In the present work, Cs aberration corrected and monochromated scanning transmission electron microscopy electron energy loss spectroscopy STEM-EELS has been used to explore experimental set-ups that allows bandgaps of high refractive index materials to be determined. 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 and the tail of the zero loss peak. The bandgap of several GaAs-based materials were successfully determined by direct inspection and without any background subtraction of the EEL spectra. The presented set-up does not require that the acceleration voltage is set to below the Cerenkov limit and can be applied over the entire acceleration voltage range of modern TEMs and for a wide range of specimen thicknesses.
@article{arxiv.1609.09778,
title = {Bandgap measurement of high refractive index materials by off-axis EELS},
author = {Maryam Vatanparast and Ricardo Egoavil and Turid W. Reenaas and Johan Verbeeck and Randi Holmestad and Per Erik Vullum},
journal= {arXiv preprint arXiv:1609.09778},
year = {2016}
}