Transcript Folie 1
TEM-investigation of CrSiON sample of Lorenzo Castaldi (EMPA) EMEZ-Proposal 0190 Elisabeth Müller, 31.10.2007 Zeitaufwand: Mikroskop Auswertung/Bericht 3h30 3h30 If any of the TEM-data is used for a publication, I would certainly like to read the publication before submission 1. Materials and methods A. Problem Structural characterisation of the a CrSiON layer B. Sample preparation Tripod polishing, done at EMPA by Lorenzo Castaldi C. Methods HAADF-STEM: The sample has so far been investigated by scanning transmission electron microscopy and EDX ( in STEM mode). For the STEM imaging I used the high angular annular dark field detector (HAADF). For this nealy fully incoherent imaging technique the electron beam is highly focused instead of being a plane wave. As in an SEM this beam is scanned over the sample area. In a homogenuous, defect-free material the image contrast is just reversed to that of conventional TEM: the brightness increases for thick areas and heavier atomic species, the vacuum part appears as black area. Since this technique is analogous to Rutherford-scattering but with electrons The intensities seen in the images show about a linear dependence on the sample thickness and nearly a square one from the atomic number Z. Therefore this technique is highly chemically sensitive and is also called Z-contrast imaging. For this work the Tecnai F30ST (FEI) transmission electron microscope (operated at 300kV, point to point resolution 0.19nm) was used. EDX: (Energy dispersive X-ray spectroscopy) When interacting with matter electrons may deposit some of there energy in the material (inelastic scattering processes). This energy is at some later time released by the emission of an X-ray. Since the energy of these X-rays is specific for each type of atom, it is a means to determine qualitatively the composition of the material under investigation. The sensitivity of the EDXdetector, however, is rather low for the low energies e.g. emitted from light elements. Due to re-absorption of X-rays still within the material and subsequent re-emission of a new X-ray as well as due to several other effects, a quantification of the composition of the material is not very reliable. Since all values given in this report are not calibrated by a calibration sample of well known composition, but just calculated theoretically, these values are not reliable in a quantitative way. They just give a hint to some higher or lower concentration of neighbouring areas within the sample. HAADF STEM image: STEM-48k-edge-2_particle.jpg HAADF STEM image: STEM-68k-edge-2-particle.jpg particle Columnar structure??? compositional modulations The layer consists of different areas: There are parts of the layer with a structure reminding of large grains (possibly with some columnar structure, but possibly also effect of another particle, which is just not contained in the lamella any more) In these areas a modulation of the concentration appears to be present. The layer, however, is desturbed by the presence of a particlelike structure: these particles have a diameter of up to several 100nm. In the surroundings of the particles the lattice is highly disturbed. The density in the direct neighbourhoud appears to be much lower (probably there are voids) on top of them the lattice looks rather polycrystalline. defective structure void? particles-34k-EDXspectrum area-2.jpg HAADF STEM image showing the areas from which the EDX-measurements are taken: particles-34k_EDX_image.jpg particles-34k-EDXspectrum area-4.jpg Particles34k_EDX Area-1 Area-2 Area-3 Area-4 Element Atomic % Atomic % Atomic % Atomic % C(K) 12 21 28 12 N(K) 2 14 20 1 O(K) 6 9 14 9 Si(K) 24 3 4 8 Cr(K) 55 53 34 70 Two things can be deduced from these EDX-measurements: • There may be different types of particles in the film, one seems to be Si-rich, an other one Cr-rich. The higher brightness of the Cr-rich particle (larger Z value) is in agreement with this interpretation. • In the area of the particle the N-concentration appears to be low. The numerical values, however, I would never take for serious, because the peaks at this energy are very small and the error bars are almost as large as the numbers. STEM-270k-modulation-EDX-series.jpg STEM-270k-modulation-EDX-series-plot.jpg The compositional modulations have to be checked with elemental maps by using the energy filtered electrons, because EDX is not really conclusive on this level of spatial resolution and very small compositional changes. The strongest modulations in the EDX-signal concerns the Cr-concentration. Most probably the contrast variation in the HAADF STEM image is due to this. In the Cr-depleted area the N- and O-signal show both an opposite tendencies compared to the Cr-signal. Therefore it might (!) be, that a small increase of O and N are present at a STEM-270k-modulation-EDX-series-plot.jpg decrease of Cr. In the Cr-enriched area it is rather Si, which shows the opposite behaviour compared to Cr. But I would like to emphasize, that the changes in the spectra are extremely small and I only see a chance for more conclusive results for this phenomenon with using energy filtered TEM.