Hexagonal Silicon Formation and Its Phase Transformability

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.