Assessing a growth anomaly in ion-beam sputtered non-stoichiometric NiOx

Nonstoichiometric NiOx thin films were grown on single crystal substrates of c-plane (0001) sapphires by ion-beam sputter-deposition (IBSD) of a Ni metal target in a mixed argon and oxygen atmosphere. Structural characterization was carried out by X-ray diffraction and scanning electron microscopy. All samples grew (111)-oriented out-of-plane and with a defined in-plane orientation relationship relative to the crystalline substrate. The chemical bonding information of the films was examined by X-ray photoelectron spectroscopy showing that the composition x could be varied by adjusting the oxygen-to-argon ratio in the IBSD process. However, a growth anomaly was detected for a certain range of synthesis parameters, standing out due to an enhanced growth rate, nickel excess, and unusually elongated surface structures. With joint solid-state and plasma diagnostic tools, the underlying processes on the atomic scale were studied. An increased proportion of atomic oxygen species in the intermediate range of the oxygen-to-argon gas flux ratio was identified to be responsible for an enhanced generation of NiOx species. Optical emission spectroscopy was found to be a tool especially well-suited, since the in situ examination of various locations of interest (plasma, ion beam, and vicinity of the target) is feasible nonintrusively. Published under license by AIP Publishing.