Cross-Sectional TEM Studies of Indentation-Induced Phase Transformations in Si: Indenter Angle Effects
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Cross-Sectional TEM Studies of Indentation-Induced Phase Transformations in Si: Indenter Angle Effects Songqing Wen1, James Bentley2, Jae-il Jang1, and G.M. Pharr1,2 1 The University of Tennessee, Dept. of Materials Science & Engr., Knoxville TN 37996 2 Oak Ridge National Libratory, Metals & Ceramics Division, Oak Ridge TN 37831 ABSTRACT Nanoindentations were made on a (100) single crystal Si wafer at room temperature with a series of triangular pyramidal indenters having centerline-to-face angles ranging from 35° to 85°. Indentations produced at high (80 mN) and low (10 mN) loads were examined in plan-view by scanning electron microscopy and in cross-section by transmission electron microscopy. Microstructural observations were correlated with the indentation load-displacement behavior. Cracking and extrusion are more prevalent for sharp indenters with small centerline-to-face angles, regardless of the load. At low loads, the transformed material is amorphous silicon for all indenter angles. For Berkovich indentations made at high-load, the transformed material is a nanocrystalline mix of Si-I and Si-III/Si-XII, as confirmed by selected area diffraction. Extrusion of material at high loads for the cube-corner indenter reduces the volume of transformed material remaining underneath the indenter, thereby eliminating the pop-out in the unloading curve.
INTRODUCTION The generally recognized sequence for pressure-induced phase transformation of silicon as established in diamond anvil tests is that normal diamond cubic Si (Si-I) transforms to the metallic β-tin structure (Si-II) at a hydrostatic pressure of 11GPa, or at lower pressures when aided by shear stresses [1]. Upon decompression, the β-tin structure transforms to amorphous silicon (a-Si) at high release rates, or to a variety of metastable crystalline forms at low rates 1. Nanoindentation has also proven useful in the characterization of the pressure induced phase transformations of Si [2-14]. Since the hardness of Si is dominated by the pressure needed to induce the phase transformations, metastable phases are frequently observed within and around nanoindentation hardness impressions. Indentation-induced phase transformations in Si have been studied mostly with spherical indenters and the Berkovich indenter - a triangular pyramid with a centerline-to-face angle of 65.3°. For these indenters, there is general agreement that the "elbow" observed in unloading curves at relatively low maximum loads is caused by the formation of amorphous silicon, whereas the "pop-out" observed at higher loads results from the nucleation and growth of metastable crystalline phases such as Si-III and Si-XII. Numerous studies have documented the post indentation structures of the deformed region of spherical, Vickers and Berkovich indentations using plan-view and cross-sectional TEM [5-13]. Recently, a systematic study by Jang et al. revealed that pop-out is absent when very sharp triangular pyramidal indenters are unloaded from high loads (figure 1a), even though the
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