An analysis was made of the electronic structures of crystals which were undoped, or were doped with Er. It was concluded that the Er ion tended to form bonds with 3 ligands. The role which was played by O in the passivation of dangling bond was considered. Studies of the electric, magnetic, and optical properties of Er-doped material which had been grown by using the Czochralski method showed that Er2O complexes which existed in the crystals became electrically and optically active only when they were located near to the P shallow donor.

N.T.Bagraev, D.E.Onopko, A.I.Ryskin: Solid State Phenomena, 1996, 47-48, 307-12