Optical Absorption in Hexagonal-Diamond Si and Ge Nanowires - Insights from STEM-EELS Experiments and Ab Initio Theory
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-05-06 |
| Journal | Nano Letters |
| Authors | Luiz H. G. Tizei, Michele Re Fiorentin, Thomas Dursap, Tom Berg, Marc TĂșnica |
| Institutions | Centre National de la Recherche Scientifique, Université Paris-Saclay |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study presents the first comprehensive experimental and theoretical analysis of optical absorption in high-quality hexagonal-diamond (2H) Silicon (Si) and Germanium (Ge) nanowires (NWs), crucial materials for next-generation Group IV photonics.
- Methodology: High-quality 2H-Si and 2H-Ge NWs were grown in situ via Vapor-Liquid-Solid (VLS) methods and characterized using high-resolution STEM combined with monochromated Electron Energy Loss Spectroscopy (EELS) in both intersecting (bulk) and aloof-beam (surface) configurations.
- 2H-Si Performance: 2H-Si NWs show significant absorption in the visible range, with a marked onset above 2.5 eV and a dominant peak at 3.7 eV. This confirms the materialâs potential for photovoltaic applications due to its reduced direct bandgap (1.75 eV) compared to cubic Si.
- 2H-Ge Direct Bandgap Weakness: Intersecting-beam EELS confirmed that the predicted direct bandgap transition in 2H-Ge (0.26 eV) is extremely weak, showing no detectable absorption signature near this energy due to minimal oscillator strength (dipole forbidden transition).
- Surface Shell Identification: Aloof-beam EELS on 2H-Ge revealed a strong peak at 2 eV, which was successfully attributed to the presence of a thin (~1.5 nm) cubic (3C-Ge) shell covering the 2H-Ge core, formed during radial vapor-solid overgrowth.
- Contradiction Resolved: The findings clarify the structure-optical response relationship, confirming that the intrinsic direct bandgap emission in pure 2H-Ge is minimal, resolving previous discrepancies with reports of strong photoluminescence (PL) from ensemble 2H-Ge samples.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| 2H-Si Direct Bandgap (Î point) | 1.75 | eV | Reduced compared to 3C-Si (3.4 eV). |
| 2H-Si Fundamental Bandgap (Indirect) | 0.92 | eV | Theoretical value (Î-M line). |
| 2H-Si Absorption Onset (EELS) | 2.77 | eV | Measured in intersecting configuration. |
| 2H-Si Dominant Absorption Peak | 3.7 | eV | Confirmed by EELS and ab initio theory. |
| 2H-Ge Direct Bandgap (Î point) | 0.26 | eV | Theoretical value; transition is dipole forbidden. |
| 2H-Ge Absorption Onset (EELS) | ~0.9 | eV | Measured in intersecting configuration. |
| 3C-Ge Shell Thickness (on 2H-Ge) | ~1.5 | nm | Estimated thickness causing the 2 eV aloof-beam peak. |
| Ge Stem Nanobranch Diameter | ~100 | nm | Typical nanowire dimension. |
| Si Nanobranch Diameter | ~350 | nm | Typical nanowire dimension. |
| EELS Spectral Resolution | < 10 | meV | Used for low-loss optical response measurement. |
| STEM Operating Voltage | 200 | keV | Used for structural and chemical analysis. |
Key Methodologies
Section titled âKey MethodologiesâThe study employed a highly controlled synthesis and advanced characterization workflow to isolate the intrinsic dielectric response of the hexagonal phases:
- In Situ VLS Growth: High-quality, single-crystalline 2H-Si and 2H-Ge nanowires were synthesized using the Vapor-Liquid-Solid (VLS) growth mode. The NWs were grown as nanobranches on wurtzite GaAs stems.
- Real-Time Control: The growth process was carried out in situ in a modified TEM for Molecular Beam Epitaxy (MBE) growth, enabling real-time tuning and control of nanowire morphology and crystal phase.
- Structural Verification: As-grown samples were characterized using High-Resolution TEM (HRTEM) and Scanning TEM (STEM) to confirm excellent structural quality (strain-free, minimal defects) and composition (Energy Dispersive X-ray Spectroscopy, EDS).
- Monochromated EELS: Optical absorption properties were measured using Electron Energy Loss Spectroscopy (EELS) in the low-loss regime (< 10 meV spectral resolution), which is proportional to the materialâs dielectric response.
- Dual EELS Configuration:
- Intersecting-specimen EELS: Electron beam penetrates the NW, probing the bulk loss function (Im(-1/Δ(Ï))). Used to determine intrinsic absorption onset.
- Aloof-beam EELS: Electron beam passes near the surface (near-field spectroscopy), highly sensitive to surface morphology and thin shells. Used to detect the 3C-Ge shell response.
- Ab Initio Modeling: Experimental results were compared against first-principles calculations of electronic and optical properties, including macroscopic dielectric functions and loss functions, calculated using many-body perturbation theory (accounting for excitonic, non-collinear, and local-field effects).
Commercial Applications
Section titled âCommercial ApplicationsâThe clarification of optical properties in hexagonal-diamond Si and Ge is critical for developing advanced Group IV semiconductor devices:
- Group IV-Based Lasers: The primary application is the development of high-Ge-content hexagonal alloys for efficient, electrically pumped lasers operating in the infrared range (1.5 ”m to 3.4 ”m), addressing a long-standing challenge in silicon photonics.
- Integrated Photonics: These materials enable the creation of monolithically integrated electron-photon and spin-photon interfaces, crucial for high-speed data communication and quantum computing integrated onto silicon platforms.
- Photovoltaics (PV): 2H-Si NWs show enhanced visible light absorption compared to standard cubic Si. This suggests potential for use as efficient absorber materials in next-generation solar cells, particularly when engineered into 2H/3C heterostructures.
- Quantum Electronics: The ability to modulate intrinsic material properties through crystal phase tuning (2H vs. 3C) allows for engineering stronger, allowed optical transitions at the bandgap, beneficial for quantum electronic devices.
- Pseudo-Direct Bandgap Engineering: The insights gained regarding the weak direct bandgap transition in 2H-Ge are valuable for understanding and engineering other pseudo-direct bandgap nanostructures, such as wurtzite GaP, GaAs, and InP nanowires.
View Original Abstract
Hexagonal-diamond (2H) group IV nanowires are key for advancing group IV-based lasers, quantum electronics, and photonics. Understanding their dielectric response is crucial for performance optimization, but their optical absorption properties remain unexplored. We present the first comprehensive study of optical absorption in 2H-Si and 2H-Ge nanowires combining high-resolution STEM, monochromated EELS, and <i>ab initio</i> simulations. The nanowires, grown <i>in situ</i> in a TEM as nanobranches on GaAs stems, show excellent structural quality: single crystalline, strain-free, minimal defects, and no substrate contamination, enabling access to intrinsic dielectric response. 2H-Si exhibits enhanced absorption in the visible range compared to cubic Si, with a marked onset above 2.5 eV. 2H-Ge shows absorption near 1 eV but no clear features at the direct bandgap, as predicted by <i>ab initio</i> simulations. A peak at around 2 eV in aloof-beam spectra is attributed to a thin 3C-Ge shell. These findings clarify the structure-optical response relationships in 2H materials.