Electronic structure and optical properties of Al0.25Ga0.75N with point defects and Mg-defect complexes

Abstract

To study the influence of point defects on optoelectronic properties of Al0.25Ga0.75N, models of intrinsic and Mg doped Al0.25Ga0.75N crystals with various defects were built. Based on first-principles calculations, formation energies, electronic structure, and optical properties were calculated. Results show that N vacancy (VN) owns the smallest formation energy in p-type condition, and it always acts as a donor. Ga vacancy (VGa) always acts as an acceptor and owns the smallest formation energy in n-type condition. Interstitial N defect (Ni), Ga antisite defect (GaN), and N antisite defect (NGa) act as donors in p-type condition while they act as acceptors in n-type condition. N-type defects VN and Gai form complexes with p-type Mg impurity, and the complexes exist stably in Al0.25Ga0.75N crystal with high binding energy. The Fermi level of crystal with MgGa–VN moves into band gap and that with MgGa–Gai enters into conduction band, showing that their p-type property is damaged. Compared with intrinsic crystal without defects, the peaks of imaginary part of dielectric function for the defective crystals all shift to lower energy region. Besides, the metal reflective regions and the absorption edges of crystals with defects shift to lower energy region significantly, showing that the band gaps of defective crystals are smaller.