Advanced Transmission Electron Microscopy for Nanooptics

摘要

Transmission electron microscopy and simulations have been used to characterize nanostructures in the wide band-gap, semiconducting crystal SiC and in the 1D photonic crystals Si/Mo and Cr/Sc. Convergent beam electron diffraction has been used to measure accurately the lattice parameters of about 20nm thin cubic SiC layers periodically arranged in 4H-SiC. It was shown that a rhombohedrally strained layer has been formed as a consequence of pseudomorphic growth. It demonstrates the potential of the method to determine such a small strain state from a nanosized object. The nanocrystal formation process after high-dose ion implantation and annealing has been considered in detail. Directly after high dose and high temperature Er and Ge implantation no nanocrystals have been formed and the foreign atoms are statistically distributed within the defective SiC matrix. The prevailing defects are short interstitial loops. The Ge site occupancy was determined using EDX under electron channelling conditions for the as-implanted material and after annealing at 1200℃. In the latter case, diffusion is enhanced and Ge was found to be located preferentially on interstitial sites. Annealing at 1600℃ results in the growth of nanocrystals whose size distributions are very similar for nanocrystals formed after Er and after Ge implantation. Atomic resolution Z-contrast dark-field STEM was used to visualize the nanocrystal formation, e.g. from the birth of the nanocrystals in form of single Er atom columns, trapped at the cores of dislocations, over lines, planes to fully-developed nanocrystals. Similarities and differences between the formation of nanocrystals after Er and Ge implantation were identified. Properties such as crystallographic structure and the orientation-relationship with the matrix have been determined from the high-resolution image. EDX-analysis of the nanocrystals created after Ge implantation show that their content is not uniform and consists on average of 80 % Ge. The structure of the GeSi nanocrystals was found to be hexagonal. Molecular dynamics simulations show that the structure correspond to minima in the potential energy of the matrix-nanocrystal system. The electronic structure of GeSi nanocrystals was determined using atomic resolution Z-contrast dark-field imaging combined with electron energy loss spectroscopy. The results were indicative of quantum confinement in 5nm and 3.5nm Ge_(0.8)Si_(0.2) nanocrystals. The new understanding of the role of interstitial loops in the formation of clusters may open the possibility of the control of the dimensionality of nanocrystals, by matching the density of the interstitial loops to the dopant atoms, thereby controlling the physical properties of the resultant nanocrystals. For the case of Si/Mo and Cr/Sc multilayers their single layer structure has been identified by advanced TEM techniques and the effect of defects (precipitates) and single layer roughness on the reflectivity of the whole system has been shown.