  AlN thin films have been grown on Si (100) substrates by reactive ion beam
sputter deposition (IBSD) at different substrate temperatures varying from room
temperature (RT) to 500oC. Substrate temperature induced microstructural
transition from amorphous at RT, nanocrystalline at 300oC to microcrystalline
at 400oC has been observed by Transmission Electron Microscopy (TEM). Average
surface roughness (Ra) and morphology has been explored by using Atomic Force
Microscopy (AFM). UV-VIS spectrophotometry has been employed to probe the
substrate temperature induced changes in optical band-gap (Eg) of grown thin
films in reflectance mode. It was found that Eg was increased from 5.08 to 5.21
eV as substrate temperature was increased from RT to 500oC. Urbach energy tail
(Eu) along with weak absorption tail (WAT) energy (Et) have been estimated to
account for the optical disorder which was found to decrease with associated
increase in Eg.
