Abstract
Nanoindentation of silicon and germanium is of interest not only for the measurement of their mechanical properties but more importantly for the fact that they undergo a series of phase transformations under applied pressure. Indeed, after complete pressure release the material does not return to the starting diamond cubic phase but several metastable phases are possible, depending on the indentation conditions. In silicon both crystalline (diamond cubic) and amorphous phases undergo a phase transformation to a dense metallic phase at around 11 GPa, a deformation process that defines the hardness of these materials. On pressure release either a mixture of a rhombohedral (r8) phase and a body centred cubic (bc8) phase or a pressure-induced amorphous silicon structure result. The mixed r8/bc8 phase is stable to 200 oC and has been shown to have properties of a narrow band gap semiconductor and can be doped both n- and p-type. In germanium, the deformation processes under indentation are more complex with both plastic deformation by slip and twinning as well as phase transformation observed for diamond cubic germanium, depending on the indentation conditions. Amorphous germanium is easier to phase transform since slip-induced processes are avoided. Both crystalline and amorphous forms of germanium can be transformed to a high density metallic phase under pressure but several different transformation pathways are possible on pressure release, with the r8, hexagonal diamond and simple tetragonal end phases obtained under specific conditions. These deformation and phase transformation processes under indentation are reviewed in this chapter and compared with the behavior of these materials under diamond anvil cell pressure.
