Antisite Pairs Suppress the Thermal Conductivity of BAs
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2018-09-06 |
| Journal | Physical Review Letters |
| Authors | Qiang Zheng, Carlos A. Polanco, MaoâHua Du, Lucas Lindsay, Miaofang Chi |
| Institutions | University of Tennessee at Knoxville, Oak Ridge National Laboratory |
| Citations | 52 |
| Analysis | Full AI Review Included |
Defect Suppression of Thermal Conductivity in High-$\kappa$ Materials: An Antisite Pair Analysis in BAs
Section titled âDefect Suppression of Thermal Conductivity in High-$\kappa$ Materials: An Antisite Pair Analysis in BAsâThis technical document analyzes recent research concerning the dramatic suppression of thermal conductivity ($\kappa$) in Boron Arsenide (BAs) due to specific lattice defects. While the study focuses on BAs, the findings underscore the critical importance of material purity and defect controlâa core competency and value proposition for 6CCVDâs CVD Diamond products in thermal management, electronics, and quantum applications.
Executive Summary
Section titled âExecutive Summaryâ- Problem Identification: BAs, predicted to rival diamond with $\kappa$ > 2000 $\text{Wm}^{-1} \text{ K}^{-1}$, consistently demonstrated measured $\kappa$ values an order of magnitude lower ($\sim 140 \text{ Wm}^{-1} \text{ K}^{-1}$).
- Defect Mechanism Identified: Aberration-corrected STEM and DFT calculations identified the primary cause of thermal suppression as concentrated $\text{As}\text{B}$-$\text{B}\text{As}$ antisite pairs (an As atom on a B site paired with a B atom on an As site).
- Defect Concentration: The estimated concentration of these antisite pairs was measured via STEM imaging to be extremely high, approximately $6.6 \pm 3 \times 10^{20} \text{ cm}^{-3}$ (1.8% molar concentration).
- Modeling Validation: Thermal transport calculations utilizing this measured defect concentration yielded $\kappa$ values between 65-100 $\text{Wm}^{-1} \text{ K}^{-1}$, confirming that the antisite pairs are responsible for the low observed thermal conductivity.
- Kinetic Control Required: DFT results indicate a high defect formation energy (1.95 $\text{eV}$) for the antisite pair, suggesting the defects are kinetically trapped during vapor transport synthesis, emphasizing that achieving ultra-high $\kappa$ requires precise control of kinetic growth factors.
- Implication for Industry: The research validates that reliable ultra-high thermal conductivity ($> 2000 \text{ Wm}^{-1} \text{ K}^{-1}$) requires materials like high-purity Single Crystal Diamond (SCD), where inherent defects are minimized through advanced CVD techniques.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data was extracted from the analysis of BAs single crystals and computational models:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Predicted Thermal Conductivity ($\kappa$) | 2000 | $\text{Wm}^{-1} \text{ K}^{-1}$ | First-principles BAs (Perfect Lattice, RT) |
| Measured $\kappa$ (As-grown BAs) | $\sim$ 140 | $\text{Wm}^{-1} \text{ K}^{-1}$ | Room Temperature (RT) |
| Calculated $\kappa$ (w/ Defects) | 65-100 | $\text{Wm}^{-1} \text{ K}^{-1}$ | Based on STEM-estimated defect concentration |
| Antisite Pair Concentration ($\text{N}_{\text{ASB-BAS}}$) | $6.6 \pm 3 \times 10^{20}$ | $\text{cm}^{-3}$ | Estimated from HAADF STEM images (1.8%) |
| $\text{AS}\text{B}$-$\text{B}\text{As}$ Formation Energy | 1.95 | $\text{eV}$ | Lowest formation energy for native point defects |
| BAs Lattice Parameter ($a$) | 4.7776 | $\text{Ă }$ | Zinc blende cubic structure |
| STEM Accelerating Voltage | 100 | $\text{kV}$ | Nion UltraSTEM 100TM operation |
| Crystal Growth Temperature | 850 | $\text{°C}$ | Used for thermal equilibrium concentration calculations |
| Thickness of STEM Imaging Region | $\sim 1.7$ | $\text{nm}$ | $\sim 3.6$ unit cells |
Key Methodologies
Section titled âKey MethodologiesâThe investigation employed a highly integrated approach combining advanced microscopy and first-principles theoretical transport modeling.
- Crystal Growth: BAs single crystals were grown using the vapor transport method, utilizing iodine ($\text{I}_2$) as the chemical transport agent.
- Structural and Elemental Analysis:
- Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) confirmed high material purity (no foreign atoms detected).
- Electron Energy Loss Spectroscopy (EELS) was used to calculate the precise thickness of the imaging regions ($\sim 1.7 \text{ nm}$ to $\sim 4.7 \text{ nm}$).
- Atomic-Scale Defect Imaging (STEM):
- Aberration-Corrected Scanning Transmission Electron Microscopy (STEM) was performed at 100 kV.
- High-Angle Annular Dark-Field (HAADF) images, whose intensity is roughly proportional to $Z^{2}$ (atomic number squared), were analyzed along the [001] projection.
- Observed enhanced intensity in B columns and weakened intensity in neighboring As columns confirmed the presence of the paired $\text{As}\text{B}$-$\text{B}\text{As}$ defects.
- Image Simulation and Quantification:
- HAADF image simulations (using QSTEM software) were crucial to distinguish single $\text{As}_\text{B}$ defects from multiple defects per column, ultimately leading to the quantification of the $6.6 \times 10^{20} \text{ cm}^{-3}$ concentration.
- Density Functional Theory (DFT) Calculations:
- DFT (implemented in VASP) used the HSE hybrid functional to calculate defect formation energies, identifying the $\text{As}\text{B}$-$\text{B}\text{As}$ pair as the most energetically preferred defect (1.95 $\text{eV}$).
- Thermal Transport Modeling:
- Thermal conductivity ($\kappa$) was calculated by solving the Peierls-Boltzmann transport equation, incorporating both three-phonon scattering and crucial phonon-defect scattering using a parameter-free ab initio Greenâs function methodology.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research on BAs powerfully demonstrates the catastrophic effect that subtle, kinetically trapped lattice defects can have on intrinsic material properties, confirming that absolute defect control is paramount for thermal management applications.
While BAs research continues to strive for the theoretical $2000 \text{ Wm}^{-1} \text{ K}^{-1}$ limit, 6CCVD Single Crystal Diamond (SCD) already achieves this benchmark reliably through superior MPCVD growth control. We provide the material solution needed by engineers and scientists who demand proven, defect-minimized performance.
Applicable Materials for High-$\kappa$ Applications
Section titled âApplicable Materials for High-$\kappa$ Applicationsâ| 6CCVD Material Grade | Thermal Property Focus | Relevance to BAs Research |
|---|---|---|
| Optical Grade SCD | $\kappa > 2000 \text{ Wm}^{-1} \text{ K}^{-1}$ | Direct replacement for high-demand thermal substrates, eliminating the risks associated with kinetically trapped antisite defects found in BAs. Guaranteed high purity (low Nitrogen). |
| High-Purity PCD | $\kappa$ up to $\sim 1500 \text{ Wm}^{-1} \text{ K}^{-1}$ | Suitable for large-area thermal spreader applications (wafers up to 125 $\text{mm}$) where maximizing surface area outweighs single-crystal requirements. |
| Boron-Doped Diamond (BDD) | Excellent electrochemical stability | Used in sensing/electrochemistry. Unlike BAsâ unintentional structural defects, BDD offers controlled, intentional doping for specific functional properties. |
Customization Potential for Advanced Research
Section titled âCustomization Potential for Advanced ResearchâTo replicate or extend the advanced material synthesis techniques (like MBE or flux growth) suggested for BAs, high-quality diamond substrates and precision engineering are essential. 6CCVD provides comprehensive support:
- Custom Dimensions and Substrates: We offer SCD and PCD substrates up to $125 \text{ mm}$ with precision laser cutting capabilities, crucial for integrating materials into experimental devices (e.g., custom sizes for STEM/EEL sample preparation).
- Ultra-Smooth Polishing: Our SCD materials achieve surface finishes down to $\text{Ra} < 1 \text{ nm}$, and inch-size PCD achieves $\text{Ra} < 5 \text{ nm}$, critical for minimizing surface defects that can interfere with epitaxy (like MBE growth suggested for BAs films).
- Custom Metalization Schemes: The ability to deposit highly conductive contacts (e.g., Au, Pt, Ti) is fundamental for measuring electrical and thermal properties. We offer in-house metalization services, supporting complex geometries and multi-layer stacks required for device prototypes.
Engineering Support & Global Logistics
Section titled âEngineering Support & Global Logisticsâ- Defect Control Expertise: 6CCVDâs in-house PhD team specializes in CVD diamond growth kinetics and defect characterization, offering expert assistance with material selection for similar high thermal conductivity and microelectronic packaging projects.
- Supply Chain Reliability: We offer guaranteed global shipping (DDU default, DDP available), ensuring the rapid and reliable delivery of high-purity diamond materials needed to drive next-generation electronics research.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
BAs was predicted to have an unusually high thermal conductivity with a room temperature value of 2000 W m<sup>-1</sup> K<sup>-1</sup>, comparable to that of diamond. However, the experimentally measured thermal conductivity of BAs single crystals is still lower than this value. To identify the origin of this large inconsistency, we investigate the lattice structure and potential defects in BAs single crystals at the atomic scale using aberration-corrected scanning transmission electron microscopy (STEM). Rather than finding a large concentration of As vacancies (<em>V</em><sub>As</sub>), as widely thought to dominate the thermal resistance in BAs, our STEM results show an enhanced intensity of some B columns and a reduced intensity of some As columns, suggesting the presence of antisite defects with As<sub>B</sub> (As atom on a B site) and B<sub>As</sub> (B atom on an As site). Additional calculations show that the antisite pair with As<sub>B</sub> next to B<sub>As</sub> is preferred energetically among the different types of point defects investigated and confirm that such defects lower the thermal conductivity for B<sub>As</sub>. Using a concentration of 1.8(8)% (6.6 ± 3.0 à 10<sup>20</sup> cm<sup>-3</sup> in density) for the antisite pairs estimated from STEM images, the thermal conductivity is estimated to be 65-100 W m<sup>-1</sup> K<sup>-1</sup>, in reasonable agreement with our measured value. Our study suggests that As<sub>B</sub>-B<sub>As</sub> antisite pairs are the primary lattice defects suppressing thermal conductivity of B<sub>As</sub>. Possible approaches are proposed for the growth of high-quality crystals or films with high thermal conductivity. In conclusion by employing a combination of state-of-the-art synthesis, STEM characterization, theory, and physical insight, this work models a path toward identifying and understanding defect-limited material functionality.