A plasma enhanced atomic layer deposition (PEALD) process has been used to deposit crystalline AlN thin films at 250 degrees C using nitrogen 5% hydrogen plasma and trimethylaluminum precursors. Films grown on single crystal silicon and sapphire substrates are crystalline with strong (100) preferred orientation. Mass density measured by x-ray reflectivity (XRR) was 2.94 g cm(-3), compared to 325 g cm(-3) for the bulk materials. Photoelectron and infrared studies were used to investigate the elemental analysis and the exact bonding environment of the constituents. The optical band gap, measured using spectroscopic ellipsometry, is 6.04 eV, and the refractive index, measured at 632 nm wavelength, is 1.96. The optical properties were thickness-dependent below 30 nm for refractive index and below 15 nm for band gap. (C) 2015 Elsevier B.V. All rights reserved,