High pressure crystalline phase formation during nanoindentation: Amorphous versus crystalline silicon

Phase transformations induced by indentation at different unloading rates have been studied in crystalline and amorphous silicon via Raman microspectroscopy and transmission electron microscopy. Unloading was performed at a "slow" rate of ∼0.9 mN/s which is known to create volumes of high pressure phases (Si-III and Si-XII) in crystalline silicon as well as "rapid" unloading (∼1000 mN/s), where amorphous phases are expected. Stark differences between the resulting structures are observed depending on whether the starting material is amorphous or crystalline silicon. Interestingly, amorphous silicon transforms to high pressure phases much more readily than crystalline silicon even after rapid unloading.