Theoretical investigation of native defects, impurities, and complexes in aluminum nitride

We have performed density-functional pseudopotential calculations to investigate the electronic structure, atomic configurations, and formation energies of native point defects and impurities in AlN. For the native defects, the nitrogen vacancy has the lowest formation energy in p-type material and the aluminum vacancy has the lowest formation energy in n-type material. Under n-type conditions the formation energy of the nitrogen vacancy is high, indicating that it will not occur in high concentrations. We find that the nitrogen vacancy exhibits a different behavior in the zinc-blende and wurtzite structures with respect to the higher-lying defect-induced level: in zinc-blende materials, this level is a resonance in the conduction band causing the vacancy to act as a shallow donor, while in wurtzite the level lies well below the conduction-band edge causing the vacancy to act as a deep donor. In the zinc-blende structure we find, in addition, that the aluminum interstitial has a low formation energy in p-type material. The results indicate that these defects could be important compensation centers; we discuss this in relation to the dopant impurities O, Si, and Mg. We also investigate MgO and Mg2O2 impurity complexes. A comparison between results obtained using the local-density approximation and the generalized-gradient approximation for the exchange-correlation functional shows that the results are qualitatively very similar.