Direct observation of phase transformation anisotropy in indented silicon studied by confocal Raman spectroscopy

The theoretically predicted anisotropic nature of the indentation phase transformation in silicon (Si) is observed directly in experiments using hyperspectral, confocal Raman microscopy. The anisotropy is reflected in the two-dimensional distribution of the residual diamond cubic Si-I phase and high-pressure phases in indented Si(001), Si(110), and Si(111) surfaces, and is linked to the number and orientation of the {111}110 slip systems of the diamond cubic phase that are activated during indentation. Key to the observation of the anisotropic phase transformation is the local preservation of Si-I in the residual contact impression, which is controlled by the magnitude of the applied indentation strain, εI. The condition for islands of Si-I to be retained after indentation is 0.04 εI 0.01; strains outside of this range either do not result in a residual contact impression or completely transform the contacted material, rendering the underlying anisotropy of the phase transformation unobservable.