Helium incorporation induced direct-gap silicides
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-06-10 |
| Journal | npj Computational Materials |
| Authors | Shicong Ding, Jingming Shi, Jiahao Xie, Wenwen Cui, Pan Zhang |
| Institutions | Jilin University, Jiangsu Normal University |
| Citations | 12 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research investigates a novel high-pressure synthesis route using Helium (He) to create direct-gap silicon (Si) compounds, aiming to overcome the inherent indirect band gap limitation of conventional Si for thin-film photovoltaics.
- Core Value Proposition: He incorporation stabilizes four new Si2He host-guest structures, two of which exhibit direct band gaps near the optimal Shockley-Queisser limit (1.34 eV).
- Key Candidates: The mC12-Si2He phase is the most promising, possessing a direct band gap of 1.34 eV and a dipole-allowed transition, indicating superior solar absorption capacity compared to cubic diamond Si (CD-Si).
- Synthesis Pathway: He acts as a temporary intermediate. Due to weak Si-He interactions and low He migration barriers (as low as 0.01 eV), He atoms can be easily removed via thermal degassing.
- Resulting Materials: The removal of He yields pure, metastable Si allotropes (oC24-Si, tP6-Si, mC8-Si), providing a viable chemical route for synthesizing these high-energy structures.
- Structural Mechanism: The Si2He phases adopt channel-like Si frameworks filled with He guest atoms, with the Si framework potentially retained upon He removal to ambient pressure.
- Stability: The oP36-Si2He phase is predicted to be the most energetically stable Si2He structure up to 18 GPa.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Optimal Band Gap (SQ Limit) | 1.34 | eV | Shockley-Queisser limit for maximum solar conversion efficiency (33.7%). |
| mC12-Si2He Band Gap | 1.34 | eV | Direct, dipole-allowed transition; optimal for thin-film PV. |
| oP36-Si2He Band Gap | 1.24 | eV | Direct band gap, high absorption capacity. |
| tP9-Si2He Band Gap | 2.18 | eV | Indirect band gap (not suitable for PV). |
| mC8-Si Band Gap (Pure Si) | 0.84 | eV | Indirect band gap of the pure Si allotrope resulting from mC12-Si2He degassing. |
| He Migration Barrier (tP9-Si2He) | 0.01 | eV | Lowest energy barrier, indicating easiest He removal via degassing. |
| He Migration Barrier (mC12-Si2He) | 0.37 | eV | Low barrier, indicating comparative ease of He removal. |
| He Migration Barrier (mC18-Si2He) | 1.51 | eV | High barrier, suggesting He is difficult to remove from this phase. |
| Si-He Charge Transfer | 0.03 to 0.09 | electrons | Slight charge transfer from Si framework to He atoms (Bader analysis). |
| Pressure Stability (oP36-Si2He) | 0 to 18 | GPa | Energetically most stable phase in this pressure range (static-lattice). |
| MD Simulation Temperature | 300 | K | Used to confirm thermodynamic stability at ambient conditions. |
Key Methodologies
Section titled âKey MethodologiesâThe study relies entirely on first-principles calculations and structure prediction methods, focusing on computational materials design rather than experimental synthesis.
- Structure Prediction: Used the CALYPSO (Crystal structure Analysis by Particle Swarm Optimization) method to search for stable Si2He phases, primarily at 10 GPa, maximizing eight formula units per simulation cell.
- Structural Optimization: Employed Density Functional Theory (DFT) as implemented in VASP, utilizing the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional.
- Electronic and Optical Property Calculation: The HSE06 (Heyd-Scuseria-Ernzerhof) hybrid functional was used to accurately calculate electronic band structures and band gaps, correcting the known underestimation by standard DFT functionals.
- Dynamic Stability Verification: Phonon dispersion calculations were performed using the supercell approach (PHONOPY code) to confirm the dynamic stability of the predicted Si2He phases at 0 and 10 GPa.
- He Removal Feasibility Assessment: The CI-NEB (Climbing Image Nudged Elastic Band) method was used to calculate the energy barriers for He migration along the Si channels, determining the feasibility of thermal degassing.
- Absorption Analysis: The imaginary part of the dielectric function (Δ2) and the square of the transition dipole moment (P2) were calculated to assess solar absorption capacity and confirm the dipole-allowed nature of the direct band gaps.
Commercial Applications
Section titled âCommercial ApplicationsâThe successful computational design of direct-gap Si compounds opens pathways for significant advancements in energy and semiconductor technology.
- Photovoltaic Energy Generation:
- High-Efficiency Thin-Film Solar Cells: The primary application. Materials like mC12-Si2He (or the resulting mC8-Si allotrope) offer direct band gaps (1.34 eV) that are ideal for maximizing solar conversion efficiency, enabling the use of ultra-thin Si layers.
- Flexible Electronics: Thin-film materials are crucial for flexible solar panels and integrated building photovoltaics (BIPV).
- Advanced Semiconductor Materials:
- Novel Si Allotrope Synthesis: The He incorporation/degassing method provides a practical, two-step chemical route to synthesize metastable Si allotropes (oC24-Si, tP6-Si, mC8-Si) that are otherwise difficult to obtain directly due to their high energy relative to CD-Si.
- High-Pressure Materials Science:
- Intermediate Synthesis Medium: Confirms the utility of inert noble gases (like He) under high pressure as a temporary âscaffoldingâ element to stabilize open-framework structures, a technique potentially transferable to synthesizing other novel functional materials.
View Original Abstract
Abstract The search of direct-gap Si-based semiconductors is of great interest due to the potential application in many technologically relevant fields. This work examines the incorporation of He as a possible route to form a direct band gap in Si. Structure predictions and first-principles calculations show that He and Si, at high pressure, form four dynamically stable phases of Si 2 He (oP36-Si 2 He, tP9-Si 2 He, mC18-Si 2 He, and mC12-Si 2 He). All phases adopt host-guest structures consisting of a channel-like Si host framework filled with He guest atoms. The Si frameworks in oP36-Si 2 He, tP9-Si 2 He, and mC12-Si 2 He could be retained to ambient pressure after removal of He, forming three pure Si allotropes. Among them, oP36-Si 2 He and mC12-Si 2 He exhibit direct band gaps of 1.24 and 1.34 eV, respectively, close to the optimal value (~1.3 eV) for solar cell applications. Analysis shows that mC12-Si 2 He with an electric dipole transition allowed band gap possesses higher absorption capacity than cubic diamond Si, which makes it to be a promising candidate material for thin-film solar cell.