[Zurück]

M. Stöger-Pollach, L. Kachtik, B. Miesenberger, P. Retzl:
"Electron beam induced radiation explored by CL and EELS";
Poster: 13th Multinational Congress on Microscopy, Rovinj, Kroatien; 24.09.2017 - 29.09.2017; in: "13th Multinational Congress on Microscopy", (2017), S. 217 - 218.

When the electron beam passes through the specimen in a transmission electron microscope (TEM), several photon emission mechanisms are activated. The most common are the excitation if X-rays being detected within energy or wave length dispersive X-ray spectroscopy (EDX or WDX) and the excitation of IR-VIS-UV light detected by cathodoluminescence. The latter is generated if a valence electron was excited into the conduction band and recombines with its hole in the valence band or of a radiative surface mode of a non-periodic nanostructure is excited. Additionally Cerenkov radiation and transition radiation are generated by the swift electron. The emission probability per incoming electron for transition radiation (TR) is measured by employing electron energy losses spectrometry (EELS) and cathodoluminescence (CL) in a transmission electron microscope (TEM) using beam energies varying from 20 - 200 keV. We further demonstrate that TR is excited only at the sample surfaces, because the emitted intensity and the respective energy loss are independent of the sample thickness. As specimen we use an aluminium single crystal, because TR is known to be strongest on metals. For studying Cerenkov radiation we record CL spectra of a crystalline MgO specimen. Due to its low refractive index the Cerenkov limit can be determined to be approximately 90 keV (1). CL spectra were recorded using a GATAN VULCAN system, corrected for the spectrometer response and integrated over the whole visible range. Using EELS we explored the angular behaviour of the transition radiation losses (TRLs) and verified the emission probability by observing the corresponding inelastic contributions in the EELS spectrum. Summing up it is the first time that within a TEM transition radiation of aluminium
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was studied by means of EELS and CL. We proved experimentally that the angular dispersion of the TRLs is within the light cone, that TR is emitted from the surfaces only, and we determined the photon emission probability with respect to the beam energy. Due to the fact, that the probability for the emission of TR is very low, even for aluminium, it is even smaller in the case of semiconductors, where low loss EELS is used for determining the local dielectric behaviour.