Simulations of indentation-induced phase transformations in crystalline and amorphous silicon

The pressure- and indentation-induced phase transformations in crystalline (cd) and amorphous (a) silicon are studied by using molecular dynamics simulations based on the modified Tersoff potential. The sp(3)s(star) tight-binding scheme is employed to gain insight into the origin of the change in conductivity during nanoindentation. The Gibbs free energy calculations predict the following pressure-induced phase transitions: cd-Si ->beta-tin Si(beta-Si) (11.4 GPa); cd-Si -> high density amorphous phase (HDA) (22.5 GPa); a-Si ->beta-Si (2.5 GPa); a-Si -> HDA (8.4 GPa). Simulations of nanoindentation of crystalline silicon reveal discontinuities in the load-displacement curves. In the loading curves of the cd-Si (100) substrate, the pop-in is assigned to the appearance of the beta-tin Si phase. During unloading, the pop-out is due to the formation of a low-density amorphous phase a-Si. The a-Si -> HDA transformation takes place during nanoindentation of a-Si in loading regime. Upon unloading the a-Si phase is preserved. The structural transformations in cd-Si and a-Si during nanoindentation are treated in terms of triaxial and uniaxial compressions of the respective bulk samples. A change in conductivity from semiconducting to metallic during nanoindentation of the cd-Si (100) and a-Si slabs is explained in terms of a transformation of the localized electronic states in the band gap region. The results are compared to those of available theoretical models and experiments.