Oral: Origin of n-type doping in AlN and Al-rich AlGaN

The n-type doping of AlN and Al-rich AlGaN by Si and Ge is limited by the formation of DX centers. In this configuration, the bond between the donor and a neighboring N atom breaks, causing the N to buckle in the opposite direction, forming a deep defect level that traps two electrons. However, experiments have observed suppressed formations of DX centers in samples doped by ion implantation. Here, we apply predictive atomistic calculations to understand the mechanism of shallow-donor formation in AlN. While the DX center is most stable near the conduction band, Fermi energies near mid-gap lead donors to adopt shallow geometry without bond severance. Moreover, once equilibrium is reestablished in n-type AlN and the Fermi level shifts closer to the conduction band, the severance of Si-N bond is inhibited by an energy barrier on the order of 1 eV, preventing conversion of these metastable shallow Si donors to DX centers. Furthermore, Al interstitials produced by implantation adopt multiple geometries and exhibit shallower –1/+1 transition levels, enabling the shallow doping of AlN. Our results explain the recent experimental observation of shallow donor formation in Si- and Ge-implanted AlN and underline the important technological applications of high-conductivity n-type AlN.