Micro-Raman Mapping and Analysis of Indentation-Induced Phase Transformations in Germanium
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Micro-Raman Mapping and Analysis of Indentation-Induced Phase Transformations in Germanium Jae-il Jang 1, M.J. Lance 2, Songqing Wen 1, J.J. Huening3, R.J. Nemanich3, and G.M. Pharr 1,2 1 The University of Tennessee, Dept. of Mater. Sci. & Eng., Knoxville, TN 37996-2200, USA. 2 Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831, USA. 3 North Carolina State University, Dept. of Physics, Raleigh, NC 27695, USA.
ABSTRACT Although it has been confirmed by diamond anvil cell experiments that germanium transforms under hydrostatic pressure from the normal diamond cubic phase (Ge-I) to the metallic β-tin phase (Ge-II) and re-transforms to Ge-III (ST12 structure) or Ge-IV (BC8 structure) during release of the pressure, there are still controversies about whether the same transformations occur during nanoindentation. Here, we present new evidence of indentationinduced phase transformations in germanium. Nanoindentation experiments were performed on a (100) Ge single crystal using two triangular pyramidal indenters with different tip angles - the common Berkovich and the sharper cube-corner. Although the indentation load-displacement curves do not show any of the characteristics of phase transformation that are well-known for silicon, micro-Raman spectroscopy in conjunction with scanning electron microscopy reveals that phase transformations to amorphous and metastable crystalline phases do indeed occur. However, the transformations are observed reproducibly only for the cube-corner indenter. INTRODUCTION Together with silicon, germanium is one of the most important materials in the electronic industry, and thus its mechanical behavior is of a considerable interest from both scientific and engineering viewpoints. Following a number of theoretical studies and diamond anvil cell experiments (see review articles [1-2]), it is now well recognized that pristine Ge-I with its diamond cubic structure transforms to the metallic β-tin phase (Ge-II) at about 10 - 11 GPa, and, upon unloading, the Ge-II transforms to Ge-III (ST12, a simple tetragonal structure with 12 atoms in the unit cell) or Ge-IV (BC8, a body-centered cubic structure with 8 atoms in the unit cell), depending on the pressure release rate. Since these transformations are broadly analogous to those occurring in silicon, one might expect the well-known indentation-induced phase transformations for silicon to also occur in Ge. The structural similarities of these two materials also support this expectation. However, whether or not a phase transformation can actually occur during the nanoindentation of Ge is still controversial. While some experimental evidence for transformation has been reported for high load indentations performed mainly using a Vickers indenter [3-6], transformed phases have very rarely been observed reproducibly in Ge nanoindentations, as reviewed by Domnich and Gogotsi [1]. Moreover, using cross-sectional transmission electron microscopy, Bradby et al. [7] have found that severe twinning, rather than pha
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