Spatial resolution of strain state analysis by convergent nano beam electron diffraction

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Development of methods for measurement of strain with sub-nanometer spatial resolution is of high relevance for material science as well as industrial microelectronics production. Local strain fields influence mechanic and electronic properties of semiconductor nanostructures and govern kinetic processes of epitaxial growth. For convergent nano-beam electron diffraction the specimen surface is scanned with a focused electron beam whose diameter lies in the sub-nanometer range. This leads to formation of diffraction disks in the back focal plane of the objective lens. The distance of a diffracted disk from the undiffracted one depends on the local lattice parameter and thus is given by the local lattice strain. Using this principle, the authors of this proposal developed and published the SANBED (strain analysis by nano beam electron diffraction) method for strain state analysis, which uses an electron beam with 0.5-0.7 nm width. This method yields a very high precision which is comparable with the best values published in literature using transmission electron microscopy. However, analysing a semiconductor nanostructure close to an interface, it is possible that a part of the tail of the electron beam extends beyond the interface. Although this part of the electron beam has a small local intensity, this effect reduces the spatial resolution of strain measurement in a complex manner. Aim of this proposal is measurement and optimisation of the spatial resolution of the strain analysis with the SANBED method. It is planned to use three semiconductor nanostructure samples with published well-known composition profiles as reference samples to measure strain profiles with the published standard-SANBED method. Then, the experimental results will be compared with image simulation. To this end, various approximations with different complexity will be tested, aiming at high accuracy at sufficiently small computation time. In the second part of the project we will investigate improvement of the spatial resolution of the method by optimizing the experimental parameters as well as the evaluation method. Experimental parameters include selection and excitation of different reflections and the geometry of the condenser aperture. Concerning the evaluation procedure we intend to test additional filtering and alternative methods to measure positions of diffraction disks. To perform the optimizations in an efficient way, both experimental measurement and image simulation will be applied. In the third part of the project the beam semi convergence angle will be increased so that the diffraction disks start to overlap, which leads to a smaller diameter of the illuminating electron beam. We expect that this also improves the spatial resolution of the strain evaluation. On the other hand, the method for measurement of disk position must be adapted so that it can be applied to overlapping diffraction disks.