ABSTRACT
The valence state of rare earth ions in semiconductors is important from both scientific and practical points of view, because it is strongly related to the optical and magnetic properties. It has been well known that some rare earth ions can have various valence states in semiconductors [1-3]. Although most of rare earth ions are present as trivalent, the divalent state is sometimes observed in Π-VI compounds. Delerue and Lannoo calculated the 4f occupancy levels of rare earth impurities in semiconductors by a selfconsistent tight-binding Green’s function technique and found the general trend that the 4f energy levels decrease with the ionicity of the host semiconductor [4]. They predicted that Eu and Yb ions become divalent in Π-VI semiconductors, whereas they are trivalent in ΙΠ-V compounds. On the other hand, other lanthanide ions were found to be always in trivalent states both in Π-VI and ΠΙ-V materials. Their results seem to be almost consistent with previous experimental results, but the dependence of the valence states of the Eu ions on the ionicity of the host semiconductor has not been confirmed, because Eu has never been doped into ΠΙ-V materials, to our knowledge.
