On the validity of the Čerenkov limit as a criterion for precise band gap measurements by VEELS

The generation of Čerenkov photons, contributions from retardation and surface effects, and the excitation of guided light modes can complicate the analysis of valence electron energy-loss spectroscopy (VEELS) data. In recent works it was argued that working below the so-called Čerenkov limit unwanted spectral contributions can be avoided, simplifying the reliable measurement of band gap energies and the extraction of the dielectric function by the Kramers-Kronig analysis. Here, relativistic simulations of valence electron energy-loss spectra of GaAs are employed in order to identify different spectral contributions. It is found that for electron energies below the Čerenkov limit the spectra can still be affected by the excitation of guided light modes and retardation effects. These spectral contributions can influence the position of the apparent band gap signal and can be problematic for the Kramers-Kronig analysis when working below or above the Čerenkov limit.