Aluminum nitride (AlN) films have been deposited on glass substrates at various nitrogen flow ratios by rf reactive magnetron sputtering. The AlN film deposited at 10% of nitrogen flow ratio shows a strongly c-axis preferred orientation with a crystalline size of 100 nm, thickness of 1100 nm, and band gap energy of 4.38 eV. The optimum crystallographic structure occurs at a nitrogen flow ratio of 100%, where a considerable crystallinity enhancement of the AlN film is observed. The band gap energies E(g) calculated by the Tauc model and parabolic bands are well described by a relationship, E(g) = 10(-3)X+4.39, where X is the nitrogen flow ratio. The results suggest that the nitrogen flow ratio plays a key role in growing the high-quality AlN films. (C) 2011 Elsevier B.V. All rights reserved.