Microstructural analysis of plasticity in low-load contact damage of silicon and germanium
Morris, Jonathan Cordell
Doctor of Philosophy
The mechanisms responsible for microplasticity in silicon and germanium subjected to low-load contact damage have been investigated. Microstructural characterization has been carried out primarily by electron microscopy and electron diffraction. The particular contact damage processes that have been studied are indentation, scratching, and single-point diamond turning (ductile-regime machining). The substantial plasticity that results from the initial stages of low-load indentation in silicon and germanium is not controlled by a traditional mechanism such as dislocation activity. Evidence of plasticity includes a permanent indentation impression and ductile extrusions emanating from the indentation interior. Rather, the plasticity is a result of a ductile metallic phase that forms under the indenter from a pressure-induced metallization of the semiconductor (Mott transition). A metastable amorphous phase is formed when the pressure on the metallic phase drops as the indenter is lifted from the surface. This amorphous phase makes up the indentation interior and the ductile extrusions. The extent of the transformed zone in plan-view equates with that of the contact area of the indenter. Observed dislocation activity results from high stress fields ahead of crack tips. Plasticity induced by low-load scratching of silicon is likewise the result of a pressure-induced metallization of the semiconductor. Scratches show the same microstructural features as indentations, namely, a permanent plastic impression with a ductile morphology, ductile extrusions, and an amorphous remnant. The extent of the phase transformed zone in plan-view is exactly that of the contact area. The extent of the transformed zone in cross-section is between 200 to 300 nm depending on load. Minor dislocation activity is and/or cracking is found under the transformed zone. The "ductile regime" in silicon and germanium that allows for ductile material removal in precision machining processes is also a result of the same pressure-induced metallization. The chips from a germanium machining experiment are completely or largely amorphous and show machining marks and shear instability morphology. The ductile morphology and amorphous structure of the chips lead to the conclusion that the metallization and back-transformation is again responsible for plasticity. Morphological and crystallographic analysis of the machining chips allow for the location on the original workpiece, from which certain chips originated, to be known.
Engineering; Materials science