Highlights

Nanoindentation-induced Amorphous Silicon Characterized by FEM

Scientific Achievement

In nanoindentation experiments crystalline silicon (c-Si) is known to transform under certain unloading conditions to amorphous silicon (a-Si), the so-called indentation induced a-Si, a phase transformation that has not been observed with diamond-anvil cells. So far the physical structure and state of this form of a-Si had not been characterized. Thus the aim of the project was to investigate for the first time the state of indentation-induced a-Si using fluctuation electron microscopy (FEM).

When studying different types of a-Si with FEM it has been found that only relaxed a-Si approaches a perfect continuous random network, while all the other types of a-Si exhibit a certain degree of medium range order. In addition the regions of indentation-induced a-Si are highly localized (1-2 mm) and thus FEM is the only viable technique to characterize the degree of order and hence the state of the new form of a-Si. In order to put the state of indentation-induced a-Si into context it was compared with the state of as-prepared and relaxed (annealed) ion-implanted a-Si.

Figure 1 shows clearly differences between the MRO in ion-implanted and indentation-induced a-Si. The state of relaxed indentation-induced and relaxed ion-implanted a-Si seems to be almost identical. In contrast, as-implanted a-Si displays a higher MRO, whereas as-indented a-Si shows a lower MRO than any other type of a-Si. The differing degrees of MRO in these types of amorphous silicon reflect the very different displacement processes leading to their formation. We have also correlated the MRO of these types of amorphous silicon with different preparation techniques and thermal histories to the deformation behaviour under indentation and find that materials that will transform to crystalline high pressure phases, e.g. both relaxed cases, seem to have a common state.

Significance

The state of indentation-induced a-Si has been characterized for the very first time and clear differences between the different types of a-Si have been revealed. To fully understand this type of a-Si additional diffraction experiments have been commenced which will yield information about the short range order. Thereafter, modeling will be performed in order to fully understand the amorphous network of indentation-induced a-Si. This understanding will hopefully tell us about the kinetics of the crystalline-to-amorphous phase transformation process induced by indentation, a process that is not well understand but significant to many machining and device building problems.

Performers