Examining pressure-induced phase transformations in silicon by spherical indentation and Raman spectroscopy: A statistical study

Unloading rate and maximum load have been previously shown to affect the response of silicon to sharp indentation, but no such study exists for spherical indentation. In this work, a statistical analysis of over 1900 indentations made with a 13.5-mum radius spherical indenter on a single-crystal silicon wafer over a range of loads (25-700 mN) and loading/unloading rates (1-30 mN/s) is presented. The location of pop-in and Pop-out events, most likely due to pressure-induced phase transformations, is noted, as well as pressures at which they occur. Multiple occurrences of pop-in and pop-out events are reported. Raman micro-spectroscopy shows a higher intensity of metastable silicon phases at some depth under the surface of the residual impression, where the highest shear stresses are present. A stability range for Si-II is demonstrated and compared with previous results for Berkovich indentation.