Observation of a nanoscale phase separation in blue-emitting Ce-doped SiO1.5 thin films

Abstract

Both the optical and structural properties of Ce-doped SiO_1.5 thin films were investigated. The Ce-related blue luminescence, which can be seen even at room temperature for as-grown films, exhibits a rather complex evolution with the annealing temperature. In particular, a strong decrease is observed when the films are annealed at 900 °C. Structural characterizations combining scanning transmission electron microscopy and atom probe tomography reveal the formation of Si- and Ce-rich clusters at this temperature, thus demonstrating that the decreasing Ce-related luminescence is due to concentration induced quenching. For annealing temperatures higher than 900 °C, the Ce-related luminescence increases. The different structural characterizations provide clear experimental evidence of a phase separation occurring at the nanoscale between pure Si nanocrystals and Ce-rich clusters having a stoichiometry close to the cerium silicates Ce₂Si₂O₇ or Ce_4.667(SiO₄)₃O. The latter compounds are optically active thereby explaining the increased Ce-related luminescence observed at the highest annealing temperature.