  The phases in the ZnO half of the ZnO-In2O3 binary system are natural
superlattices consisting of periodic stacking of single InO2 sheets separated
by indium doped ZnO blocks with a spacing that depends on the composition
according to the relationship In2O3(ZnO)k. Characterization by combined, atomic
resolution, aberration-corrected scanning transmission electron microscopy
(STEM) and electron energy loss spectroscopy (EELS) analysis, indicates that
the atomic structure of each InO2 layer consists of a single continuous sheet
of octahedrally-coordinated InO2. The sheets also are crystallographic
inversion boundaries. Analysis of the electrical conductivity, thermal
conductivity and Seebeck coefficient data at 800 oC indicates that the InO2
sheets not only decrease thermal conductivity by phonon scattering but also
create potential barriers to electron transport. The origin of the potential
barriers, the role of piezoelectric effects and their dependence on
superlattice spacing is discussed qualitatively. It is also argued that the
crystallographically-aligned InO2 sheets within individual grains are also
transport barriers in randomly oriented polycrystalline materials.
