Morphology, structure, and nucleation of out-of-phase boundaries (OPBs) in epitaxial films of layered oxides

Abstract

Out-of-phase boundaries (OPBs) are translation boundary defects characterized by a misregistry of a fraction of a unit cell dimension in neighboring regions of a crystal. Although rarely observed in the bulk, they are common in epitaxial films of complex crystals due to the physical constraint of the underlying substrate and a low degree of structural rearrangement during growth. OPBs can strongly affect properties, but no extensive studies of them are available. The morphology, structure, and nucleation mechanisms of OPBs in epitaxial films of layered complex oxides are presented with a review of published studies and new work. Morphological trends in two families of layered oxide phases are described. The atomic structure at OPBs is presented. OPBs may be introduced into a film during growth via the primary mechanisms that occur at film nucleation (steric, nucleation layer, a-bmisfit, and inclined-cmisfit) or after growth via the secondary nucleation mechanism (crystallographic shear in response to loss of a volatile component). Mechanism descriptions are accompanied by experimental examples. Alternative methods to the direct imaging of OPBs are also presented.

Atomic vapor deposition of bismuth titanate thin films

“…20 There, a peak splitting was observed in the (008), (0014), and (0020) peaks, but in the BTO system, only the (004) and (0012) peaks show splitting. Zurbuchen et al 20 proposed that in SBT, the XRD peak splitting gives a qualitative measure of the density of OPBs, a large peak splitting meaning a high OPB density and vice-versa. The same kind of splitting pattern was observed in our XRD data for the BTO thin films.…”

“…OPBs were previously observed in MBE growth of BTO by Pan et al 19 and are common in epitaxial films of Aurivillius compounds. 20 For example, the epitaxial SrTa 2 Bi 2 O 9 (SBT) system also shows OPBs. 20 An OPB is a boundary between two parts of crystal relatively displaced with respect to one another, such that the magnitude of displacement parallel to the z-direction is "c/x" 20 ("c" is the lattice parameter).…”

“…20 For example, the epitaxial SrTa 2 Bi 2 O 9 (SBT) system also shows OPBs. 20 An OPB is a boundary between two parts of crystal relatively displaced with respect to one another, such that the magnitude of displacement parallel to the z-direction is "c/x" 20 ("c" is the lattice parameter). Under condition x ¼ 2, the OPBs are called anti-phase boundaries where displacement is exactly half of c. These defects frequently occur 20 in layer-by-layer growth mode.…”

“…20 An OPB is a boundary between two parts of crystal relatively displaced with respect to one another, such that the magnitude of displacement parallel to the z-direction is "c/x" 20 ("c" is the lattice parameter). Under condition x ¼ 2, the OPBs are called anti-phase boundaries where displacement is exactly half of c. These defects frequently occur 20 in layer-by-layer growth mode. This is because in this kind of growth modes, slight change in stoichiometry causes change in local crystal structure, which in turn gives rise to these defects.…”

“…20 One of the reasons for the OPBs to occur can be the atomic steps on the STO substrate surface, as these steps act as nucleation sites for OPBs. The origination of OPBs at substrate can be clearly seen in Figure 5.…”