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viii 4. Shape and size of the PbTe Dots 31 4.1. Available Samples 31 4.2. Precipitation steps 33 4.3. Precipitation from different layer thicknesses 36 4.3.1. Annealed 5nm layer 36 4.3.2. Annealed 1nm layer 38 4.4. Equilibrium shape 40 5. Interpretation of HRTEM Images 44 5.1. JEMS Simulation Package 44 5.1.1. Multi-Slice Approach 46 5.1.2. Simulation Procedure 47 5.2. Image simulation of the CdTe/PbTe interfaces 50 6. Interface characterization 52 6.1. Available Samples 52 6.1.1. Specimens for HRTEM imaging 52 6.1.2. Beam Damage 53 6.2. HRTEM investigation of PbTe nanocrystals 54 6.2.1. HRTEM images of single QDs 54 6.2.2. Total energy calculations 56 6.2.3. The (001) interface 58 6.2.4. The (110) interface 65 6.2.5. The (111) interface 73 7. Conclusions and Outlook 77 Appendix A Structure factors of CdTe and PbTe 82 Appendix B Simulated HR-maps of CdTe/PbTe interfaces 84 References 91 Curriculum vitae 95 List of Publications 96 Acknowledgment 98
Chapter 1 Introduction 1.1 Materials for mid-infrared devices and applications Optical devices have a broad field of applications. The physical principle behind most applications is the recombination of an electron-hole pair over a direct band gap in semiconductor materials. Band gap design is a key to produce materials with fitting band gaps. Further tuning of the optical properties is done by micro- and nano- structuring of the optical active materials. The approach of low dimensional systems like quantum dots (QDs) or quantum wires leads to confined electrons. Electrons confined in a certain potential show quantized properties and behave like the electrons in atoms or molecules with certain excitation levels [1, 2]. The confinement yields to a blueshift and is controlled by the size of the structures used for confinement. Optoelectronic device based on QDs are realized for different application, e.g QDs lasers [3, 4] or single photon sources [5, 6]. But the band gaps of typical direct band gap semiconductors are not perfectly suitable for the mid-infrared frequency range (3- 30μm). Lead salts are used for applications in this frequency range and further nano- structuring would improve the performance of these devices. The mid infrared frequency range can be found e.g. at the absorption energies of CO2 or other molecules important for environmental monitoring or medical diagnosis. Improved semiconductor-lasers and detectors would allow efficient application in these fields. QDs can be produced by using self-organisation schemes during epitaxial growth of strained semiconductor heterostructures. The best-known self-organisation scheme is the Stranski-Krastanov (SK) growth mode [7-10]. This growth mode is governed by surface energy minimisation and strain relaxation. SK growth is always starting with the formation of a wetting layer to minimise the surface, which covers the entire substrate. If the stored strain energy gets larger than the energy benefit of layer-by- layer growth, the formation of QDs starts. A capping layer is needed for an implementation of these dots, so that the dots are surrounded by a host material. During the overgrowth, alloying and shape transitions appear [11-13]. An often 1
Page 1: Transmission Electron Microscopy of
Page 5: Kurzfassung Nanostrukturierung erla
Page 9: Contents Preface i Eidesstattliche
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 and 34: kind of imaging is called Dark fiel
Page 35 and 36: Centred Dark Field One disadvantage
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
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interface through the whole specime
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Figure 6.1: A schematic sketch of t
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Figure 6.3: Two HRTEM examples of d
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PbTe and CdTe, respectively, are co
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simulation a continuous Te matrix.
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Figure 6.8: HR simulation of the (0
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Figure 6.10: The images on the righ
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6.2.4 The (110) interface The main
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Figure 6.14: Two example of the (11
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Figure 6.15c and 6.16b show example
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figure 6.15. This displacement can
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6.2.5 The (111) interface It is dif
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Figure 6.20: HR-map of the model co
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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
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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
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References [1] R.C. Ashoori, Nature
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Curriculum Vitae 1 March 1979 born
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Contributed talks: “The coherent
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All member of the institute for the
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