Phonon Bridge Engineering for Enhanced Interfacial Thermal Transport in Si/Diamond Heterostructure

Phonon Bridge Engineering for Enhanced Interfacial Thermal Transport in Si/Diamond Heterostructures

1. Background

When two dissimilar materials such as silicon (Si) and diamond come into contact, a significant phonon spectral mismatch occurs. Because phonons—the quanta of heat—cannot efficiently transmit across mismatched spectra, a large portion of phonons are reflected or scattered at the interface, resulting in interfacial thermal resistance (ITR).
In high-power electronic devices, such as GaN-on-diamond systems, interfacial thermal resistance can contribute to over 40% of total temperature rise, making it a key bottleneck in thermal management.

To overcome this limitation,  a “phonon bridge” strategy, where an intermediate layer possessing phonon frequencies between those of the two materials is inserted at the interface. This layer facilitates spectral overlap and energy transfer, enabling otherwise non-transmissive phonons to participate in heat conduction.

2.Building a Phonon Bridge

A three-layer Si/SiC/Diamond model was designed, using first-principles calculations and Monte Carlo simulations to investigate interfacial heat transport mechanisms.
Silicon carbide (SiC) was identified as an optimal bridge material because its phonon frequency distribution lies between Si and diamond, allowing it to couple effectively with both. This overlapping spectrum builds a continuous phonon transmission channel, fundamentally improving interfacial heat flow.

3. Key Findings

(1) Thickness Dependence

When a 40 nm SiC interlayer is introduced, the interfacial thermal conductance (ITC) increases by ~46.6%, reaching 202.3 W·m⁻²·K⁻¹.

• Too thin (<5 nm): enhanced phonon scattering reduces ITC.

• Too thick (>100 nm): SiC bulk resistance offsets the improvement.
  Thus, an optimal thickness (~40 nm) balances phonon-bridge effects and intrinsic layer resistance, demonstrating strong size-dependent thermal behavior.

(2) Spectral and Mechanistic Analysis

Without a bridge, Si/Diamond interfaces show a large temperature jump—interfacial resistance accounts for ~⅔ of total resistance.
Spectral analysis reveals limited overlap below 15 THz, with optical phonons (9–15 THz) as the main contributors to cross-interface transport.
The SiC bridge broadens the effective phonon conduction range to 3–13 THz, filling the spectral gap and significantly enhancing phonon transmission.

(3) Comparative Analysis of Intermediate Layers

Thirteen candidate materials (SiC, AlN, α-Si₃N₄, β-Si₃N₄, AlₓGa₁₋ₓN, etc.) were evaluated:

• SiC: best performer, +46.6% ITC improvement.

• AlN: moderate improvement (~21.9%).

• Al₀.₁Ga₀.₉N: good phonon spectrum compatibility.
  Enhancement depends on three coupled factors:

1. Phonon spectral matching,

2. Bridge thickness (size effect), and

3. Intrinsic thermal conductivity of the interlayer.

4. Conclusions

1. Phonon Bridge Mechanism: Introducing a properly selected interlayer (e.g., SiC) effectively bridges phonon spectra and reduces interfacial thermal resistance.

2. Optimal Thickness: A 40 nm SiC interlayer provides the best balance between spectral coupling and intrinsic resistance.

3. Design Criteria: Enhanced ITC requires simultaneous optimization of spectral overlap, size-dependent scattering, and intrinsic conductivity.