N vacancy, substitutional O, and Al defects in the bandgap of composition-tunable nonstoichiometric AlN powder

AlN powders are prepared by direct nitridation via Al liquid and vapor phases in mixed atmospheres of N₂ and NH₃ with different NH₃/N₂ ratios. The reaction analysis reveals that NH₃ acts as catalyst for N₂ dissociation and the transportations of N, O, and Al in the liquid phase are different from those in the vapor phase. Accordingly, the products are Al-rich and composition-tunable nonstoichiometric AlN in which N, O, and Al content values change with nitridation atmosphere and temperature, leading to the variation of the relevant defect concentration. Therefore, the AlN powders exhibit prominent absorption bands around 5.30, 3.40, and 1.50 eV, which are tentatively assigned to V_N, O_N donors, and Al_N acceptor respectively. Furthermore, a new donor named V_N-O_N complex is predicted at 4.40 eV within the 5.90 eV bandgap. It is demonstrated that the optical spectra of nonstoichiometric AlN are preferable to the nominal stoichimometric one for the identification of the defects energy level.