On the origin of an additional Raman mode at 275 cm−1 in N-doped ZnO thin films

A systematic investigation on the optical properties of N-doped ZnO thin films was performed in order to understand the origin of an additional Raman mode at 275 cm−1. This Raman peak was observable only at N2 pressures lower than 30 Pa during pulsed laser deposition. Its intensity decreased with an increase of N2 pressures and eventually vanished at pressures above 30 Pa. N substitution of O (NO) was identified by photoluminescence and x-ray photoelectron spectroscopy and correlated well with the Raman intensity. The electrical measurements showed significant changes in resistivity, charge carrier concentration, and mobility due to the presence of N acceptors. Investigations on undoped ZnO films grown in Ar without N2 further confirm that N doping plays a key role in the Raman scattering. The experimental data indicate that the Raman mode originates from NO related complexes, likely in the form of Zni-NO. These investigations help to understand the doping mechanisms and underlying physics of the additional Raman mode in the ZnO films.