The character and formation energy of defects in semiconductors are of utmost importance in dictating the nature of the semiconductor material, and hence its performance in device applications. We have performed density-functional theory pseudopotential calculations to investigate the electronic structure, atomic configurations, and formation energies of native point defects (and impurities) in AlN.[C. Stampfl and C. G. Van de Walle, Phys. Rev. B 65, 155212 (2002)]

The various native defects (N-vacancy, N-antisite, N-interstitital, Al-vacancy, Al-antisite, and Al-interstitial; from left to right, top to bottom) are illustrated below:


The calculated formation energies are depicted (right).The nitrogen vacancy, V_N,  has the lowest formation energy in p-type material (near E_F=0, i.e., near the valence band maximum) and the aluminum vacancy, V_Al,  has the lowest formation energy in n-type material (near E_F=5, i.e. closer to the conduction band minimum).  The nitrogen vacancy is a triple donor and the aluminium vacancy is a triple acceptor. Due to the low formation energies, the triple donors V_N³⁺  and Al_i³⁺  may act as compensating centers when trying to achieve p-type doping (e.g., with Mg). If oxygen contaminants are present in the material, then also substitutional Mg_AlO_N  pairs may contribute to the compensation.
In n-type material, V_Al^3-  may act as a compensating center, thus limiting the electron concentration provided by Si and O impurities which we find are shallow donors. The formation energy of the nitrogen vacancy is  high in n-type AlN, indicating that this defect is not expected to occur in high concentrations.