Single layer In-O atomic sheets as phonon and electron barriers in ZnO-In2O3 natural superlattices: Implications for thermoelectricity

Abstract

The phases in the ZnO half of the ZnO-In2O3 binary system are natural superlattices consisting of a 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 and electron energy loss spectroscopy analysis indicates that the atomic structure of each InO2 layer consists of a single continuous sheet of octahedrally coordinated InO2. The sheets are also crystallographic inversion boundaries. Analysis of the electrical conductivity, thermal conductivity, and Seebeck coefficient data at 800 °C 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 are discussed qualitatively. It is also argued that the crystallographically aligned InO2 sheets within individual grains are also transport barriers in randomly oriented polycrystalline materials.