Role of shallow bound states in emission processes of rare-earth doped semiconductors

Participation of shallow states in the energy transfer processes between th e rare-earth ions and a host semiconductor matrix is discussed. It is argue d that shallow levels link the atomic-like states of the inner core of the rare-earth ion with the band structure of the host crystal. For the two mos t investigated systems InP:Yb and Si:Er the important role of these states at various stages of excitation and de-excitation mechanisms is shown. Mode ls of the energy transfer process with a formation of a shallow intermediat e state are discussed. It is shown that the formation of such a state is es sential for the RE ion core activation. Experimental results supporting the involvement of shallow states are reviewed. Temperature dependencies of th e photoluminescence activation and decay time constants are presented; thes e show an important role of shallow doping in the activation of Auger proce sses relevant to the mechanism of rare-earth ion photoluminescence. For the Si:Er system direct evidence for the two-stage excitation mechanism is dis cussed.