materials Science and Engineering
Degree:: Doctor of Philosophy
Document Type:: Dissertation
Date of Defense:: March 29, 2002
Committee Members:: Darrell G Schlom, Committee Chair/Co-Chair
Altaf H Carim, Committee Member
Ying Liu, Committee Member
Earle Richard Ryba, Committee Member
Karl Eugene Spear Ii, Committee Member
James Patrick Runt, Committee Member
Keywords:: ferroelectric domains
out-of-phase boundaries
OPBs
domains
ferroelectrics
microstructure
epitaxial films
transmission electron microscopy
defects
epitaxy
Abstract:: Three types of domains in epitaxial films of layered perovskites have been investigated by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The phases studied belong to two homologous series, Aurivillius phases (SrBi2Nb2O9 and SrBi2Ta2O9) and Ruddlesden-Popper phases (Sr2RuO4). Both contain perovskite crystallographic units, alternating along c, with close-packed Bi2O22+ or AO rock-salt layers, respectively, for the two series. Films grown on SrTiO3, LaAlO3, and NdGaO3 substrates, as well as on epitaxial SrRuO3 bottom electrodes, with orientations of (100), (110), and (111), were studied. Two domain types, growth twin domains and out-of-phase domains, exist in epitaxial films of layered complex oxides, and the ferroelectric phases also have ferroelectric domains. The morphology, nucleation mechanisms, and effects on properties of two types of domains, growth twin domains and out-of-phase domains, were determined. Growth twin domains occur in complex oxide films grown on substrates for which multiple, equivalent epitaxial orientations exist, and are the result of local continuation of the perovskite sublattice of the substrate, resulting in one, two, and three possible growth twin variants on (001), (110), and (111) perovskite substrates. Out-of-phase domains are common in complex oxides due to their highly anisotropic crystal structure. Out-of-phase domain boundaries (OPBs) are shown to impact material properties, and to have a range of possible offsets as a result of multiple nucleation mechanisms, three during growth and one post-growth, in layered complex oxides. Steps on substrates or at internal surfaces can cause misregistry along the long unit cell direction between neighboring regions of an epitaxial film, resulting in the generation of an OPB. In phases with multiple structurally-equivalent occurrences of the preferred nucleation layer, OPBs are generated at the film substrate interface by the preferred nucleation-growth order mechanism. Substrate surface features of only one or two atomic layers can result in the generation of OPB defects that propagate through an entire film of over 0.5 µm in thickness. In epitaxial films with the long unit cell axis inclined to the substrate surface, mismatch of the coincident-site lattice generates OPBs. Post-growth, loss of a volatile component through evaporation at elevated temperature leads to the generation of OPBs by crystallographic shear. The ferroelectric domain structure of SrBi2Nb2O9, a unique zero-strain oxide ferroelectric, has also been determined. The ferroelastic distortion of SrBi2Nb2O9 is four orders of magnitude less than that of prototypical oxide ferroelectrics. Thus, it serves as an end point in the spectrum of oxide ferroelectric materials. Imaging of the ferroelectric domain structure has allowed the first experimental separation of the energetic terms controlling ferroelectric domain structure: strain, space charge, and dipole-dipole interactions.

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