Glancing-angle XAFS of buried ultrathin films

Abstract

The techniques for glancing-angle XAFS of ultrathin multilayer systems have been analyzed, with an emphasis on the conditions of detection modes under which distortion- free ne structure data are obtainable. The XANES of a multilayer system obtained in the total reection geometry can be distorted because of anomalous dispersion effects in the sample layer and in adjacent media. The former plays a negligible role in ultrathin lm cases. The latter, caused by the interference of the evanescent waves from the interfaces involved, cannot always be ignored. The distortion depends on the glancing angle, the composition of the layered structure (mainly the thickness of the layer of interest and the material of other layers), the energy from the absorption edge, and the detection modes adopted. Calculated results of these effects are presented and analytical expressions are provided in closed form. Experimental XANES data of a buried 8 monolayer (ML) Cu lm, representing the thin limit, and 37 ML Ni, representing the medium thickness are used to illustrate the theory. In the thin limit, ltered uorescence XANES data are independent of angle and are not distorted. However, total electron-yield and reectivity data do depend on angle and are signicantly distorted near the critical angle. The thickness dependence of the EXAFS measured at glancing angles is also analyzed. It is shown that both the distortions in EXAFS amplitude and phase caused by the anomalous scattering effects are strongly dependent on the sample lm thickness, but when the sample thickness is in the range of 10 atomic layers the distortions are practically negligible for all X-ray incident angles. Experimental data for 8 ML bct Cu grown on Ag(001) substrate indicate that the phase and amplitude of the data have no detectable dependence on the X-ray incident angle through the critical-angle region, in agreement with the calculated results.