Anodic aluminum oxide films formed in mixed electrolytes of oxalic and sulfuric acid and their optical constants

Porous anodic aluminum oxide (AAO) films were fabricated electrochemically in the mixed electrolytes with various volume ratios of 0.3 M C₂H₂O₄ and 0.3 M H₂SO₄. The transmission spectra with the interference fringes were measured and the modified Swanepoel method was used to determine the optical constants of the free standing AAO films. The calculated thickness agrees well with the measured thickness from the FE-SEM images of the cross section, which indicates that the modified Swanepoel method is very fit for the determination of the optical constants of the free standing AAO films. Meantime, with the decrease of the volume ratio of C₂H₂O₄ and H₂SO₄, the refractive index and thickness of AAO films increase, but the extinction coefficient decreases. The optical band gap is appropriately fitted to the direct transition model proposed by Tauc in the strong-absorption region of investigated films, and is derived from Tauc's extrapolation. The reasons were investigated.