Stacking faults along the {111} planes seed pressure-induced phase transformation in single crystal silicon

We explore the onset of phase transformation, at the nanoscale, in single-crystal diamond-cubic silicon (dc-Si) subjected to pressures of 13 GPa using a diamond anvil cell with a methanol-ethanol pressure medium. Transmission electron microscopy reveals two distinct structural features along {111} planes: (1) thin bands of defective dc-Si and (2) thicker bands of body-centered cubic silicon (bc8), surrounded by defective dc-Si. We propose that these features are consistent with shear bands that have been formed by slip along the low energy {111} planes and have a range of thicknesses depending on how much plastic deformation has occurred. The presence of bc8-Si within the thicker bands can be explained by localized regions of high pressure or energy at their center facilitating phase transformation to the metastable metallic β-Sn phase, which in turn, transforms to bc8 on pressure release. Our observations reveal that phase formation in silicon can be shear-activated, the transformation is not nucleation-limited, and its sluggish nature may be due to the slow growth of the metallic phase.