Study of native point defects in Al0.5Ga0.5N by first principles calculations

To explore the formation mechanism of native point defects in high Al content AlGaN film, the first principles methods are applied to study the native point defects in Al_0.5Ga_0.5N. The different kinds of vacancies, interstitials, and antisites are investigated. The formation energies of Al_0.5Ga_0.5N with native point defects under different charge states and growth conditions are analyzed and compared. Then, the preferable charge state, donor and acceptor properties of Al_0.5Ga_0.5N with different native point defects are explicitly obtained. The results show that Al_N, Ga_N, Al_i, and V_N exhibit donor properties in p-type condition while V_Ga, V_Al, V_N, and N_Ga exhibit acceptor properties and can play roles as compensating center in n-type Al_0.5Ga_0.5N under metal rich condition. For N rich condition,V_Ga, V_Al, N_Ga, and N_Al are favorable acceptors in n-type Al_0.5Ga_0.5N. Meanwhile, the charge distribution and bonding state of Al_0.5Ga_0.5N with native point defects are explored. It is found the N atoms in N_i and N_Ga form covalent bond and ionic bond with atoms in Al_0.5Ga_0.5N. Moreover, the band calculation reveals that the removal of N atom in Al_0.5Ga_0.5N makes the conduction band fatter and the effective masses of electrons increase while the introduction of N point defects make the valence band flatter, increasing the effective masses of holes. Furthermore, the corresponding thermodynamic transition energy levels of defects under different charge states are summarized. It is found that the thermodynamic transitions for V_N, N_i, and N_Al may likely to happen under certain conditions. The above studies yield a detailed and quantitative description of native point defects in Al_0.5Ga_0.5N, which helps to get a deeper insight to the growth and doping of AlGaN.