Phase stability of silicon during indentation at elevated temperature: evidence for a direct transformation from metalli
- PDF / 279,826 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 13 Downloads / 235 Views
apid Communications
Phase stability of silicon during indentation at elevated temperature: evidence for a direct transformation from metallic Si-II to diamond cubic Si-I S.K. Bhuyan, J.E. Bradby, S. Ruffell, B. Haberl, C. Saint, and J.S. Williams, Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra, ACT 0200, Australia P. Munroe, Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia Address all correspondence to J.E. Bradby at [email protected] (Received 29 August 2011; accepted 3 November 2011)
Abstract Nanoindentation-induced phase transformations in both crystalline silicon (c-Si) (100) and ion-implanted amorphous silicon have been studied at temperatures up to 200 °C. The region under the indenter undergoes rapid volume expansion at temperatures above 125 °C during unloading, which is indicated by “bowing” behavior in the load–displacement curve. Polycrystalline Si-I is the predominant end phase for indentation in crystalline silicon whereas high-pressure Si-III/Si-XII phases are the result of indentation in amorphous silicon. We suggest that the Si-II phase is unstable in a c-Si matrix at elevated temperatures and can directly transform to Si-I during the early stages of unloading.
The phase transformation behavior of silicon under contact loading has been the focus of research for many years. During nanoindentation, crystalline silicon (c-Si), that is the diamond cubic Si-I phase, transforms to a metallic β-Sn phase at a pressure of ∼11 GPa, involving a 22% increase in density.[1] During unloading, Si-II undergoes further phase transformation (depending on unloading conditions) to amorphous silicon (a-Si) and/or to the high-pressure crystalline phases Si-III and Si-XII.[2] Details of load–unload curves can be directly correlated with the induced structural changes. A change in slope of the unloading curve may involve either an “elbow” or a “pop-out”. The former is predominant when an amorphous phase is observed during rapid unloading while a pop-out corresponds to the formation of high-pressure phases.[3] Some recent work on ion-implanted a-Si suggests that it behaves in a similar way provided it is in a relaxed state, which is achieved by annealing at 450 °C following implantation but prior to indentation.[4] To date, very little work has been reported on the phase transformation behavior of Si during nanoindentation at elevated temperature. Previous elevated temperature indentation studies have been exclusively used to measure the temperature dependence of the mechanical properties of materials,[5–8] whereas few attempts have been made to understand phase transformation pathways.[7,9] Previous work showed that nucleation of Si-III/Si-XII on unloading is enhanced with increasing temperature but the final composition of the phasetransformed zone is also dependent on the thermal stability of phases in their respective matrices.[10] However, the situation
is not clear and the current paper has
Data Loading...
