Abstract
The pivotal role of silicon (Si) in semiconductor technology is well-established; however, its hexagonal diamond (hd) crystal structure remains underexplored. This study addresses the paucity of microscopic evidence concerning the formation and phase transformation behavior of hd-Si up to 1000 °C. Utilizing instrumented nanoindentation and subsequent annealing, the hd-Si phase is obtained from a rhombohedral (R8) and body-centered cubic (BC8) mixture within a diamond cubic silicon (dc-Si) wafer. In situ characterization reveals that hd-Si undergoes a reversible phase transition to a metallic β-tin (Sn) phase under indentation loading, reverting to an R8/BC8 mixture upon unloading, thereby providing experimental confirmation of prior theoretical predictions. Thermal stability assessments indicate that hd-Si remains stable beyond ≈700 °C and transitions to dominantly dc-Si at 1000 °C, with traces of hd-Si persisting. Notably, annealing at 500 and 700 °C yields large-area textured hd-Si nanocrystals with slight misorientations featuring 2H, 4H, and 6H polytypes. Interestingly, the dc-Si formed from hd-Si upon annealing at 1000 °C also transforms to a metallic β-Sn phase during Berkovich indentation and reverts to an R8/BC8 mixture upon unloading. This work provides critical insights into the high-pressure phases of Si, paving the way for future studies on phase engineering and stabilization for advanced semiconductor applications and material innovations.
