TEM analysis of Textured Silicon Polymorph Crystals obtained via Nanoindentation and Annealing

Silicon polymorphs exhibit unique properties, such as the combination of low thermal conductivity with high thermoelectric efficiency, or high carrier mobility with superconductivity. Some polymorphs are also predicted to possess a direct bandgap. Among these, hexagonal (hd) silicon is particularly attractive due to its potential use as a template for the epitaxial growth of hexagonal SiGe alloys. These alloys can maintain a tunable direct bandgap across a broad compositional range, offering significant flexibility for optoelectronic applications. In our recent work [1], we demonstrated the formation of micrometer-sized, textured hexagonal silicon crystals through nanoindentation followed by rapid post-indentation annealing. This approach offers a simpler alternative to more complex synthesis methods, such as those involving nanowires or sophisticated deposition systems. Under mechanical pressure, diamond cubic silicon (dc-Si) transforms into a metallic β-Sn phase (Si-II) at high pressures. Upon pressure release, and by optimizing the nanoindentation unloading process, we observed—via Raman spectroscopy—the formation of a mixed body-centered cubic (bc8, Si-III) and rhombohedral (r8, Si-XII) phase. In this study, we investigate nanoindented diamond cubic silicon (100) pits prior to annealing to gain deeper crystallographic insight into the intermediate metastable Si phases. The system was analyzed using advanced characterization techniques, including polarized Raman spectroscopy and transmission electron microscopy (TEM). TEM micrographs revealed extensive regions of r8/bc8 phases well-aligned with the Si (001) substrate. As an alternative to rapid annealing for producing hexagonal silicon, we also explored the effects of slow annealing. This process led to the formation of other polymorphs, potentially attributable to Si-XIII, a phase with an as-yet unidentified crystal structure. The authors acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 104 published on 2.2.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– Project Title ”SiGe Hexagonal Diamond Phase by nanoIndenTation (HD-PIT)” – Prot. 2022EYXJ5X – CUP B53D23004120006 ( CUP Master H53D23000780001 ) – Grant Assignment Decree No. 957 adopted on 30.06.2023 by the Italian Ministry of University and Research (MUR).