A comparison of transmission electron microscopy methods to measure wetting layer thicknesses to sub-monolayer precision

Abstract

Electron microscopic imaging methods usually provide only indirect chemical information on thin layers. Energy-dispersive X-ray spectroscopy in a transmission electron microscope is a more direct analytical method of microanalysis but lacks atomic spatial resolution. Hence, information on ultra-thin layers must be interpreted carefully. Two approaches are compared here. They are based on either recording, in scan mode, line profiles across thin layers or, in transmission imaging mode, series of spectra with different electron beam diameters centred on the same position. The accuracy of quantification is judged using Monte Carlo simulations of electron beam broadening within a GaAs specimen. It is shown for a 0.28 nm thin InAs layer that the indirect approach based on linear regression analysis of series of spectra reliably yields sub-monolayer accuracy in the effective chemical layer width.