Defect induced activation of Raman silent modes in rf co-sputtered Mn doped ZnO thin films

We study the influence of Mn doping on the vibrational properties of rf sputtered ZnO thin films. Raman spectra of the Mn doped ZnO samples reveal two additional vibrational modes, in addition to the host phonon modes, at 252 and 524?cm?1. The intensity of the additional modes increases continuously with Mn concentration in ZnO and can be used as an indication of Mn incorporation in ZnO. The modes are assigned 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 is observed with very high intensity in the Raman spectra of undoped ZnO thin film and is attributed to the built-in electric field at the grain boundaries.