3 - Heat transport in polycrystalline diamond from the meso to the nano scale

Abstract

One of the most promising approaches for thermal management of gallium nitride (GaN) electronics is the integration of diamond as a heat spreader, which is grown directly via chemical vapor deposition (CVD). CVD diamond has a complex grain structure with a high density of grain boundaries in the “near-junction” region. Understanding how the polycrystalline microstructure impacts thermal transport is crucial for enabling high-performance GaN-on-diamond devices. In this chapter, we present a physics-based approach that examines heat transport in polycrystalline diamond from the meso to the nano scale. First, at the device level on the μm length scale, modulated optical measurements reveal a distinct anisotropy and thickness-dependent inhomogeneity in the thermal conductivity. A geometric model is derived, which captures the essential physics of grain competition, and the resulting evolution in phonon conduction. Next, zooming in to the level of a single grain on the nm length scale, we address a fundamental question that is at the heart of this research: how do phonon scattering events near a grain boundary modulate thermal transport locally? We discuss the development of a novel correlative imaging technique, which combines spatial maps of thermal conductivity with those of the microstructure, using ultrafast time-domain thermoreflectance and electron backscatter diffraction, respectively. Diffuse phonon scattering at disorder-rich grain boundaries leads to a strong local suppression in heat flux, an effect that is directly visualized in the experiments. This approach paves the way for a new line of inquiry into the fundamental mechanisms underlying phonon–defect interactions, which can be applied broadly to other crystalline materials. Taken together, the experimental and theoretical concepts described here will serve as a starting point for identifying the fundamental limits of (thermal) performance in GaN-on-diamond high electron mobility transistors.