The optical properties and stress in thin films of aluminum nitride have been studied as a function of ion energy and the ion-to-vapor arrival ratio (J(i)/J(v)) for the N(2)(+) ion-assisted deposition of aluminum. The ion energy was varied between 75 and 1000 eV. The refractive index was found to depend on the ion energy and flux. The highest index was 2.10 at 633 nm. The films were found to be highly transparent over the wavelength region 275-800 nm. The films were found to be free of major oxide contamination, and x-ray photoelectron spectroscopy studies revealed the presence of excess nitrogen in films prepared at high J(i)/J(v) values. The film stress was also found to be related to the ion energy, and an anomalously high compressive stress of -5.0 GPa was found for 100-eV N(2)(+)-assisted depositions. By comparison, films prepared by magnetron deposition were found to have lower refractive indices (1.97-1 99, n633) and higher stress (-8 to -12 GPa) when deposited in pure nitrogen.