Excited state impurity band conductivity in Y2(SiO4)O: Ce3+

To properly describe the optical properties of doped insulators, one must include the host valence and conduction bands in addition to the energy levels of the impurity ion. Thus, the impurity-host system can be treated in a donor-acceptor model, similar to the semiconductor approach. Here we report another parallel between doped semiconductors and insulators: the formation of an impurity band. In Y-2(SiO4)O : Ce3+, the lowest 5d band of the Ce3+ ion lies about 500 meV below the conduction band. We observe conductivity in heavily doped samples while exciting the lowest 4f-5d transition. This effect is attributed to the formation of an impurity sub-band. In contrast to the semiconductor case, where the impurity ground state levels form a sub-band due to the large Bohr radii of the impurities, the Ce3+ ground state 4f wave functions are very compact and only for the more extended 5d states is an impurity band found. (C) 1999 Elsevier Science B.V. All rights reserved.