We investigated the influence of Al contents on the structural, electrical and optical properties of Al-doped ZnO (AZO) polycrystalline films with the columnar grain structures. In addition, we demonstrated an effective way for achieving improved Hall mobility AZO films at any given Al contents. We deposited 500-nm-thick AZO films on glass substrates at a substrate temperature of 200 degrees C by direct-current magnetron sputtering with a direct-current power of 200 W. The oxide targets were high-density sintered AZO targets with Al2O3 contents from 0.5 to 3.0 wt%. Analysis of the data obtained by X-ray diffraction and Hall effect measurements showed that an increase in the Al2O3 contents resulted in an increase in volume fraction of (0001) orientation (V-(0001)) from 85 to 98% and in carrier concentration of the AZO films. The use of a 10-nm-thick Ga-doped ZnO film deposited by ion plating with direct-current arc discharge as an interface layer between the glass substrate and a thick AZO film drastically changed the above behavior of the orientation distribution limited by the Al2O3 contents. We found the V-(0001) of about 99% at any given Al2O3 content. The resulting well-defined (0001) crystallographic orientation led to very small contribution of grain boundary scattering to carrier transport. Hall mobility of AZO films with the very thin interface layer was improved with enhanced carrier concentration as a result.