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.