Valence-Band and Chemical-State Analyses of Zr and O in Thermally Grown Thin Zirconium-Oxide Films: An XPS Study

In situ X-ray photoelectron spectroscopy (XPS) was applied to investigate the valence-band spectrum and to analyze the local chemical states of Zr and O in thin (thickness <10 nm) oxide films, grown on bare single-crystalline Zr surfaces by dry thermal oxidation in the temperature range of T = 300-450 K. The oxide films grown at T <= 400 K are predominantly amorphous. The measured upper valence band (UVB) region of the grown oxide films shows pronounced changes in shape with increasing oxidation temperature, which can be attributed to the gradual formation of a tetragonal ZrO2-like phase at oxidation temperatures T > 400 K. The resolved Zr 3d(5/2) and O 1s photoelectron lines and the Zr M45N1N23, Zr M45N23N23, Zr M45N23V, and O KL23L23 Auger transitions were combined to construct Wagner plots for Zr and O in the oxide films. The observed decreases of the Zr and O Auger-parameter values with increasing oxidation temperature evidence a lowering of the electronic polarizability around core-ionized Zr and O atoms. It was concluded that the amorphous-to-crystalline transition of the oxide films with increasing oxidation temperature is accompanied with an increase in the Zr-O bond ionicity and changes in the first coordination spheres of both Zr and O. The results obtained for the amorphous-to-crystalline transition of zirconium-oxide films were compared with those for aluminum-oxide films.