Boron nitride thin films were grown on sapphire and Si substrates by atomic layer deposition from triethylborane (TEB) and NH3 precursors in the temperature range of 500 to 900 degrees C. By varying the TEB exposure the film thickness can be controlled with<1 nm precision. At 600 degrees C the process is self-limiting, but films are found to be amorphous. Films grown at higher temperatures were identified as sp(2) BN, but the process is no longer self-limiting. From Raman and IR absorption spectroscopy films deposited at 900 degrees C were identified as nanocrystalline sp(2) BN with crystallite sizes in the range of 3 to 8 nm depending on the NH3 dosage. Films deposited at lower temperatures had broad red shifted IR absorption peaks indicating the lack of long range ordering. The visible and UV optical properties of these films were characterized by UV-Vis transmission measurements over the range of 800 to 190 nm. Nanocrystalline films are highly transparent over this range up to the band gap, which was measured to be in the range of 5.83 to 5.65 eV depending on the NH3 dosage. Published by Elsevier B.V.