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In cyclic nanoindentation of single-crystal silicon, an interesting phenomenon of a secondary pop-out event that closely follows the first pop-out event but with a larger critical load than the first is presented. Cyclic nanoindentation experiments under various loading/unloading rates and various maximum indentation loads were performed to verify the generality of the phenomenon of two pop-out events. Raman spectroscopy results indicate that the secondary pop-out does not induce any new phase, and the dominated end phases after the two pop-out events are still a mixture of Si-XII/Si-III phases. According to average contact pressure analysis, the phase transformation paths and the formation mechanism for the secondary pop-out event are discussed from the viewpoint of crystal nucleation and growth. The results indicate that phase transformations from the Si-I phase to Si-XII/Si-III phases are completed by two pop-out events in two adjacent indentation cycles, and the Si-XII/Si-III phases formed in previous indentation cycles strongly affect the phase transformations in subsequent loading/unloading processes.

I. INTRODUCTION

Nanoindentation-induced phase transformation behaviors in single-crystal silicon (c-Si) have been commonly observed in previous publications.1–10 Corresponding to different phase transformation mechanisms, three types of phenomena, pop-ins in the loading process, pop-outs, and elbows in the unloading process, could appear in load–displacement curves. Pop-ins and pop-outs lead to discontinuities of load–displacement curves, whereas elbows change the slope of the unloading curve suddenly. By combining Raman microspectroscopy,2,3,6,8,11,12 cross-sectional transmission electron microscopy (XTEM),5,7,8,10,13,14 and in situ electrical characterization,13,15 the phase transformation mechanisms of these three types of phenomena have been widely investigated. It has been commonly accepted that in the loading process, the diamond cubic Si-I phase transforms into a metallic Si-II phase at a pressure of ;11 GPa. In the unloading process, pop-outs and elbows occur depending on the unloading conditions such as the indentation load and unloading rate. Usually, a large indentation load and a low unloading rate will promote the appearance of popouts, whereas a small indentation load and a high unloading rate will lead to elbows. When the Si-II phase formed during the loading process transforms into a mixture of high-pressure Si-XII/Si-III phases during the unloading process, a pop-out appears. When the Si-II Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.120 J. Mater. Res., Vol. 30, No. 11, Jun 14, 2015

phase transforms into an amorphous phase (a-Si), an elbow appears, and the dominated end phase underneath the residual indent will be predominantly a-Si. However, the nanoindentation-induced phase transformation in c-Si is a very complex issue, and some phase transformation paths have not been clarified. Recently, in situ transmi