5 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 Scanning Transmission Electron Microscopy (STEM) We now come to the second part of our talk, which deals with scanning TEM. In conventional TEM, we already explained that lenses are used to retrieve local information by superimposing diffracted beams to an image. Due to the lens errors connected with this imaging process, only those electrons scattered to angles smaller than, say, 20mrad are used to form the image. In STEM, the image formation process is completely different. The probe is sharply focused onto the specimen plane with a diameter in the Angstromrange. The signal on the (ring-shaped) high angle annular dark field (HAADF) detector comes from electrons scattered to large angles well above 30mrad. Local information is finally retrieved by mapping the HAADF signal against the position of the probe. Therefore, the probe is rastered over the specimen. On the left, we can see the HAADF detector just above the viewing screen and the part where the scan coils are located near the condenser system. On the right, it is illustrated (strongly symplified), how we can imagine the HAADF intensity recording: When the probe position coincides with the position of an atomic column, a strong scattering potential leads to a large amount of electrons detected at high scattering angles. Between the atoms, the scattering potential is of course much weaker, so that the HAADF detector registers less electrons. The next question is: Which physical properties determine the HAADF signal? If we look at the diffraction pattern here and make an intensity line scan through it, it seems that only Bragg scattering might be present and we do not expect significant intensity at angles larger than 30mrad. However, this diffraction pattern clearly shows that there is a diffuse background in between the Bragg spots. If we plot the line profile in logarithmic scale, it becomes obvious that the intensity at the HAADF detector -shown in grey- is strongly influenced by the diffuse background, which decreases more slowly with increasing scattering angle. Diffuse intensity in a diffraction pattern indicates that the strict periodicity in our specimen is violated, for example due to thermal movements and static 10/13

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 displacements of atoms. Because of this loss of translational symmetry, it is a demanding task to simulate HAADF signals. This is why we will not go into details here but refer to the talk of Mr. Schowalter. As the scattering power depends on the composition in our specimen, we can see so called Z-contrast (corresponding to the nuclear charge) in this image. Three InGaAs quantum wells appear as bright stripes between GaAs barriers, and an up-to-date topic is the question: How can we derive the local chemical composition from STEM-HAADF images? Usually, we use atomically resolved images and average the intensity perpendicular to the growth direction. This gives a line intensity profile depicted up top right. By comparison with extensive simulations, the chemical composition can be derived, as shown in a preliminary result bottom left. We see that the maximum concentration of about 11% indium is the same as we get from the CELFA technique. However, we expect considerable improvement from focused ion beam sample preparation, which makes especially the determination of the specimen thickness easier. Energy dispersive X-ray (EDX) analysis Besides EELS, energy dispersive X-ray (EDX) analysis is another method to investigate the chemical composition in a specimen from spectral analysis. The origin of the EDX signal is -as in EELS- an inelastic scattering event. An incident electron is inelastically scattered at an atom of the specimen and excites, say, a core electron. An electron from a higher orbital recombines with the developed hole. The energy difference is released in form of X-rays, whose energy is characteristic for the transition in the respective target atom. The X-rays are detected spectrally resolved (the number of counts can be translated to the energy of the registered X-rays) with a pin-diode in the EDX detector. The specimen investigated for this talk is a sapphire nanotube, which was placed on a carbon coated film. TEM and STEM-HAADF overviews are shown in the upper half of the slide. The EDX spectrum in the lower half shows aluminium and oxygen signals 11/13

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