Photoelectron energy loss spectroscopy: a versatile tool for material science

Abstract

International audienceX-ray Photoelectron Spectroscopy (XPS) used in quantitative chemical analysis of solid surfaces requires subtraction of a broad background, arising from various energy loss mechanisms, to obtain reliable core level peak intensities. Besides single electron excitation, collective electron oscillations (plasmons) can be excited in the bulk and at the surface. Photoelectron energy loss spectroscopy (XPS-PEELS) is a non-destructive tool useful for both process control and thin film metrology. This review emphasizes its versatility to elucidate material research issues. The energy loss function (ELF) is useful for thin film growth optimization since it gives insight in valence electron density, hardness, optical band gap and interface properties such as adhesion and wetting. XPS-PEELS also provides depth and width of implanted atom profiles in solid targets, e.g. Ar nanobubbles in Al. Special emphasis is given to the retrieval of electronic properties from XPS-PEELS data. Since the ELF, q, is related to the q-averaged dielectric function, q, the latter can be obtained by taking into account multiple bulk and surface plasmon excitations. This task is rather simple in wide band gap materials where the ELF and the no-loss peak are clearly separated, as illustrated by amorphous silicon, amorphous carbon or Al oxide data. In contrast, in metals or small band gap materials, the broad asymmetric photoemission peak overlaps the ELF and low energy features in the ELF may be lost. A Fourier Transform (FT) method is proposed to analyze PEELS data, with the objective of retrieving such low energy excitations, e.g. inter band transitions. This FT method is compared with an empirical method based on a smooth cutoff of the zero-loss peak, using PEELS data obtained from Al2O3. Current developments of a quantum mechanical theory are crucial to obtain the respective contributions of intrinsic and extrinsic plasmon excitation (along with their interference) and to assess some approximations performed in classical treatments