Determination of core structure periodicity and point defect density along dislocations

Understanding the structure of defects in crystalline materials is essential for modern materials science. While most defect characterization involves the inverse and non-unique problem of fitting passible structural models to experimental data, which are in many cases average spectra from a variety of defects (Electron Paramagnetic Resonance (EPR) Spectroscopy, Deep Level Transient Spectroscopy (DLTS), Photoluminescense Spectroscopy (PL), etc.), being able to directly probe the atomic structure of single defects would also provide their electronic as well as mechanical properties, since those can be determined computationally, once the structure is known. This work will report on a new electron diffraction technique to directly determine the periodicity of dislocation core structures as well as a way to greatly enhance the accuracy of the forbidden reflection lattice imaging (FRLI) technique to image individual structural point defects along partial dislocations.; Electron microdiffraction experiments with Silicon samples oriented along the [110] direction will be described, which will give direct experimental evidence for the double period reconstruction of the 90° and 30° partial dislocations. Also, Silicon and β-Silicon Carbide samples with atomically flat (111) surfaces have been prepared. Perfectly smooth surfaces are shown to be essential for imaging point defects such as kinks along partial dislocations in these materials.; In addition to these experiments, advances in the theory of electron diffraction will be reported. A new imaging technique called “Atomic String Holography” will be introduced, as well as a solution to the inversion problem in dynamical scattering theory based on a new expansion of the matrix exponential of two non-commuting matrices, one of which is diagonal.