Bandgap measurement of high refractive index materials by off-axis EELS

TL;DR

This study demonstrates a novel off-axis EELS method using aberration-corrected STEM to accurately measure bandgaps of high refractive index materials across various TEM settings without background subtraction.

Contribution

It introduces an off-axis EELS technique that suppresses zero loss peak effects, enabling direct bandgap measurement in high refractive index materials without low voltage constraints.

Findings

Successful bandgap determination of GaAs-based materials

Method works across a wide range of acceleration voltages

No background subtraction needed for spectra

Abstract

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.