Abstract: There is considerable controversy over the deformation behavior of germanium (Ge) under nanoindentation using a sharp diamond tip, with a diverse range of observations that suggest competing mechanisms. Here we show the deformation mechanism of Ge can be controlled by the rate of applied load. Loading rate is varied over three orders of magnitude using depth-sensing nanoindentation. At slow loading rates, shear-induced plasticity is observed. At rapid loading rates (>100 mN sâ1), pressure-induced phase transformations are detected by ex situ micro-Raman spectroscopy and transmission electron microscopy. This switch in the deformation mechanism is due to the differing rate sensitivities of the respective deformation modes, shear-induced plasticity or pressure-induced phase transformation.
Abstract: We have investigated nanoindentation-induced plastic deformation in amorphous germanium (a-Ge) prepared by high-energy self-ion implantation. Using cross-sectional transmission electron microscopy, micro-Raman spectroscopy, and force-displacement curve analysis, we find strong evidence for a pressure-induced metallic phase transformation during indentation. Crystalline diamond-cubic Ge-I is observed in residual indents. Relaxed and unrelaxed structural states of a-Ge exhibit similar behavior on loading, but transform at different pressures on unloading. Both forms are markedly softer mechanically than crystalline Ge. These results assist in furthering the understanding of the intriguing phenomenon known as âexplosive crystallization.â
Abstract: Most indentation studies to date on crystalline germanium (c-Ge) and related covalent semiconductors have been carried out on pristine defect-free material. This paper addresses the paucity of studies on imperfect crystalline materials by exploring the impact of defects generated by ion implantation, prior to contact damage, upon the mechanical properties of c-Ge. Implantation with Ge ions is carried out to generate a layer of highly defective but still-crystalline Ge. Under nanoindentation with a sharp diamond tip, enhanced plasticity is observed relative to pristine material. Characterization by cross-sectional transmission electron microscopy, atomic force microscopy, and load curve analysis shows softening, quasiductile extrusion, and cracking suppression taking place. These changes can be explained by the high density of defects, and dangling bonds in particular, created by ion implantation and revealed by positron-annihilation spectroscopy, and are proportional to the fraction of âmissing bondsâ or vacancies in the material. A thermal annealing step at 200 °C is sufficient to restore the mechanical response of pristine material, despite incomplete recovery of the original pristine crystal structure.
Abstract: Giant âpop-inâ displacements are observed in crystalline silicon and germanium during high-load nanoindentation with a spherical diamond tip. These events are consistent with material removal triggered by lateral cracking during loading, which poses a hazard to microelectromechanical systems (MEMS) operation. We examine the scaling of the pop-in displacements as a function of peak indentation load and demonstrate a correlation with the depth of the plastic contact zone. We argue that giant pop-ins may occur in a broad range of highly brittle materials.
Abstract: We investigate the mechanical response of 50-600 nm epitaxial Ge films on a Si substrate using nanoindentation with a nominally spherical (R~4.3um) diamond tip. The inelastic deformation mechanism is found to depend critically on the film thickness. Sub-100 nm Ge films deform by pressure-induced phase transformation, whereas thicker films deform only by shear-induced dislocation slip and twinning. Nanoindentation fracture response is similarly dependent on film thickness. Elastic stress modelling shows that differing stress modes vary in their spatial distribution, and consequently the film thickness governs the stress state in the film, in conjunction with the radius of the nanoindenter tip. This opens the prospect of tailoring the contact response of Ge and related materials in thin film form by varying film thickness and indenter radius.
Abstract: Sudden excursions of unusually large magnitude (>1 µm), âgiant pop-ins,â have been observed in the force-displacement curve for high load indentation of crystalline germanium (Ge). A range of techniques including Raman microspectroscopy, focused ion-beam cross sectioning, and transmission electron microscopy, are applied to study this phenomenon. Amorphous material is observed in residual indents following the giant pop-in. The giant pop-in is shown to be a material removal event, triggered by the development of shallow lateral cracks adjacent to the indent. Enhanced depth recovery, or âelbowing,â observed in the force-displacement curve following the giant pop-in is explained in terms of a compliant response of plates of material around the indent detached by lateral cracking. The possible causes of amorphization are discussed, and the implications in light of earlier indentation studies of Ge are considered.
