Erbium in GaAs: Coupling with native defects

We calculated the total energy of Er point defects in GaAs and of Er defects coupled with native defects in GaAs by the ab initio pseudopotential method. The total-energy calculation indicates that various coupled defects comprising an Er atom and native defects will be formed depending on the growth conditions and the Fermi-level position. By investigating the valence charge distribution, it was found that an Er atom forms a strong bond with an As atom. This chemical feature and the lattice relaxation around the coupled defect are the main factors that stabilize the coupled states. The intra-$4f$-shell luminescence spectrum of Er in GaAs is generally complicated and strongly depends on sample preparation methods and growth conditions. We propose that this tendency is due to the sample-dependent concentration of various defects that form complexes with Er.