Nanoindentation of ion-implanted crystalline germanium

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 ``missing bonds'' or vacancies in the material. A thermal annealing step at $200\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ is sufficient to restore the mechanical response of pristine material, despite incomplete recovery of the original pristine crystal structure.