Size-dependent phase transformations during point loading of silicon
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J.B. Pethica Department of Materials, Oxford University, Oxford, OX1 3PH, United Kingdom
T.P. Weihs Department of Materials Science & Engineering, The Johns Hopkins University, Baltimore, Maryland 21218 (Received 14 February 2000; accepted 22 May 2000)
Using a unique combination of in situ electrical and acoustical measurements and ex situ transmission electron microscopy, the phase transformations of silicon during point loading were shown to exhibit a strong dependence on the size of the deformed volume. For nanometer-size volumes of silicon, the final phase was the body centered cubic structure BC8, but for larger volumes it was amorphous. The size dependence was explained by considering how shear stress fields vary with contact size and how interfacial effects between the silicon substrate and the BC8 phase determine its stability. For both small and large contacts the presence of a nonmetallic phase (assumed to be the Rhombohedral structure R8) was observed. I. INTRODUCTION
There have been many theoretical and experimental investigations of the high-pressure phases of silicon1–6 since the identification of its metallic –Sn phase in the 1960s.7–10 The occurrence of these high-pressure phases under point contacts has generated considerable interest and controversy in recent years.11–19 Data recorded in situ during the loading and unloading cycles have shown the occurrence of a discontinuity in the unloading portion of the load/displacement curve. Ex situ electron microscopy analysis of the deformed region has shown the presence of amorphous (␣) Si in large indentations11,13,19 and in very small nanoindentations the presence of a mosaic structure in the indentation center.13,18 Extrusions at the edge of the contact have also been observed with electron microscopy.12,16 Another ex situ technique that has been used to examine large indentations is Raman spectroscopy. These results suggest that BC8, R8, and ␣-silicon may be present at ambient pressures after microscale contacts have been unloaded.19 In situ electrical measurements between two metal pads on a Si surface with indentations straddling the gap between the pads have indicated that a transformation to a more conductive phase of Si occurs during loading of diamond cubic (DC) Si. Overall the data until now have been explained by the transformation of DC Si to the metallic –Sn phase during loading, and then during unloading the –Sn phase has been assumed to transform to ␣–Si, possibly with small volumes of crystalline phases being formed at the edge of the contact. 1754
J. Mater. Res., Vol. 15, No. 8, Aug 2000
In this paper, we present the results of a unique combination of experiments which have enabled us to identify both in situ and ex situ the occurrence of multiple, size-dependent phase transformations during the loading and unloading of nanoscale contacts on DC Si. In this way we have resolved many of the previous controversies regarding the deformation of silicon during pointloading. The results include novel in situ measurements of contact res
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