Amorphous GaN thin films have been deposited using an ion-assisted deposition technique. These films are typically 100400 nm in thickness and show promising optoelectronic properties. X-ray absorption near edge spectroscopy reveals that a significant amount of molecular nitrogen is trapped near the substrate-film interface, where the films show the least microcrystalline structure. Secondary ion mass spectrometry (SIMS) depth profiles through this interface reveal a complex structure where nitrogen concentration peaks at the interface and oxygen contamination of the film peaks a short distance into the films. Crystallinity increases with film thickness and with decreasing oxygen levels. Control of oxygen levels in the films reveals that indeed the amorphous nature of the films is tied to this oxygen content. (c) 2004 Published by Elsevier B.V.