First-principles study of alloying effects on fluorine incorporation in Al x Ga 1-x N alloys

Incorporation of fluorine (F) into the AlGaN layer is crucial to the fabrication of enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistors (HEMTs). However, the understanding of properties of F doping in AlGaN alloys is rather limited. Using first-principles calculations and the special quasirandom structure (SQS) approach, we investigate the alloying effects on the doping properties of F-incorporated Al x Ga1−x N alloys. We find that substitutional F on N sites (FN) and interstitial F (Fi) are dominant defects for F in Al x Ga1−x N alloys. For these two types of defects, both the global composition x and the local motif surrounding the dopant play important roles. On contrary, the incorporation of substitutional F on Ga sites (FGa) or Al sites (FAl) are affected only by the composition x. We also find that there exists a large asymmetric bowing for the effective formation energies of FN and Fi. These results are explained in terms of local structural distortion and electronic effects. The mechanism discussed in this paper can also be used in understanding doping in other semiconductor alloys.