Microstructural Evidence of Hall Mobility Anisotropy in c-Axis Textured Al-Doped ZnO

The high electrical conductivity, 1150 S/cm at room temperature, in the ab-plane of c-axis textured Al-doped ZnO is attributed to its high Hall mobility that is almost double the mobility in the c-axis direction. Temperature-independent mobility in the ab-plane below 200 K suggests that ionized impurity dominates the scattering of electron transport, which reasonably agrees with a modified Brooks-Herring-Dingle model taking into account nonparabolic E-k dispersion. However, the pronounced anisotropy between ab-plane and c-axis cannot be expected based on the model. Detailed observations of the grain boundary (GB) by means of high-resolution transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy revealed the existence of an Al-enriched, Zn-deficient layer near the GB traversing the c-axis direction. In contrast, the highly conductive direction encompasses a tilt grain boundary, in which coincident sites were observed and Al segregation was barely evident. We conclude that such a preferential segregation in the GB and/or GB structure itself are responsible for the anisotropy of mobility in the textured Al-doped ZnO.