Unrevealing The Unexpected Large Thermal Boundary Resistance induced by Inelastic Heat Carrier Trapping at GaN-Diamond Interface

The rapid advancements in the wide band gap semiconductors like gallium nitride (GaN), silicon carbide (SiC), and gallium oxide (Ga₂O₃), have revolutionized various fields, including electric vehicles, mobile devices, and next-generation telecommunications. With the increase in power and the reduction in device size, the elevating heat density via self-heating makes thermal management a significant issue. One of the most promising solutions is integrating the devices with high thermal conductivity diamond substrates. However, this will introduce additional thermal boundary resistance (TBR) at the semiconductor/diamond interface and impedes heat dissipation.<br/>In such kinds of interfaces, the substantial differences in Debye temperature between diamond and semiconductors like SiC, GaN, or Si would lead to a significant mismatch in phonon modes at the interface, where the traditional acoustic mismatch model (AMM) and diffuse mismatch model (DMM) are no longer applicable as the inelastic phonon scattering is non-negligible. Moreover, this interface is typically composed of amorphous layers when using room temperature bonding techniques, which is essential to avoid thermal damage and thermal-induced strain in the devices. The amorphous layers not only bring large TBR but also increase the complexity of the behavior of heat carriers at the interface because the heat conduction in amorphous materials differs from crystalline materials and involves different heat carriers: the delocalized heat carriers (propagons and diffusons) and localized heat carriers (locons). The mechanism of heat conduction at such an interface is still to be elucidated, posing difficulty in interfacial structure design for reducing TBR for the semiconductor-on-diamond systems.<br/>This study investigated the thermal boundary resistance (TBR) in the GaN/diamond system, a representative case of semiconductor-on-diamond structures. We employed an innovative room-temperature surface-activated bonding (SAB) technique to control the thickness and structure of the interfacial layer precisely. As a result, a multi-layer heterogeneous interfacial layer was generated, bonding the GaN to the diamond substrate. This interface includes a SiO_x-enriched region sandwiched between two diffusive regions as characterized by transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX) analysis. We then used time-domain thermoreflectance (TDTR) measurement to investigate the TBR. The increase in the total thickness of the interfacial layer leads to a nonlinear variation in TBR. More surprisingly, the increase in the thickness of the interfacial layer provides counterintuitively huge thermal resistance, far exceeding its diffusive limit. To unravel this phenomenon, we performed molecular dynamics (MD) simulations to analyze the density of state (DOS) of heat carriers and their spectral heat conduction at the interfacial, which shows the existence of the local low-frequency states in the SiO_x-enriched region and the resultant conversion of phonon frequencies through an inelastic process in this region. This discovery challenges the traditional belief that interfacial inelastic scattering consistently improves heat conduction by opening up new inelastic pathways, while the pathways were limited in our specific case instead. This phenomenon can be further intensified by increasing the thickness of either the SiO_x-enriched region or the two diffusive regions, ultimately promoting the TBR of the GaN/diamond interface. Leveraging these innovative insights into the physics of interfacial heat conduction, a record low TBR of 8.3 m²-K/GW among direct bonded GaN/diamond interface was achieved. This breakthrough has significant implications for efficient interfacial heat dissipation