GROWTH AND CHARACTERIZATION OF CADMIUM-SULFIDE NANOCRYSTALS EMBEDDED IN SILICON DIOXIDE FILMS

In this communication we report on the growth and the optical characte risation of CdS quantum dots in the diameter ran,at of 4-10 nm embedde d in silicon dioxide glass films grown by magnetron If-sputtering tech nique with post-deposition annealing. Optical, transmission measuremen ts display a marked blue shift of the absorption band edge due to the quantum confinement of the electrons and holes in the CdS nanocrystals . The mean diameter of the nanocrystals wins estimated from absorption spectra using a standard theoretical model. A study of the effect of the nanocrystal size an the Raman spectra has been made from an analys is of the first order LO mode lineshape. The increase in the linewidth observed with decreasing crystal size is in agreement with the result s of a calculation based on the phonon confinement model. The asymmetr ic lineshape is well reproduced by the model, but they fail to reprodu ce the large low-frequency tail. The origin of this tail is dir;cussed considering several possible sources proposed in the literature and i s under study. Room temperature infrared reflectance measurements have also been obtained showing a diffuse band at about 230-300 cm(-1), wh ich closely corresponds to CdS optical polar phonons observed in the b ulk crystals. However, we are unable to fit the observed spectra by as suming just this contribution. Instead, by including an additional con tribution due to CdO we are able to well simulate the experimental spe ctra using a model based on the Maxwell-Garnet theory. (C) 1998 Elsevi er Science S.A.