It was argued that the vacancy–O (VO) complex (A center) in Ge had 3 charge states: doubly negative, singly negative and neutral. The corresponding energy levels were located at Ec–0.21eV (VO–/–) and Ev + 0.27eV (VO–/0). An absorption line at 716/cm was

attributed to the asymmetrical stretching vibration mode of the doubly negatively charged VO complex.

Electronic Properties of Vacancy–Oxygen Complex in Ge Crystals. V.P.Markevich, I.D.Hawkins, A.R.Peaker, V.V.Litvinov, L.I.Murin, L.Dobaczewski, J.L.Lindström: Applied Physics Letters, 2002, 81[10], 1821-3