A decrease was observed in the number of effective emission centers, Mn2+, in aged Mn-doped material, by using electron paramagnetic resonance techniques. During the operation of an electroluminescent device, the isolated Mn could easily diffuse into another site and form a cluster. Another possibility was that the Mn2+ could change into Mn+ or Mn3+ by transfer of the electronic charge of the isolated Mn2+ to neighboring Mn ions via S and/or S vacancies. As a result, the luminescence was lowered due to a decrease in the number of efficient emission centers of isolated Mn2+.

Y.H.Lee, D.H.Kim, B.K.Ju, M.H.Song, T.S.Hahn, S.H.Choh, M.H.Oh: Journal of Applied Physics, 1995, 78[6], 4253-7