Assessing the Role of Fluorine in the Performance of AlxGa1-xN/GaN High-Electron-Mobility Transistors from First-Principles Calculations

Doping fluorine (F) into the AlxGa1-xN layer is critical to the performance of enhancement-mode AlxGa1-xN/GaN high-electron-mobility transistors (HEMTs). However, the understanding of the role of F in AlxGa1-xN/GaN HEMTs is rather limited. Using the first-principles-calculated defect formation energies and transition energy levels, combined with the special quasirandom structure approach and the detailed balance theory, we investigate the interaction between F and native defects and impurities, as well as its effect on the Fermi energy of the AlxGa1-xN alloy. Our results suggest that F is incorporated as F-i(-) in the AlxGa1-xN layer, which exhibits auto n-type conductivity because of unintentionally induced oxygen (O). F doping causes the redistribution of the charge states of intrinsic defects and impurities, and thus the Fermi energy of the AlxGa1-xN layer. The charge-redistribution depends on the difference between the concentrations of F and O. Finally, we reveal the mechanism for the change of the electronical performance of AlxGa1-xN/GaN HEMTs after F doping. The positive shift of the threshold voltage is related to the negatively charged F-i. Only when the concentration of F is higher than that of unintentionally induced O in AlxGa1-xN, F begins to increase the surface potential and the Schottky barrier height of AlxGa1-xN/GaN HEMTs.