Optical constants and electron inelastic mean-free paths of SiO₂, HfO₂ and CaF₂ extracted from reflection electron energy loss spectra

Abstract

Reflection electron energy loss spectroscopy (REELS) spectra were measured for three inorganic compounds (SiO<inf>2</inf>, HfO<inf>2</inf> and CaF<inf>2</inf>). VIS-XUV optical constants were extracted from the measured REELS spectra after elimination of multiple inelastic scattering by deconvolution. The derived normalised single scattering loss distributions were fitted to a Drude-type dielectric function. The resulting optical constants reasonably satisfy the Kramers–Kronig and (Formula presented.) -sum rules. Values for the energy band gap were also determined from the deconvoluted loss spectra. Special attention is paid to the extension of the optical constants onto the complex plane and its relation to the kinematics in an inelastic collision for a material with a given electronic structure. The analysis suggests differences in the elementary scattering kinematics in a REELS experiment between insulators and metals. Describing the dispersion of the loss features on the complex plane by the dispersion constant (Formula presented.), we find that the dispersion constant (Formula presented.) in the analysis of REELS data should be treated as a fit parameter or, alternatively, use of a vanishing value for insulators and (Formula presented.) for conductors is recommended. For evaluating the inelastic mean-free path (IMFP) from optical constants, (Formula presented.) should always be taken to be unity.