Nanoindentation-Induced Phase Transformations in Ge Studied by Electron Microscopy and Raman Spectroscopy

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The sequences of hydrostatic pressure-induced phase transformations in Si and Ge have been established in diamond anvil studies. At sufficient pressure the normal diamond-cubic structure Si-I/Ge-I transforms to the metallic {beta}-tin structure; upon unloading, several metastable phases can form depending on the maximum load and/or unloading rate. In the case of Ge, the metastable phases are Ge-III(ST12) or Ge-IV(BC8). Since microstructural investigations of Si indents have revealed metastable crystalline phases, it was expected that Ge indents could demonstrate similar phenomena. However, there is no prior evidence of phase transformations from indentation load-displacement (P-h) curves or electron microscopy. In this study, nanoindentations were made on undoped (100) Ge at room temperature with triangular pyramidal indenters with centerline-to-face angles of 35{sup o} (cube-corner) and 65{sup o} (Berkovich) at loads of 10, 50 and 80 mN and a loading/unloading rate of 5mN/s. The indents were examined by SEM, micro-Raman analyses and TEM of cross-sections prepared by dual-beam FIB milling.