Abstract
Variable-period x-ray standing wave (VPXSW) studies have been carried out using 3 keV x rays and photoelectron detection. Two model surfaces have been used, a native $\mathrm{Si}{\mathrm{O}}_{2}$ layer (20 \AA{} thick) on bulk silicon, and a purpose-built multilayer surface comprising a chloroform/water marker layer (12 \AA{} thick) on an ionic liquid spacer layer (211 \AA{} thick) deposited on a $\mathrm{Si}{\mathrm{O}}_{2}$/Si substrate at 90 K. By using photoelectron detection, both chemical and elemental sensitivity were achieved. The surfaces were modeled using dynamic x-ray scattering for x-ray intensity, and attenuation of photoelectrons transmitted through the layers, to produce simulations which accurately reproduced the experimental VPXSW measurements. VPXSW measurements made using the substrate, spacer layer, and marker layer photoelectron signatures produced consistent structural values. This work demonstrates that VPXSW can be used to determine chemically specific layer thicknesses within thick ($\ensuremath{\lesssim}300\phantom{\rule{0.16em}{0ex}}\AA{}$) surface structures composed of the light elements B, C, N, O, F, and Cl with an accuracy of 10 to 15 \AA{}, perpendicular to the surface.
Highlights
The techniques available for determining the composition of thin films 1-50 nm thick, as a function of depth, are currently rather limited
Variable period X-ray standing wave experiments have been carried out using photoelectron detection. 3 keV X-rays were reflected from a polished silicon surface over an angular range of 2.5 ̊
Two surfaces layer structures were studied: the native oxide of silicon on a bulk silicon substrate, and a purpose built model surface consisting of a the native oxide/silicon as a substrate, surmounted by a spacer layer of solid [OMIM][BF4], which terminated in a marker layer composed of CHCl3 and H2O
Summary
The techniques available for determining the composition of thin films 1-50 nm thick, as a function of depth, are currently rather limited. In this work we demonstrate that it is possible to quantitatively determine the structure of a thick film ( ≲ 250 Å) using several periods of an X-ray standing wave set up by reflection from a buried interface, in the manner of GI-XSW, using photoelectron detection rather than X-ray fluorescence to achieve chemical state specificity via the photoelectron chemical shift 38 This eliminates the need for a crystalline substrate, greatly lengthens the analysis range and provides more structural information than the single period from Bragg diffraction. This demonstrates that adlayers of the order of 100's Å can be structurally analysed with an accuracy of 10-15 Å
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