Quantifying The Effects Of Amorphous Layers on Image Contrast Using Energy Filtered Transmission Electron Microscopy
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C. J. HUMPHREYS
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, U.K.
ABSTRACT High resolution images of a block oxide, (Nb2 0 5 )9(VW0 3 )8, with and without a superposed carbon film are compared both energy filtered and including the inelastic scattering. The differences between the images are quantified on an absolute intensity scale and possible origins of the differences in atomic level contrast are assessed using multislice simulations.
INTRODUCTION If high resolution imaging is to become truly useful in the characterisation of structure then the images obtained need to be fully quantified. While vectorial analysis of the local intensity and contrast in a characteristic area, as well as the Fourier analysis of such templates, facilitates image quantification, it is as yet not widely recognised that matches of such images with the equivalent conventional simulations are generally poor [1], and particularly so when the imaging parameters and the thickness of the specimen are determined by independent means. Part of the problem lies in the contributions to the image detail caused by inelastic scattering [e.g. 2, 3] so that filtered images, now obtainable using both Zeiss filtered microscopes (912) as well as high resolution electron microscopes fitted with added filters such as the Gatan Imaging Filter, allow a major step forward. Nonetheless a further problem derives from the effect of the contamination and damage on a specimen, particularly when it has been made using ion beam thinning methods. Not only is the angular distribution of the beam entering the specimen changed as a function of the thickness of the contamination layer above it but so are the phases of the electrons scattered by the generally amorphous material. In previous work on the effects of contamination on AIAslGaAs structures we have demonstrated that for hollow cone imaging the consequences can be a change in the atomic level contrast rather than simply the superposition of random noise on the image [4]. This previous work was carried out using unfiltered images and it was thus difficult to separate the effects on the image detail caused by the contributions from the increased angular spread due to elastic scattering in the contamination layer and by the subsequent elastic scattering of the electrons which had lost energy in the contamination layer. Here we attempt to separate the distinct contributions. The approach we have taken has been to compare quantitatively both a filtered and an unfiltered through focal series of (001) images of a test sample of the tetragonal block oxide, (Nb2 O5 )9(WO 3)8 , ground and suspended from a holey carbon support film, in areas of each of the images where the crystal was superposed by a carbon overlayer as well as where it was not. The microscope used was a JEOL 4000FX with a modified objective lens, designed by K. Tsuno of JEOL, with a measured Cc of 1.4mm and a C, of more approximately 2mm allowing somewhat enhanced resolution relative to t
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