Origin of carrier scattering in polycrystalline Al-doped ZnO films

We observed the carrier transport phenomena in polycrystalline Al-doped ZnO (AZO) films with carrier densities ranging from 2.0 × 10¹⁹ to 1.1 × 10²¹ cm^-3. A comparison of the optical carrier density and Hall carrier density indicates that the conduction band in AZO films is nonparabolic above 2.0 × 10²⁰ cm^-3. A transition from grain boundary scattering to ionized impurity scattering is observed at a doping level of >4.0 × 10²⁰ cm^-3. The trap density at the grain boundary increases with increasing Al concentration in the films, implying that the doping level plays a decisive role in the trap density. The excellent fitting of the optical mobility and carrier density using the Brooks-Herring model shows that the acceptor concentration increases with increasing doping level.