First-principles study of native defects in CdGeAS2

First-principles results are presented for various native defects in CdGeAS(2) as function of the relevant elements' chemical potentials. The defect formation energies were calculated using fully relaxed 64 atom supercells by means of the full-potential linearized muffin-tin orbital implementation of the density-functional theory in the local-density approximation (LDA). The LDA band gap is adjusted using the LDA + U approach by introducing a semiempirical orbital dependent U shift to the s orbitals of Cd and Ge and the d orbitals of Cd. The transition energies of the vacancies V(Cd), V(Ge) and V(As) and antisites Ge(Cd), Cd(Ge) Ge(As) and AS(Ge) are calculated. Defect levels are interpreted in a simple-molecular orbital theory picture and the relation between Kohn-Sham band structures and transition levels is discussed. The vacancies are generally found to have higher energy of formation than the antisites. In particular, the somewhat deeper acceptor VG, is found to have the highest energy of formation among the defects studied. Among the three shallow acceptors (V(Cd), Cd(Ge), Ge(As)), the lowest energy of formation is found for Cd(Ge), but only the Ge(As) antisite is expected to be active in electron paramagnetic resonance (EPR). This is consistent with experimental data, establishing a link between the EPR-active center and the shallow acceptor responsible for optical absorption. Both Ge(Cd) and AS(Ge) are found to be deep donors.