Determination of bandgaps and optical properties using electron energy loss spectroscopy (EELS) has attracted interest since monochromated transmission electron microscopes (TEM) with excellent spatial resolution and an energy resolution that is in the tens of meV range have become available. .
However, for small bandgap materials (like Si, GaAs) Čerenkov losses changes the valence EEL spectrum because of the large refractive index . The phase velocity of light in the sample material scales with the refractive index, and beam electrons which move faster than this phase velocity of light will suffer. The occurrence of Čerenkov losses causes unwanted spectral artifacts which affect the interpretation of low-loss EELS and complicates precise measurements of bandgap values. In addition to Čerenkov losses, contributions from retardation and surface effects, and the excitation of guided light modes can complicate the analysis of valence EELS data. For determination of optical properties, one has to ensure that no relativistic effects impact the low-loss signal .
In our previous work , we presented an experimental set-up that allows bandgaps of high refractive index materials to be determined. In this method, semi-convergence and -collection angles in the micro-radian range were combined with off-axis or dark field EELS to avoid relativistic losses and guided light modes in the low loss range to contribute to the acquired EEL spectra. Off-axis EELS further suppressed the zero loss peak. The bandgap of several GaAS-based materials were successfully determined by simple regression using this method.
In this work we have continued band gap measurements of GaAs-based materials to improve our methodology and to transfer it to slightly different compositions. Materials used in this work are all potential candidate structures for use in intermediate band solar cell applications. On the top of a (001) GaAs substrate, one of the samples has a periodic stack of thin Inx Gax-1As quantum well layers grown by molecular beam epitaxy. The spacer layers in between consist of 50 nm GaAs. In the other sample, on top of the substrate a stack of AlxGa1-xAs layers with 30 nm thick layers are grown. A double corrected Titan TEM with monochromator was used, operating at 60 kV in all experiments.
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 M. Vatanparast, R. Egoavil, T.W. Reenaas, J. Verbeek, R. Holmestad and P.E. Vullum, Ultramicroscopy, 182 (2017) 92-98
The Norwegian Research Council is acknowledged for funding the HighQ-IB project under contract no. 10415201