Strain Analysis by Nano-Beam Electron Diffraction using convergent electron nanoprobes

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The measurement of strain fields in semiconductor crystals is a prerequisite for understanding physical processes in novel GeSi/Si or (Al)GaN/sapphire based heterostructures. This proposal firstly deals with the strain characterization in GeSi/Si islands for a basic understanding of Ge growth on Si. Secondly, strain fields and local Ge composition in GeSi-MOSFETs, being the basic module of high-performance computer processors, are to be measured as electronic properties drastically depend on the strain distribution. Thirdly, GaN/sapphire pseudosubstrates will be investigated with respect to strain since the efficiency of stress reduction in this layer highly determines efficiency and growth of subsequent InGaN/GaN LED active layers.In order to access these strain fields reliably, a scanning transmission electron microscopy (STEM) method for strain measurement is to be developed which exploits the positions of diffracted beams. In particular, a STEM probe is scanned over the specimen while a convergent beam electron diffraction (CBED) pattern is acquired at each scan point. With dedicated algorithms the disc-shaped CBED reflections can be detected accurately, leading to precisions of 7x10-4. To reduce the amount of experimental data drastically, these algorithms are to be implemented in a software for controlling the FEI Titan microscope used, so as to perform in-situ CBED pattern processing while the STEM probe is scanning. This not only enables 2D strain mapping but also provides the strain result immediately after data acquisition. One main prerequisite to exploit the full capability of the method for 2D mapping as to the spatial resolution of 0.5nm already achieved before is a drastic acceleration of CBED pattern acquisition which is not possible with conventional cameras. Therefore, test, modification and final installation of a delay-line detector are planned at the beginning of the project.To study strain fields of large extensions in the micron range, a STEM-precession method will be developed experimentally and in simulations. In precession mode, the incidence of the STEM probe is rotating on a cone while the resulting shift of the diffraction pattern is compensated for. In this way, strong variations of CBED intensities can be eliminated. In addition, the high strain sensitivity of high order reflections can be exploited as they appear brighter.Furthermore, distortions of the diffraction pattern due to post-magnifying lenses are to be measured and accounted for in strain measurements. This is important for the subsequent determination of the full strain tensor including shear.Finally, this technique is to be combined with the so-called Z-contrast signal, being sensitive to the chemical composition and to strain of the specimen. Consequently, the simultaneous measurement of both strain and Z-contrast will improve the precision of composition measurements since strain- and composition-induced Z-contrast variations can be separated.