CdSe and CdS nanocrystals (NCs) are efficiently used for numerous applications (LEDs, photovoltaic devices, memory, biomedical experiments, etc.). They can be fabricated and embedded in dielectric media by a variety of techniques. Here we present a Raman study of the laser-induced formation of CdSe and CdS NCs in thin amorphous arsenic chalcogenide films.
Thin (1–2 mm) As2S3 and As2Se3 films doped with Cd (the nominal content of 1 to 10%) were prepared by thermal evaporation on silicate glass and Si substrates. The film morphology was checked by atomic force microscopy showing a uniform film surface with a roughness below 1 nm. X-ray photoelectron spectroscopy (XPS) measurements performed using ESCALAB 250Xi XPS Microprobe with a monochromatized Al Kα X-ray source showed the Cd content to decrease rapidly from the film surface into its depth. Micro-Raman and photoluminescence (PL) measurements were performed at room temperature using a LabRAM spectrometer and solid-state (488.0 nm and 514.7 nm) or He-Ne (632.8 nm) laser.
The formation of CdSe and CdS NCs is evidenced by Raman spectroscopy. With increasing laser power densities new sharp peaks related to CdSe or CdS LO and 2LO phonons appear in addition to the broad maxima of amorphous As2Se3 or As2S3. High-Pexc illumination leads to a photoinduced (non-thermal) drastic drop of the film viscosity and enhanced diffusion in the amorphous As2S(Se)3 film. This effect results in a radial mass transfer from the laser spot and facilitates the II–VI NC formation in the area of the laser spot. The NC formation is confirmed by the appearance of a PL maximum enabling their average size to be estimated.