Nanoindentation-induced subsurface phase engineering in oxide-capped silicon

The controlled formation of high-pressure silicon polymorphs beneath an oxide layer offers a new paradigm for subsurface phase engineering. We systematically compared sharp Berkovich and spherical nanoindentation on 285 nm SiO₂-capped Si(100) using Raman spectroscopy and cross-sectional electron microscopy to reveal how contact geometry and oxide constraint govern phase transformation. Sharp indentation initiates R8 (rhombohedral)/BC8 (body-centered-cubic) phase formation at low loads (42 mN), but the high stress concentration promotes early oxide fracture and radial cracking, limiting the continuous crystalline volume. In contrast, spherical indentation delays observable transformation until ∼92 mN, distributing stress more uniformly. Crucially, we identify a "critical loading window" for optimization. While moderate spherical loads (∼200 mN) facilitate highly ordered crystalline recovery with intact interfaces, excessive loads (∼500 mN) exceed the oxide's confinement capacity, favoring collapse into a disordered amorphous state and localized fracture due to the significant volumetric expansion of the intermediate β-Sn phase. Our results confirm that the oxide modulates stress-relaxation kinetics without altering the fundamental 11-12 GPa transformation threshold. These findings explicitly define the operational limits for dielectric confinement, providing a versatile pathway for engineering subsurface crystalline phases with enhanced carrier mobility and sub-bandgap optical absorption for next-generation silicon photonic and sensing platforms.