Structural and Functional Properties of Si and Related Semiconducting Materials Processed by High-Pressure Torsion

At a Glance

Metadata	Details
Publication Date	2023-03-30
Journal	MATERIALS TRANSACTIONS
Authors	Yoshifumi Ikoma
Institutions	Kyushu University, Materials Science & Engineering
Citations	15
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Pressure Torsion of Si and SiGe

Reference Paper: Ikoma, Y. (2023). Structural and Functional Properties of Si and Related Semiconducting Materials Processed by High-Pressure Torsion. Materials Transactions, 64(7), 1346-1352.

Executive Summary

This overview analyzes the creation of novel, functional metastable phases (Si-III, Si-XII, Si-IV, bc8-Si_0.5Ge_0.5) in Silicon and Silicon-Germanium alloys using Severe Plastic Deformation (High-Pressure Torsion, HPT).

Novel Phase Formation: HPT successfully induced strain-driven phase transformations (Si-I $\rightarrow$ Si-II $\rightarrow$ Si-III/XII) at low nominal pressures (6 GPa), overcoming the limitations of hydrostatic high-pressure cells.
Semimetallic Properties: The formation of semimetallic Si-III/XII phases significantly reduced electrical resistivity to $7 \times 10^{-3}$ $\Omega$m, indicating potential for narrow-gap semiconductor devices.
Extreme Thermal Reduction: Thermal conductivity ($\kappa$) was drastically reduced from $\sim 140$ W m^-1K^-1 (bulk Si) to $\sim 3$ W m^-1K^-1, primarily due to grain refinement and the presence of metastable phases, offering intrinsic thermal insulation.
Quantum Confinement: Annealing HPT-processed Si at 873 K resulted in a weak, broad photoluminescence (PL) peak in the visible light region, attributed to quantum confinement effects in Si-I nanograins.
Scalability for Devices: The HPT method enables the preparation of these functional materials on a millimeter-to-centimeter scale, making them viable for integration into novel electronic and optoelectronic devices.
6CCVD Relevance: The extreme processing conditions (HPT) and the need for superior thermal management in resulting low-$\kappa$ devices highlight the necessity for high-performance diamond tooling and heat spreaders, core specialties of 6CCVD.

Technical Specifications

Parameter	Value	Unit	Context
HPT Nominal Pressure (Si)	6, 24	GPa	Used for 5 mm and 10 mm diameter disks.
HPT Rotations (N)	10, 50, 100	N/A	Correlates directly with imposed shear strain ($\gamma$).
Minimum Resistivity ($\rho$)	$7 \times 10^{-3}$	$\Omega$m	Achieved in HPT-processed Si (6 GPa, N=100) due to semimetallic Si-III formation.
Minimum Thermal Conductivity ($\kappa$)	$\sim 3$	W m^-1K^-1	Achieved in HPT-processed Si (6 GPa, N $\ge$ 50), a reduction of $\sim 47$x from bulk Si ($\sim 140$ W m^-1K^-1).
Si-III/XII Volume Fraction	$\sim 0.5$	N/A	Achieved in Si (6 GPa, N=100). Si-XII saturates at $\sim 0.2$.
Si-IV Formation Temperature	473	K	Observed during in-situ annealing of 24 GPa HPT-processed Si.
PL Annealing Temperature	873	K	Required to induce visible light photoluminescence from Si-I nanograins.
bc8-Si_0.5Ge_0.5 Resistivity	$5 \times 10^{-4}$	$\Omega$m	Resistivity of as-grown TLZ Si_0.5Ge_0.5 was $9 \times 10^{-5}$ $\Omega$m. HPT (N=10) increased it to $5 \times 10^{-4}$ $\Omega$m.
Crystallite Size (Si-XII)	< 10	nm	Achieved when N $\ge$ 50, confirming nanograin formation.

Key Methodologies

The research utilized High-Pressure Torsion (HPT) to induce severe plastic deformation, followed by thermal annealing and comprehensive material characterization.

Sample Preparation:
- Starting materials: Single-crystal Si(100) wafers and bulk-crystal Si_0.5Ge_0.5 grown by the Traveling Liquidus-Zone (TLZ) method.
- Samples were cut into disks (5 mm or 10 mm diameter, 0.25 mm thickness).
High-Pressure Torsion (HPT) Processing:
- Tooling: Lower anvil made of Tungsten Carbide (WC).
- Conditions: Nominal pressures set at 6 GPa or 24 GPa. Rotational speed set at 1 rpm. Experiments conducted at Room Temperature.
- Shear Strain Control: The number of anvil rotations (N=10, 50, 100) was varied to control the imposed shear strain ($\gamma$).
Thermal Processing:
- In-situ annealing of HPT-processed Si was performed up to 473 K to observe phase transformation kinetics (Si-III/XII disappearance, Si-IV appearance).
- Ex-situ annealing was performed at 473 K (2h, N₂ atmosphere) and 873 K (1h, N₂ atmosphere) for structural and optical analysis.
Characterization Techniques:
- Structural Analysis: In-situ synchrotron high-energy X-ray Diffraction (XRD) (61.4 keV) and high-resolution Transmission Electron Microscopy (HRTEM) with Fast Fourier Transforms (FFT).
- Electrical Properties: Four-probe method used to measure electrical resistivity ($\rho$).
- Thermal Properties: Laser flash method used to measure thermal diffusivity ($\alpha$), which was used to calculate thermal conductivity ($\kappa = \alpha \rho c$).
- Optical Properties: Photoluminescence (PL) measurements performed at room temperature using a 488 nm laser.

6CCVD Solutions & Capabilities

The findings presented in this paper—particularly the use of extreme mechanical processing to create functional metastable phases and the resulting ultra-low thermal conductivity—present significant opportunities for 6CCVD to provide enabling materials for next-generation devices.

Applicable Materials for Advanced HPT Research

To replicate or extend this research, particularly in scaling up HPT processing or integrating the resulting low-$\kappa$ materials into high-power devices, 6CCVD recommends the following specialized diamond materials:

6CCVD Material	Recommended Grade	Application in HPT/Device Integration
SCD Diamond	Thermal Grade (k > 2000 W m^-1K^-1)	Essential for heat spreading layers to manage thermal hotspots when integrating the ultra-low $\kappa$ Si/SiGe phases into functional devices.
PCD Diamond	Mechanical/Tooling Grade	Superior replacement for Tungsten Carbide (WC) anvils used in HPT, offering higher hardness, stiffness, and wear resistance for large-scale or ultra-high-pressure processing (up to 125 mm diameter capability).
Boron-Doped Diamond (BDD)	Heavy Boron Doped (Conductive)	Ideal for high-performance electrodes or contacts for characterizing the semimetallic Si-III/XII phases, offering chemical inertness and high conductivity.
Optical Grade SCD	High Purity (Low Nitrogen)	Used as high-pressure windows for in-situ optical or synchrotron measurements (e.g., replacing Be windows or used in diamond anvil cells).

Customization Potential for Device Integration

The successful application of HPT to create functional materials requires precise control over geometry and integration interfaces. 6CCVD’s custom capabilities directly address these engineering challenges:

Custom Dimensions: While the paper used 5 mm and 10 mm disks, 6CCVD offers Polycrystalline Diamond (PCD) plates up to 125 mm in diameter, enabling the large-scale HPT processing mentioned as a future goal by the author.
Precision Thickness Control: We provide SCD and PCD wafers with thicknesses ranging from 0.1 µm to 500 µm, allowing researchers to precisely control the thickness ($t$) parameter critical to calculating shear strain ($\gamma = 2\pi r N / t$).
Advanced Metalization: The integration of these novel Si/SiGe phases into electronic devices requires robust, low-resistance contacts. 6CCVD offers in-house custom metalization (Au, Pt, Pd, Ti, W, Cu) tailored to specific semiconductor interfaces, crucial for measuring the low resistivity of the semimetallic Si-III phase.
Ultra-Low Roughness Polishing: For high-quality interface bonding (e.g., wafer bonding Si/SiGe to diamond heat spreaders), 6CCVD guarantees Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD.

Engineering Support

The creation of novel narrow-gap semiconductors via severe plastic deformation is a complex materials science challenge. 6CCVD’s in-house PhD team specializes in the physical and electronic properties of wide-bandgap materials and can assist researchers with:

Material Selection: Consulting on the optimal diamond grade (SCD vs. PCD, thermal vs. mechanical) for high-pressure tooling and subsequent device integration.
Thermal Modeling: Designing diamond heat spreaders to manage the extreme thermal gradients created when integrating ultra-low thermal conductivity materials ($\sim 3$ W m^-1K^-1) with high-power electronics.
Custom BDD Electrode Design: Developing highly conductive BDD structures for advanced electrical characterization of semimetallic Si-III/XII and bc8-Si_0.5Ge_0.5 phases.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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 ∼3 W m−1K−1 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.

Tech Support

Original Source

DOI: https://doi.org/10.2320/matertrans.mt-mf2022031