Isotope Effect in Thermal Conductivity of Polycrystalline CVD-Diamond - Experiment and Theory
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-03-24 |
| Journal | Crystals |
| Authors | A. V. Inyushkin, А. Н. Талденков, Victor Ralchenko, A. P. Bolshakov, А. В. Хомич |
| Institutions | Institute of Radio-Engineering and Electronics, Harbin Institute of Technology |
| Citations | 5 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This analysis focuses on the experimental and theoretical investigation of the isotope effect on the thermal conductivity (κ) of high-quality polycrystalline Chemical Vapor Deposition (CVD) diamond.
- Record Thermal Conductivity: The isotopically enriched 12C polycrystalline diamond (99.96% 12C) achieved a thermal conductivity of 25.1 ± 0.5 W cm-1 K-1 at room temperature (298 K).
- Performance Benchmark: This measured value is higher than the thermal conductivity reported for the most perfect natural and synthetic single-crystalline diamonds of natural isotopic composition (natC).
- Isotope Effect Magnitude: The enrichment resulted in a 35% increase in κ at 298 K compared to the natC sample, with the relative difference peaking at 75% at 150 K.
- Limiting Factors Identified: Using a Callaway theory model, structural defects (vacancies, dislocations, and grain boundaries) were found to substantially reduce the potential isotope effect.
- Dominant Resistance: Phonon scattering by grain boundaries and intergrain regions is the most critical resistive process determining κ(T) below approximately 250 K.
- Theoretical Potential: If non-isotopic point defects and dislocations were eliminated, the model predicts the isotope effect could reach up to 37% at room temperature.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| 12C Isotopic Purity | 99.96 | at.% | Enriched sample composition |
| Thermal Conductivity (12C) | 25.1 ± 0.5 | W cm-1 K-1 | At Room Temperature (298 K) |
| Thermal Conductivity (natC) | 18.6 | W cm-1 K-1 | At Room Temperature (298 K) |
| Maximum Isotope Effect (Δκ/κ) | 75 | % | Observed at 150 K |
| Predicted Ideal Isotope Effect | 37.5 | % | Defect-free, dislocation-free 12C polycrystal |
| Measurement Temperature Range | 5 to 410 | K | Full experimental range |
| Mean Grain Size (Growth Side) | ~80 | µm | Larger size due to material removal |
| Mean Grain Size (Nucleation Side) | ~20 | µm | Polished side |
| Intergrain Region Thickness (tgg) | less than 2 | nm | Approximately the same for both samples |
| Single Substitutional Nitrogen ([N]) (12C) | ~1.9 | ppm | Determined by UV absorption |
| Estimated Vacancy Concentration (natC) | ~1.8 | ppm | Major non-isotopic point defect scatterer |
| Estimated Dislocation Density (natC) | ~1 x 1010 | cm-2 | Derived from Adisl fit parameter |
| Casimir Length (lc) (12C) | 0.75 | mm | External boundary scattering geometry |
Key Methodologies
Section titled “Key Methodologies”- Diamond Synthesis: High-quality polycrystalline diamond wafers were grown using Microwave Plasma Chemical Vapor Deposition (MPCVD) in an ARDIS-100 reactor on silicon substrates (57 mm diameter).
- Gas Precursors: Growth utilized CH4-H2 gas mixtures. The enriched sample used 12CH4 derived from 12CO, purified via cryogenic rectification to achieve 99.96% 12C content.
- Growth Recipe: Parameters were nominally identical for both natC and 12C samples: 1000 sccm total gas flow, 1.2% CH4 content, 87 Torr pressure, 820 °C substrate temperature, and 4.4 kW microwave power.
- Sample Preparation: Rectangular parallelepipeds were laser-cut for in-plane (κ||) thermal measurement. A heavily defective layer was removed from the nucleation side (50 µm for natC, 130 µm for 12C).
- Thermal Measurement Technique: Steady-state longitudinal heat flow method was employed across 5 K to 410 K. Heat flux (Q) was applied via a heater (H), and the temperature gradient (ΔT) was measured using two Cernox resistive thermometers (T1, T2) separated by a distance ΔL (~8.5 mm).
- Data Calculation: Thermal conductivity was determined by the formula κ = QΔL/ΔT. Measurements were performed in vacuum with a multilayered radiation shield above 95 K to minimize error (total error < 3%).
- Theoretical Modeling: Experimental data were fitted using a phenomenological model based on the full version of Callaway theory, incorporating resistive scattering rates for:
- Three-phonon Normal (N) and Umklapp (U) processes.
- Point defects (isotopes 13C and vacancies).
- Dislocations (static strain field scattering).
- Grain boundaries and intergrain regions (Klemens’ theory).
- Charge carriers bound to dopant centers (holes bound to acceptors).
Commercial Applications
Section titled “Commercial Applications”The achievement of ultra-high thermal conductivity in polycrystalline CVD diamond, surpassing natural single crystals, is critical for high-performance thermal management in several demanding engineering fields:
- High-Power Semiconductor Devices: Essential as heat spreaders for wide band-gap semiconductors like Gallium Nitride (GaN) and Silicon Carbide (SiC) used in 5G/6G infrastructure, radar, and high-frequency power amplifiers.
- High-Energy Optics and Lasers: Used as output windows and heat sinks in high-power laser systems to prevent thermal runaway, thermal lensing, and catastrophic failure.
- Advanced Microelectronics Packaging: Enables effective heat dissipation in dense integrated circuits, 3D stacked chips, and microchannel cooling systems, improving reliability and clock speeds.
- Cryogenic Technology: The maximized isotope effect at low temperatures (e.g., 150 K) makes 12C diamond ideal for specialized cryogenic components and sensors requiring superior thermal transport.
- Thermal Metrology and Standards: High-purity, isotopically enriched diamond serves as a reference material for calibrating thermal measurement equipment due to its predictable and extreme thermal properties.
View Original Abstract
We measured the thermal conductivity κ(T) of polycrystalline diamond with natural (natC) and isotopically enriched (12C content up to 99.96 at.%) compositions over a broad temperature T range, from 5 to 410 K. The high quality polycrystalline diamond wafers were produced by microwave plasma chemical vapor deposition in CH4-H2 mixtures. The thermal conductivity of 12C diamond along the wafer, as precisely determined using a steady-state longitudinal heat flow method, exceeds much that of the natC sample at T>60 K. The enriched sample demonstrates the value of κ(298K)=25.1±0.5 W cm−1 K−1 that is higher than the ever reported conductivity of natural and synthetic single crystalline diamonds with natural isotopic composition. A phenomenological theoretical model based on the full version of Callaway theory of thermal conductivity is developed which provides a good approximation of the experimental data. The role of different resistive scattering processes, including due to minor isotope 13C atoms, defects, and grain boundaries, is estimated from the data analysis. The model predicts about a 37% increase of thermal conductivity for impurity and dislocation free polycrystalline chemical vapor deposition (CVD)-diamond with the 12C-enriched isotopic composition at room temperature.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 1990 - Thermal diffusivity of isotopically enriched 12C diamond [Crossref]
- 1992 - Some aspects of the thermal conductivity of isotopically enriched diamond single crystals [Crossref]
- 1993 - Thermal conductivity of diamond between 170 and 1200 K and the isotope effect [Crossref]
- 1993 - Thermal conductivity of isotopically modified single crystal diamond [Crossref]
- 1991 - Effect of 13C isotopes on the diamond thermal conduction in the approximation of the dominant role of normal phonon-scattering processes
- 1992 - Thermal conductivity of isotopically enriched diamonds [Crossref]
- 1992 - Lattice dynamics and Raman spectra of isotopically mixed diamond [Crossref]
- 1998 - Interpretation of the thermal conductivity of isotopically depleted diamonds [Crossref]
- 2002 - Kinetic coefficients in isotopically disordered crystals [Crossref]
- 2002 - Estimation of the isotope effect on the lattice thermal conductivity of group IV and group III-V semiconductors [Crossref]