A new method for studying 2-electron states in DX centers at atmospheric pressure was reported. It was based upon the idea of co-doping a GaAs0.6P0.4 sample with a uniform background of Te shallow donors (as a source of free carriers), and with a Gaussian distribution of S DX centers, by ion implantation. By using both capacitance-voltage profiling and deep-level transient spectroscopic measurements, it was demonstrated that the ground state of the S DX center trapped 2 electrons and therefore had negative U.

M.F.Li, Y.Y.Luo, P.Y.Yu, E.R.Weber, H.Fujioka, A.Y.Du, S.J.Chua, Y.T.Lim: Physical Review B, 1994, 50[11], 7996-9