Quantum size effects in CdS thin films

Resonant Raman spectroscopy is used to study quantum size effects in CdS films. The lattice softening of the CdS LO-phonon mode in a CdS film with a thickness less than 800 Å is observed. As the thickness is less than 410 Å, the TO-phonon mode is observed at 4880 Å excitation wavelength, which is above the band gap of bulk CdS (2.42 eV) at room temperature. These phenomena are attributed to the size quantization effects of the grain size and the low-dimensional thin-film structure. The quantum size effects cause a blueshift of the band gap in the as-deposited CdS thin film. The peak of the TO-phonon-mode Raman line of the CdS film is observed around 220 ${\mathrm{cm}}^{\mathrm{-}1}$, which has a shift of 8 ${\mathrm{cm}}^{\mathrm{-}1}$ from the Raman line of the most active TO-phonon mode of bulk CdS. The magnitude of softening energy of the TO-phonon mode is observed to be independent of the film thickness.