Yang-Ming Lu

Effects of substrate temperature on the resistivity of non-stoichiometric sputtered NiOx films

Abstract Thin films of nickel oxide (NiO) were deposited on Corning 7059 glass substrates by RF magnetron sputtering. The relationship between substrate temperature and resistivity and the microstructural defects of the NiO films were investigated. Crystalline NiO film with (111) orientation was obtained in this study. A resistivity of 0.22 Ω cm and a hole concentration of 4.4×10 19 cm −3 were obtained for non-doped NiO films prepared at a substrate temperature of 300 °C in pure oxygen sputtering gas. As the substrate temperature was increased from 300 to 400 °C, the resistivity changed from 0.22 to 0.70 Ω cm. The mechanism of electrical conductivity for the NiO films is discussed from the viewpoint of defect chemistry and was confirmed by X-ray photoelectron (XPS) and energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) data.

Point defects in sputtered NiO films

The dominant point defects in p-type NiO films were determined by analyzing the coordination number (CN) change with various annealing temperatures and the composition profile of double-layer films deposited individually in oxygen and in argon atmospheres. The results show that the nonstoichiometry of sputtered NiO film is determined by the number of nickel atoms rather than by the number of oxygen atoms. It is concluded that nickel vacancies are the dominant point defects that result in the electrical conductivity of NiO films.

Local geometric and electronic structures of gasochromic VOx films

VOx films were deposited by radio-frequency reactive magnetron sputtering from a vanadium target at room temperature. Local atomic and electronic structures of the films were then modified by thermal annealing. The oxidation state and structural and gasochromic properties of the films were elucidated by X-ray absorption spectroscopy. Analytical results indicate that the as-deposited VOx films were amorphous with mixed V(4+) and V(5+) valences. The amorphous VOx had a disordered and expanded lamellar structure resembling that of polymer-intercalated V2O5 gels. VOx films were crystallized into orthorhombic V2O5 at 300 °C, and the lamellar structure was eliminated at 400 °C. Additionally, the gasochromic reaction reduced the vanadium valence via intervalence transitions between V(5+) and V(3+). Moreover, removing the lamellar structure reduced the gasochromic rate, and the gasochromic reaction transformed the V2O5 crystalline phase irreversibly into an H1.43V2O5 phase. Based on the results of this study, amorphous VOx with a lamellar structure is recommended for use in H2 gas sensors.

A study of thermal decomposition of sputtered NiO films

NiO films were deposited by radio-frequency reactive magnetron sputtering from a NiO target. A decomposition reaction was found as the films annealed at 400 °C in vacuum. The decomposition increased with increasing annealing temperature and the non-stoichiometry of the film. The electronic and atomic structure of the annealed films were further investigated by X-ray absorption near-edge structure (XANES) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The results show the decomposition of sputtered NiO is attributed to excess vacancies that formed during sputtering, which destroyed the atomic bonds in the NiO crystalline. The EXAFS results indicate that there is a transition stage of surplus Ni ions filling anion sites at the beginning of the decomposition reaction. The surplus Ni ions act as a link between the central atom and the Ni in the second shell, which may be responsible for the filamentary conducting paths proposed in resistive random access memory (ReRAM).