Temperature dependence of silicon hardness: Experimental evidence of phase transformations

The hardness of silicon is known to be nearly independent of temperature below a certain transition point, and to decrease steeply thereafter. Based on high-temperature Berkovich nanoindentation at 25-500 °C and Raman microanalysis of Vickers indentations produced in single-crystal silicon at 25-750 °C, we present evidence of a transformation into a high-pressure metallic Si phase during indentation at temperatures up to about 350 °C. We show that this transformation pressure determines silicon hardness below the transition temperature. We also report the temperature stability ranges of different metastable phases of silicon, including a new Si-XIII phase.