The origin of the ∼274 cm−1 additional Raman mode induced by the incorporation of N dopants and a feasible route to achieve p-type ZnO:N thin films

Nitrogen doped ZnO films (ZnO:N) were deposited on quartz glass substrates by radio-frequency magnetron sputtering technique with various N2 flow rate mixed with Ar. Raman measurements indicate that the intensity of ∼274 cm−1 mode of ZnO:N films exhibits an anomalous variation, which neither depends on N2 flow rate nor on NO acceptor content based on X-ray photoelectron spectroscopy (XPS) analysis. Combined with defect formation energy calculations, it is demonstrated that the ∼274 cm−1 mode is attributed to Zni defects which can be increased by the incorporation of NO acceptors, but suppressed by the presence of (N2)O double donors. XPS and optical absorption spectra suggest that ZnO:N film prepared under specific N2 flow rate (Ar:N2 = 3:1), has high concentration of acceptor NO with shallow states and the absence of shallow donor (N2)O defects, could be most likely to achieve p-type conductivity. But, at the same time, such specific ZnO:N film is the presence of shallow donor Zni defects bonded with NO acceptors. Performing density functional calculations in conjunction with the climbing image nudged elastic band method shows that Zni could be dissociated from Zni-NO complexes by post-annealing treatment and NO acceptors would be activated to p-type ZnO:N. This is confirmed by our further Hall investigation, indicating that p-type ZnO:N can be achieved by choosing appropriate post-annealing treatment.