The topological order and its correlation to self-atom transport in amorphous materials: silicon and germanium as model systems
Final Report Abstract
Within the framework of this project, the local structure and dynamics of amorphous silicon (a-Si) and amorphous germanium (a-Ge) have been examined. The Fluctuation Electron Microscopy (FEM) experiments carried out for characterizing the structures of a-Si and a-Ge contribute to enhancing the methodology. The results show that several aspects influence the normalized variance, which is a quantity derived from FEM measurements used for structural interpretation. A disturbing factor to avoid during FEM measurements are modifications of the structure due to the electron beam. For a-Si and a-Ge, it is essential to choose the electron energy as low as possible to minimize atomic displacements caused by the beam and other effects. It has also been shown that a frequently used method applied in the past for analyzing the normalized variance, known as ’pair persistence analysis’, proved to be very sensitive to measurement settings. This makes precise comparison between different datasets difficult or impossible. An alternative, more robust method has been confirmed experimentally. Measurements on the structural dynamics of a-Si and a-Ge using electron correlation microscopy (ECM) have revealed that both materials are highly susceptible to the electron beam. Due to this, unwanted crystallization occurs faster in irradiated areas than in non-irradiated ones. Furthermore, both a-Si and a-Ge undergo step-by-step etching under electron radiation, causing them to evaporate after some time in the irradiated regions. Another undesirable effect is contamination resulting from sample preparation with a focused ion beam (FIB) device, primarily involving platinum and carbon. This could be avoided if samples were prepared without FIB. The evaluated ECM data indicate that the measured dynamics in a-Si and a-Ge exhibit structural heterogeneity. Both materials contain nanometer-sized domains with relatively slow dynamics surrounded by interconnected networks exhibiting faster dynamics. Moreover, surprisingly, the ECM- measured dynamics of a-Si and a-Ge display no temperature dependence, contradicting expectations. This suggests that most of the observed dynamics are driven by the electron beam rather than thermal activation. Purely thermally activated dynamics would occur at much slower rates within the measured temperature range, making the ECM measurement completely dominated by beam-driven dynamics and artifacts. Molecular dynamics simulations (MD) performed to investigate solid phase epitaxial recrystallization (SPER) and self-diffusion (SD) in a-Si employed Stillinger-Weber potentials and Tersoff potentials. Calculations reveal significant similarities between activation energies for these two processes, indicating identical mechanisms driving SPER and SD in a-Si. Using a modified three-body-parameter within the Stillinger-Weber potential improves agreement with experimental findings. The mechanism of both process is characterized by local bond rearrangements or nearest neighbor exchanges. Diffusion studies were performed on amorphous isotopically modified germanium layers followed by depth profiling using time-of-flight secondary-ion mass spectrometry (ToF-SIMS). The temperature dependence of SD in a-Ge follows an Arrhenius law. The activation energy match those for SPER, suggesting an identical mechanism. These findings imply that self-diffusion in a-Ge may also be mediated through local bond rearrangements. Molecular dynamic simulations employing a modified Stillinger-Weber potential support the proposed mechanism.
Publications
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Comparison of Experimental STEM Conditions for Fluctuation Electron Microscopy. Microscopy and Microanalysis, 26(6), 1100-1109.
Radić, Dražen; Hilke, Sven; Peterlechner, Martin; Posselt, Matthias; Wilde, Gerhard & Bracht, Hartmut
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Focused Ion Beam Sample Preparation for In Situ Thermal and Electrical Transmission Electron Microscopy. Microscopy and Microanalysis, 27(4), 828-834.
Radić, Dražen; Peterlechner, Martin & Bracht, Hartmut
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Atomic mechanisms of self-diffusion in amorphous silicon. AIP Advances, 12(11).
Posselt, Matthias; Bracht, Hartmut; Ghorbani-Asl, Mahdi & Radić, Drazen
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Atomistic simulations on the relationship between solid-phase epitaxial recrystallization and self-diffusion in amorphous silicon. Journal of Applied Physics, 131(3).
Posselt, M.; Bracht, H. & Radić, D.
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The Impact of Energy Filtering on Fluctuation Electron Microscopy. Microscopy and Microanalysis, 29(1), 189-195.
Radić, Dražen; Peterlechner, Martin; Posselt, Matthias & Bracht, Hartmut
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Treating Knock-On Displacements in Fluctuation Electron Microscopy Experiments. Microscopy and Microanalysis, 28(6), 2036-2046.
Radić, Dražen; Peterlechner, Martin; Posselt, Matthias & Bracht, Hartmut
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Challenges of Electron Correlation Microscopy on Amorphous Silicon and Amorphous Germanium. Microscopy and Microanalysis, 29(5), 1579-1594.
Radić, Dražen; Peterlechner, Martin; Spangenberg, Katharina; Posselt, Matthias & Bracht, Hartmut
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Fluctuation Electron Microscopy on Amorphous Silicon and Amorphous Germanium. Microscopy and Microanalysis, 29(2), 477-489.
Radić, Dražen; Peterlechner, Martin; Posselt, Matthias & Bracht, Hartmut
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The Medium Range Order Structure and Structural Dynamicsof Ion Implanted Amorphous Silicon and Germanium. PhD thesis (University of Munster, 2023)
Radić, D.
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Experimental and theoretical studies on self-diffusion in amorphous germanium. AIP Advances, 14(6).
Böckendorf, Tim; Kirschbaum, Jan; Kipke, Felix; Bougeard, Dominique; Hansen, John Lundsgaard; Larsen, Arne Nylandsted; Posselt, Matthias & Bracht, Hartmut
