Chemical bonding of nitrogen formed by nitridation of crystalline and amorphous aluminum oxide studied by X-ray photoelectron spectroscopy.

Numerous efforts have already been made to optimize nitridation of crystalline sapphire (c-AlO) substrates whereas very little attention has been paid to nitridation of amorphous aluminum oxide layers (a-AlO ). An extensive analysis of the reaction of amorphous aluminum oxide films with nitrogen species is thus needed to clarify the mechanisms of nitrogen incorporation into such layers and to control their properties. In this work X-ray photoelectron spectroscopy was used to determine the chemical state of nitrogen formed by nitrogen plasma treatment of c-AlO and 15 nm thick a-AlO layers grown by atomic layer deposition on Si and sapphire substrates. The results show that the nitridation proceeds significantly different for c-AlO and a-AlO samples, which we correlate with the initial stoichiometry of both materials. At the surface of sapphire O vacancies were found, which are necessary for the formation of AlN-type bonding diffusion limited replacement of oxygen by nitrogen. This process was slow and involved formation of oxinitride AlN-O. After 80 min of nitridation only ∼3.4 at% of N was incorporated. In contrast, in a-AlO layers Al vacancies were present before nitridation. This opened a new, more effective path for nitrogen incorporation accumulation of N in the cation-deficient lattice and creation of the Al(NO ) phase, followed by AlN and AlN-O formation. This scenario predicts more effective nitrogen incorporation into a-AlO than c-AlO, as indeed observed. It also explains our finding that more N was incorporated into a-AlO on Si than on sapphire due to supply of oxygen from the sapphire substrate.