4 years ago

Advanced Methods in Transmission Electron Microscopy

Advanced Methods in Transmission Electron Microscopy

Knut Müller,

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 Electron energy loss spectroscopy (EELS) Another method to investigate the chemical composition of a material is electron energy loss spectroscopy (EELS). The principle of signal development is as follows: An incident electron is inelastically scattered by an atom of the specimen, for example it excites a core electron of an atom in the specimen. This energy loss is characteristic for the scattering atom and can thus be used for elemental analysis. To measure the energy loss a Gatan imaging filter is used which works roughly as follows: Electrons transmitted through specimen will pass the magnetic prism, where they are dispersed according to their energy loss by the Lorentz force. Finally, a CCD camera detects the electrons. An energy-selecting slit may be inserted to select electrons that have suffered a certain energy loss (energy window). A sequence of lenses is then used to form an image using these electrons in the so-called energy-filtered TEM (EFTEM) mode. In spectroscopy mode, the spectral energy distribution of the electrons can be visualised. Here we see an unfiltered TEM image of an AlGaN/GaN Distributed Bragg reflector (DBR) top left. The spectrum of the electrons transmitted through this area is shown top right. The most intense peak is the zero-loss peak, which originates from elastically scattered electrons. The next peak is the plasmon peak, which stems from plasmon excitations. The tail at higher energy losses consists of background and contains especially the core losses. Three spectral windows are magnified in the lower half of the slide which correspond to the ionisation edges of the three chemical components nitrogen (K-edge), gallium (L-edge) and aluminium (K-edge). These edges can be used to perform energy filtered TEM and to create elemental maps of the N-, Ga- and Al-distribution. For this purpose the edge signal of the respective element has to be separated from the background of the full spectrum, which leads us to the EFTEM part of our tutorial. 8/13

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 Energy-filtered TEM (EFTEM) A method which is frequently applied to subtract the background is the three window method. The first two images (pre-edge 1 and pre-edge 2) are recorded with energy losses that are lower than the loss at the edge. The background is then estimated (solid blue lines) using these two images and extrapolated according to a power law (dashed blue line). The resulting background curve is then subtracted from the image taken directly after the edge (post-edge). So, the pure element signal is remaining und can be used for energy filtered imaging. The three resulting images for every window are shown in the lower half of the slide. A sketch of the mapped signal is also shown. Here you can see the elemental maps for nitrogen, gallium and aluminium. In the nitrogen map the signal is almost evenly distributed according to the homogeneous distribution of nitrogen in the specimen. In contrast to nitrogen, the gallium and aluminium maps show a signal according to the layered structure in the DBR. From the gallium signal in the gallium map we can derive the aluminium concentration in the AlGaN layer by assuming a linear dependence of the EFTEM signal on the elemental concentration. The ratio of the minima and maxima of the gallium–Lsignal in the line profile is 0.57, so the gallium content in the AlGaN layer is 57%, because the signal in the GaN layer corresponds to pure GaN. Therefore the aluminium content in the AlGaN layers is approximately 43%. It is possible to determine the aluminium content also using scanning transmission electron microscopy (STEM). This yields an Al content of 45±5%. The following part of our talk will now give insight to the current state-of-the-art of quantitative chemical analysis using STEM. 9/13

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