Cross-sectional TEM studies of indentation-induced phase transformations in Si: Indenter angle effects

Nanoindentations were made on a (100) single crystal Si wafer at room temperature with a series of triangular pyramidal indenters having centerline-to-face angles ranging from 35 degrees to 85 degrees. Indentations produced at high (80 mN) and low (10 mN) loads were examined in plan-view by scanning electron microscopy and in cross-section by transmission electron microscopy. Microstructural observations were correlated with the indentation load-displacement behavior. Cracking and extrusion are more prevalent for sharp indenters with small centerline-to-face angles, regardless of the load. At low loads, the transformed material is amorphous silicon for all indenter angles. For Berkovich indentations made at high-load, the transformed material is a nanocrystalline mix of Si-I and Si-III/Si-XII, as confirmed by selected area diffraction. Extrusion of material at high loads for the cube-corner indenter reduces the volume of transformed material remaining underneath the indenter, thereby eliminating the pop-out in the unloading curve.