Exploring possibilities of band gap measurement with off-axis EELS in TEM

TL;DR

This paper presents a novel off-axis EELS technique in TEM using a Bessel aperture to accurately measure band gaps in dielectric materials with high refractive index, suppressing unwanted signals and enhancing spectral clarity.

Contribution

The study introduces a new off-axis EELS method with a Bessel aperture that improves band gap measurement accuracy and signal-to-noise ratio in dielectric materials.

Findings

Effective suppression of Cherenkov losses and surface modes.

High-quality band gap signals obtained for diamond, GaN, and AlN.

Enhanced spatial resolution in band gap measurements.

Abstract

A technique to measure the band gap of dielectric materials with high refractive index by means of energy electron loss spectroscopy (EELS) is presented. The technique relies on the use of a circular (Bessel) aperture and suppresses Cherenkov losses and surface-guided light modes by enforcing a momentum transfer selection. The technique also strongly suppresses the elastic zero loss peak, making the acquisition, interpretation and signal to noise ratio of low loss spectra considerably better, especially for excitations in the first few eV of the EELS spectrum. Simulations of the low loss inelastic electron scattering probabilities demonstrate the beneficial influence of the Bessel aperture in this setup even for high accelerating voltages. The importance of selecting the optimal experimental convergence and collection angles is highlighted. The effect of the created off-axis acquisition conditions on the selection of the transitions from valence to conduction bands is discussed in detail on a simplified isotropic two band model. This opens the opportunity for deliberately selecting certain transitions by carefully tuning the microscope parameters. The suggested approach is experimentally demonstrated and provides good signal to noise ratio and interpretable band gap signals on reference samples of diamond, GaN and AlN while offering spatial resolution in the nm range.