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 N-2 flow rate mixed with Ar. Raman measurements indicate that the intensity of similar to 274 cm(-1) mode of ZnO:N films exhibits an anomalous variation, which neither depends on N-2 flow rate nor on N-O acceptor content based on X-ray photoelectron spectroscopy (XPS) analysis. Combined with defect formation energy calculations, it is demonstrated that the similar to 274 cm(-1) mode is attributed to Zn-i defects which can be increased by the incorporation of N-O acceptors, but suppressed by the presence of (N-2)(O) double donors. XPS and optical absorption spectra suggest that ZnO:N film prepared under specific N-2 flow rate (Ar:N-2 = 3:1), has high concentration of acceptor N-O with shallow states and the absence of shallow donor (N-2)(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 Zn-i defects bonded with N-O acceptors. Performing density functional calculations in conjunction with the climbing image nudged elastic band method shows that Zn-i could be dissociated from Zn-i-N-O complexes by post-annealing treatment and N-O 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. (C) 2014 Elsevier B.V. All rights reserved.