| Group-III nitrides have emerged as one of the most promising wide-bandgap semiconductors for devices used in power and optoelectronic applications. The knowledge of the thermal conductivity in group-III nitrides is important for both managing self-heating in high power applications and for proper operation in high temperature environments. Despite the popularity of the nitrides, the current knowledge of the thermal conductivity is still incomplete due to the complex dependence on the material properties stemming from growth. This work focuses on how properties resulting from growth such as material quality, intentional/unintentional impurity concentration, and alloying elements affect the thermal conductivity in group-III nitrides and oxides.;The thermal conductivity of n-type GaN was measured using the 3u method and compared to calculations from a modified Callaway model for understanding the contribution of the different scattering mechanisms. Samples were grown by hydride phase vapor epitaxy with Si concentrations varying from 1.6x10 16 -- 7x1018 cm-3 for the purpose of modulating the electrical conductivity. At room temperature, the thermal conductivity was found to decrease with increasing Si concentration from the maximum value of 245 W/mK for the undoped sample, which was attributed to increased phonon-impurity and phonon-free electron scattering. In the range of 295-400K, the thermal conductivity was found to decrease with increasing temperature. However, increasing Si concentration was found to decrease this temperature dependence and it was suggested to be due to an increase in the free electron contribution to thermal conductivity.;Semi-insulating material was then grown and the impact of increasing Fe concentration on the thermal conductivity was characterized. The thermal conductivity was found to decrease with increased Fe dopant concentration similar to Si, however the effect was found to be less pronounced due to the lack of a contribution from phonon-free electron scattering. The reduction in thermal conductivity at elevated temperatures was described by a simple power law, which decreased from a power of -0.94 for the undoped sample to -0.55 for the sample with the highest Fe concentration.;Next, the effect of alloying elements on thermal conductivity was studied by measuring AlGaN films on GaN substrates with varying alloy compositions and film thicknesses using the differential 3o method. Small additions of alloying elements were found to cause a pronounced decrease in the thermal conductivity, which was explained by scattering caused by the large mass difference between the Al and Ga atoms. The effect increasing AlGaN film thickness, and therefore reducing the defect density, on thermal conductivity was then studied and was found to logarithmically increase the thermal conductivity up to the predicted bulk value, which was approached at thicknesses of 25mum.;Finally, gallium oxide (beta-Ga2O3) was examined by aiming to determine the thermal conductivity of undoped, n-type (Sn doped), and semi-insulating (Fe doped) bulk crystals. A novel approach was employed to extract the anisotropic thermal conductivity with the 3o slope method. The thermal conductivity was found to be highly anisotropic, with the maximum of 29 W/mK in the [010] direction being nearly double that in other crystalline directions. The addition of Sn and Fe dopants showed almost reduction in thermal conductivity from the undoped sample, however dopants were found on average to reduce the temperature dependence of thermal conductivity. |
