Abstract
Hard X-ray Photoelectron Spectroscopy (HAXPES) provides minimally destructive depth profiling into the bulk, extending the photoelectron sampling depth. Detection of deeply buried layers beyond the elastic limit is enabled through inelastic background analysis. To test the robustness of this technique, we present results on a thin (18 nm) layer of metal–organic complex buried up to 200 nm beneath organic material. Overlayers with thicknesses 25–140 nm were measured using photon energies ranging 6–10 keV at the I09 end station at Diamond Light Source, and a new fixed energy Ga Kα (9.25 keV) laboratory-based HAXPES spectrometer was also used to measure samples with overlayers up to 200 nm thick. The sampling depth was varied: at Diamond Light Source by changing the photon energy, and in the lab system by performing angle-resolved measurements. For all the different overlayers and sampling depths, inelastic background modelling consistently provided thicknesses which agreed, within reasonable error, with the ellipsometric thickness. Relative sensitivity factors were calculated, and these factors consistently provided reasonable agreement with the expected nominal stoichiometry, suggesting the calculation method can be extended to any element. These results demonstrate the potential for the characterisation of deeply buried layers using synchrotron and laboratory-based HAXPES.
Highlights
There is a need to obtain elemental and chemical state information non-destructively on length scales of 50–150 nm into the material sur face, across a wide range of materials science, physics and chemistry
We have explored use of the angle-resolved mode of the labbased Hard X-ray Photoelectron Spectroscopy (HAXPES) analyser for this purpose, and here we demonstrate that it can be used to accumulate a set of spectra at different angles of photoemission, and differing sample depths according to Eqs. (2) and (12), without moving the sample. 2D data of emission angle vs. electron kinetic energy are accumulated at the CCD camera after the analyser, and the emission angle may be converted to elastic sampling depth according to Eq (2) or to inelastic sampling depth (Eq (12))
We have demonstrated the routine and accurate application of in elastic background modelling of hard X-ray photoelectron spectra by testing a large set of high-quality samples made up of metal–organic complexes buried underneath calibrated overlayer thicknesses up to 200 nm thick
Summary
There is a need to obtain elemental and chemical state information non-destructively on length scales of 50–150 nm into the material sur face, across a wide range of materials science, physics and chemistry. This is true both in the research environment and increasingly important in industry as structures on this depth scale become prevalent in elec tronic, photonic, magnetic and even medical devices. Where L is the electron effective attenuation length This is related to the inelastic mean free path (IMFP, λ) of photoelectrons, and for the organic materials used here, where elastic scattering has a negligible influence (the single scattering albedo, ω, is approximately 0), L ≈ λ [3,4].
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