Size effects in the nanoindentation of silicon at ambient temperature

The work reported here was undertaken to explore an interesting "size effect'' in the nanoindentation of Si. Flat-topped wedges of Si were etched from single crystal wafers and indented with a three-sided pyramidal Berkovich tip with a 120nm radius. The widths of the flats varied from 100 mm to 100 nm. Samples of width >= 1.6 mm were indented under load control to a load of 20 mN. Those with widths >= 1.6 mm were indented to a load of 1 mN. The indentation of thicker specimens is accomplished by transformation-induced plasticity, as reported in prior work. On indentation Si transforms to metallic Sn-II; during relaxation it transforms either to an ultrafine-grained mixture of the Si-III and Si-XII phases (with a pop-out in the load-deflection curve) or to amorphous Si (creating a knee in the curve). A small dislocated field appears at the interface between the metastable phase volume and the Si. As the sample thickness decreases to the indentation size, dislocation plasticity becomes more important. A pop-in appears in the loading curve, slip traces appear on the sample surface, and the dislocated region within the specimen becomes larger. At sufficiently small sample width (similar to 100 nm) phase transformations disappear and the sample deforms by classic dislocation plasticity ( as we observed previously in in situ studies). The change in mechanism is due to the relative ease of nucleating dislocations in the thin sample, which is believed to be a consequence of the change in the indentation stress field.