Light emission from Ge and GeO2 nanocrystals

Ge and GeO2 nanocrystals are prepared in a-SiOx matrix by DC-magnetron sputtering and thermal annealing. An increased infrared absorption at 860 cm−1 is found in films containing GeO2 clusters. Under ultra violet excitation, both types of films luminesce near 3.1 eV, with a characteristic blue PL decay time around 1 ns, independent of the chemistry and size. The strongest PL intensity is found for the films containing GeO2 crystals and for the films containing the largest Ge crystals. The room temperature PL excitation spectra are independent of the crystal size. Although the blue luminescence is correlated with the formation of the crystals, it is not due to the radiative recombination of confined excitons. We suggest that the origin for the blue PL band is a Ge/O related defect located in the inhomogeneous strain field of the crystal surfaces.