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V. Unger:
"Non-linear finite element simulations of nanoindentation in Gallium Nitride considering fracture mechanics and plasticity";
Betreuer/in(nen): H. E. Pettermann; Institut für Leichtbau und Struktur-Biomechanik, TU Wien, 2018; Abschlussprüfung: 24.10.2018.

Modern power semiconductor materials need to withstand high power densities, while devices are getting downsized to meet the industrial demands.

In order to study the mechanical behaviour and the limitations of these brittle layered electronics materials, often nanoindentation experiments are conducted. For the purpose of supporting the instrumented indentation testing, the Finite Element Method in combination with a cohesive zone model and anisotropic Hill's plasticity is applied. Special focus is laid on Gallium Nitride (GaN) on Silicon (Si) stacks penetrated by a Berkovich indenter tip. The main goal of this thesis is to establish a non-linear nanoindentation Finite Element model to predict crack emergence and propagation in the layers including various residual stress states, that possibly arise during the wafer fabrication. The developed numerical model is able to predict both, the energy dissipation due to plastic deformation as well as material fracture.

The numerically predicted load-penetration curves show a good compliance with the experimental data, obtained by the KAI GmbH, Villach. The simulation results predict quarter-penny cracks in GaN and Si, depending on the indentation depth and residual stress state. Various influences on the mechanical behaviour, and especially the fracture behaviour are studied. Residual stresses are found to show a strong effect on the fracture behaviour of GaN. Tensile stresses lead to a temporarily unstable crack growth, whereas compressive stresses tend to mitigate the crack propagation. The non-linear simulations give the possibility to investigate different stack designs in rather short time and yield information about the stress state and sub-surface cracks during the indentation process.