Nanoindentation-induced phase transformation in crystalline silicon and relaxed amorphous silicon

Nanoindentation-induced phase transformation in both crystalline silicon (c-Si) and relaxed amorphous silicon (a-Si) have been studied. A series of nanoindentations were made with a sharp diamond Berkovich tip. During nanoindentations, maximum loads were applied from 1000 to 6000 μN, with a 1000 μN/s loading rate. A slow unloading rate at 100 μN/s was chosen to favor the formation of the high-pressure polycrystalline phases (Si-III and Si-XII). A fast unloading rate within 1 s was used to obtain a-Si end phase. The nanoindentation behavior and the structure of deformation regions were examined by load-depth characteristics curves and Raman. Large differences were observed between the transformation behavior in c-Si and that in relaxed a-Si. Indentation curves in c-Si present plastic deformation curves with elbow (no pop-out) on the unloading curves, even for loads up to 9000 μN. On the other hand, indentations in relaxed a-Si give rise to the same plastic deformation as c-Si at low loads (1000-2000 μN), whereas show clear pop-outs at high loads (above 3000 μN). Raman results suggest that high-pressure phases (HPPs) can occur more easily within a relaxed a-Si matrix than in a c-Si matrix. The results suggest a significantly different indentation behavior and phase transformation sequence in c-Si and relaxed a-Si at the nanoscale.