Since the advent of monochromated electron energy loss spectrometry (EELS) the experimental detection of band gaps in semiconducting materials is of great importance. In the non-relativistic limit of this technique the onset of the inelastic signal represents the band gap. But due to relativistic energy losses, like Čerenkov losses and the corresponding light guiding modes, appearing at high beam energies the band gap is usually hidden. The highest beam energy, which does not excite relativistic losses in a certain material, is called the Čerenkov limit of the material. In this work the low loss EELS signals of Si, GaAs and GaP are measured at various beam energies and the calculated Čerenkov limits are experimentally confirmed.

Institute of Physical Engineering Brno University of Technology Technická 2 616 69 Brno Czech Republic

University Service Center for Transmission Electron Microscopy Technische Universität Wien Wiedner Hauptstraße 8 10 1040 Wien Austria

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Analytical electron microscopy analysis of insulating and metallic phases in nanostructured vanadium dioxide