Structural and Functional Properties of Si and Related Semiconducting Materials Processed by High-Pressure Torsion
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2023-03-30 |
| Journal | MATERIALS TRANSACTIONS |
| Authors | Yoshifumi Ikoma |
| Institutions | Materials Science & Engineering, Kyushu University |
| Citations | 15 |
| Analysis | Full AI Review Included |
Structural and Functional Properties of Si and Related Semiconducting Materials Processed by High-Pressure Torsion
Section titled âStructural and Functional Properties of Si and Related Semiconducting Materials Processed by High-Pressure TorsionâExecutive Summary
Section titled âExecutive SummaryâThis research utilizes High-Pressure Torsion (HPT) to induce severe plastic deformation (SPD) in Si and Si0.5Ge0.5, achieving novel metastable phases and significantly altered functional properties suitable for advanced devices.
- Metastable Phase Formation: HPT successfully induced the strain-driven phase transformation sequence Si-I â Si-II â Si-III/XII in Si at a nominal pressure of 6 GPa.
- Si-IV Synthesis: In-situ synchrotron XRD confirmed that annealing HPT-processed Si up to 473 K resulted in the formation of the hexagonal-diamond Si-IV phase from Si-III/XII.
- Electrical Enhancement: Resistivity of Si decreased significantly (from ~0.2 Ωm to ~7 x 10-3 Ωm) with increasing shear strain, attributed to the formation of semimetallic Si-III.
- Thermal Management: Thermal conductivity (Îș) of Si was drastically reduced from the bulk value (~140 W m-1K-1) to approximately 3 W m-1K-1, primarily due to grain refinement.
- SiGe Alloy Discovery: HPT processing of Si0.5Ge0.5 formed the metastable bc8-Si0.5Ge0.5 phase, which exhibits semimetallic properties, analogous to Si-III.
- Optical Properties: A weak, broad photoluminescence (PL) peak appeared in the visible light region after annealing, indicating the presence of Si-I nanograins exhibiting quantum confinement effects.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| HPT Nominal Pressure (Si) | 6, 24 | GPa | Used for 10 mm and 5 mm diameter disks, respectively. |
| HPT Rotational Speed | 1 | rpm | Standard processing speed for all HPT experiments. |
| Minimum Thermal Conductivity (Si) | ~3 | W m-1K-1 | Achieved after HPT processing (N â„ 50), representing a 98% reduction from bulk Si (~140 W m-1K-1). |
| Minimum Resistivity (Si) | ~7 x 10-3 | Ωm | Achieved at N=100 rotations, indicating semimetallic behavior due to Si-III formation. |
| Si-III Bandgap | 30 | meV | Potential for narrow gap semiconductor applications. |
| Si-XII Bandgap | 0.24 | eV | Potential for narrow gap semiconductor applications. |
| Si-IV Formation Temperature | 473 | K | Maximum annealing temperature observed for Si-IV formation via in-situ synchrotron XRD. |
| bc8-Si0.5Ge0.5 Lattice Constant | 0.678 | nm | Metastable phase formed in HPT-processed Si0.5Ge0.5. |
| Si-I Crystallite Size (N â„ 50) | <10 | nm | Estimated average size for Si-I, Si-III, and Si-XII phases after HPT. |
| Si-I Volume Fraction (N=100, 6 GPa) | ~0.5 | N/A | Remaining diamond-cubic phase after maximum deformation. |
Key Methodologies
Section titled âKey Methodologiesâ- Material Preparation: Single-crystal Si(100) wafers and bulk Si0.5Ge0.5 crystals (grown by the Traveling Liquidus-Zone, TLZ, method) were cut into disks (5 or 10 mm diameter).
- High-Pressure Torsion (HPT): Samples were processed at room temperature using a tungsten carbide lower anvil with a 0.25 mm deep cavity. Nominal pressures were 6 GPa or 24 GPa, applied at 1 rpm rotational speed for up to 100 rotations (N).
- In-situ Synchrotron XRD: Phase transformations during annealing were monitored using high-energy XRD (61.4 keV) at SPring-8 (BL04B1). Samples were annealed up to 473 K to observe the formation of Si-IV.
- Microstructural Characterization: High-Resolution Transmission Electron Microscopy (HRTEM) was used on ex-situ annealed samples (473 K for 2 h) to confirm the presence of Si-I and Si-IV nanograins.
- Electrical Property Measurement: Electrical resistivity was determined using a standard four-probe method.
- Thermal Property Measurement: Thermal diffusivity (α) was measured via the laser flash method. Thermal conductivity (Îș) was calculated using the formula Îș = αΔÏ, where c (heat capacity) was fixed at 0.71 J g-1K-1.
- Optical Property Measurement: Photoluminescence (PL) spectra were collected at room temperature using a 488 nm laser to detect quantum confinement effects from Si nanograins.
Commercial Applications
Section titled âCommercial Applicationsâ- Narrow Gap Semiconductor Devices: Utilizing the metastable Si-III (30 meV bandgap) and Si-XII (0.24 eV bandgap) phases for infrared detection, low-power electronics, and specialized sensors where conventional Si is unsuitable.
- High-Efficiency Photovoltaics (Solar Cells): Leveraging Si-I nanograins for quantum confinement effects (visible light PL) and the potential light-absorbing properties of Si-IV (0.95 eV indirect bandgap) to enhance solar energy conversion efficiency, potentially utilizing Multiple Exciton Generation (MEG).
- Advanced Thermoelectrics: The extreme reduction in thermal conductivity (Îș ~3 W m-1K-1) in HPT-processed Si and SiGe alloys is critical for improving the figure of merit (ZT) in thermoelectric generators, enabling efficient waste heat recovery.
- Novel Semimetallic Components: Utilizing the semimetallic nature of Si-III and bc8-Si0.5Ge0.5 for specialized interconnects or electronic components requiring high carrier mobility or unique electrical interfaces.
- Bulk Metastable Material Production: HPT enables the synthesis of these high-pressure metastable phases in bulk form (millimeter to centimeter scale), facilitating industrial scaling compared to traditional diamond anvil cell methods.
View Original Abstract
We report on high-pressure torsion (HPT) processing of Si and related semiconducting materials, and discuss their phase transformations and electrical, thermal, and optical properties. In-situ synchrotron x-ray diffraction revealed that the metastable bc8-structure Si-III and r8-structure Si-XII in the HPT-processed Si samples gradually disappeared and hexagonal-diamond Si-IV appeared during annealing up to 473 K. The formation of Si-III/XII in the samples processed at a nominal pressure of 6 GPa indicated the strain-induced phase transformation from diamond-cubic Si-I to a high-pressure tetragonal Si-II phase during HPT processing, and a following phase transformation from Si-II to Si-III/XII upon decompression. The resistivity decreased with increasing the number of anvil rotations due to the formation of semimetallic Si-III. The thermal conductivity of Si was reduced to âŒ3 W mâ1Kâ1 after HPT processing. A weak and broad photoluminescence peak associated with Si-I nanograins appeared in the visible light region after annealing. Metastable bc8-Si0.5Ge0.5 with a semimetallic property was formed by HPT processing of a traveling-liquidus-zone-grown Si0.5Ge0.5 crystal. These results indicate that the application of HPT processing to Si and related semiconductors paves the way to novel devices utilizing nanograins and metastable phases.