Indentation-induced phase transformations in silicon nanostructures

Анотація

The pressure- and indentation-induced phase transformations in crystalline (cd), nano-crystalline (nc), nano-layered (nl) and amorphous (a) silicon are studied by using molecular dynamics simulations based on the modified Tersoff potential. The sp3s tight-binding scheme is employed to gain insight into the origin of the change in conductivity during nano-indentation. The Gibbs free energy calculations predict the following pressure-induced phase transitions: cd-Si -Si (11 GPa); cd-Si HDA (22.5 GPa); nc-Si -Si (5.2 GPa); nc-Si HDA (13.3 GPa); a-Si -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) and (110) substrates, the pop-in is assigned to the appearance of -tin Si and high-density amorphous (HDA) phases, respectively. During unloading, the pop-out is due to the formation of a low-density amorphous phase a-Si. The a-Si HDA and HDA a-Si + HDA transformations take place during nanoindentation of a-Si in loading and unloading regimes, respectively. At high loads, the strength of nc-Si and nl-Si is not higher than the one of the crystalline counterpart. However, at moderate loads, these materials with an optimal grain or nanolayer orientation can exhibit nanohardness that will be higher than for crystalline or amorphous silicon. A change in conductivity from semiconducting to metallic during nanoindentation of the cd-Si (100) slab is explained in terms of a transformation of the localized electronic states in the band gap region. The results are compared with those of available theoretical models and experiments.