Cesium doped and undoped ZnO nanocrystalline thin films: a comparative study of structural and micro-Raman investigation of optical phonons

Undoped and cesium-doped zinc oxide (ZnO) thin films have been deposited on sapphire substrate (0001) using the sol-gel method. Films were preheated at 300 degrees C for 10 min and annealed at 600 and 800 degrees C for 1 h. The grown thin films were confirmed to be of wurtzite structure using X-ray diffraction. Surface morphology of the films was analyzed using scanning electron microscopy. The photoluminescence (PL) spectra of ZnO showed a strong ultraviolet (UV) emission band located at 3.263 eV and a very weak visible emission associated with deep-level defects. Cesium incorporation induced a blue shift of the optical band gap and quenching of the near-band-edge PL for nanocrystalline thin film at room temperatures because of the band-filling effect of free carriers. A shift of about 10-15 cm(-1) is observed for the first-order longitudinal-optical (LO) phonon Raman peak of the nanocrystals when compared to the LO phonon peak of bulk ZnO. The UV resonant Raman excitation at RI shows multiphonon LO modes up to fifth order. Copyright (C) 2010 John Wiley & Sons, Ltd.