Mixed-valent states of rare-earth dopants in IV-VI semiconductors

Mixed-valent states of rare-earth dopants in narrow-gap IV-VI semiconductor s are analyzed. A theoretical approach to describing rare-earth-doped IV-VI semiconductors is proposed in which the dopant is considered a strongly co rrelated system, with electronic states coupled to conduction- and valence- band states. The results are illustrated by the example of the Yb dopant, h aving an almost-filled f shell. It is shown that the system can be describe d by a narrow resonance level, whose position and width depend on the chemi cal potential and band gap. The possibility of the formation of mixed-valen t states is demonstrated.