[Paper Review] Small-mass atomic defects enhance vibrational thermal transport at disordered interfaces with ultrahigh thermal boundary conductance

Ashutosh Giri, Sean W. King+12 more|arXiv (Cornell University)|Oct 25, 2017

Thermal properties of materials3 citations

TL;DR

This study demonstrates that small-mass atomic defects—specifically nitrogen dopants—at disordered amorphous SiOC:H/SiC:H interfaces enhance vibrational thermal transport, achieving a thermal boundary conductance (TBC) approaching 1 GW m⁻² K⁻¹, the highest ever measured for a diffusive interface. The enhancement arises from interfacial vibrational modes unique to the defect-rich interface, which increase energy transfer efficiency beyond conventional phonon transport theories.

ABSTRACT

The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer, decreasing the thermal boundary conductance (TBC).$^1$ However, recent computational works have suggested that interfacial defects can enhance this thermal boundary conductance through emergence of unique vibrations that are intrinsic to the material interface and defect atoms,$^{2-6}$ a finding that contradicts traditional theory and conventional understanding. By manipulating the local heat flux of atomic vibrations that comprise these interfacial modes, in principle, the TBC can be increased. In this work, we provide evidence that interfacial defects can enhance the TBC across interfaces through the emergence of unique high frequency vibrational modes that arise from atomic mass defects at the interface with relatively small masses. We demonstrate ultrahigh TBC at amorphous SiOC:H/SiC:H interfaces, approaching 1 GW m$^{-2}$ K$^{-1}$, that is further increased through the introduction of nitrogen defects. The fact that disordered interfaces can exhibit such high conductances, which can be further increased with additional defects offers a unique direction in controlling interfacial thermal transport that becomes important in manipulating heat transfer across materials with high densities of interfaces.

Motivation & Objective

• To investigate the role of interfacial defects in enhancing thermal boundary conductance (TBC) at disordered interfaces, challenging the conventional view that defects impede heat transfer.
• To experimentally validate the theoretical prediction that unique interfacial vibrational modes arising from atomic mass defects can significantly increase TBC.
• To demonstrate that ultrahigh TBC values—approaching 1 GW m⁻² K⁻¹—can be achieved in amorphous multilayers (AMLs) of a-SiC:H/a-SiOC:H through controlled nitrogen doping.
• To establish a materials engineering strategy for tuning interfacial thermal transport by manipulating defect chemistry at nanoscale interfaces.
• To resolve the long-standing discrepancy between theoretical predictions of enhanced TBC via interfacial modes and the lack of experimental evidence.

Proposed method

• Fabricated amorphous multilayer (AML) structures with alternating layers of hydrogenated amorphous silicon carbide (a-SiC:H, k ≈ 6.5 W m⁻¹ K⁻¹) and hydrogenated amorphous silicon oxycarbide (a-SiOC:H, k ≈ 3.2 W m⁻¹ K⁻¹) via plasma-enhanced chemical vapor deposition (PECVD).
• Used time-domain thermoreflectance (TDTR) to measure thermal conductivity and extract the thermal boundary conductance (TBC) across the a-SiC:H/a-SiOC:H interfaces.
• Performed in situ plasma nitrogen doping to introduce light-mass nitrogen defects at and near the interfaces, followed by re-measurement of thermal transport properties.
• Employed X-ray reflectivity (XRR) and X-ray scanning electron microscopy (XSEM) to confirm periodic layer structure and thicknesses across the multilayer stacks.
• Conducted transmission Fourier-transform infrared (FTIR) spectroscopy to analyze chemical bonding and defect incorporation, with substrate interference effects removed via rigorous optical modeling.
• Combined molecular dynamics (MD) simulations and vibrational spectroscopy to identify and validate the emergence of high-frequency interfacial modes due to mass-defect engineering.

Experimental results

Research questions

• RQ1 Can atomic mass defects at disordered interfaces enhance thermal boundary conductance beyond the limits predicted by conventional phonon transport theories?
• RQ2 What is the role of light-mass dopants (e.g., nitrogen) in modifying interfacial vibrational modes and promoting energy transfer across amorphous heterointerfaces?
• RQ3 Does the presence of interfacial defects lead to a measurable increase in thermal boundary conductance in amorphous multilayers, even when interface density is varied?
• RQ4 Can ultrahigh thermal boundary conductance (approaching 1 GW m⁻² K⁻¹) be experimentally achieved and stabilized in disordered, amorphous systems through defect engineering?
• RQ5 How do interfacial vibrational modes unique to defect-rich interfaces contribute to the breakdown of the traditional assumption that disorder reduces thermal conductance?

Key findings

• The thermal boundary conductance (TBC) at amorphous a-SiC:H/a-SiOC:H interfaces reaches approximately 1 GW m⁻² K⁻¹, the highest value ever measured for a diffusive interface.
• Incorporation of nitrogen defects via in situ plasma treatment eliminates interfacial resistance, rendering the thermal conductivity of the multilayer independent of interface density.
• Vibrational spectroscopy and molecular dynamics simulations confirm the emergence of high-frequency interfacial modes due to light-mass nitrogen defects, which enhance energy transfer across the interface.
• The thermal conductivities of the individual a-SiC:H and a-SiOC:H layers were measured as 1.48 ± 0.12 W m⁻¹ K⁻¹ and 0.75 ± 0.06 W m⁻¹ K⁻¹, respectively, with volumetric heat capacities of 1.9 ± 0.3 J cm⁻³ K⁻¹ and 1.3 ± 0.2 J cm⁻³ K⁻¹.
• The observed TBC enhancement cannot be explained by classical phonon transport models, indicating that defect-induced interfacial modes play a dominant role in thermal transport at disordered interfaces.