XPS STUDY OF THALLIUM OXIDATION-STATES IN PRECURSOR TLBACACUO HTSC THIN-FILMS

The Tl oxidation state in the precursor of Tl2Ba2CaCu2O8 superconducting films plays an important role in the post-deposition process and has been carefully examined by X-ray photoelectron spectroscopy (XPS). The Tl4f5/2 and Tl4f7/2 peaks in the XPS spectra have been deconvoluted into several peaks corresponding to different oxidation states. Under normal deposition conditions for laser ablation, it is found that mixtures of Tl-I (+ 1 oxidation state) and Tl-III (+ 3) are present with various percentages. The study of time evolution and depth profile data suggests that the surface oxidation state is a function of time and 200 angstrom of film should be removed before collecting data. The Tl-oxidation state depends strongly on the oxygen background pressure during deposition and the optimal pressure is found to be about 180 mTorr. Negatively charged Tl (presumably due to alloying with other metal constituents) is found when deposited with no background oxygen pressure.