Understanding the effect of impurities and grain boundaries on mechanical behavior of Si via nanoindentation of (110)/(1
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Xuegong Yu Department of Materials Science Engineering and State Key Lab of Silicon Materials, Zhejiang University, 310027 Hangzhou, People’s Republic of China
Mike Seacrist MEMC Electronic Materials, Inc., St. Peters, Missouri 63376
George Rozgonyi Department of Material Science and Engineering, North Carolina State University, Raleigh, North Carolina 27606 (Received 8 April 2011; accepted 2 August 2011)
Nanoindentation was used to examine the impact of impurities and grain boundaries on the mechanical properties of a “model” (110)/(100) grain boundary (GB) interface prepared using direct silicon bonding via the hybrid orientation technique of (110) and (100) p-type silicon wafers. Remarkable differences were found between the mechanical behavior of Fe- and Cu-contaminated samples. The direct silicon bonded wafers contaminated with either Fe or Cu showed opposite effects on mechanical properties, with Fe enhancing the silicon hardness, while Cu contamination induces a gradual weakening. High-resolution transmission electron microscopy was used to verify that the abrupt hardness changes observed during increasing nanoindentation loading is attributed to local deformation induced by the GB interface, Cu precipitate colony induced dislocations, and the abrupt crystallographic orientation change across the GB. The resulting dislocation loop generation facilitated the deformation process during nanoindentation and therefore softened the material.
I. INTRODUCTION
The fracture properties of silicon are crucial in determining the manufacturing yield, device performance, and operational reliability of photovoltaic solar cells, very large scale integrated circuits, and microelectromechanical systems. Since it is well known that impurities (metallic and light element), structural defects, crystal orientation, and residual stresses all affect the mechanical properties of single- and multicrystalline silicon,1–4 achieving control of silicon fracture will ultimately depend on our fundamental understanding of the role of extended defect/impurity interactions and intragrain versus grain boundary (GB) sources of mechanical failure. Unfortunately, generally available multicrystalline silicon wafers consist of several microstructural factors which can impact the mechanical behavior of silicon wafers, e.g., grain orientation, nature of GB, intragrain defects (dislocations, inclusions, precipitates, etc.), and their interaction with impurities. For instance, the mechanical strength of silicon is impacted by the density of mobile dislocations, while immobilizaa)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.265 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
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tion or retardation of dislocation motion by the controlled introduction of light element impurities N or O has been shown to improve the strength and hardness significantly.2,3,5–7 However, to fully understand the strength and breakage behavior (crack initiation/propagatio
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