Metastable Shallow Donors and Origin of n-type Doping in AlN and AlGaN

<b>The n-type doping of AlN and Al-rich AlGaN alloys through the incorporation of group-IV donors is constrained by the formation of compensating DX centers. Within the DX configuration, the bond connecting the donor atom to its adjacent N atom is disrupted, consequently causing the N atom to undergo a buckling distortion in the reverse direction. This results in the creation of a deep defect level within the band gap that is capable of trapping two electrons. Nevertheless, recent experimental works [1-4] have observed shallow donor formation and high conductivity and in Si- or Ge-implanted AlN. However, the nature of these shallow donors and the mechanism of doping remain unclear. Here, we apply predictive atomistic calculations to understand the formation of shallow donors and compensating defects in n-type AlN. We first identify that the high conductivity of Si- and Ge-implanted AlN arises from Al interstitials introduced by the ion damage. At the same time, the substitutional implanted Si and Ge ions form complexes with Al vacancies, which maintain the group-IV donors in the shallow configuration and prevent compensation. The presence of these complexes has been identified experimentally through their characteristic peaks in photoluminescence. Moreover, although the DX-center geometry is the most stable configuration of Si and Ge dopants for Fermi levels close to the conduction band, Fermi levels near the middle of the gap cause donors to adopt the shallow-donor geometry, in which Al atoms are substituted by the dopants without bond severance or strong bond distortions. This indicates that Fermi-level engineering during the material growth or processing can stabilize donors in the shallow geometry and prevent the formation of DX centers. 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 Si donors into DX centers over a sufficiently long time. Our results explain the mechanisms for the efficient n-type doping of AlN and Al-rich AlGaN, and identify Fermi-level engineering strategies to further increase the doping efficiency and improve the conductivity.</b><br/><br/><b>References: </b><br/><b>[1] B. Neuschl et al. Physica Status Solidi B, 249(3): 511–515, 2012.</b><br/><b>[2] M. Hayden Breckenridge et al. Applied Physics Letters, 116(17), 2020.</b><br/><b>[3] R. Vermeersch et al. Applied Physics Letters, 119(26), 2021.</b><br/><b>[4] P. Bagheri et al. Applied Physics Letters, 122(14), 2023.</b>