Identification of Nitrogen Chemical States in N-Doped ZnO via X-ray Photoelectron Spectroscopy

Nitrogen-doped films of ZnO grown by two methods, metalorganic chemical vapor deposition (MOCVD) and reactive sputtering, were studied with x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Systematic differences in the N chemical states were observed between films grown by sputtering and MOCVD: only two N chemical states were observed in films grown by reactive sputtering, whereas four N chemical states were observed in MOCVD films. To aid in the assignment of the N chemical states, photoemission data from the polycrystalline films were compared with data taken on N ₂ ⁺-implanted Zn metal and N ₂ ⁺-implanted ZnO. High-resolution core level spectra of the N 1s region indicated that nitrogen can occupy at least four different chemical environments in ZnO; these include the N _O acceptor, the double donor (N ₂) _O, and two carbon-nitrogen species. Valence band spectra indicate that the Fermi energy of all films studied was near the conduction band minimum, implying that the films remained n-type after nitrogen doping. Analysis of the relative amounts of acceptors and donors identified by XPS in the sputter-grown films provides clues as to why only a small percentage of incorporated nitrogen is found to contribute to carriers, and points toward possible paths to higher quality ZnO:N films.