Thermal conduction in gallium nitride composite substrates [electronic resource]
Keywords: Mechanical Engineering.

The thermal management challenge posed by gallium nitride (GaN) high-electron-mobility transistor (HEMT) technology has received much attention in the past decade. The peak amplification power density of these devices is limited by heat transfer at the device, substrate, package, and system levels. Thermal resistances within micrometers of the transistor junction can limit efficient heat spreading from active device regions into the substrate and can dominate the overall temperature rise. Gallium nitride composite substrates, which consist of AlGaN/GaN heterostructures with thickness of a few microns on a thicker non-GaN substrate, govern the thermal resistance associated with the "near-junction" region. Silicon and silicon carbide have been widely used as a substrate material, but the performance of GaN devices grown on these substrates is still severely limited by thermal constraints and associated reliability issues. The importance of effective junction-level heat conduction has motivated the development of composite substrates containing high-thermal-conductivity diamond, but these composites require careful attention to the thermal interface resistance between the GaN and diamond. The present doctoral research investigates thermal conduction in GaN composite substrates. Special attention is paid to the thermal conductivity of the GaN buffer layer and the thermal interface resistance between the GaN buffer layer and different types of substrates (Si, SiC, and diamond). In the first part of this work, time-domain thermoreflectance is applied to multiple GaN thicknesses to simultaneously determine the GaN thermal conductivity and the GaN-substrate thermal interface resistance for both GaN-Si and GaN-SiC composite substrates. The GaN-Si and GaN-SiC interface resistances measured in this work (7--10 and 4--6 m^2 K GW^--1, respectively) are much lower than those reported previously by micro-Raman thermometry, and the temperature dependence is considerably weaker than that of the past Raman data. Phonon transport arguments based on the Boltzmann transport equation suggest that point defects within the AlN transition layer and near-interface defects around the AlN interfaces are responsible for the temperature trend of the interface resistance. This research extends time-domain thermoreflectance methodologies to GaN-diamond composite substrates. A systematic approach—using a careful sample design and a combination of time-domain thermoreflectance, nanosecond transient thermoreflectance, and electrical joule heating and thermometry—is employed to determine the GaN-diamond thermal interface resistance for three generations of GaN-diamond composite substrates. A substantial reduction in the GaN-diamond thermal interface resistance is demonstrated from the first generation (~110 m^2 K GW^--1) to the latest generation (29 m^2 K GW^--1) by etching the resistive transition layer and achieving a direct contact between the GaN and diamond. Finally, this work examines thermal conduction normal to polycrystalline silicon films on diamond, a particularly important material system for heat sink applications in high power electronic devices, including GaN HEMTs. Time-domain thermoreflectance and phonon transport theory are used to extract the cross-plane thermal conductivity of the polysilicon films, as well as the thermal boundary resistance between the polysilicon and diamond. Nonuniform thermal conductivity values are observed between different thickness samples (11--15 and 24--25 W m^--1 K^--1 for the 79 nm and 630 nm films, respectively) owing to the impact of spatially varying defect concentrations along the direction normal to the film.