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Cheng, Zhe
Graham, Samuel
Cola, Baratunde A.
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The development of ultrawide and wide bandgap semiconductors enables a variety of applications in power and RF electronics, including energy infrastructure, wireless communication, self-driving cars, and radar systems for defense. With the increasing power and frequency of these applications, Joule-heating induced hot-spots in the device channel degrade the device performance and reliability. Thermal management of these devices plays a very important role in achieving stable device operation and long lifetime, and correspondingly improving energy efficiency and reducing cost. The basic component, GaN HEMTs, is usually integrated with high thermal conductivity substrates such as SiC and CVD diamond to extract the generated heat. For instance, GaN is grown on SiC with an AlN transition layer. CVD diamond is grown on GaN with an interfacial dielectric layer. The AlN layer and the low quality of the GaN layer near the interface induces additional thermal resistance. The nanocrystalline nature of the CVD diamond near the GaN-diamond interface results in significantly reduced thermal conductivity and large thermal stress due to the high growth temperature and large thermal expansion coefficient mismatch. Additionally, great attention has been focused on β-Ga2O3 recently due to the potential of affordable large-area wafers for homo-epitaxial growth, large breakdown voltages, and its ultrawide bandgap. However, its thermal conductivity is more than one order of magnitude lower than the other wide bandgap semiconductors. A disproportionally small amount of work has been done to address the thermal issues compared to analogous demonstrations of related devices. The understanding of heat transport mechanisms in nanostructures and interfaces, solution to address the thermal management challenges are in demand. The grand challenge of thermal management of power and RF electronics lies in placing the hot-spot area of GaN/AlGaN and Ga2O3 devices close to heat sinks or heat spreaders with small thermal resistance and low stress. Thermal boundary resistance accounts for a large or even dominant part of the total thermal resistance in these devices. This thesis studied the TBC of five technologically important interfaces: GaN-SiC, GaN-diamond, diamond-Si, (Al0.1Ga0.9)2O3-Ga2O3, Ga2O3-diamond. (1) Instead of including a defective AlN transition layer between GaN and SiC in direct growth method, a room-temperature surface-activated bonding technique is used to integrate GaN with SiC which brings high-quality GaN directly to the GaN-SiC interface. The measured GaN thermal conductivity is higher than the MBE-grown GaN on SiC substrates. Moreover, a very high GaN-SiC TBC is observed for the bonded GaN-SiC interface, especially for the annealed interface whose TBC (~230 MWm-2K-1) is close to the highest reported value of GaN-SiC interfaces in the literature. (2) Unlike the growth of CVD diamond on GaN which has a nucleation layer with low thermal conductivity, GaN is heterogeneously integrated with single crystalline diamond substrates with two modified room-temperature surface-activated bonding techniques for thermal management of GaN-on-diamond applications. The measured TBC of the bonded GaN-diamond interfaces is among the highest values reported in the literatures and is affected by the thickness of the interfacial bonding layer. Device modeling shows a relatively large GaN-diamond TBC value (>50 MW/m2-K) achieved in this work could enable device designers to take full advantage of the high thermal conductivity of single crystalline diamond. (3) To improve the low TBC of diamond related interfaces because of the large phonon density of states mismatch of diamond and other semiconductors, the TBC at semiconductor-dielectric interfaces isincreased by nanoscale graphoepitaxy. By growing CVD diamond on nanopatterned silicon wafers, a general strategy is provided to significantly reduce the thermal resistance of both a diamond layer and diamond-substrate interface simultaneously. The diamond-silicon TBC could increase by 65% comparing with that of a flat diamond-silicon interface. (4) To understand the phonon transport mechanisms across β-(Al0.1Ga0.9)2O3-Ga2O3 interfaces, temperature-dependent measurement on thermal conductivity of β-(Al0.1Ga0.9)2O3/Ga2O3 superlattices is reported from 80 K to 480 K. Significantly reduced thermal conductivity is observed (5.7 times reduction) at room temperature comparing with bulk Ga2O3, which highlights the importance of thermal management of related devices. The estimated minimum TBC of β-(Al0.1Ga0.9)2O3/Ga2O3 interfaces is found to be larger than the Ga2O3 maximum TBC, which shows that some phonons could transmit through several interfaces before scattering with other phonons or structural imperfections, as possible evidence of phonon coherence. (5) To develop cooling strategies of Ga2O3-related devices, Ga2O3 is integrated with single crystal diamond with exfoliation-transferring and ALD-growth. The Van der Waals Ga2O3-diamond TBC was measured to be 17 -1.7/+2.0 MW/m2-K while the TBC calculated with a Landauer approach and DMM is 312 MW/m2-K, which sheds light on the possible TBC which can be achieved. The measured TBC of the grown ultra-clean interface is 179 MW/m2-K, about 10 times higher than TBC of a Van der Waals bonded Ga2O3-diamond interface, suggesting that covalent bonding facilitates interfacial heat transport better than Van der Waals interfacial bonding. Integration of Ga2O3 and single crystal diamond could be a solution to cool Ga2O3-related devices.
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