Nanoscale heat transport and interface properties in group III nitride semiconductive heterostructures

High-electron-mobility transistors (HEMTs), light emitting diodes (LEDs) and power electronics require efficient thermal management due to the large amount of heat generated during operation. Nitride semiconductors are used in these devices and thermal transport is dominated by phonons. Phonon scattering at the interfaces as well as with grain boundaries and impurities largely impedes thermal transport through the nanoscale structures and can affect their performance and lifetime. To understand the overall heat dissipation rate, both the thermal conductivities of nitride thin films and the interface conductance between different layers in LED architectures were measured over a 100-400K temperature range using the 3-omega method, complemented by a pump-probe laser based technique called frequency domain thermoreflectance (FDTR).;Knowledge of the heat conduction and phonon transport across thin films as well as interfaces within the LED structures was determined by fitting experimental results with basic phonon transport models including the Callaway Model and the Born von Karman Slack Model. I measured and analyzed different nitride layers of the LED one by one, and found that the highest thermal resistance comes from the aluminum nitride (AlN) nucleation layer that is grown directly on non-native SiC or sapphire substrates. After that, a more detailed analysis of the AlN layer was carried out. Transmission electron microscope (TEM) images revealed a high dislocation density within the AlN layer and planar defects near the interface. The effects of film thickness, substrate properties, and surface roughness on thermal transport in AlN were then studied parametrically. This work was executed by 3-omega, however, in order to isolate the interface conductance between certain layers, FDTR was used since it is more sensitive to the near surface interfaces than 3-omega. This approach enables us to isolate both the thin film thermal conductivities and interface conductance. The main thermal resistance was not the film itself but instead the interface between the AlN and the SiC/sapphire substrates. I further found that this resistance increases with surface roughness. Atomic resolution TEM images of the interface confirmed that the density of near-interface planar defects in the AlN films increased with SiC surface roughness, suggesting the origin of the increased thermal resistance in rougher films.;Impurity concentration and alloying can also impact phonon transport. Multiple samples of AlxGal1-xN with x = 0.039, 0.049, 0.055, 0.144 and 0.24 were epitaxially grown on GaN and then polished by Kyma Technologies for our measurements of their thermal conductivies as well as future measurements of their phonon mean free path spectra. Through changing the aluminum concentrations we could analyze how impurity scattering would affect phonons' contribution to thermal conductivity at different mean free paths. This work is still in progress, though I have completed thermal conductivity measurements and the results compare favorably with predictions from the virtual crystal model.