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2. EXPERIMENTAL METHODS contact between the sample, i.e. thin film, and the underlying sample holder. For metals in general the issue does not appear. However for semiconducting and insulating materials it is necessary to ensure the contact between bottom of the sample and top of the holder, as well as provide additional ’tracks’ for electrons by applying conducting silver paste glue at sample edges or even glue the sample to the holder. Because below the MI transition the films were semiconducting it was neces- sary to prevent the positive charge build-up at the films’ surfaces. A low incidence stream of low energy electrons was used to compensate for the charge of ejected photoelectrons. The compensation was verified by looking most often at carbon C 1s or oxygen O 1s spectral line. Both the emission current and electron energy were adjusted. The multiplet spectra of nickel were most often measured with pass energy 23.5 eV. Fitting of spectra was made with a constant number of lines. For main multiplet line it were 4 components, while for the 2p3/2 and 2p1/2 satellites - 3 components were necessary. The same set of components was used for fitting spectra obtained at every temperature. At initial stages of each fitting procedure constraints were set for the multiplet splitting energy and ratio between 2p3/2 and 2p1/2 lines . The background was fitted with iterated Shirley model. Due to charging effects it was sometimes necessary to use the low energy electron gun. This however was not always sufficient to neutralize the measured surface. Therefore a post-processing procedure involving spectra shift with respect to a certain XPS line was done. The reference peak was either the C1s line of carbon or Sm 3d5/2 line of samarium which is not expected to vary with temperature. For each fitted nickel dataset the line position, FWHM, Ni3−δ/Ni3+δ area ratio, Ni3+δ- Ni3−δ energy displacement, spin-orbit splitting, were extracted and compared. Obtaining good quality nickel spectra of SmNiO3 thin films was not an easy task due to a layer of contaminations of the surface. Long several-hour experi- ments, especially at lower temperatures, were necessary to increase the statistics and signal-to-noise ratio. Temperature was kept constant during each experiment. Prolonged spectra aquisition time and irradiation by X-rays have at times caused charging effects that were adjusted for before the measurement. At the beginning of each experiment any changes were monitored and if necessary, additional ad- justments were made. It was necessary for some of the samples to mechanically modify the surface by scraping with an alumina blade. 52PDF Image | Investigation of metal-insulator transition in magnetron sputtered samarium nickelate thin films
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