Crystalline aluminum nitride (AlN) thin films were formed on various substrates by using RF magnetron sputtering of an Al target in a nitrogen plasma and also by ion-assisted molecular beam epitaxy (IAMBE). Basal-oriented AlN/(111) Si showed a degradation of crystallinity with increased substrate temperature from 550 to 770 degrees C, while the crystallinity of AlN/(0001) Al2O3 samples improved from 700 to 850 degrees C. The optical absorption characteristics of the AlN/(0001) Al2O3 films as grown by both deposition methods revealed a decrease in sub-band gap absorption with increased substrate temperature.