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24 Figure 3.6: (a) BF image of a PbTe/CdTe specimen, recorded slightly away of the [001] direction. The images show the mass-thickness contrast. The PbTe dots appear darker than the surrounding CdTe host material (mass contrast). A thickness contrast can be seen at the top and the bottom of image (a). The diffraction contrast is dominating in the DF image (b). The (020) spot, which is a forbidden reflex for the zb lattice, was used for image formation. Thus, the PbTe dots appear bright in a dark background. In addition, the dots in the thicker areas, which are not visible in image (a) are visible in image (b). (c) shows a diffraction pattern, that was recorded slightly away from the zone axis, so that the (020) reflex is the brightest of the {200} spots. Figure 3.7: Sketch of the diffraction condition for CDF. (A) Common two beam condition. The spot +g(hkl) and 000 are excited. If now the +g(hkl) spot is moved to the position of the 000 spot it becomes weak and the +g(hkl) spot is strong (B). If the opposite –g(hkl) spot is tilted toward the 000 position (C) the reverse two-beam condition is now applied to this spot. [32]
Centred Dark Field One disadvantage of the straight-forward DF imaging procedure is increased lens aberration. Scattered e-beams are not near the optical axis of lenses, so they are stronger bent towards this axis and it is difficult to form high quality real space images. To record DF images in the same quality as BF images it is necessary to bring the e-beam of interest on the optical axis and into a two beam condition. That can be done by tilting the incident beam in such a way that the spot of interest replaces the (000) spot. This kind of DF imaging is called Centered Dark Field (CDF) mode. The tilting of the incident beam changes the diffraction conditions for the electrons of the hkl- plane and the applied two-beam condition is destroyed. Both conditions can be reached if first the symmetric spot of the diffraction spot is tilted to the two-beam condition and after that the incident beam is tilted. This procedure is sketch in Figure 3.7, [32]. Most TEM are able to save the alignments for the beam tilts, so it is possible to switch simply between DF and BF imaging, once the objective aperture is well aligned. 3.4 High Resolution Transmission Electron Microscopy Another kind of investigation of specimens is High Resolution Transmission Electron Microscopy (HRTEM). The main difference to DF and BF imaging is the number of collected beams for image formation. To form a HRTEM image more than one beam is collected by the objective aperture (see Fig. 3.8). If the resolution of the microscope is high enough, so-called phase contrast can be obtained. This phase contrast contains the spatial information of the lattice planes of the sample and can be exploited to explore the crystal structure. 3.4.1 Requirements for HRTEM There are some essential requirements for the microscope to obtain high resolution TEM images. [33] • High mechanical stability, low vibrations • A goniometer of high precision • High enough acceleration voltage to reach the necessary resolution • High gun brightness 25
Page 1: Transmission Electron Microscopy of
Page 5: Kurzfassung Nanostrukturierung erla
Page 9 and 10: Contents Preface i Eidesstattliche
Page 11 and 12: Chapter 1 Introduction 1.1 Material
Page 13 and 14: 1.2 Quantum Dot precipitation in so
Page 15 and 16: Chapter 2 The material system and i
Page 17 and 18: of PbTe is one order of magnitude s
Page 19 and 20: an intermediate case with a strain
Page 21 and 22: therefore it can be Te or Cd termin
Page 23 and 24: Chapter 3 The Transmission Electron
Page 25 and 26: screen. These are the two basic mod
Page 27 and 28: This leads with Cs = 1mm and λ = 2
Page 29 and 30: F(θ) is the structure factor of th
Page 31 and 32: 3.3 Contrast Enhancement with Brigh
Page 33: kind of imaging is called Dark fiel
Page 37 and 38: Whereas A 2 and B 2 are the intensi
Page 39 and 40: Figure 3.9: Plots of the sin x(u) t
Page 41 and 42: Chapter 4 Shape and size of the PbT
Page 43 and 44: 4.2 Precipitation steps TEM images
Page 45 and 46: lines approaches the diameter of th
Page 47 and 48: Figure 4.6: The distribution of the
Page 49 and 50: Figure 4.8: The image at the top sh
Page 51 and 52: Figure 4.10: Length of the {111} in
Page 53 and 54: Figure 4.12: Different shapes of cu
Page 55 and 56: (Centre Interdisciplinare de Micros
Page 57 and 58: 5.1.2 Simulation Procedure The stan
Page 59 and 60: The DP of the materials is very hel
Page 61 and 62: interface through the whole specime
Page 63 and 64: Figure 6.1: A schematic sketch of t
Page 65 and 66: Figure 6.3: Two HRTEM examples of d
Page 67 and 68: PbTe and CdTe, respectively, are co
Page 69 and 70: simulation a continuous Te matrix.
Page 71 and 72: Figure 6.8: HR simulation of the (0
Page 73 and 74: Figure 6.10: The images on the righ
Page 75 and 76: 6.2.4 The (110) interface The main
Page 77 and 78: Figure 6.14: Two example of the (11
Page 79 and 80: Figure 6.15c and 6.16b show example
Page 81 and 82: figure 6.15. This displacement can
Page 83 and 84: 6.2.5 The (111) interface It is dif
Page 85 and 86:
Figure 6.20: HR-map of the model co
Page 87 and 88:
Chapter 7 Conclusions and Outlook C
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Figure 7.1: (a) QDs interfaces stit
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segregate at the surface, build lat
Page 93 and 94:
A1 CdTe structure factor CdTe has z
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Figure B2: HR-map for the Te termin
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B2 The (110) interface The tree HR-
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Figure B8: HR-map of the (110) inte
Page 101 and 102:
References [1] R.C. Ashoori, Nature
Page 105 and 106:
Curriculum Vitae 1 March 1979 born
Page 107 and 108:
Contributed talks: “The coherent
Page 109:
All member of the institute for the
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