VEELS band gap measurements using monochromated electrons

With the development of monochromators for transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) has become a powerful technique to study the band structure of materials with high spatial resolution. However, artefacts such as Cerenkov radiation and surface effects pose a limit for interpretation of the low-loss spectra; also the inelastic delocalisation restricts the spatial resolution on band gap mapping. For direct semiconductors, spectra acquired at thin regions can efficiently minimize the Cerenkov effects. Examples of h-GaN spectra acquired at different thickness showed that a correct band gap onset value can be obtained for sample thicknesses up to about 60 nm. For indirect semiconductors, the correct band gap onset can be obtained in the dark-field mode when the required momentum transfer for indirect transition is satisfied. At low energy-loss range the spatial resolution of this technique, which is mainly limited by the inelastic delocalisation, can be improved by dark-field VEELS at high collection angles.