Comparison of Si and Ge low-loss spectra to interpret the Ge contrast in EFTEM images of Si1−x Gex nanostructures

Recently, an EFTEM imaging method, exploiting the inelastically scattered electrons in the 60–90 eV energy range, was proposed to visualise Ge in SiGe alloys [Pantel, R., Jullian, S., Delille, D., Dutartre, D., Chantre, A., Kermarrec, O., Campidelli, Y., Kwakman, L.F.T.Z., 2003. Inelastic electron scattering observation using Energy Filtered Transmission Electron Microscopy for silicon-germanium nanostructures imaging. Micron 34, 239–247]. This method was proven to be highly more efficient in terms of noise, drift and exposure time than the imaging of the weak and delayed ionization GeL2,3 edge at 1236 eV. However, the physical phenomenon behind this Ge contrast was not clearly identified. In this work, we explain the origin of this Ge contrast, by comparing in details EELS low-loss spectra (<100 eV) recorded from pure Si and Ge crystals. High resolved low-loss experiments are performed using analytical Field Emission Gun Transmission Electron Microscopes fitted or not with a monochromator. Low-loss spectra (LLS) are then deconvoluted from elastic/quasi-elastic and plural scattering effects. The deconvolution procedure is established from Si spectra recorded with the monochromated machine. The absence of second plasmon and the measurement of a band gap (1.12 eV) on the Si single scattering distribution (SSD) spectrum allowed us to control the accuracy of the deconvolution procedure at high and low energy and to state that it could be reliably applied to Ge spectra. We show that the Ge-M4,5 ionisation edge located at 29 eV, which is shadowed by the high second plasmon in the unprocessed Ge spectrum, can be clearly separated in the single scattering spectrum. We also show that the front edge of Ge-M4,5 is rather sharp which generates a high intensity post edge tail on several tens of eV. Due to this tail, the Si and Ge EELS signals in the 60 to 100 eV energy window are very different and the monitoring of this signal gives information about the Ge concentration inside SiGe alloys. It is now evident that the EFTEM imaging technique proposed to quantify Ge (90 eV/60 eV image ratio) in Si–Ge nanostructures is valid and is a relevant way of exploiting the Ge-M4-5 ionisation edge.