Hybrid functional calculations of DX centers in AlN and GaN

Using hybrid functional calculations, we investigate the formation of DX centers in GaN and AlN. We find that O, Si, and Ge are shallow donors in GaN, but form stable DX centers in AlN for Fermi-level positions near the conduction band. Using a linear interpolation, we estimate the composition at which the onset of DX behavior will occur in AlGaN alloys. Based on these predictions, we identify Si as the most effective donor for high Al-content AlGaN, and explain a number of seemingly conflicting experimental results for Si-doped AlGaN. Although based on size matching Ge was expected to be superior to Si as a shallow donor, it actually turns out to be more prone to DX-center formation.