Indentation Rate Effect on Pop-out in Silicon Studied by Cross-sectional TEM

Abstract

It is well known that diamond cubic Si-I transforms to a metallic state under high hydrostatic (and shear) stress, and that the subsequent reverse transformations to other phases can occur during release of the pressure [1]. When metastable crystalline phases such as Si-III and Si-XII form during nanoindentation unloading, pop-out (a subtle change of slope on the indentation load-displacement (p-h) curve) will be detected [2]. The pop-out is believed to result from the nucleation and growth of metastable crystalline phases which are about 10% less dense than the metallic silicon phase. Previous nanoindenation studies show that the indentation-induced phase transformations in silicon at medium (30mN) to low (10mN) maximum load may depend largely on indentation unloading rate; slower rates promote nucleation and growth [3]. In this study, microstructures of the transformed zone under the nanoidenter were studied through cross-sectional TEM and correlated to their pop-out behavior. The indentations were made at room temperature on undoped (100) single crystal silicon with a triangular pyramidal indenter with a centerline-to-face angle of 65 and loads of 10 mN at two loading/unloading rates, 5mN/s and 0.05mN/s. Cross-section TEM specimens were prepared by dual-beam focused ion beam (FIB) milling and examined with an FEI Tecnai 20. Bright-field (BF) and dark-field (DF) imaging combined with selected area diffraction (SAD) and supplemented by high-resolution imaging (HREM) were used to characterize the transformed zone under the indenter and identify different crystallographic phases.