Co-doped (Yb and Er) monocrystalline films were grown by means of molecular beam epitaxy, involving YbF3, ErF3 and CaF2 evaporation onto CaF2 substrates. The luminescence and excitation luminescence at 25K were measured in order to investigate the effect of Yb3+ co-doping and the effect of the molecular beam epitaxy conditions upon the incorporation of active rare-earth species. It was found that the energies of the levels of the Er3+ ions were not affected by the double-doping, although mixed complex centers were formed. Excitation and absorption spectra showed that the lowest component of the Er:4I11/2 manifold had an energy which was higher than that of Yb:2F5/2.

E.Daran, R.Legros, P.Pernas, C.Fontaine: Journal of Applied Physics, 1997, 81[2], 679-84