On the cyclic indentation behavior of crystalline silicon with a sharp tip
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M.V. Swain Biomaterials Science Research Unit, Faculty of Dentistry, University of Sydney, United Dental Hospital, Surry Hills, NSW 2010, Australia (Received 26 November 2006; accepted 20 February 2007)
Detailed cyclic indentation experiments of crystalline silicon in this study show interesting behavior depending on the end phase from the previous cycle. To enable the behavior of these phases to be studied on reloading, the cyclic indentation response of the material is examined under conditions where the pressure-induced Si-II phase transforms either to amorphous (a-Si) or high pressure Si-XII/Si-III phases on unloading. For an amorphous end phase the subsequent reloading is hysteretic, and for high pressure crystalline end phases it is elastic. This indicates that, whereas a-Si re-transforms readily to Si-II upon reloading, Si-XII/Si-III does not retransform to Si-II even at the maximum indentation load. Based on the concept of the effective indenter shape and stresses induced in the material, we show that Si-XII/Si-III has a greater critical hydrostatic pressure for retransformation to Si-II than that of the diamond cubic Si-I. I. INTRODUCTION
There has been considerable interest in pressureinduced phase transformation in crystalline silicon (c-Si) because of its wide range of technological applications. High pressure diamond-anvil studies have shown that the diamond cubic Si-I phase transforms to a metallic Si-II phase at a pressure of ∼11 GPa.1,2 Under slow decompression, Si-II retransforms to crystalline high pressure phase Si-XII, which then partly transforms to Si-III upon further pressure release, resulting in a mixture of crystalline high pressure phases Si-XII and Si-III following full pressure release.3,4 Despite the lack of in situ indentation diffraction data, there are a number of papers that provide indirect evidence of the Si-I to Si-II transformation on indentation loading.5 Upon unloading, in the case of indentation, the Si-II phase further transforms to either an amorphous phase (a-Si) or to a mixture of the metastable crystalline high pressure phases Si-XII and SiIII,6–10 depending on unloading conditions. A statistical correlation has been established between the occurrence of a sudden decrease of the indentation displacement or pop-out during unloading and the formation of Si-XII/ Si-III phases in the unloaded material, as identified by a)
Address all correspondence to this author: e-mail: [email protected] DOI: 10.1557/JMR.2007.0406 2992 J. Mater. Res., Vol. 22, No. 11, Nov 2007 http://journals.cambridge.org Downloaded: 18 Mar 2015
post-indentation Raman microspectroscopy.6,8 If no popout event occurred during unloading, the final structure of the unloaded material has been found to be predominantly amorphous by Raman microspectroscopy6–8 and transmission electron microscopy (TEM).7,10 The popout is a probabilistic event,6,8 and the formation of SiXII/Si-III phases has been hypothesized elsewhere to be kinetically limited by the nucleation of Si-XII/Si-III from Si-II.7,9 The use
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