Identification of nitrogen chemical states in N-doped ZnO via x-ray photoelectron spectroscopy

Abstract

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 N2+-implanted Zn metal and N2+-implanted ZnO. High-resolution core level spectra of the N1s region indicated that nitrogen can occupy at least four different chemical environments in ZnO; these include the NO acceptor, the double donor (N2)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.