Abstract: A CVD process for producing a rare earth-doped, epitaxial semiconductor layer on a substrate is disclosed. The process utilizes a silane or germane and a rare earth compound in the gas phase. By this method single phase, rare earth-doped semiconductor layers, supersaturated in the rare earth, are produced. The preferred rare earth is erbium and the preferred precursors for depositing erbium by CVD are erbium hexafluoroacetylacetonate, acetylacetonate, tetramethylheptanedionate and flurooctanedionate. The process may be used to produce optoelectronic devices comprising a silicon substrate and an erbium-doped epitaxial silicon film.