Computational Analysis of Metastable Shallow Donors: Unveiling the Mechanisms of n-type Doping in AlN and AlGaN

N-type doping of AlN and Al-rich AlGaN using group-IV donors has been limited by the formation of DX centers, which disrupt the donor-N bond and create a deep level in the band gap that traps two electrons. Recent experiments observed shallow donor formation in Si- and Ge-implanted AlN. However, the theory behind the nature of shallow donors and doping mechanisms remains ambiguous. Using first-principles calculations, we find that the high conductivity could be induced by Ali from ion damage. Simultaneously, SiAl and GeAl can form defect complexes with VAl, keeping group-IV donors shallow and preventing compensation. Moreover, while the DX-center geometry is stable for Fermi levels near the conduction band, mid-gap Fermi levels favor the shallow-donor geometry without significant bond disruptions. Furthermore, once equilibrium is re-established in n-type AlN and the Fermi level shifts closer to the conduction band, the disruption of the Si-N bond is inhibited by an energy barrier of ~1 eV, preventing the conversion of the metastable shallow donors into DX centers. Our results explain mechanisms for efficient n-type doping of AlN and Al-rich AlGaN, and identify Fermi-level engineering strategies to further improve the doping efficiency and conductivity.