Al-doped ZnO thin films (AZO) were prepared on Si (100) substrates by using sub-molecule doping technique. The Al content was controlled by varying Al sputtering time. The as-prepared samples were annealed in vacuum chamber at 800 degrees C for 30 min. From the XRD observations, it is found that all films exhibit only the (002) peak, suggesting that they have c-axis preferred orientation. The average transmittance of the visible light is above 80%. Spectroscopic ellipsometry was used to extract the optical constants of the films. The absorption coefficient and the energy gap were then calculated. The results show that the absorption edge initially blue-shifts and then red-shifts with increase of Al content. (C) 2008 Published by Elsevier B.V.