A metastable electron paramagnetic resonance signal was observed in Se-doped material. When cooled in the dark, the sample was non-paramagnetic. After illumination with white light from a W source, a strong persistent electron paramagnetic resonance signal with an isotropic g-factor of 1.949 was observed. A metastability of the signal was consistent with earlier results on DX-like centers. The absence of electron paramagnetic resonance when samples were cooled in the dark was suggested to constitute direct evidence for the negative-U model. The electron paramagnetic resonance arose from the effective-mass state of the defect, which was not filled at thermal equilibrium. A detailed analysis of the line-shape revealed that the line-width was governed by hyperfine interactions.

P.Stallinga, W.Walukiewicz, E.R.Weber, P.Becla, J.Lagowski: Physical Review B, 1995, 52[12], R8609-12