Formation of intrinsic point defects in AlN: a study of donor and acceptor characteristics using hybrid QM/MM techniques

The wide-gap material aluminium nitride (AlN) is gaining increasing attention for its applications in optoelectronics, energy, and quantum computing, making the investigation of its defect properties crucial for effective use in these fields. This study employs a hybrid quantum mechanical/molecular mechanical (QM/MM) embedded cluster method and uses three different hybrid-level density functional theory (DFT) functionals (B97-2, PBE0, and BB1K) to investigate systematically the thermodynamic stability, electronic properties, and donor/acceptor nature of intrinsic charged point defects in AlN. Our approach allows for the examination of defects within a significantly larger simulation cell, enhancing the reliability of the calculations. Our findings highlight the stable structures of defects including the symmetry-breaking reconstruction of octahedral-centred nitrogen interstitial and nitrogen split-interstitial defects, as well as the potential of nitrogen vacancies and aluminium interstitial defects as sources of shallow donor species. Additionally, we compute equilibrium defect concentrations at different annealing temperatures up to 2800 K, elucidating the roles of nitrogen and aluminium vacancies and interstitials in the conductivity of undoped, n-type doped, and p-type doped AlN. This comprehensive study advances the understanding of defect stability and electronic properties in AlN, paving the way for its enhanced application in advanced technologies.