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Advanced Methods in Transmission Electron Microscopy

Advanced Methods in Transmission Electron Microscopy

Knut Müller,

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 As a third example, we'd like to present an analogon to the second one, except that a Cs-value of 5µm was used to simulate the contrast transfer function. The most modern microscopes like our Titan have an aberration corrector shown here, which is situated below the objective lens and allows to set also Cs to a desired value. In the focus series, you can again see the passbands developping like before, except that they are spread over a much wider range of spatial frequencies -especially at Scherzer condition-. However, also the information limit increases drastically, meaning that much more information can be accessed in principle. To sum this part up: We demonstrated that an imperfect lens can be used to translate phase information in an object to image intensity contrast. Nevertheless it is impossible to transfer all spatial frequencies with good contrast at the same time, which leads to image distortions since some Fourier components are missing to built up the image. This means on the one hand that the full information of the object exit wave cannot be recorded in a single image. On the other hand, different images recorded with different defocus settings subsequently sample a very large range of spatial frequencies. One way to retrieve the full information at the object exit surface is the reconstruction of a focal series. This series shows the same area in InGaN/GaN recorded with different defoci, and again we see that not only the atom positions, but also the contrast changes drastically, indicating once more that we should evaluate neither the interatomic distances nor the contrast from one of these images alone. 6/13 1 0 . 8 0 . 6 0 . 4 0 . 2 0 - 0 . 2 - 0 . 4 - 0 . 6 - 0 . 8 - 1 Δ f = - 2 0 0 n m 0 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 1 0 x 1 0 1 0 . 8 0 . 6 0 . 4 0 . 2 0 - 0 . 2 - 0 . 4 - 0 . 6 - 0 . 8 - 1 1 0 . 8 0 . 6 0 . 4 0 . 2 0 - 0 . 2 - 0 . 4 - 0 . 6 - 0 . 8 - 1 Δ f = - 2 2 n m 0 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 Δ f = 2 0 0 n m 1 0 x 1 0 0 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 1 0 x 1 0

Knut Müller, Katharina Gries: TEM Tutorial Riezlern 09/2008 However, here we see amplitude and phase of the object exit wave reconstructed from a focal series. In these images, it is now allowed to evaluate the interatomic distances quantitavely. In combination with elasticity theory, the local indium-concentration can be calculated and mapped, showing an InGaN island on top of GaN. Two other established methods require less technical effort. For example, optimised conditions for the specimen tilt and the defocus can be found, under which either the distances or the contrast is correctly transferred by the lens for one certain spatial frequency. In this example for InGaN again, we use only the 0002 with the primary beam to form an image of the 0002 lattice planes. In this image, the fringe periodicity is neglegibly distorted by lens errors, thus we can use the distances -which are depicted colour-coded on the left- of the 0002 fringes to compute the indium concentration map from an elasticity theory approach an Vegard's rule again, resulting in the InGaN island representation on the right hand side, which is a colour-coded Inconcentration map. A method which evaluates the contrast is the CELFA method, standing for composition evaluation by lattice fringe analysis. Again, the 002 lattice fringes are shown here, but in this case for InGaAs. In such zinc blende crystals, the intensity of the 002 reflection in the diffraction pattern strongly depends on chemical composition, which we call chemical sensitivity. Therefore, the local indium content can be derived from the amplitude of the intensity oscillation of the 002 fringe image by comparison with simulations. This results in an indium distribution depicted in the quantum well image on the right. From this, segregation profiles can be derived, for example. 7/13

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