Optically active vibrations of extrinsic iron defects in zinc oxide

In the paper, using Fe³⁺ ions in a non-centrosymmetric ZnO lattice as an example, we present a theoretical study of impurity-induced vibrations. The modeling was carried out within the framework of density functional theory using the generalized gradient approximation and the potential-based method. The C_3v lattice distortions around a trivalent impurity were computed. Independent calculation methods give similar results, which indicates their reliability. We calculated local symmetrized phonon densities of states in Fe-doped ZnO and determined the frequencies of the impurity vibrations of different symmetry types induced by charged Fe ions. The results of lattice-dynamic calculations were used to interpret the structure of the phonon sideband that accompanies the zero-phonon line in the polarized emission spectra associated with intracenter transitions of Fe³⁺. We believe that the approach used allows us to objectively evaluate the contribution of charged impurities with a weak electron-phonon coupling and surrounding ions to the formation of the main peaks observed in the vibronic spectrum of crystals.