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Light momentum turns pure silicon from
an indirect to a direct bandgap semiconductor
https://phys.org/news/2024-09-momentum-pure-silicon-indirect-bandgap.html

Original Paper
https://arxiv.org/pdf/2304.14521

Photon Momentum-Enabled Light Absorption in Bulk Silicon

Source: Kharintsev, S. S. et al. Photon momentum enabled light absorption in bulk silicon. Science Advances, 10, eadf5997 (2024).

Central Theme: This research demonstrates a novel method to dramatically enhance light absorption in silicon, an indirect bandgap semiconductor, by exploiting the momentum of confined photons. This challenges the traditional reliance on plasmonic or light-trapping mechanisms for improving silicon's optical properties.

Key Findings:

• Photon Momentum as a Driving Force: Confining photons to scales below 3 nm significantly increases their momentum, enabling direct optical transitions in silicon that would otherwise be forbidden due to momentum mismatch. "Photons do not carry sufficient momentum to induce indirect optical transitions in semiconducting materials such as silicon, necessitating the assistance of lattice phonons to conserve momentum… This work introduces an alternative strategy to fulfill the momentum-matching requirement in indirect optical transitions."

• Enhanced Absorption Across Broad Spectrum: This effect substantially increases silicon's absorption coefficient across a wide spectral range, from the UV to the near-IR, effectively transforming it into a direct bandgap semiconductor for the confined photons.

• Experimental Validation: Tip-enhanced Raman scattering (TERS) experiments demonstrate significant heating and melting of silicon AFM tips when placed near 1-2 nm gold structures on a substrate. This is attributed to enhanced absorption due to confined photon momentum. "We observe an inverse relationship between particle size and optical heating, as revealed by both Raman (Figures 3b2-3b5) and phase measurements (Figures 3c1-3c5)."

• Reflectance measurements on silicon wafers decorated with 1-2 nm gold nanoparticles show reduced reflection, indicating enhanced absorption, particularly near the silicon's band edge. "The difference in the reflectance spectra, between the bare wafer and the covered wafer, shows an abrupt reduction of light reflection at the band edge, at 1100 nm, and a lower but otherwise flat response throughout the UV-IR. The reduced reflectance can be explained as direct absorption…"

• Photocurrent measurements on silicon photodiodes with gold-coated photosensitive layers exhibit increased responsivity, signifying enhanced light absorption and photocarrier generation.

• Mechanism Outperforming Plasmonics: The observed enhancement cannot be explained by plasmonic or geometry-dependent field enhancement alone. The inverse relationship between structure size and absorption enhancement further supports the dominant role of confined photon momentum.
• Potential Impact: This discovery opens up new possibilities for significantly boosting the efficiency of silicon-based devices in various fields:

• Photovoltaics: Thinner and cheaper silicon solar cells with enhanced light harvesting capabilities.

• Photodetection: Improved sensitivity and responsivity of silicon photodetectors.

• Optoelectronics: More efficient light emission and manipulation in silicon-based devices.

Conclusion: This research highlights the importance of photon momentum in light-matter interactions at th