In-situ high temperature micro-Raman investigation of annealing behavior of high-pressure phases of Si

Among the 13 polymorphic phases of Si, the ambient temperature stable body-centered cubic (bc8) and rhombohedral (r8) polymorphs have gained significant interest due to their attractive optical and electronic properties suitable for photovoltaic applications. Though ex situ methods were extensively employed previously to understand the pressure-induced phase transformation kinetics of Si, the limited number of available in situ studies has significantly improved the knowledge in this field and clarified uncertainties. Similarly, in this article, we attempt to understand the thermal annealing behavior of nanoindentation-induced r8 and bc8 phases of Si and their volume dependence using in situ high temperature micro-Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM). A spherical diamond indenter of ∼20-μm radius was chosen to indent diamond cubic (dc) Si (100) at different peak loads (Pmax) to create different volumes of high-pressure phases. The Raman spectra, Raman imaging, and XTEM of the pre- and postannealed indents confirm complete annealing of r8/bc8 phases at 200 ± 10 °C, irrespective of the volume of indents. In contrast to the previous ex situ studies, no signature of the presence of the hexagonal diamond (hd)-Si phase was found at elevated temperatures and the overall transformation observed is directly from r8 → polycrystalline dc-Si and bc8 → polycrystalline dc-Si rather than through other metastable phases such as Si-XIII/hd-Si. The present systematic in situ study provides evidence for a few earlier predictions and clarifies ambiguities involved in understanding the annealing behavior and transformation pathways of two high-pressure phases of Si at elevated temperatures.