UCLA

Heat management problem limits the potential of further development of high-power devices. Diamond, with the highest thermal conductivity among all the materials, can be integrated near the hot spot region as a heat management component. The goal of this dissertation is to study grain morphology and thermal properties in polycrystalline diamond across boundaries using advanced characterization techniques. This work is anticipated to lead to an understanding of how to control and improve thermal transport in diamond. Diamond in device integration is achieved by chemical vapor deposition (CVD) at 750˚C with methane as the carbon source. A hydrogen plasma is used in the CVD process to remove any non-diamond carbon. In this research, transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used for grain morphology analysis. Both of the techniques can be used to separate the individual grains, which leads to an accurate size determination. In-plane thermal conductivity was measured with Raman thermography. Cross-plane thermal properties, including thermal boundary resistance (TBR), were measured with Time-domain thermoreflectance (TDTR). The combination of the advanced techniques has been done in several aspects to improve the diamond integration for better thermal transport.

The challenging issue for polycrystalline CVD diamond is the relatively lower thermal conductivity compared with single crystal diamond due to phonon scattering at grain boundaries, and the thermal conductivity continues to decrease closer to the nucleation region. At the interface, where the grain starts to nucleate and grow, the diamond structure is composed of small size, randomly oriented diamond grains. In this research, a larger diamond seed size is shown to increase the average diamond grain size near the nucleation region, which leads to a higher in-plane thermal transport for 1 μm diamond films. Also, with the combination of EBSD and spatially resolved TDTR, ~ 60% of thermal conductivity reduction is directly observed at the grain boundaries. While in the traditional model, the grain boundaries act as discrete thermal boundary resistances, this research has pointed out the impact is not restricted to the grain boundaries and can be observed in up to ~ 10 μm nearby region. Another challenge exists when integrating diamond on GaN-based high-power devices. The presence of hydrogen plasma at a high temperature can etch into the GaN substrate and cause a rough Diamond/GaN interface, which could increase the TBR value. In this research, an ultra-thin SiN is shown to be an effective protection layer, and a low TBR value of 9.5 m2K/GW is achieved.