Molecular dopants in LiGaO₂: N₂, NO and O₂ in Ga and Li vacancies

LiGaO₂ is an ultrawide band gap tetrahedrally bonded semiconductor (E_g = 5.6 eV) which was recently shown by first-principles calculations to be n-type dopable with Si or Ge. Here we investigate using N₂, NO, and O₂ molecules placed in either Ga or Li-vacancies as potential acceptors for p-type doping. Their optimal position and orientation relative to the lattice, as well as their transition levels and formation energy, are investigated. We use a realistic oxygen chemical potential based on the growth condition. The energy of formation of the (N₂)_Li is lower than that of (N₂)_Ga, which reflects the similar ordering of the vacancy formation energies.  Both (N₂)_Ga and (NO)_Ga show a negative U behavior, with transition levels of 1-/3 and 0/2-, respectively. Unfortunately, all transition levels are quite deep, with the lowest 0/1- transition level for the (N₂)_Li case occurring at approximately 1.57 eV above the VBM. As a result, p-type doping is impossible to achieve with these molecular dopants. We discuss their energy levels in relation to the corresponding Ga and Li vacancies.