A study was made of the influence of Mn doping upon the vibrational properties of radio-frequency sputtered ZnO thin films. Raman spectra of the Mn doped ZnO samples revealed 2 additional vibrational modes, in addition to the host phonon modes, at 252 and 524/cm. The intensity of the additional modes increased continuously with Mn concentration in ZnO and could be used as an indication of Mn incorporation in ZnO. The modes were attributed to the activation of ZnO silent modes due to relaxation of Raman selection rules produced by the breakdown of the translational symmetry of the crystal lattice with the incorporation of Mn at the Zn site. Furthermore, the A1 (LO) mode was observed with very high intensity in the Raman spectra of undoped ZnO thin film and was attributed to the built-in electric field at the grain boundaries.

Defect Induced Activation of Raman Silent Modes in RF Co-Sputtered Mn Doped ZnO Thin Films. Yadav, H.K., Sreenivas, K., Katiyar, R.S., Gupta, V.: Journal of Physics D, 2007, 40[19], 6005-9