Aluminium doped zinc oxide thin films (4 at.% Al) were deposited by reactive magnetron sputtering technique and characterized by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), four point probe technique and optical spectrophotometry. High heterogeneities were observed as a function of sample position in the chamber. The chemical analyses did not reveal significant change in composition. Optical investigation showed a strong variation of the density of free carriers, through the Burstein-Moss effect, suggesting that Al dopants were partially inactivated.